GCC(1) GNU GCC(1)
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-pedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...
Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options as gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall options" allow you to
stop this process at an intermediate stage. For example, the -c option says not to run the linker. Then the output consists
of object files output by the assembler.
Other options are passed on to one stage of processing. Some options control the preprocessor and others the compiler
itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to
use any of them.
Most of the command-line options that you can use with GCC are useful for C programs; when an option is only useful with
another language (usually C++), the explanation says so explicitly. If the description for a particular option does not
mention a source language, you can use that option with all supported languages.
The gcc program accepts options and file names as operands. Many options have multi-letter names; therefore multiple single-
letter options may not be grouped: -dv is very different from -d -v.
You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter when you
use several options of the same kind; for example, if you specify -L more than once, the directories are searched in the
order specified. Also, the placement of the -l option is significant.
Many options have long names starting with -f or with -W---for example, -fmove-loop-invariants, -Wformat and so on. Most of
these have both positive and negative forms; the negative form of -ffoo would be -fno-foo. This manual documents only one of
these two forms, whichever one is not the default.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations are in the following sections.
Overall Options
-c -S -E -o file -no-canonical-prefixes -pipe -pass-exit-codes -x language -v -### --help[=class[,...]]
--target-help --version -wrapper @file -fplugin=file -fplugin-arg-name=arg -fdump-ada-spec[-slim] -fdump-go-spec=file
C Language Options
-ansi -std=standard -fgnu89-inline -aux-info filename -fallow-parameterless-variadic-functions -fno-asm -fno-builtin
-fno-builtin-function -fhosted -ffreestanding -fopenmp -fms-extensions -fplan9-extensions -trigraphs -no-integrated-cpp
-traditional -traditional-cpp -fallow-single-precision -fcond-mismatch -flax-vector-conversions -fsigned-bitfields
-fsigned-char -funsigned-bitfields -funsigned-char
C++ Language Options
-fabi-version=n -fno-access-control -fcheck-new -fconserve-space -fconstexpr-depth=n -ffriend-injection
-fno-elide-constructors -fno-enforce-eh-specs -ffor-scope -fno-for-scope -fno-gnu-keywords -fno-implicit-templates
-fno-implicit-inline-templates -fno-implement-inlines -fms-extensions -fno-nonansi-builtins -fnothrow-opt
-fno-operator-names -fno-optional-diags -fpermissive -fno-pretty-templates -frepo -fno-rtti -fstats
-ftemplate-depth=n -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ -fno-default-inline
-fvisibility-inlines-hidden -fvisibility-ms-compat -Wabi -Wconversion-null -Wctor-dtor-privacy
-Wdelete-non-virtual-dtor -Wnarrowing -Wnoexcept -Wnon-virtual-dtor -Wreorder -Weffc++ -Wstrict-null-sentinel
-Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions -Wsign-promo
Objective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime -fno-nil-receivers -fobjc-abi-version=n
-fobjc-call-cxx-cdtors -fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -fobjc-nilcheck -fobjc-std=objc1
-freplace-objc-classes -fzero-link -gen-decls -Wassign-intercept -Wno-protocol -Wselector -Wstrict-selector-match
-Wundeclared-selector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-line] -fno-diagnostics-show-option
Warning Options
-fsyntax-only -fmax-errors=n -pedantic -pedantic-errors -w -Wextra -Wall -Waddress -Waggregate-return
-Warray-bounds -Wno-attributes -Wno-builtin-macro-redefined -Wc++-compat -Wc++11-compat -Wcast-align -Wcast-qual
-Wchar-subscripts -Wclobbered -Wcomment -Wconversion -Wcoverage-mismatch -Wno-cpp -Wno-deprecated
-Wno-deprecated-declarations -Wdisabled-optimization -Wno-div-by-zero -Wdouble-promotion -Wempty-body -Wenum-compare
-Wno-endif-labels -Werror -Werror=* -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 -Wno-format-contains-nul
-Wno-format-extra-args -Wformat-nonliteral -Wformat-security -Wformat-y2k -Wframe-larger-than=len
-Wno-free-nonheap-object -Wjump-misses-init -Wignored-qualifiers -Wimplicit -Wimplicit-function-declaration
-Wimplicit-int -Winit-self -Winline -Wmaybe-uninitialized -Wno-int-to-pointer-cast -Wno-invalid-offsetof -Winvalid-pch
-Wlarger-than=len -Wunsafe-loop-optimizations -Wlogical-op -Wlong-long -Wmain -Wmaybe-uninitialized -Wmissing-braces
-Wmissing-field-initializers -Wmissing-format-attribute -Wmissing-include-dirs -Wno-mudflap -Wno-multichar -Wnonnull
-Wno-overflow -Woverlength-strings -Wpacked -Wpacked-bitfield-compat -Wpadded -Wparentheses -Wpedantic-ms-format
-Wno-pedantic-ms-format -Wpointer-arith -Wno-pointer-to-int-cast -Wredundant-decls -Wreturn-type -Wsequence-point
-Wshadow -Wsign-compare -Wsign-conversion -Wstack-protector -Wstack-usage=len -Wstrict-aliasing -Wstrict-aliasing=n
-Wstrict-overflow -Wstrict-overflow=n -Wsuggest-attribute=[pure|const|noreturn] -Wswitch -Wswitch-default -Wswitch-enum
-Wsync-nand -Wsystem-headers -Wtrampolines -Wtrigraphs -Wtype-limits -Wundef -Wuninitialized -Wunknown-pragmas
-Wno-pragmas -Wunsuffixed-float-constants -Wunused -Wunused-function -Wunused-label -Wunused-local-typedefs
-Wunused-parameter -Wno-unused-result -Wunused-value -Wunused-variable -Wunused-but-set-parameter
-Wunused-but-set-variable -Wvariadic-macros -Wvector-operation-performance -Wvla -Wvolatile-register-var -Wwrite-strings
-Wzero-as-null-pointer-constant
C and Objective-C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-parameter-type -Wmissing-prototypes -Wnested-externs
-Wold-style-declaration -Wold-style-definition -Wstrict-prototypes -Wtraditional -Wtraditional-conversion
-Wdeclaration-after-statement -Wpointer-sign
Debugging Options
-dletters -dumpspecs -dumpmachine -dumpversion -fdbg-cnt-list -fdbg-cnt=counter-value-list -fdisable-ipa-pass_name
-fdisable-rtl-pass_name -fdisable-rtl-pass-name=range-list -fdisable-tree-pass_name -fdisable-tree-pass-name=range-list
-fdump-noaddr -fdump-unnumbered -fdump-unnumbered-links -fdump-translation-unit[-n] -fdump-class-hierarchy[-n]
-fdump-ipa-all -fdump-ipa-cgraph -fdump-ipa-inline -fdump-passes -fdump-statistics -fdump-tree-all
-fdump-tree-original[-n] -fdump-tree-optimized[-n] -fdump-tree-cfg -fdump-tree-vcg -fdump-tree-alias -fdump-tree-ch
-fdump-tree-ssa[-n] -fdump-tree-pre[-n] -fdump-tree-ccp[-n] -fdump-tree-dce[-n] -fdump-tree-gimple[-raw]
-fdump-tree-mudflap[-n] -fdump-tree-dom[-n] -fdump-tree-dse[-n] -fdump-tree-phiprop[-n] -fdump-tree-phiopt[-n]
-fdump-tree-forwprop[-n] -fdump-tree-copyrename[-n] -fdump-tree-nrv -fdump-tree-vect -fdump-tree-sink -fdump-tree-sra[-n]
-fdump-tree-forwprop[-n] -fdump-tree-fre[-n] -fdump-tree-vrp[-n] -ftree-vectorizer-verbose=n -fdump-tree-storeccp[-n]
-fdump-final-insns=file -fcompare-debug[=opts] -fcompare-debug-second -feliminate-dwarf2-dups
-feliminate-unused-debug-types -feliminate-unused-debug-symbols -femit-class-debug-always -fenable-kind-pass
-fenable-kind-pass=range-list -fdebug-types-section -fmem-report -fpre-ipa-mem-report -fpost-ipa-mem-report
-fprofile-arcs -frandom-seed=string -fsched-verbose=n -fsel-sched-verbose -fsel-sched-dump-cfg
-fsel-sched-pipelining-verbose -fstack-usage -ftest-coverage -ftime-report -fvar-tracking -fvar-tracking-assignments
-fvar-tracking-assignments-toggle -g -glevel -gtoggle -gcoff -gdwarf-version -ggdb -grecord-gcc-switches
-gno-record-gcc-switches -gstabs -gstabs+ -gstrict-dwarf -gno-strict-dwarf -gvms -gxcoff -gxcoff+
-fno-merge-debug-strings -fno-dwarf2-cfi-asm -fdebug-prefix-map=old=new -femit-struct-debug-baseonly
-femit-struct-debug-reduced -femit-struct-debug-detailed[=spec-list] -p -pg -print-file-name=library
-print-libgcc-file-name -print-multi-directory -print-multi-lib -print-multi-os-directory -print-prog-name=program
-print-search-dirs -Q -print-sysroot -print-sysroot-headers-suffix -save-temps -save-temps=cwd -save-temps=obj
-time[=file]
Optimization Options
-falign-functions[=n] -falign-jumps[=n] -falign-labels[=n] -falign-loops[=n] -fassociative-math -fauto-inc-dec
-fbranch-probabilities -fbranch-target-load-optimize -fbranch-target-load-optimize2 -fbtr-bb-exclusive -fcaller-saves
-fcheck-data-deps -fcombine-stack-adjustments -fconserve-stack -fcompare-elim -fcprop-registers -fcrossjumping
-fcse-follow-jumps -fcse-skip-blocks -fcx-fortran-rules -fcx-limited-range -fdata-sections -fdce -fdelayed-branch
-fdelete-null-pointer-checks -fdevirtualize -fdse -fearly-inlining -fipa-sra -fexpensive-optimizations -ffat-lto-objects
-ffast-math -ffinite-math-only -ffloat-store -fexcess-precision=style -fforward-propagate -ffp-contract=style
-ffunction-sections -fgcse -fgcse-after-reload -fgcse-las -fgcse-lm -fgraphite-identity -fgcse-sm -fif-conversion
-fif-conversion2 -findirect-inlining -finline-functions -finline-functions-called-once -finline-limit=n
-finline-small-functions -fipa-cp -fipa-cp-clone -fipa-matrix-reorg -fipa-pta -fipa-profile -fipa-pure-const
-fipa-reference -fira-algorithm=algorithm -fira-region=region -fira-loop-pressure -fno-ira-share-save-slots
-fno-ira-share-spill-slots -fira-verbose=n -fivopts -fkeep-inline-functions -fkeep-static-consts -floop-block
-floop-flatten -floop-interchange -floop-strip-mine -floop-parallelize-all -flto -flto-compression-level
-flto-partition=alg -flto-report -fmerge-all-constants -fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves
-fmove-loop-invariants fmudflap -fmudflapir -fmudflapth -fno-branch-count-reg -fno-default-inline -fno-defer-pop
-fno-function-cse -fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole -fno-peephole2
-fno-sched-interblock -fno-sched-spec -fno-signed-zeros -fno-toplevel-reorder -fno-trapping-math
-fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls -fpartial-inlining
-fpeel-loops -fpredictive-commoning -fprefetch-loop-arrays -fprofile-correction -fprofile-dir=path -fprofile-generate
-fprofile-generate=path -fprofile-use -fprofile-use=path -fprofile-values -freciprocal-math -free -fregmove
-frename-registers -freorder-blocks -freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop
-freschedule-modulo-scheduled-loops -frounding-math -fsched2-use-superblocks -fsched-pressure -fsched-spec-load
-fsched-spec-load-dangerous -fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n] -fsched-group-heuristic
-fsched-critical-path-heuristic -fsched-spec-insn-heuristic -fsched-rank-heuristic -fsched-last-insn-heuristic
-fsched-dep-count-heuristic -fschedule-insns -fschedule-insns2 -fsection-anchors -fselective-scheduling
-fselective-scheduling2 -fsel-sched-pipelining -fsel-sched-pipelining-outer-loops -fshrink-wrap -fsignaling-nans
-fsingle-precision-constant -fsplit-ivs-in-unroller -fsplit-wide-types -fstack-protector -fstack-protector-all
-fstrict-aliasing -fstrict-overflow -fthread-jumps -ftracer -ftree-bit-ccp -ftree-builtin-call-dce -ftree-ccp -ftree-ch
-ftree-copy-prop -ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre
-ftree-loop-if-convert -ftree-loop-if-convert-stores -ftree-loop-im -ftree-phiprop -ftree-loop-distribution
-ftree-loop-distribute-patterns -ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize -ftree-parallelize-loops=n
-ftree-pre -ftree-partial-pre -ftree-pta -ftree-reassoc -ftree-sink -ftree-sra -ftree-switch-conversion -ftree-tail-merge
-ftree-ter -ftree-vect-loop-version -ftree-vectorize -ftree-vrp -funit-at-a-time -funroll-all-loops -funroll-loops
-funsafe-loop-optimizations -funsafe-math-optimizations -funswitch-loops -fvariable-expansion-in-unroller
-fvect-cost-model -fvpt -fweb -fwhole-program -fwpa -fuse-linker-plugin --param name=value -O -O0 -O1 -O2 -O3 -Os
-Ofast
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -dD -dI -dM -dN -Dmacro[=defn] -E -H -idirafter dir -include file
-imacros file -iprefix file -iwithprefix dir -iwithprefixbefore dir -isystem dir -imultilib dir -isysroot dir -M -MM
-MF -MG -MP -MQ -MT -nostdinc -P -fdebug-cpp -ftrack-macro-expansion -fworking-directory -remap -trigraphs -undef
-Umacro -Wp,option -Xpreprocessor option
Assembler Option
-Wa,option -Xassembler option
Linker Options
object-file-name -llibrary -nostartfiles -nodefaultlibs -nostdlib -pie -rdynamic -s -static -static-libgcc
-static-libstdc++ -shared -shared-libgcc -symbolic -T script -Wl,option -Xlinker option -u symbol
Directory Options
-Bprefix -Idir -iplugindir=dir -iquotedir -Ldir -specs=file -I- --sysroot=dir
Machine Dependent Options
Adapteva Epiphany Options -mhalf-reg-file -mprefer-short-insn-regs -mbranch-cost=num -mcmove -mnops=num -msoft-cmpsf
-msplit-lohi -mpost-inc -mpost-modify -mstack-offset=num -mround-nearest -mlong-calls -mshort-calls -msmall16
-mfp-mode=mode -mvect-double -max-vect-align=num -msplit-vecmove-early -m1reg-reg
ARM Options -mapcs-frame -mno-apcs-frame -mabi=name -mapcs-stack-check -mno-apcs-stack-check -mapcs-float
-mno-apcs-float -mapcs-reentrant -mno-apcs-reentrant -msched-prolog -mno-sched-prolog -mlittle-endian -mbig-endian
-mwords-little-endian -mfloat-abi=name -mfpe -mfp16-format=name -mthumb-interwork -mno-thumb-interwork -mcpu=name
-march=name -mfpu=name -mstructure-size-boundary=n -mabort-on-noreturn -mlong-calls -mno-long-calls -msingle-pic-base
-mno-single-pic-base -mpic-register=reg -mnop-fun-dllimport -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns
-mpoke-function-name -mthumb -marm -mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking
-mcallee-super-interworking -mtp=name -mtls-dialect=dialect -mword-relocations -mfix-cortex-m3-ldrd -munaligned-access
AVR Options -mmcu=mcu -maccumulate-args -mbranch-cost=cost -mcall-prologues -mint8 -mno-interrupts -mrelax -mshort-calls
-mstrict-X -mtiny-stack
Blackfin Options -mcpu=cpu[-sirevision] -msim -momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer -mspecld-anomaly
-mno-specld-anomaly -mcsync-anomaly -mno-csync-anomaly -mlow-64k -mno-low64k -mstack-check-l1 -mid-shared-library
-mno-id-shared-library -mshared-library-id=n -mleaf-id-shared-library -mno-leaf-id-shared-library -msep-data
-mno-sep-data -mlong-calls -mno-long-calls -mfast-fp -minline-plt -mmulticore -mcorea -mcoreb -msdram -micplb
C6X Options -mbig-endian -mlittle-endian -march=cpu -msim -msdata=sdata-type
CRIS Options -mcpu=cpu -march=cpu -mtune=cpu -mmax-stack-frame=n -melinux-stacksize=n -metrax4 -metrax100 -mpdebug
-mcc-init -mno-side-effects -mstack-align -mdata-align -mconst-align -m32-bit -m16-bit -m8-bit
-mno-prologue-epilogue -mno-gotplt -melf -maout -melinux -mlinux -sim -sim2 -mmul-bug-workaround
-mno-mul-bug-workaround
CR16 Options -mmac -mcr16cplus -mcr16c -msim -mint32 -mbit-ops -mdata-model=model
Darwin Options -all_load -allowable_client -arch -arch_errors_fatal -arch_only -bind_at_load -bundle -bundle_loader
-client_name -compatibility_version -current_version -dead_strip -dependency-file -dylib_file -dylinker_install_name
-dynamic -dynamiclib -exported_symbols_list -filelist -flat_namespace -force_cpusubtype_ALL -force_flat_namespace
-headerpad_max_install_names -iframework -image_base -init -install_name -keep_private_externs -multi_module
-multiply_defined -multiply_defined_unused -noall_load -no_dead_strip_inits_and_terms -nofixprebinding -nomultidefs
-noprebind -noseglinkedit -pagezero_size -prebind -prebind_all_twolevel_modules -private_bundle -read_only_relocs
-sectalign -sectobjectsymbols -whyload -seg1addr -sectcreate -sectobjectsymbols -sectorder -segaddr
-segs_read_only_addr -segs_read_write_addr -seg_addr_table -seg_addr_table_filename -seglinkedit -segprot
-segs_read_only_addr -segs_read_write_addr -single_module -static -sub_library -sub_umbrella -twolevel_namespace
-umbrella -undefined -unexported_symbols_list -weak_reference_mismatches -whatsloaded -F -gused -gfull
-mmacosx-version-min=version -mkernel -mone-byte-bool
DEC Alpha Options -mno-fp-regs -msoft-float -malpha-as -mgas -mieee -mieee-with-inexact -mieee-conformant
-mfp-trap-mode=mode -mfp-rounding-mode=mode -mtrap-precision=mode -mbuild-constants -mcpu=cpu-type -mtune=cpu-type
-mbwx -mmax -mfix -mcix -mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data -mlarge-data -msmall-text
-mlarge-text -mmemory-latency=time
DEC Alpha/VMS Options -mvms-return-codes -mdebug-main=prefix -mmalloc64
FR30 Options -msmall-model -mno-lsim
FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float -msoft-float -malloc-cc -mfixed-cc -mdword -mno-dword
-mdouble -mno-double -mmedia -mno-media -mmuladd -mno-muladd -mfdpic -minline-plt -mgprel-ro -multilib-library-pic
-mlinked-fp -mlong-calls -malign-labels -mlibrary-pic -macc-4 -macc-8 -mpack -mno-pack -mno-eflags -mcond-move
-mno-cond-move -moptimize-membar -mno-optimize-membar -mscc -mno-scc -mcond-exec -mno-cond-exec -mvliw-branch
-mno-vliw-branch -mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec -mno-nested-cond-exec -mtomcat-stats -mTLS
-mtls -mcpu=cpu
GNU/Linux Options -mglibc -muclibc -mbionic -mandroid -tno-android-cc -tno-android-ld
H8/300 Options -mrelax -mh -ms -mn -mint32 -malign-300
HPPA Options -march=architecture-type -mbig-switch -mdisable-fpregs -mdisable-indexing -mfast-indirect-calls -mgas
-mgnu-ld -mhp-ld -mfixed-range=register-range -mjump-in-delay -mlinker-opt -mlong-calls -mlong-load-store
-mno-big-switch -mno-disable-fpregs -mno-disable-indexing -mno-fast-indirect-calls -mno-gas -mno-jump-in-delay
-mno-long-load-store -mno-portable-runtime -mno-soft-float -mno-space-regs -msoft-float -mpa-risc-1-0 -mpa-risc-1-1
-mpa-risc-2-0 -mportable-runtime -mschedule=cpu-type -mspace-regs -msio -mwsio -munix=unix-std -nolibdld -static
-threads
i386 and x86-64 Options -mtune=cpu-type -march=cpu-type -mfpmath=unit -masm=dialect -mno-fancy-math-387
-mno-fp-ret-in-387 -msoft-float -mno-wide-multiply -mrtd -malign-double -mpreferred-stack-boundary=num
-mincoming-stack-boundary=num -mcld -mcx16 -msahf -mmovbe -mcrc32 -mrecip -mrecip=opt -mvzeroupper -mmmx -msse -msse2
-msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx -mavx2 -maes -mpclmul -mfsgsbase -mrdrnd -mf16c -mfma -msse4a -m3dnow
-mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop -mlzcnt -mbmi2 -mlwp -mthreads -mno-align-stringops -minline-all-stringops
-minline-stringops-dynamically -mstringop-strategy=alg -mpush-args -maccumulate-outgoing-args -m128bit-long-double
-m96bit-long-double -mregparm=num -msseregparm -mveclibabi=type -mvect8-ret-in-mem -mpc32 -mpc64 -mpc80 -mstackrealign
-momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs -mcmodel=code-model -mabi=name -m32 -m64 -mx32
-mlarge-data-threshold=num -msse2avx -mfentry -m8bit-idiv -mavx256-split-unaligned-load -mavx256-split-unaligned-store
i386 and x86-64 Windows Options -mconsole -mcygwin -mno-cygwin -mdll -mnop-fun-dllimport -mthread -municode -mwin32
-mwindows -fno-set-stack-executable
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic -mvolatile-asm-stop -mregister-names -msdata
-mno-sdata -mconstant-gp -mauto-pic -mfused-madd -minline-float-divide-min-latency -minline-float-divide-max-throughput
-mno-inline-float-divide -minline-int-divide-min-latency -minline-int-divide-max-throughput -mno-inline-int-divide
-minline-sqrt-min-latency -minline-sqrt-max-throughput -mno-inline-sqrt -mdwarf2-asm -mearly-stop-bits
-mfixed-range=register-range -mtls-size=tls-size -mtune=cpu-type -milp32 -mlp64 -msched-br-data-spec -msched-ar-data-spec
-msched-control-spec -msched-br-in-data-spec -msched-ar-in-data-spec -msched-in-control-spec -msched-spec-ldc
-msched-spec-control-ldc -msched-prefer-non-data-spec-insns -msched-prefer-non-control-spec-insns
-msched-stop-bits-after-every-cycle -msched-count-spec-in-critical-path -msel-sched-dont-check-control-spec
-msched-fp-mem-deps-zero-cost -msched-max-memory-insns-hard-limit -msched-max-memory-insns=max-insns
IA-64/VMS Options -mvms-return-codes -mdebug-main=prefix -mmalloc64
LM32 Options -mbarrel-shift-enabled -mdivide-enabled -mmultiply-enabled -msign-extend-enabled -muser-enabled
M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops -mno-align-loops -missue-rate=number -mbranch-cost=number
-mmodel=code-size-model-type -msdata=sdata-type -mno-flush-func -mflush-func=name -mno-flush-trap -mflush-trap=number -G
num
M32C Options -mcpu=cpu -msim -memregs=number
M680x0 Options -march=arch -mcpu=cpu -mtune=tune -m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 -m68060
-mcpu32 -m5200 -m5206e -m528x -m5307 -m5407 -mcfv4e -mbitfield -mno-bitfield -mc68000 -mc68020 -mnobitfield
-mrtd -mno-rtd -mdiv -mno-div -mshort -mno-short -mhard-float -m68881 -msoft-float -mpcrel -malign-int
-mstrict-align -msep-data -mno-sep-data -mshared-library-id=n -mid-shared-library -mno-id-shared-library -mxgot
-mno-xgot
MCore Options -mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates -mno-relax-immediates -mwide-bitfields
-mno-wide-bitfields -m4byte-functions -mno-4byte-functions -mcallgraph-data -mno-callgraph-data -mslow-bytes
-mno-slow-bytes -mno-lsim -mlittle-endian -mbig-endian -m210 -m340 -mstack-increment
MeP Options -mabsdiff -mall-opts -maverage -mbased=n -mbitops -mc=n -mclip -mconfig=name -mcop -mcop32 -mcop64 -mivc2
-mdc -mdiv -meb -mel -mio-volatile -ml -mleadz -mm -mminmax -mmult -mno-opts -mrepeat -ms -msatur -msdram -msim
-msimnovec -mtf -mtiny=n
MicroBlaze Options -msoft-float -mhard-float -msmall-divides -mcpu=cpu -mmemcpy -mxl-soft-mul -mxl-soft-div
-mxl-barrel-shift -mxl-pattern-compare -mxl-stack-check -mxl-gp-opt -mno-clearbss -mxl-multiply-high -mxl-float-convert
-mxl-float-sqrt -mxl-mode-app-model
MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64 -mips64r2
-mips16 -mno-mips16 -mflip-mips16 -minterlink-mips16 -mno-interlink-mips16 -mabi=abi -mabicalls -mno-abicalls
-mshared -mno-shared -mplt -mno-plt -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfp64 -mhard-float -msoft-float
-msingle-float -mdouble-float -mdsp -mno-dsp -mdspr2 -mno-dspr2 -mfpu=fpu-type -msmartmips -mno-smartmips
-mpaired-single -mno-paired-single -mdmx -mno-mdmx -mips3d -mno-mips3d -mmt -mno-mt -mllsc -mno-llsc -mlong64
-mlong32 -msym32 -mno-sym32 -Gnum -mlocal-sdata -mno-local-sdata -mextern-sdata -mno-extern-sdata -mgpopt
-mno-gopt -membedded-data -mno-embedded-data -muninit-const-in-rodata -mno-uninit-const-in-rodata
-mcode-readable=setting -msplit-addresses -mno-split-addresses -mexplicit-relocs -mno-explicit-relocs
-mcheck-zero-division -mno-check-zero-division -mdivide-traps -mdivide-breaks -mmemcpy -mno-memcpy -mlong-calls
-mno-long-calls -mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp -mfix-24k -mno-fix-24k -mfix-r4000 -mno-fix-r4000
-mfix-r4400 -mno-fix-r4400 -mfix-r10000 -mno-fix-r10000 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 -mno-fix-vr4130
-mfix-sb1 -mno-fix-sb1 -mflush-func=func -mno-flush-func -mbranch-cost=num -mbranch-likely -mno-branch-likely
-mfp-exceptions -mno-fp-exceptions -mvr4130-align -mno-vr4130-align -msynci -mno-synci -mrelax-pic-calls
-mno-relax-pic-calls -mmcount-ra-address
MMIX Options -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu -mabi=mmixware -mzero-extend -mknuthdiv
-mtoplevel-symbols -melf -mbranch-predict -mno-branch-predict -mbase-addresses -mno-base-addresses -msingle-exit
-mno-single-exit
MN10300 Options -mmult-bug -mno-mult-bug -mno-am33 -mam33 -mam33-2 -mam34 -mtune=cpu-type -mreturn-pointer-on-d0
-mno-crt0 -mrelax -mliw -msetlb
PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mbcopy -mbcopy-builtin -mint32 -mno-int16
-mint16 -mno-int32 -mfloat32 -mno-float64 -mfloat64 -mno-float32 -mabshi -mno-abshi -mbranch-expensive
-mbranch-cheap -munix-asm -mdec-asm
picoChip Options -mae=ae_type -mvliw-lookahead=N -msymbol-as-address -mno-inefficient-warnings
PowerPC Options See RS/6000 and PowerPC Options.
RL78 Options -msim -mmul=none -mmul=g13 -mmul=rl78
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -mpower -mno-power -mpower2 -mno-power2
-mpowerpc -mpowerpc64 -mno-powerpc -maltivec -mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt
-mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb -mpopcntd -mno-popcntd -mfprnd -mno-fprnd -mcmpb
-mno-cmpb -mmfpgpr -mno-mfpgpr -mhard-dfp -mno-hard-dfp -mnew-mnemonics -mold-mnemonics -mfull-toc -mminimal-toc
-mno-fp-in-toc -mno-sum-in-toc -m64 -m32 -mxl-compat -mno-xl-compat -mpe -malign-power -malign-natural -msoft-float
-mhard-float -mmultiple -mno-multiple -msingle-float -mdouble-float -msimple-fpu -mstring -mno-string -mupdate
-mno-update -mavoid-indexed-addresses -mno-avoid-indexed-addresses -mfused-madd -mno-fused-madd -mbit-align
-mno-bit-align -mstrict-align -mno-strict-align -mrelocatable -mno-relocatable -mrelocatable-lib -mno-relocatable-lib
-mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian -mdynamic-no-pic -maltivec -mswdiv -msingle-pic-base
-mprioritize-restricted-insns=priority -msched-costly-dep=dependence_type -minsert-sched-nops=scheme -mcall-sysv
-mcall-netbsd -maix-struct-return -msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt
-mblock-move-inline-limit=num -misel -mno-isel -misel=yes -misel=no -mspe -mno-spe -mspe=yes -mspe=no -mpaired
-mgen-cell-microcode -mwarn-cell-microcode -mvrsave -mno-vrsave -mmulhw -mno-mulhw -mdlmzb -mno-dlmzb -mfloat-gprs=yes
-mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double -mprototype -mno-prototype -msim -mmvme -mads -myellowknife
-memb -msdata -msdata=opt -mvxworks -G num -pthread -mrecip -mrecip=opt -mno-recip -mrecip-precision
-mno-recip-precision -mveclibabi=type -mfriz -mno-friz -mpointers-to-nested-functions -mno-pointers-to-nested-functions
-msave-toc-indirect -mno-save-toc-indirect
RX Options -m64bit-doubles -m32bit-doubles -fpu -nofpu -mcpu= -mbig-endian-data -mlittle-endian-data -msmall-data
-msim -mno-sim -mas100-syntax -mno-as100-syntax -mrelax -mmax-constant-size= -mint-register= -mpid
-msave-acc-in-interrupts
S/390 and zSeries Options -mtune=cpu-type -march=cpu-type -mhard-float -msoft-float -mhard-dfp -mno-hard-dfp
-mlong-double-64 -mlong-double-128 -mbackchain -mno-backchain -mpacked-stack -mno-packed-stack -msmall-exec
-mno-small-exec -mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug -mesa -mzarch -mtpf-trace -mno-tpf-trace
-mfused-madd -mno-fused-madd -mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard
Score Options -meb -mel -mnhwloop -muls -mmac -mscore5 -mscore5u -mscore7 -mscore7d
SH Options -m1 -m2 -m2e -m2a-nofpu -m2a-single-only -m2a-single -m2a -m3 -m3e -m4-nofpu -m4-single-only -m4-single
-m4 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al -m5-64media -m5-64media-nofpu -m5-32media -m5-32media-nofpu
-m5-compact -m5-compact-nofpu -mb -ml -mdalign -mrelax -mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas
-mnomacsave -mieee -mno-ieee -mbitops -misize -minline-ic_invalidate -mpadstruct -mspace -mprefergot -musermode
-multcost=number -mdiv=strategy -mdivsi3_libfunc=name -mfixed-range=register-range -madjust-unroll -mindexed-addressing
-mgettrcost=number -mpt-fixed -maccumulate-outgoing-args -minvalid-symbols -msoft-atomic -mbranch-cost=num -mcbranchdi
-mcmpeqdi -mfused-madd -mpretend-cmove
Solaris 2 Options -mimpure-text -mno-impure-text -pthreads -pthread
SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -mmemory-model=mem-model -m32 -m64 -mapp-regs
-mno-app-regs -mfaster-structs -mno-faster-structs -mflat -mno-flat -mfpu -mno-fpu -mhard-float -msoft-float
-mhard-quad-float -msoft-quad-float -mlittle-endian -mstack-bias -mno-stack-bias -munaligned-doubles
-mno-unaligned-doubles -mv8plus -mno-v8plus -mvis -mno-vis -mvis2 -mno-vis2 -mvis3 -mno-vis3 -mfmaf -mno-fmaf
-mpopc -mno-popc -mfix-at697f
SPU Options -mwarn-reloc -merror-reloc -msafe-dma -munsafe-dma -mbranch-hints -msmall-mem -mlarge-mem -mstdmain
-mfixed-range=register-range -mea32 -mea64 -maddress-space-conversion -mno-address-space-conversion -mcache-size=cache-
size -matomic-updates -mno-atomic-updates
System V Options -Qy -Qn -YP,paths -Ym,dir
TILE-Gx Options -mcpu=cpu -m32 -m64
TILEPro Options -mcpu=cpu -m32
V850 Options -mlong-calls -mno-long-calls -mep -mno-ep -mprolog-function -mno-prolog-function -mspace -mtda=n
-msda=n -mzda=n -mapp-regs -mno-app-regs -mdisable-callt -mno-disable-callt -mv850e2v3 -mv850e2 -mv850e1 -mv850es
-mv850e -mv850 -mbig-switch
VAX Options -mg -mgnu -munix
VxWorks Options -mrtp -non-static -Bstatic -Bdynamic -Xbind-lazy -Xbind-now
x86-64 Options See i386 and x86-64 Options.
Xstormy16 Options -msim
Xtensa Options -mconst16 -mno-const16 -mfused-madd -mno-fused-madd -mforce-no-pic -mserialize-volatile
-mno-serialize-volatile -mtext-section-literals -mno-text-section-literals -mtarget-align -mno-target-align -mlongcalls
-mno-longcalls
zSeries Options See S/390 and zSeries Options.
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions -fnon-call-exceptions -funwind-tables
-fasynchronous-unwind-tables -finhibit-size-directive -finstrument-functions
-finstrument-functions-exclude-function-list=sym,sym,... -finstrument-functions-exclude-file-list=file,file,...
-fno-common -fno-ident -fpcc-struct-return -fpic -fPIC -fpie -fPIE -fno-jump-tables -frecord-gcc-switches
-freg-struct-return -fshort-enums -fshort-double -fshort-wchar -fverbose-asm -fpack-struct[=n] -fstack-check
-fstack-limit-register=reg -fstack-limit-symbol=sym -fno-stack-limit -fsplit-stack -fleading-underscore
-ftls-model=model -ftrapv -fwrapv -fbounds-check -fvisibility -fstrict-volatile-bitfields
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order.
GCC is capable of preprocessing and compiling several files either into several assembler input files, or into one assembler
input file; then each assembler input file produces an object file, and linking combines all the object files (those newly
compiled, and those specified as input) into an executable file.
For any given input file, the file name suffix determines what kind of compilation is done:
file.c
C source code that must be preprocessed.
file.i
C source code that should not be preprocessed.
file.ii
C++ source code that should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the libobjc library to make an Objective-C program work.
file.mi
Objective-C source code that should not be preprocessed.
file.mm
file.M
Objective-C++ source code. Note that you must link with the libobjc library to make an Objective-C++ program work. Note
that .M refers to a literal capital M.
file.mii
Objective-C++ source code that should not be preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (default), or C, C++ header file
to be turned into an Ada spec (via the -fdump-ada-spec switch).
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code that must be preprocessed. Note that in .cxx, the last two letters must both be literally x. Likewise,
.C refers to a literal capital C.
file.mm
file.M
Objective-C++ source code that must be preprocessed.
file.mii
Objective-C++ source code that should not be preprocessed.
file.hh
file.H
file.hp
file.hxx
file.hpp
file.HPP
file.h++
file.tcc
C++ header file to be turned into a precompiled header or Ada spec.
file.f
file.for
file.ftn
Fixed form Fortran source code that should not be preprocessed.
file.F
file.FOR
file.fpp
file.FPP
file.FTN
Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).
file.f90
file.f95
file.f03
file.f08
Free form Fortran source code that should not be preprocessed.
file.F90
file.F95
file.F03
file.F08
Free form Fortran source code that must be preprocessed (with the traditional preprocessor).
file.go
Go source code.
file.ads
Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a
generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration).
Such files are also called specs.
file.adb
Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies.
file.s
Assembler code.
file.S
file.sx
Assembler code that must be preprocessed.
other
An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.
You can specify the input language explicitly with the -x option:
-x language
Specify explicitly the language for the following input files (rather than letting the compiler choose a default based on
the file name suffix). This option applies to all following input files until the next -x option. Possible values for
language are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input f95 f95-cpp-input
go
java
-x none
Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as
they are if -x has not been used at all).
-pass-exit-codes
Normally the gcc program will exit with the code of 1 if any phase of the compiler returns a non-success return code. If
you specify -pass-exit-codes, the gcc program will instead return with numerically highest error produced by any phase
that returned an error indication. The C, C++, and Fortran frontends return 4, if an internal compiler error is
encountered.
If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to start, and one
of the options -c, -S, or -E to say where gcc is to stop. Note that some combinations (for example, -x cpp-output -E)
instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in
the form of an object file for each source file.
By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly, are ignored.
-S Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file for
each non-assembler input file specified.
By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with .s.
Input files that don't require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source
code, which is sent to the standard output.
Input files that don't require preprocessing are ignored.
-o file
Place output in file file. This applies regardless to whatever sort of output is being produced, whether it be an
executable file, an object file, an assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable file in a.out, the object file for source.suffix in source.o,
its assembler file in source.s, a precompiled header file in source.suffix.gch, and all preprocessed C source on standard
output.
-v Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number
of the compiler driver program and of the preprocessor and the compiler proper.
-###
Like -v except the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or
"./-_". This is useful for shell scripts to capture the driver-generated command lines.
-pipe
Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on
some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble.
--help
Print (on the standard output) a description of the command-line options understood by gcc. If the -v option is also
specified then --help will also be passed on to the various processes invoked by gcc, so that they can display the
command-line options they accept. If the -Wextra option has also been specified (prior to the --help option), then
command-line options that have no documentation associated with them will also be displayed.
--target-help
Print (on the standard output) a description of target-specific command-line options for each tool. For some targets
extra target-specific information may also be printed.
--help={class|[^]qualifier}[,...]
Print (on the standard output) a description of the command-line options understood by the compiler that fit into all
specified classes and qualifiers. These are the supported classes:
optimizers
This will display all of the optimization options supported by the compiler.
warnings
This will display all of the options controlling warning messages produced by the compiler.
target
This will display target-specific options. Unlike the --target-help option however, target-specific options of the
linker and assembler will not be displayed. This is because those tools do not currently support the extended
--help= syntax.
params
This will display the values recognized by the --param option.
language
This will display the options supported for language, where language is the name of one of the languages supported in
this version of GCC.
common
This will display the options that are common to all languages.
These are the supported qualifiers:
undocumented
Display only those options that are undocumented.
joined
Display options taking an argument that appears after an equal sign in the same continuous piece of text, such as:
--help=target.
separate
Display options taking an argument that appears as a separate word following the original option, such as: -o output-
file.
Thus for example to display all the undocumented target-specific switches supported by the compiler the following can be
used:
--help=target,undocumented
The sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to display all binary
warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use:
--help=warnings,^joined,^undocumented
The argument to --help= should not consist solely of inverted qualifiers.
Combining several classes is possible, although this usually restricts the output by so much that there is nothing to
display. One case where it does work however is when one of the classes is target. So for example to display all the
target-specific optimization options the following can be used:
--help=target,optimizers
The --help= option can be repeated on the command line. Each successive use will display its requested class of options,
skipping those that have already been displayed.
If the -Q option appears on the command line before the --help= option, then the descriptive text displayed by --help= is
changed. Instead of describing the displayed options, an indication is given as to whether the option is enabled,
disabled or set to a specific value (assuming that the compiler knows this at the point where the --help= option is
used).
Here is a truncated example from the ARM port of gcc:
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]
The output is sensitive to the effects of previous command-line options, so for example it is possible to find out which
optimizations are enabled at -O2 by using:
-Q -O2 --help=optimizers
Alternatively you can discover which binary optimizations are enabled by -O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
-no-canonical-prefixes
Do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when generating a relative
prefix.
--version
Display the version number and copyrights of the invoked GCC.
-wrapper
Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma
separated list.
gcc -c t.c -wrapper gdb,--args
This will invoke all subprograms of gcc under gdb --args, thus the invocation of cc1 will be gdb --args cc1 ....
-fplugin=name.so
Load the plugin code in file name.so, assumed to be a shared object to be dlopen'd by the compiler. The base name of the
shared object file is used to identify the plugin for the purposes of argument parsing (See -fplugin-arg-name-key=value
below). Each plugin should define the callback functions specified in the Plugins API.
-fplugin-arg-name-key=value
Define an argument called key with a value of value for the plugin called name.
-fdump-ada-spec[-slim]
For C and C++ source and include files, generate corresponding Ada specs.
-fdump-go-spec=file
For input files in any language, generate corresponding Go declarations in file. This generates Go "const", "type",
"var", and "func" declarations which may be a useful way to start writing a Go interface to code written in some other
language.
@file
Read command-line options from file. The options read are inserted in place of the original @file option. If file does
not exist, or cannot be read, then the option will be treated literally, and not removed.
Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding the
entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the
character to be included with a backslash. The file may itself contain additional @file options; any such options will
be processed recursively.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use
.hh, .hpp, .H, or (for shared template code) .tcc; and preprocessed C++ files use the suffix .ii. GCC recognizes files with
these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually
with the name gcc).
However, the use of gcc does not add the C++ library. g++ is a program that calls GCC and treats .c, .h and .i files as C++
source files instead of C source files unless -x is used, and automatically specifies linking against the C++ library. This
program is also useful when precompiling a C header file with a .h extension for use in C++ compilations. On many systems,
g++ is also installed with the name c++.
When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in
any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++
programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that
the compiler accepts:
-ansi
In C mode, this is equivalent to -std=c90. In C++ mode, it is equivalent to -std=c++98.
This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C++
(when compiling C++ code), such as the "asm" and "typeof" keywords, and predefined macros such as "unix" and "vax" that
identify the type of system you are using. It also enables the undesirable and rarely used ISO trigraph feature. For
the C compiler, it disables recognition of C++ style // comments as well as the "inline" keyword.
The alternate keywords "__asm__", "__extension__", "__inline__" and "__typeof__" continue to work despite -ansi. You
would not want to use them in an ISO C program, of course, but it is useful to put them in header files that might be
included in compilations done with -ansi. Alternate predefined macros such as "__unix__" and "__vax__" are also
available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that, -pedantic is required in
addition to -ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi option is used. Some header files may notice this macro and
refrain from declaring certain functions or defining certain macros that the ISO standard doesn't call for; this is to
avoid interfering with any programs that might use these names for other things.
Functions that would normally be built in but do not have semantics defined by ISO C (such as "alloca" and "ffs") are not
built-in functions when -ansi is used.
-std=
Determine the language standard. This option is currently only supported when compiling C or C++.
The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of those standards, such as gnu90
or gnu++98. By specifying a base standard, the compiler will accept all programs following that standard and those using
GNU extensions that do not contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible
with ISO C90, such as the "asm" and "typeof" keywords, but not other GNU extensions that do not have a meaning in ISO
C90, such as omitting the middle term of a "?:" expression. On the other hand, by specifying a GNU dialect of a standard,
all features the compiler support are enabled, even when those features change the meaning of the base standard and some
strict-conforming programs may be rejected. The particular standard is used by -pedantic to identify which features are
GNU extensions given that version of the standard. For example -std=gnu90 -pedantic would warn about C++ style //
comments, while -std=gnu99 -pedantic would not.
A value for this option must be provided; possible values are
c90
c89
iso9899:1990
Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C
code.
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. Note that this standard is not yet fully supported; see <http://gcc.gnu.org/gcc-4.7/c99status.html> for
more information. The names c9x and iso9899:199x are deprecated.
c11
c1x
iso9899:2011
ISO C11, the 2011 revision of the ISO C standard. Support is incomplete and experimental. The name c1x is
deprecated.
gnu90
gnu89
GNU dialect of ISO C90 (including some C99 features). This is the default for C code.
gnu99
gnu9x
GNU dialect of ISO C99. When ISO C99 is fully implemented in GCC, this will become the default. The name gnu9x is
deprecated.
gnu11
gnu1x
GNU dialect of ISO C11. Support is incomplete and experimental. The name gnu1x is deprecated.
c++98
The 1998 ISO C++ standard plus amendments. Same as -ansi for C++ code.
gnu++98
GNU dialect of -std=c++98. This is the default for C++ code.
c++11
The 2011 ISO C++ standard plus amendments. Support for C++11 is still experimental, and may change in incompatible
ways in future releases.
gnu++11
GNU dialect of -std=c++11. Support for C++11 is still experimental, and may change in incompatible ways in future
releases.
-fgnu89-inline
The option -fgnu89-inline tells GCC to use the traditional GNU semantics for "inline" functions when in C99 mode.
This option is accepted and ignored by GCC versions 4.1.3 up to but not including 4.3. In GCC versions 4.3 and later
it changes the behavior of GCC in C99 mode. Using this option is roughly equivalent to adding the "gnu_inline" function
attribute to all inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for "inline" when in C99 or gnu99 mode (i.e.,
it specifies the default behavior). This option was first supported in GCC 4.3. This option is not supported in
-std=c90 or -std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to check which semantics are in
effect for "inline" functions.
-aux-info filename
Output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit,
including those in header files. This option is silently ignored in any language other than C.
Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the
declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respectively, in the first character
after the line number and the colon), and whether it came from a declaration or a definition (C or F, respectively, in
the following character). In the case of function definitions, a K&R-style list of arguments followed by their
declarations is also provided, inside comments, after the declaration.
-fallow-parameterless-variadic-functions
Accept variadic functions without named parameters.
Although it is possible to define such a function, this is not very useful as it is not possible to read the arguments.
This is only supported for C as this construct is allowed by C++.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that code can use these words as identifiers. You can use
the keywords "__asm__", "__inline__" and "__typeof__" instead. -ansi implies -fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm" and "inline" are standard keywords. You may want to
use the -fno-gnu-keywords flag instead, which has the same effect. In C99 mode (-std=c99 or -std=gnu99), this switch
only affects the "asm" and "typeof" keywords, since "inline" is a standard keyword in ISO C99.
-fno-builtin
-fno-builtin-function
Don't recognize built-in functions that do not begin with __builtin_ as prefix.
GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to
"alloca" may become single instructions which adjust the stack directly, and calls to "memcpy" may become inline copy
loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you
cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different
library. In addition, when a function is recognized as a built-in function, GCC may use information about that function
to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still
contains calls to that function. For example, warnings are given with -Wformat for bad calls to "printf", when "printf"
is built in, and "strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in function function is disabled. function must not begin with
__builtin_. If a function is named that is not built-in in this version of GCC, this option is ignored. There is no
corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or
-ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fhosted
Assert that compilation takes place in a hosted environment. This implies -fbuiltin. A hosted environment is one in
which the entire standard library is available, and in which "main" has a return type of "int". Examples are nearly
everything except a kernel. This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation takes place in a freestanding environment. This implies -fno-builtin. A freestanding
environment is one in which the standard library may not exist, and program startup may not necessarily be at "main".
The most obvious example is an OS kernel. This is equivalent to -fno-hosted.
-fopenmp
Enable handling of OpenMP directives "#pragma omp" in C/C++ and "!$omp" in Fortran. When -fopenmp is specified, the
compiler generates parallel code according to the OpenMP Application Program Interface v3.0 <http://www.openmp.org/>.
This option implies -pthread, and thus is only supported on targets that have support for -pthread.
-fgnu-tm
When the option -fgnu-tm is specified, the compiler will generate code for the Linux variant of Intel's current
Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature whose
interface may change in future versions of GCC, as the official specification changes. Please note that not all
architectures are supported for this feature.
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions).
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
In C++ code, this allows member names in structures to be similar to previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};
Some cases of unnamed fields in structures and unions are only accepted with this option.
-fplan9-extensions
Accept some non-standard constructs used in Plan 9 code.
This enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions that expect
pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef.
This is only supported for C, not C++.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std options for strict ISO C conformance) implies -trigraphs.
-no-integrated-cpp
Performs a compilation in two passes: preprocessing and compiling. This option allows a user supplied "cc1", "cc1plus",
or "cc1obj" via the -B option. The user supplied compilation step can then add in an additional preprocessing step after
normal preprocessing but before compiling. The default is to use the integrated cpp (internal cpp)
The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.
-traditional
-traditional-cpp
Formerly, these options caused GCC to attempt to emulate a pre-standard C compiler. They are now only supported with the
-E switch. The preprocessor continues to support a pre-standard mode. See the GNU CPP manual for details.
-fcond-mismatch
Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression
is void. This option is not supported for C++.
-flax-vector-conversions
Allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types. This
option should not be used for new code.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be. It is either like "unsigned char" by default or like
"signed char" by default.
Ideally, a portable program should always use "signed char" or "unsigned char" when it depends on the signedness of an
object. But many programs have been written to use plain "char" and expect it to be signed, or expect it to be unsigned,
depending on the machines they were written for. This option, and its inverse, let you make such a program work with the
opposite default.
The type "char" is always a distinct type from each of "signed char" or "unsigned char", even though its behavior is
always just like one of those two.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Likewise, the option
-fno-signed-char is equivalent to -funsigned-char.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned, when the declaration does not use either "signed" or
"unsigned". By default, such a bit-field is signed, because this is consistent: the basic integer types such as "int"
are signed types.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful for C++ programs; but you can also use most of the
GNU compiler options regardless of what language your program is in. For example, you might compile a file "firstClass.C"
like this:
g++ -g -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs; you can use the other options with any language
supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. Version 2 is the version of the C++ ABI that first appeared in G++ 3.4. Version 1 is the
version of the C++ ABI that first appeared in G++ 3.2. Version 0 will always be the version that conforms most closely
to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change as ABI bugs are fixed.
The default is version 2.
Version 3 corrects an error in mangling a constant address as a template argument.
Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.
Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types,
decltype of a plain decl, and use of a function parameter in the declaration of another parameter.
Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of
template argument packs, const/static_cast, prefix ++ and --, and a class scope function used as a template argument.
See also -Wabi.
-fno-access-control
Turn off all access checking. This switch is mainly useful for working around bugs in the access control code.
-fcheck-new
Check that the pointer returned by "operator new" is non-null before attempting to modify the storage allocated. This
check is normally unnecessary because the C++ standard specifies that "operator new" will only return 0 if it is declared
throw(), in which case the compiler will always check the return value even without this option. In all other cases,
when "operator new" has a non-empty exception specification, memory exhaustion is signalled by throwing "std::bad_alloc".
See also new (nothrow).
-fconserve-space
Put uninitialized or run-time-initialized global variables into the common segment, as C does. This saves space in the
executable at the cost of not diagnosing duplicate definitions. If you compile with this flag and your program
mysteriously crashes after "main()" has completed, you may have an object that is being destroyed twice because two
definitions were merged.
This option is no longer useful on most targets, now that support has been added for putting variables into BSS without
making them common.
-fconstexpr-depth=n
Set the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to detect endless
recursion during constant expression evaluation. The minimum specified by the standard is 512.
-fdeduce-init-list
Enable deduction of a template type parameter as std::initializer_list from a brace-enclosed initializer list, i.e.
template <class T> auto forward(T t) -> decltype (realfn (t))
{
return realfn (t);
}
void f()
{
forward({1,2}); // call forward<std::initializer_list<int>>
}
This deduction was implemented as a possible extension to the originally proposed semantics for the C++11 standard, but
was not part of the final standard, so it is disabled by default. This option is deprecated, and may be removed in a
future version of G++.
-ffriend-injection
Inject friend functions into the enclosing namespace, so that they are visible outside the scope of the class in which
they are declared. Friend functions were documented to work this way in the old Annotated C++ Reference Manual, and
versions of G++ before 4.1 always worked that way. However, in ISO C++ a friend function that is not declared in an
enclosing scope can only be found using argument dependent lookup. This option causes friends to be injected as they
were in earlier releases.
This option is for compatibility, and may be removed in a future release of G++.
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of
the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all
cases.
-fno-enforce-eh-specs
Don't generate code to check for violation of exception specifications at run time. This option violates the C++
standard, but may be useful for reducing code size in production builds, much like defining NDEBUG. This does not give
user code permission to throw exceptions in violation of the exception specifications; the compiler will still optimize
based on the specifications, so throwing an unexpected exception will result in undefined behavior.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a for-init-statement is limited to the for loop itself,
as specified by the C++ standard. If -fno-for-scope is specified, the scope of variables declared in a for-init-
statement extends to the end of the enclosing scope, as was the case in old versions of G++, and other (traditional)
implementations of C++.
The default if neither flag is given to follow the standard, but to allow and give a warning for old-style code that
would otherwise be invalid, or have different behavior.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this word as an identifier. You can use the keyword
"__typeof__" instead. -ansi implies -fno-gnu-keywords.
-fno-implicit-templates
Never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit
instantiations.
-fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so
that compiles with and without optimization will need the same set of explicit instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions controlled by #pragma implementation. This will cause
linker errors if these functions are not inlined everywhere they are called.
-fms-extensions
Disable pedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via
non-standard syntax.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include "ffs", "alloca", "_exit",
"index", "bzero", "conjf", and other related functions.
-fnothrow-opt
Treat a "throw()" exception specification as though it were a "noexcept" specification to reduce or eliminate the text
size overhead relative to a function with no exception specification. If the function has local variables of types with
non-trivial destructors, the exception specification will actually make the function smaller because the EH cleanups for
those variables can be optimized away. The semantic effect is that an exception thrown out of a function with such an
exception specification will result in a call to "terminate" rather than "unexpected".
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor", "compl", "not", "or" and "xor" as synonyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such diagnostic issued
by G++ is the one for a name having multiple meanings within a class.
-fpermissive
Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using -fpermissive will allow some
nonconforming code to compile.
-fno-pretty-templates
When an error message refers to a specialization of a function template, the compiler will normally print the signature
of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. "void f(T) [with
T = int]" rather than "void f(int)") so that it's clear which template is involved. When an error message refers to a
specialization of a class template, the compiler will omit any template arguments that match the default template
arguments for that template. If either of these behaviors make it harder to understand the error message rather than
easier, using -fno-pretty-templates will disable them.
-frepo
Enable automatic template instantiation at link time. This option also implies -fno-implicit-templates.
-fno-rtti
Disable generation of information about every class with virtual functions for use by the C++ run-time type
identification features (dynamic_cast and typeid). If you don't use those parts of the language, you can save some space
by using this flag. Note that exception handling uses the same information, but it will generate it as needed. The
dynamic_cast operator can still be used for casts that do not require run-time type information, i.e. casts to "void *"
or to unambiguous base classes.
-fstats
Emit statistics about front-end processing at the end of the compilation. This information is generally only useful to
the G++ development team.
-fstrict-enums
Allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the
enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits
needed to represent all the enumerators). This assumption may not be valid if the program uses a cast to convert an
arbitrary integer value to the enumerated type.
-ftemplate-depth=n
Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to
detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a
maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, as the compiler can run out of stack
space before hitting 1024 in some situations.
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics.
You can use this option to reduce code size slightly in code that doesn't need to be thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with the "__cxa_atexit" function rather than the "atexit"
function. This option is required for fully standards-compliant handling of static destructors, but will only work if
your C library supports "__cxa_atexit".
-fno-use-cxa-get-exception-ptr
Don't use the "__cxa_get_exception_ptr" runtime routine. This will cause "std::uncaught_exception" to be incorrect, but
is necessary if the runtime routine is not available.
-fvisibility-inlines-hidden
This switch declares that the user does not attempt to compare pointers to inline functions or methods where the
addresses of the two functions were taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline methods with "__attribute__ ((visibility ("hidden")))" so
that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO.
Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the
dynamic export table when the library makes heavy use of templates.
The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect
static variables local to the function or cause the compiler to deduce that the function is defined in only one shared
object.
You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example,
if you do want to compare pointers to a particular inline method, you might mark it as having default visibility.
Marking the enclosing class with explicit visibility will have no effect.
Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared
library boundary.
-fvisibility-ms-compat
This flag attempts to use visibility settings to make GCC's C++ linkage model compatible with that of Microsoft Visual
Studio.
The flag makes these changes to GCC's linkage model:
1. It sets the default visibility to "hidden", like -fvisibility=hidden.
2. Types, but not their members, are not hidden by default.
3. The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than
one different shared object: those declarations are permitted if they would have been permitted when this option was
not used.
In new code it is better to use -fvisibility=hidden and export those classes that are intended to be externally visible.
Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior.
Among the consequences of these changes are that static data members of the same type with the same name but defined in
different shared objects will be different, so changing one will not change the other; and that pointers to function
members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR
to define types with the same name differently.
-fno-weak
Do not use weak symbol support, even if it is provided by the linker. By default, G++ will use weak symbols if they are
available. This option exists only for testing, and should not be used by end-users; it will result in inferior code and
has no benefits. This option may be removed in a future release of G++.
-nostdinc++
Do not search for header files in the standard directories specific to C++, but do still search the other standard
directories. (This option is used when building the C++ library.)
In addition, these optimization, warning, and code generation options have meanings only for C++ programs:
-fno-default-inline
Do not assume inline for functions defined inside a class scope.
Note that these functions will have linkage like inline functions; they just won't be inlined by default.
-Wabi (C, Objective-C, C++ and Objective-C++ only)
Warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI. Although an effort has
been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is
generating incompatible code. There may also be cases where warnings are emitted even though the code that is generated
will be compatible.
You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may
not be binary compatible with code generated by other compilers.
The known incompatibilities in -fabi-version=2 (the default) include:
· A template with a non-type template parameter of reference type is mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}
This is fixed in -fabi-version=3.
· SIMD vector types declared using "__attribute ((vector_size))" are mangled in a non-standard way that does not allow
for overloading of functions taking vectors of different sizes.
The mangling is changed in -fabi-version=4.
The known incompatibilities in -fabi-version=1 include:
· Incorrect handling of tail-padding for bit-fields. G++ may attempt to pack data into the same byte as a base class.
For example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place "B::f2" into the same byte as"A::f1"; other compilers will not. You can avoid this
problem by explicitly padding "A" so that its size is a multiple of the byte size on your platform; that will cause
G++ and other compilers to layout "B" identically.
· Incorrect handling of tail-padding for virtual bases. G++ does not use tail padding when laying out virtual bases.
For example:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ will not place "B" into the tail-padding for "A"; other compilers will. You can avoid this problem
by explicitly padding "A" so that its size is a multiple of its alignment (ignoring virtual base classes); that will
cause G++ and other compilers to layout "C" identically.
· Incorrect handling of bit-fields with declared widths greater than that of their underlying types, when the bit-
fields appear in a union. For example:
union U { int i : 4096; };
Assuming that an "int" does not have 4096 bits, G++ will make the union too small by the number of bits in an "int".
· Empty classes can be placed at incorrect offsets. For example:
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ will place the "A" base class of "C" at a nonzero offset; it should be placed at offset zero. G++ mistakenly
believes that the "A" data member of "B" is already at offset zero.
· Names of template functions whose types involve "typename" or template template parameters can be mangled
incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations of these templates may be mangled incorrectly.
It also warns psABI related changes. The known psABI changes at this point include:
· For SYSV/x86-64, when passing union with long double, it is changed to pass in memory as specified in psABI. For
example:
union U {
long double ld;
int i;
};
"union U" will always be passed in memory.
-Wctor-dtor-privacy (C++ and Objective-C++ only)
Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has
neither friends nor public static member functions.
-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
Warn when delete is used to destroy an instance of a class that has virtual functions and non-virtual destructor. It is
unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a
virtual destructor. This warning is enabled by -Wall.
-Wnarrowing (C++ and Objective-C++ only)
Warn when a narrowing conversion prohibited by C++11 occurs within { }, e.g.
int i = { 2.2 }; // error: narrowing from double to int
This flag is included in -Wall and -Wc++11-compat.
With -std=c++11, -Wno-narrowing suppresses the diagnostic required by the standard. Note that this does not affect the
meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE context.
-Wnoexcept (C++ and Objective-C++ only)
Warn when a noexcept-expression evaluates to false because of a call to a function that does not have a non-throwing
exception specification (i.e. throw() or noexcept) but is known by the compiler to never throw an exception.
-Wnon-virtual-dtor (C++ and Objective-C++ only)
Warn when a class has virtual functions and accessible non-virtual destructor, in which case it would be possible but
unsafe to delete an instance of a derived class through a pointer to the base class. This warning is also enabled if
-Weffc++ is specified.
-Wreorder (C++ and Objective-C++ only)
Warn when the order of member initializers given in the code does not match the order in which they must be executed.
For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler will rearrange the member initializers for i and j to match the declaration order of the members, emitting a
warning to that effect. This warning is enabled by -Wall.
The following -W... options are not affected by -Wall.
-Weffc++ (C++ and Objective-C++ only)
Warn about violations of the following style guidelines from Scott Meyers' Effective C++, Second Edition book:
· Item 11: Define a copy constructor and an assignment operator for classes with dynamically allocated memory.
· Item 12: Prefer initialization to assignment in constructors.
· Item 14: Make destructors virtual in base classes.
· Item 15: Have "operator=" return a reference to *this.
· Item 23: Don't try to return a reference when you must return an object.
Also warn about violations of the following style guidelines from Scott Meyers' More Effective C++ book:
· Item 6: Distinguish between prefix and postfix forms of increment and decrement operators.
· Item 7: Never overload "&&", "||", or ",".
When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use grep -v
to filter out those warnings.
-Wstrict-null-sentinel (C++ and Objective-C++ only)
Warn also about the use of an uncasted "NULL" as sentinel. When compiling only with GCC this is a valid sentinel, as
"NULL" is defined to "__null". Although it is a null pointer constant not a null pointer, it is guaranteed to be of the
same size as a pointer. But this use is not portable across different compilers.
-Wno-non-template-friend (C++ and Objective-C++ only)
Disable warnings when non-templatized friend functions are declared within a template. Since the advent of explicit
template specification support in G++, if the name of the friend is an unqualified-id (i.e., friend foo(int)), the C++
language specification demands that the friend declare or define an ordinary, nontemplate function. (Section 14.5.3).
Before G++ implemented explicit specification, unqualified-ids could be interpreted as a particular specialization of a
templatized function. Because this non-conforming behavior is no longer the default behavior for G++,
-Wnon-template-friend allows the compiler to check existing code for potential trouble spots and is on by default. This
new compiler behavior can be turned off with -Wno-non-template-friend, which keeps the conformant compiler code but
disables the helpful warning.
-Wold-style-cast (C++ and Objective-C++ only)
Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts (dynamic_cast,
static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and much easier to search for.
-Woverloaded-virtual (C++ and Objective-C++ only)
Warn when a function declaration hides virtual functions from a base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();
will fail to compile.
-Wno-pmf-conversions (C++ and Objective-C++ only)
Disable the diagnostic for converting a bound pointer to member function to a plain pointer.
-Wsign-promo (C++ and Objective-C++ only)
Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to
an unsigned type of the same size. Previous versions of G++ would try to preserve unsignedness, but the standard
mandates the current behavior.
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, G++ will synthesize a default A& operator = (const A&);, while cfront will use the user-defined operator
=.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves.
This section describes the command-line options that are only meaningful for Objective-C and Objective-C++ programs, but you
can also use most of the language-independent GNU compiler options. For example, you might compile a file "some_class.m"
like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; you can use the other
options with any language supported by GCC.
Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the
C front-end (e.g., -Wtraditional). Similarly, Objective-C++ compilations may use C++-specific options (e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C and Objective-C++ programs:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for each literal string specified with the syntax "@"..."". The
default class name is "NXConstantString" if the GNU runtime is being used, and "NSConstantString" if the NeXT runtime is
being used (see below). The -fconstant-cfstrings option, if also present, will override the -fconstant-string-class
setting and cause "@"..."" literals to be laid out as constant CoreFoundation strings.
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of
systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including Darwin and Mac
OS X. The macro "__NEXT_RUNTIME__" is predefined if (and only if) this option is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches ("[receiver message:arg]") in this translation unit ensure that the
receiver is not "nil". This allows for more efficient entry points in the runtime to be used. This option is only
available in conjunction with the NeXT runtime and ABI version 0 or 1.
-fobjc-abi-version=n
Use version n of the Objective-C ABI for the selected runtime. This option is currently supported only for the NeXT
runtime. In that case, Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C
2.0 additions. Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0
additions. Version 2 is the modern (64-bit) ABI. If nothing is specified, the default is Version 0 on 32-bit target
machines, and Version 2 on 64-bit target machines.
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial default
constructor. If so, synthesize a special "- (id) .cxx_construct" instance method which will run non-trivial default
constructors on any such instance variables, in order, and then return "self". Similarly, check if any instance variable
is a C++ object with a non-trivial destructor, and if so, synthesize a special "- (void) .cxx_destruct" method which will
run all such default destructors, in reverse order.
The "- (id) .cxx_construct" and "- (void) .cxx_destruct" methods thusly generated will only operate on instance variables
declared in the current Objective-C class, and not those inherited from superclasses. It is the responsibility of the
Objective-C runtime to invoke all such methods in an object's inheritance hierarchy. The "- (id) .cxx_construct" methods
will be invoked by the runtime immediately after a new object instance is allocated; the "- (void) .cxx_destruct" methods
will be invoked immediately before the runtime deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the "- (id) .cxx_construct"
and "- (void) .cxx_destruct" methods.
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page.
-fobjc-exceptions
Enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C++ and Java.
This option is required to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized. This option is
available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the NeXT runtime on Mac
OS X 10.2 and earlier).
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++ programs. This option is only available with the NeXT
runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags.
-fobjc-nilcheck
For the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual
method call. This is the default and can be disabled using -fno-objc-nilcheck. Class methods and super calls are never
checked for nil in this way no matter what this flag is set to. Currently this flag does nothing when the GNU runtime,
or an older version of the NeXT runtime ABI, is used.
-fobjc-std=objc1
Conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C
additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is controlled by the separate
C/C++ dialect option flags. When this option is used with the Objective-C or Objective-C++ compiler, any Objective-C
syntax that is not recognized by GCC 4.0 is rejected. This is useful if you need to make sure that your Objective-C code
can be compiled with older versions of GCC.
-freplace-objc-classes
Emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow dyld(1) to load it
in at run time instead. This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in
question may be recompiled and dynamically reloaded in the course of program execution, without the need to restart the
program itself. Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac
OS X 10.3 and later.
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily replaces calls to "objc_getClass("...")" (when the name of
the class is known at compile time) with static class references that get initialized at load time, which improves run-
time performance. Specifying the -fzero-link flag suppresses this behavior and causes calls to "objc_getClass("...")"
to be retained. This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be
modified during program execution. The GNU runtime currently always retains calls to "objc_get_class("...")" regardless
of command-line options.
-gen-decls
Dump interface declarations for all classes seen in the source file to a file named sourcename.decl.
-Wassign-intercept (Objective-C and Objective-C++ only)
Warn whenever an Objective-C assignment is being intercepted by the garbage collector.
-Wno-protocol (Objective-C and Objective-C++ only)
If a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not
implemented by the class. The default behavior is to issue a warning for every method not explicitly implemented in the
class, even if a method implementation is inherited from the superclass. If you use the -Wno-protocol option, then
methods inherited from the superclass are considered to be implemented, and no warning is issued for them.
-Wselector (Objective-C and Objective-C++ only)
Warn if multiple methods of different types for the same selector are found during compilation. The check is performed
on the list of methods in the final stage of compilation. Additionally, a check is performed for each selector appearing
in a "@selector(...)" expression, and a corresponding method for that selector has been found during compilation.
Because these checks scan the method table only at the end of compilation, these warnings are not produced if the final
stage of compilation is not reached, for example because an error is found during compilation, or because the
-fsyntax-only option is being used.
-Wstrict-selector-match (Objective-C and Objective-C++ only)
Warn if multiple methods with differing argument and/or return types are found for a given selector when attempting to
send a message using this selector to a receiver of type "id" or "Class". When this flag is off (which is the default
behavior), the compiler will omit such warnings if any differences found are confined to types that share the same size
and alignment.
-Wundeclared-selector (Objective-C and Objective-C++ only)
Warn if a "@selector(...)" expression referring to an undeclared selector is found. A selector is considered undeclared
if no method with that name has been declared before the "@selector(...)" expression, either explicitly in an @interface
or @protocol declaration, or implicitly in an @implementation section. This option always performs its checks as soon as
a "@selector(...)" expression is found, while -Wselector only performs its checks in the final stage of compilation.
This also enforces the coding style convention that methods and selectors must be declared before being used.
-print-objc-runtime-info
Generate C header describing the largest structure that is passed by value, if any.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...). The
options described below can be used to control the diagnostic messages formatting algorithm, e.g. how many characters per
line, how often source location information should be reported. Right now, only the C++ front end can honor these options.
However it is expected, in the near future, that the remaining front ends would be able to digest them correctly.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n characters. The default is 72 characters for g++ and 0
for the rest of the front ends supported by GCC. If n is zero, then no line-wrapping will be done; each error message
will appear on a single line.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit once source location
information; that is, in case the message is too long to fit on a single physical line and has to be wrapped, the source
location won't be emitted (as prefix) again, over and over, in subsequent continuation lines. This is the default
behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same source location
information (as prefix) for physical lines that result from the process of breaking a message which is too long to fit on
a single line.
-fno-diagnostics-show-option
By default, each diagnostic emitted includes text indicating the command-line option that directly controls the
diagnostic (if such an option is known to the diagnostic machinery). Specifying the -fno-diagnostics-show-option flag
suppresses that behavior.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest
there may have been an error.
The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by
GCC.
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond that.
-fmax-errors=n
Limits the maximum number of error messages to n, at which point GCC bails out rather than attempting to continue
processing the source code. If n is 0 (the default), there is no limit on the number of error messages produced. If
-Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this option.
-w Inhibit all warning messages.
-Werror
Make all warnings into errors.
-Werror=
Make the specified warning into an error. The specifier for a warning is appended, for example -Werror=switch turns the
warnings controlled by -Wswitch into errors. This switch takes a negative form, to be used to negate -Werror for
specific warnings, for example -Wno-error=switch makes -Wswitch warnings not be errors, even when -Werror is in effect.
The warning message for each controllable warning includes the option that controls the warning. That option can then be
used with -Werror= and -Wno-error= as described above. (Printing of the option in the warning message can be disabled
using the -fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply anything.
-Wfatal-errors
This option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and
printing further error messages.
You can request many specific warnings with options beginning -W, for example -Wimplicit to request warnings on implicit
declarations. Each of these specific warning options also has a negative form beginning -Wno- to turn off warnings; for
example, -Wno-implicit. This manual lists only one of the two forms, whichever is not the default. For further, language-
specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.
When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC will emit a diagnostic stating that the
option is not recognized. However, if the -Wno- form is used, the behavior is slightly different: No diagnostic will be
produced for -Wno-unknown-warning unless other diagnostics are being produced. This allows the use of new -Wno- options with
old compilers, but if something goes wrong, the compiler will warn that an unrecognized option was used.
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden extensions, and some
other programs that do not follow ISO C and ISO C++. For ISO C, follows the version of the ISO C standard specified by
any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few will require
-ansi or a -std option specifying the required version of ISO C). However, without this option, certain GNU extensions
and traditional C and C++ features are supported as well. With this option, they are rejected.
-pedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __. Pedantic
warnings are also disabled in the expression that follows "__extension__". However, only system header files should use
these escape routes; application programs should avoid them.
Some users try to use -pedantic to check programs for strict ISO C conformance. They soon find that it does not do quite
what they want: it finds some non-ISO practices, but not all---only those for which ISO C requires a diagnostic, and some
others for which diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable
additional work and would be quite different from -pedantic. We don't have plans to support such a feature in the near
future.
Where the standard specified with -std represents a GNU extended dialect of C, such as gnu90 or gnu99, there is a
corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from -pedantic
are given where they are required by the base standard. (It would not make sense for such warnings to be given only for
features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler
supports with the given option, and there would be nothing to warn about.)
-pedantic-errors
Like -pedantic, except that errors are produced rather than warnings.
-Wall
This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or
modify to prevent the warning), even in conjunction with macros. This also enables some language-specific warnings
described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.
-Wall turns on the following warning flags:
-Waddress -Warray-bounds (only with -O2) -Wc++11-compat -Wchar-subscripts -Wenum-compare (in C/Objc; this is on by
default in C++) -Wimplicit-int (C and Objective-C only) -Wimplicit-function-declaration (C and Objective-C only)
-Wcomment -Wformat -Wmain (only for C/ObjC and unless -ffreestanding) -Wmaybe-uninitialized -Wmissing-braces -Wnonnull
-Wparentheses -Wpointer-sign -Wreorder -Wreturn-type -Wsequence-point -Wsign-compare (only in C++) -Wstrict-aliasing
-Wstrict-overflow=1 -Wswitch -Wtrigraphs -Wuninitialized -Wunknown-pragmas -Wunused-function -Wunused-label
-Wunused-value -Wunused-variable -Wvolatile-register-var
Note that some warning flags are not implied by -Wall. Some of them warn about constructions that users generally do not
consider questionable, but which occasionally you might wish to check for; others warn about constructions that are
necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning. Some of
them are enabled by -Wextra but many of them must be enabled individually.
-Wextra
This enables some extra warning flags that are not enabled by -Wall. (This option used to be called -W. The older name
is still supported, but the newer name is more descriptive.)
-Wclobbered -Wempty-body -Wignored-qualifiers -Wmissing-field-initializers -Wmissing-parameter-type (C only)
-Wold-style-declaration (C only) -Woverride-init -Wsign-compare -Wtype-limits -Wuninitialized -Wunused-parameter (only
with -Wunused or -Wall) -Wunused-but-set-parameter (only with -Wunused or -Wall)
The option -Wextra also prints warning messages for the following cases:
· A pointer is compared against integer zero with <, <=, >, or >=.
· (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.
· (C++ only) Ambiguous virtual bases.
· (C++ only) Subscripting an array that has been declared register.
· (C++ only) Taking the address of a variable that has been declared register.
· (C++ only) A base class is not initialized in a derived class' copy constructor.
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common cause of error, as programmers often forget that this type
is signed on some machines. This warning is enabled by -Wall.
-Wcomment
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a Backslash-Newline appears in a //
comment. This warning is enabled by -Wall.
-Wno-coverage-mismatch
Warn if feedback profiles do not match when using the -fprofile-use option. If a source file was changed between
-fprofile-gen and -fprofile-use, the files with the profile feedback can fail to match the source file and GCC cannot use
the profile feedback information. By default, this warning is enabled and is treated as an error.
-Wno-coverage-mismatch can be used to disable the warning or -Wno-error=coverage-mismatch can be used to disable the
error. Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very
minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended.
-Wno-cpp
(C, Objective-C, C++, Objective-C++ and Fortran only)
Suppress warning messages emitted by "#warning" directives.
-Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
Give a warning when a value of type "float" is implicitly promoted to "double". CPUs with a 32-bit "single-precision"
floating-point unit implement "float" in hardware, but emulate "double" in software. On such a machine, doing
computations using "double" values is much more expensive because of the overhead required for software emulation.
It is easy to accidentally do computations with "double" because floating-point literals are implicitly of type "double".
For example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}
the compiler will perform the entire computation with "double" because the floating-point literal is a "double".
-Wformat
Check calls to "printf" and "scanf", etc., to make sure that the arguments supplied have types appropriate to the format
string specified, and that the conversions specified in the format string make sense. This includes standard functions,
and others specified by format attributes, in the "printf", "scanf", "strftime" and "strfmon" (an X/Open extension, not
in the C standard) families (or other target-specific families). Which functions are checked without format attributes
having been specified depends on the standard version selected, and such checks of functions without the attribute
specified are disabled by -ffreestanding or -fno-builtin.
The formats are checked against the format features supported by GNU libc version 2.2. These include all ISO C90 and C99
features, as well as features from the Single Unix Specification and some BSD and GNU extensions. Other library
implementations may not support all these features; GCC does not support warning about features that go beyond a
particular library's limitations. However, if -pedantic is used with -Wformat, warnings will be given about format
features not in the selected standard version (but not for "strfmon" formats, since those are not in any version of the C
standard).
Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull.
-Wformat is included in -Wall. For more control over some aspects of format checking, the options -Wformat-y2k,
-Wno-format-extra-args, -Wno-format-zero-length, -Wformat-nonliteral, -Wformat-security, and -Wformat=2 are available,
but are not included in -Wall.
NOTE: In Ubuntu 8.10 and later versions this option is enabled by default for C, C++, ObjC, ObjC++. To disable, use
-Wformat=0.
-Wformat-y2k
If -Wformat is specified, also warn about "strftime" formats that may yield only a two-digit year.
-Wno-format-contains-nul
If -Wformat is specified, do not warn about format strings that contain NUL bytes.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess arguments to a "printf" or "scanf" format function. The C standard
specifies that such arguments are ignored.
Where the unused arguments lie between used arguments that are specified with $ operand number specifications, normally
warnings are still given, since the implementation could not know what type to pass to "va_arg" to skip the unused
arguments. However, in the case of "scanf" formats, this option will suppress the warning if the unused arguments are
all pointers, since the Single Unix Specification says that such unused arguments are allowed.
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are
allowed.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked, unless the
format function takes its format arguments as a "va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format functions that represent possible security problems. At
present, this warns about calls to "printf" and "scanf" functions where the format string is not a string literal and
there are no format arguments, as in "printf (foo);". This may be a security hole if the format string came from
untrusted input and contains %n. (This is currently a subset of what -Wformat-nonliteral warns about, but in future
warnings may be added to -Wformat-security that are not included in -Wformat-nonliteral.)
NOTE: In Ubuntu 8.10 and later versions this option is enabled by default for C, C++, ObjC, ObjC++. To disable, use
-Wno-format-security, or disable all format warnings with -Wformat=0. To make format security warnings fatal, specify
-Werror=format-security.
-Wformat=2
Enable -Wformat plus format checks not included in -Wformat. Currently equivalent to -Wformat -Wformat-nonliteral
-Wformat-security -Wformat-y2k.
-Wnonnull
Warn about passing a null pointer for arguments marked as requiring a non-null value by the "nonnull" function attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.
-Winit-self (C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables that are initialized with themselves. Note this option can only be used with the
-Wuninitialized option.
For example, GCC will warn about "i" being uninitialized in the following snippet only when -Winit-self has been
specified:
int f()
{
int i = i;
return i;
}
-Wimplicit-int (C and Objective-C only)
Warn when a declaration does not specify a type. This warning is enabled by -Wall.
-Wimplicit-function-declaration (C and Objective-C only)
Give a warning whenever a function is used before being declared. In C99 mode (-std=c99 or -std=gnu99), this warning is
enabled by default and it is made into an error by -pedantic-errors. This warning is also enabled by -Wall.
-Wimplicit (C and Objective-C only)
Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.
-Wignored-qualifiers (C and C++ only)
Warn if the return type of a function has a type qualifier such as "const". For ISO C such a type qualifier has no
effect, since the value returned by a function is not an lvalue. For C++, the warning is only emitted for scalar types
or "void". ISO C prohibits qualified "void" return types on function definitions, so such return types always receive a
warning even without this option.
This warning is also enabled by -Wextra.
-Wmain
Warn if the type of main is suspicious. main should be a function with external linkage, returning int, taking either
zero arguments, two, or three arguments of appropriate types. This warning is enabled by default in C++ and is enabled
by either -Wall or -pedantic.
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed. In the following example, the initializer for a is not
fully bracketed, but that for b is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
Warn if a user-supplied include directory does not exist.
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value
is expected, or when operators are nested whose precedence people often get confused about.
Also warn if a comparison like x<=y<=z appears; this is equivalent to (x<=y ? 1 : 0) <= z, which is a different
interpretation from that of ordinary mathematical notation.
Also warn about constructions where there may be confusion to which "if" statement an "else" branch belongs. Here is an
example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C/C++, every "else" branch belongs to the innermost possible "if" statement, which in this example is "if (b)". This
is often not what the programmer expected, as illustrated in the above example by indentation the programmer chose. When
there is the potential for this confusion, GCC will issue a warning when this flag is specified. To eliminate the
warning, add explicit braces around the innermost "if" statement so there is no way the "else" could belong to the
enclosing "if". The resulting code would look like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
Also warn for dangerous uses of the ?: with omitted middle operand GNU extension. When the condition in the ?: operator
is a boolean expression the omitted value will be always 1. Often the user expects it to be a value computed inside the
conditional expression instead.
This warning is enabled by -Wall.
-Wsequence-point
Warn about code that may have undefined semantics because of violations of sequence point rules in the C and C++
standards.
The C and C++ standards defines the order in which expressions in a C/C++ program are evaluated in terms of sequence
points, which represent a partial ordering between the execution of parts of the program: those executed before the
sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not
part of a larger expression), after the evaluation of the first operand of a "&&", "||", "? :" or "," (comma) operator,
before a function is called (but after the evaluation of its arguments and the expression denoting the called function),
and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of
subexpressions of an expression is not specified. All these rules describe only a partial order rather than a total
order, since, for example, if two functions are called within one expression with no sequence point between them, the
order in which the functions are called is not specified. However, the standards committee have ruled that function
calls do not overlap.
It is not specified when between sequence points modifications to the values of objects take effect. Programs whose
behavior depends on this have undefined behavior; the C and C++ standards specify that "Between the previous and next
sequence point an object shall have its stored value modified at most once by the evaluation of an expression.
Furthermore, the prior value shall be read only to determine the value to be stored.". If a program breaks these rules,
the results on any particular implementation are entirely unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] = b[n++]" and "a[i++] = i;". Some more complicated cases
are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found
fairly effective at detecting this sort of problem in programs.
The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules
in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC
readings page, at <http://gcc.gnu.org/readings.html>.
This warning is enabled by -Wall for C and C++.
-Wreturn-type
Warn whenever a function is defined with a return-type that defaults to "int". Also warn about any "return" statement
with no return-value in a function whose return-type is not "void" (falling off the end of the function body is
considered returning without a value), and about a "return" statement with an expression in a function whose return-type
is "void".
For C++, a function without return type always produces a diagnostic message, even when -Wno-return-type is specified.
The only exceptions are main and functions defined in system headers.
This warning is enabled by -Wall.
-Wswitch
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of the named codes
of that enumeration. (The presence of a "default" label prevents this warning.) "case" labels outside the enumeration
range also provoke warnings when this option is used (even if there is a "default" label). This warning is enabled by
-Wall.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of the named codes
of that enumeration. "case" labels outside the enumeration range also provoke warnings when this option is used. The
only difference between -Wswitch and this option is that this option gives a warning about an omitted enumeration code
even if there is a "default" label.
-Wsync-nand (C and C++ only)
Warn when "__sync_fetch_and_nand" and "__sync_nand_and_fetch" built-in functions are used. These functions changed
semantics in GCC 4.4.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of the program (trigraphs within comments are not
warned about). This warning is enabled by -Wall.
-Wunused-but-set-parameter
Warn whenever a function parameter is assigned to, but otherwise unused (aside from its declaration).
To suppress this warning use the unused attribute.
This warning is also enabled by -Wunused together with -Wextra.
-Wunused-but-set-variable
Warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This warning is
enabled by -Wall.
To suppress this warning use the unused attribute.
This warning is also enabled by -Wunused, which is enabled by -Wall.
-Wunused-function
Warn whenever a static function is declared but not defined or a non-inline static function is unused. This warning is
enabled by -Wall.
-Wunused-label
Warn whenever a label is declared but not used. This warning is enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only)
Warn when a typedef locally defined in a function is not used.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the unused attribute.
-Wno-unused-result
Do not warn if a caller of a function marked with attribute "warn_unused_result" does not use its return value. The
default is -Wunused-result.
-Wunused-variable
Warn whenever a local variable or non-constant static variable is unused aside from its declaration. This warning is
enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not used. To suppress this warning cast the unused
expression to void. This includes an expression-statement or the left-hand side of a comma expression that contains no
side effects. For example, an expression such as x[i,j] will cause a warning, while x[(void)i,j] will not.
This warning is enabled by -Wall.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you must either specify -Wextra -Wunused (note that -Wall
implies -Wunused), or separately specify -Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first being initialized or if a variable may be clobbered by a "setjmp"
call. In C++, warn if a non-static reference or non-static const member appears in a class without constructors.
If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the
-Winit-self option.
These warnings occur for individual uninitialized or clobbered elements of structure, union or array variables as well as
for variables that are uninitialized or clobbered as a whole. They do not occur for variables or elements declared
"volatile". Because these warnings depend on optimization, the exact variables or elements for which there are warnings
will depend on the precise optimization options and version of GCC used.
Note that there may be no warning about a variable that is used only to compute a value that itself is never used,
because such computations may be deleted by data flow analysis before the warnings are printed.
-Wmaybe-uninitialized
For an automatic variable, if there exists a path from the function entry to a use of the variable that is initialized,
but there exist some other paths the variable is not initialized, the compiler will emit a warning if it can not prove
the uninitialized paths do not happen at run time. These warnings are made optional because GCC is not smart enough to
see all the reasons why the code might be correct despite appearing to have an error. Here is one example of how this
can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is always initialized, but GCC doesn't know this. To suppress the
warning, the user needs to provide a default case with assert(0) or similar code.
This option also warns when a non-volatile automatic variable might be changed by a call to "longjmp". These warnings as
well are possible only in optimizing compilation.
The compiler sees only the calls to "setjmp". It cannot know where "longjmp" will be called; in fact, a signal handler
could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem because
"longjmp" cannot in fact be called at the place that would cause a problem.
Some spurious warnings can be avoided if you declare all the functions you use that never return as "noreturn".
This warning is enabled by -Wall or -Wextra.
-Wunknown-pragmas
Warn when a "#pragma" directive is encountered that is not understood by GCC. If this command-line option is used,
warnings will even be issued for unknown pragmas in system header files. This is not the case if the warnings were only
enabled by the -Wall command-line option.
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See
also -Wunknown-pragmas.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing
rules that the compiler is using for optimization. The warning does not catch all cases, but does attempt to catch the
more common pitfalls. It is included in -Wall. It is equivalent to -Wstrict-aliasing=3
-Wstrict-aliasing=n
This option is only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing
rules that the compiler is using for optimization. Higher levels correspond to higher accuracy (fewer false positives).
Higher levels also correspond to more effort, similar to the way -O works. -Wstrict-aliasing is equivalent to
-Wstrict-aliasing=n, with n=3.
Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but -fstrict-aliasing
still breaks the code, as it has very few false negatives. However, it has many false positives. Warns for all pointer
conversions between possibly incompatible types, even if never dereferenced. Runs in the front end only.
Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and
few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns
about incomplete types. Runs in the front end only.
Level 3 (default for -Wstrict-aliasing): Should have very few false positives and few false negatives. Slightly slower
than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end:
"*(int*)&some_float". If optimization is enabled, it also runs in the back end, where it deals with multiple statement
cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn
about incomplete types.
-Wstrict-overflow
-Wstrict-overflow=n
This option is only active when -fstrict-overflow is active. It warns about cases where the compiler optimizes based on
the assumption that signed overflow does not occur. Note that it does not warn about all cases where the code might
overflow: it only warns about cases where the compiler implements some optimization. Thus this warning depends on the
optimization level.
An optimization that assumes that signed overflow does not occur is perfectly safe if the values of the variables
involved are such that overflow never does, in fact, occur. Therefore this warning can easily give a false positive: a
warning about code that is not actually a problem. To help focus on important issues, several warning levels are
defined. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop will
require, in particular when determining whether a loop will be executed at all.
-Wstrict-overflow=1
Warn about cases that are both questionable and easy to avoid. For example: "x + 1 > x"; with -fstrict-overflow, the
compiler will simplify this to 1. This level of -Wstrict-overflow is enabled by -Wall; higher levels are not, and
must be explicitly requested.
-Wstrict-overflow=2
Also warn about other cases where a comparison is simplified to a constant. For example: "abs (x) >= 0". This can
only be simplified when -fstrict-overflow is in effect, because "abs (INT_MIN)" overflows to "INT_MIN", which is less
than zero. -Wstrict-overflow (with no level) is the same as -Wstrict-overflow=2.
-Wstrict-overflow=3
Also warn about other cases where a comparison is simplified. For example: "x + 1 > 1" will be simplified to "x >
0".
-Wstrict-overflow=4
Also warn about other simplifications not covered by the above cases. For example: "(x * 10) / 5" will be simplified
to "x * 2".
-Wstrict-overflow=5
Also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example:
"x + 2 > y" will be simplified to "x + 1 >= y". This is reported only at the highest warning level because this
simplification applies to many comparisons, so this warning level will give a very large number of false positives.
-Wsuggest-attribute=[pure|const|noreturn]
Warn for cases where adding an attribute may be beneficial. The attributes currently supported are listed below.
-Wsuggest-attribute=pure
-Wsuggest-attribute=const
-Wsuggest-attribute=noreturn
Warn about functions that might be candidates for attributes "pure", "const" or "noreturn". The compiler only warns
for functions visible in other compilation units or (in the case of "pure" and "const") if it cannot prove that the
function returns normally. A function returns normally if it doesn't contain an infinite loop nor returns abnormally
by throwing, calling "abort()" or trapping. This analysis requires option -fipa-pure-const, which is enabled by
default at -O and higher. Higher optimization levels improve the accuracy of the analysis.
-Warray-bounds
This option is only active when -ftree-vrp is active (default for -O2 and above). It warns about subscripts to arrays
that are always out of bounds. This warning is enabled by -Wall.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating-point division by zero is not warned about, as it can
be a legitimate way of obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files. Warnings from system headers are normally
suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler output
harder to read. Using this command-line option tells GCC to emit warnings from system headers as if they occurred in
user code. However, note that using -Wall in conjunction with this option will not warn about unknown pragmas in system
headers---for that, -Wunknown-pragmas must also be used.
-Wtrampolines
Warn about trampolines generated for pointers to nested functions.
A trampoline is a small piece of data or code that is created at run
time on the stack when the address of a nested function is taken, and
is used to call the nested function indirectly. For some targets, it
is made up of data only and thus requires no special treatment. But,
for most targets, it is made up of code and thus requires the stack
to be made executable in order for the program to work properly.
-Wfloat-equal
Warn if floating-point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as
approximations to infinitely precise real numbers. If you are doing this, then you need to compute (by analyzing the
code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when
performing comparisons (and when producing output, but that's a different problem). In particular, instead of testing
for equality, you would check to see whether the two values have ranges that overlap; and this is done with the
relational operators, so equality comparisons are probably mistaken.
-Wtraditional (C and Objective-C only)
Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that
have no traditional C equivalent, and/or problematic constructs that should be avoided.
· Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes
place within string literals, but does not in ISO C.
· In traditional C, some preprocessor directives did not exist. Traditional preprocessors would only consider a line
to be a directive if the # appeared in column 1 on the line. Therefore -Wtraditional warns about directives that
traditional C understands but would ignore because the # does not appear as the first character on the line. It also
suggests you hide directives like #pragma not understood by traditional C by indenting them. Some traditional
implementations would not recognize #elif, so it suggests avoiding it altogether.
· A function-like macro that appears without arguments.
· The unary plus operator.
· The U integer constant suffix, or the F or L floating-point constant suffixes. (Traditional C does support the L
suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern
systems, e.g. the _MIN/_MAX macros in "<limits.h>". Use of these macros in user code might normally lead to spurious
warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases.
· A function declared external in one block and then used after the end of the block.
· A "switch" statement has an operand of type "long".
· A non-"static" function declaration follows a "static" one. This construct is not accepted by some traditional C
compilers.
· The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is
only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit
patterns, are not warned about.
· Usage of ISO string concatenation is detected.
· Initialization of automatic aggregates.
· Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
· Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption
that the zero initializer in user code appears conditioned on e.g. "__STDC__" to avoid missing initializer warnings
and relies on default initialization to zero in the traditional C case.
· Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when
compiling with traditional C would cause serious problems. This is a subset of the possible conversion warnings, for
the full set use -Wtraditional-conversion.
· Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or
variadic functions because these ISO C features will appear in your code when using libiberty's traditional C
compatibility macros, "PARAMS" and "VPARAMS". This warning is also bypassed for nested functions because that
feature is already a GCC extension and thus not relevant to traditional C compatibility.
-Wtraditional-conversion (C and Objective-C only)
Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence
of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width
or signedness of a fixed-point argument except when the same as the default promotion.
-Wdeclaration-after-statement (C and Objective-C only)
Warn when a declaration is found after a statement in a block. This construct, known from C++, was introduced with ISO
C99 and is by default allowed in GCC. It is not supported by ISO C90 and was not supported by GCC versions before GCC
3.0.
-Wundef
Warn if an undefined identifier is evaluated in an #if directive.
-Wno-endif-labels
Do not warn whenever an #else or an #endif are followed by text.
-Wshadow
Warn whenever a local variable or type declaration shadows another variable, parameter, type, or class member (in C++),
or whenever a built-in function is shadowed. Note that in C++, the compiler will not warn if a local variable shadows a
struct/class/enum, but will warn if it shadows an explicit typedef.
-Wlarger-than=len
Warn whenever an object of larger than len bytes is defined.
-Wframe-larger-than=len
Warn if the size of a function frame is larger than len bytes. The computation done to determine the stack frame size is
approximate and not conservative. The actual requirements may be somewhat greater than len even if you do not get a
warning. In addition, any space allocated via "alloca", variable-length arrays, or related constructs is not included by
the compiler when determining whether or not to issue a warning.
-Wno-free-nonheap-object
Do not warn when attempting to free an object that was not allocated on the heap.
-Wstack-usage=len
Warn if the stack usage of a function might be larger than len bytes. The computation done to determine the stack usage
is conservative. Any space allocated via "alloca", variable-length arrays, or related constructs is included by the
compiler when determining whether or not to issue a warning.
The message is in keeping with the output of -fstack-usage.
· If the stack usage is fully static but exceeds the specified amount, it's:
warning: stack usage is 1120 bytes
· If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes
· If the stack usage is (partly) dynamic and not bounded, it's:
warning: stack usage might be unbounded
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler could not assume anything on the bounds of the loop indices.
With -funsafe-loop-optimizations warn if the compiler made such assumptions.
-Wno-pedantic-ms-format (MinGW targets only)
Disables the warnings about non-ISO "printf" / "scanf" format width specifiers "I32", "I64", and "I" used on Windows
targets depending on the MS runtime, when you are using the options -Wformat and -pedantic without gnu-extensions.
-Wpointer-arith
Warn about anything that depends on the "size of" a function type or of "void". GNU C assigns these types a size of 1,
for convenience in calculations with "void *" pointers and pointers to functions. In C++, warn also when an arithmetic
operation involves "NULL". This warning is also enabled by -pedantic.
-Wtype-limits
Warn if a comparison is always true or always false due to the limited range of the data type, but do not warn for
constant expressions. For example, warn if an unsigned variable is compared against zero with < or >=. This warning is
also enabled by -Wextra.
-Wbad-function-cast (C and Objective-C only)
Warn whenever a function call is cast to a non-matching type. For example, warn if "int malloc()" is cast to "anything
*".
-Wc++-compat (C and Objective-C only)
Warn about ISO C constructs that are outside of the common subset of ISO C and ISO C++, e.g. request for implicit
conversion from "void *" to a pointer to non-"void" type.
-Wc++11-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g., identifiers in ISO C++ 1998
that are keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is enabled by -Wall.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if a "const
char *" is cast to an ordinary "char *".
Also warn when making a cast that introduces a type qualifier in an unsafe way. For example, casting "char **" to "const
char **" is unsafe, as in this example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a
"char *" is cast to an "int *" on machines where integers can only be accessed at two- or four-byte boundaries.
-Wwrite-strings
When compiling C, give string constants the type "const char[length]" so that copying the address of one into a
non-"const" "char *" pointer will get a warning. These warnings will help you find at compile time code that can try to
write into a string constant, but only if you have been very careful about using "const" in declarations and prototypes.
Otherwise, it will just be a nuisance. This is why we did not make -Wall request these warnings.
When compiling C++, warn about the deprecated conversion from string literals to "char *". This warning is enabled by
default for C++ programs.
-Wclobbered
Warn for variables that might be changed by longjmp or vfork. This warning is also enabled by -Wextra.
-Wconversion
Warn for implicit conversions that may alter a value. This includes conversions between real and integer, like "abs (x)"
when "x" is "double"; conversions between signed and unsigned, like "unsigned ui = -1"; and conversions to smaller types,
like "sqrtf (M_PI)". Do not warn for explicit casts like "abs ((int) x)" and "ui = (unsigned) -1", or if the value is not
changed by the conversion like in "abs (2.0)". Warnings about conversions between signed and unsigned integers can be
disabled by using -Wno-sign-conversion.
For C++, also warn for confusing overload resolution for user-defined conversions; and conversions that will never use a
type conversion operator: conversions to "void", the same type, a base class or a reference to them. Warnings about
conversions between signed and unsigned integers are disabled by default in C++ unless -Wsign-conversion is explicitly
enabled.
-Wno-conversion-null (C++ and Objective-C++ only)
Do not warn for conversions between "NULL" and non-pointer types. -Wconversion-null is enabled by default.
-Wzero-as-null-pointer-constant (C++ and Objective-C++ only)
Warn when a literal '0' is used as null pointer constant. This can be useful to facilitate the conversion to "nullptr"
in C++11.
-Wempty-body
Warn if an empty body occurs in an if, else or do while statement. This warning is also enabled by -Wextra.
-Wenum-compare
Warn about a comparison between values of different enumerated types. In C++ enumeral mismatches in conditional
expressions are also diagnosed and the warning is enabled by default. In C this warning is enabled by -Wall.
-Wjump-misses-init (C, Objective-C only)
Warn if a "goto" statement or a "switch" statement jumps forward across the initialization of a variable, or jumps
backward to a label after the variable has been initialized. This only warns about variables that are initialized when
they are declared. This warning is only supported for C and Objective-C; in C++ this sort of branch is an error in any
case.
-Wjump-misses-init is included in -Wc++-compat. It can be disabled with the -Wno-jump-misses-init option.
-Wsign-compare
Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value is
converted to unsigned. This warning is also enabled by -Wextra; to get the other warnings of -Wextra without this
warning, use -Wextra -Wno-sign-compare.
-Wsign-conversion
Warn for implicit conversions that may change the sign of an integer value, like assigning a signed integer expression to
an unsigned integer variable. An explicit cast silences the warning. In C, this option is enabled also by -Wconversion.
-Waddress
Warn about suspicious uses of memory addresses. These include using the address of a function in a conditional
expression, such as "void func(void); if (func)", and comparisons against the memory address of a string literal, such as
"if (x == "abc")". Such uses typically indicate a programmer error: the address of a function always evaluates to true,
so their use in a conditional usually indicate that the programmer forgot the parentheses in a function call; and
comparisons against string literals result in unspecified behavior and are not portable in C, so they usually indicate
that the programmer intended to use "strcmp". This warning is enabled by -Wall.
-Wlogical-op
Warn about suspicious uses of logical operators in expressions. This includes using logical operators in contexts where
a bit-wise operator is likely to be expected.
-Waggregate-return
Warn if any functions that return structures or unions are defined or called. (In languages where you can return an
array, this also elicits a warning.)
-Wno-attributes
Do not warn if an unexpected "__attribute__" is used, such as unrecognized attributes, function attributes applied to
variables, etc. This will not stop errors for incorrect use of supported attributes.
-Wno-builtin-macro-redefined
Do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of "__TIMESTAMP__",
"__TIME__", "__DATE__", "__FILE__", and "__BASE_FILE__".
-Wstrict-prototypes (C and Objective-C only)
Warn if a function is declared or defined without specifying the argument types. (An old-style function definition is
permitted without a warning if preceded by a declaration that specifies the argument types.)
-Wold-style-declaration (C and Objective-C only)
Warn for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-class specifiers
like "static" are not the first things in a declaration. This warning is also enabled by -Wextra.
-Wold-style-definition (C and Objective-C only)
Warn if an old-style function definition is used. A warning is given even if there is a previous prototype.
-Wmissing-parameter-type (C and Objective-C only)
A function parameter is declared without a type specifier in K&R-style functions:
void foo(bar) { }
This warning is also enabled by -Wextra.
-Wmissing-prototypes (C and Objective-C only)
Warn if a global function is defined without a previous prototype declaration. This warning is issued even if the
definition itself provides a prototype. The aim is to detect global functions that are not declared in header files.
-Wmissing-declarations
Warn if a global function is defined without a previous declaration. Do so even if the definition itself provides a
prototype. Use this option to detect global functions that are not declared in header files. In C++, no warnings are
issued for function templates, or for inline functions, or for functions in anonymous namespaces.
-Wmissing-field-initializers
Warn if a structure's initializer has some fields missing. For example, the following code would cause such a warning,
because "x.h" is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This option does not warn about designated initializers, so the following modification would not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra
-Wno-missing-field-initializers.
-Wmissing-format-attribute
Warn about function pointers that might be candidates for "format" attributes. Note these are only possible candidates,
not absolute ones. GCC will guess that function pointers with "format" attributes that are used in assignment,
initialization, parameter passing or return statements should have a corresponding "format" attribute in the resulting
type. I.e. the left-hand side of the assignment or initialization, the type of the parameter variable, or the return
type of the containing function respectively should also have a "format" attribute to avoid the warning.
GCC will also warn about function definitions that might be candidates for "format" attributes. Again, these are only
possible candidates. GCC will guess that "format" attributes might be appropriate for any function that calls a function
like "vprintf" or "vscanf", but this might not always be the case, and some functions for which "format" attributes are
appropriate may not be detected.
-Wno-multichar
Do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo in the user's code, as they have
implementation-defined values, and should not be used in portable code.
-Wnormalized=<none|id|nfc|nfkc>
In ISO C and ISO C++, two identifiers are different if they are different sequences of characters. However, sometimes
when characters outside the basic ASCII character set are used, you can have two different character sequences that look
the same. To avoid confusion, the ISO 10646 standard sets out some normalization rules which when applied ensure that
two sequences that look the same are turned into the same sequence. GCC can warn you if you are using identifiers that
have not been normalized; this option controls that warning.
There are four levels of warning supported by GCC. The default is -Wnormalized=nfc, which warns about any identifier
that is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses.
Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned into NFC, are not
allowed in identifiers. That is, there's no way to use these symbols in portable ISO C or C++ and have all your
identifiers in NFC. -Wnormalized=id suppresses the warning for these characters. It is hoped that future versions of
the standards involved will correct this, which is why this option is not the default.
You can switch the warning off for all characters by writing -Wnormalized=none. You would only want to do this if you
were using some other normalization scheme (like "D"), because otherwise you can easily create bugs that are literally
impossible to see.
Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially
once formatting has been applied. For instance "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", will display just like a
regular "n" that has been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all these
into a standard form as well, and GCC will warn if your code is not in NFKC if you use -Wnormalized=nfkc. This warning
is comparable to warning about every identifier that contains the letter O because it might be confused with the digit 0,
and so is not the default, but may be useful as a local coding convention if the programming environment is unable to be
fixed to display these characters distinctly.
-Wno-deprecated
Do not warn about usage of deprecated features.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as deprecated by using the "deprecated" attribute.
-Wno-overflow
Do not warn about compile-time overflow in constant expressions.
-Woverride-init (C and Objective-C only)
Warn if an initialized field without side effects is overridden when using designated initializers.
This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-override-init.
-Wpacked
Warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size of the
structure. Such structures may be mis-aligned for little benefit. For instance, in this code, the variable "f.x" in
"struct bar" will be misaligned even though "struct bar" does not itself have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wpacked-bitfield-compat
The 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on bit-fields of type "char". This has been fixed in
GCC 4.4 but the change can lead to differences in the structure layout. GCC informs you when the offset of such a field
has changed in GCC 4.4. For example there is no longer a 4-bit padding between field "a" and "b" in this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable this warning.
-Wpadded
Warn if padding is included in a structure, either to align an element of the structure or to align the whole structure.
Sometimes when this happens it is possible to rearrange the fields of the structure to reduce the padding and so make the
structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and
changes nothing.
-Wnested-externs (C and Objective-C only)
Warn if an "extern" declaration is encountered within a function.
-Winline
Warn if a function can not be inlined and it was declared as inline. Even with this option, the compiler will not warn
about failures to inline functions declared in system headers.
The compiler uses a variety of heuristics to determine whether or not to inline a function. For example, the compiler
takes into account the size of the function being inlined and the amount of inlining that has already been done in the
current function. Therefore, seemingly insignificant changes in the source program can cause the warnings produced by
-Winline to appear or disappear.
-Wno-invalid-offsetof (C++ and Objective-C++ only)
Suppress warnings from applying the offsetof macro to a non-POD type. According to the 1998 ISO C++ standard, applying
offsetof to a non-POD type is undefined. In existing C++ implementations, however, offsetof typically gives meaningful
results even when applied to certain kinds of non-POD types. (Such as a simple struct that fails to be a POD type only by
virtue of having a constructor.) This flag is for users who are aware that they are writing nonportable code and who
have deliberately chosen to ignore the warning about it.
The restrictions on offsetof may be relaxed in a future version of the C++ standard.
-Wno-int-to-pointer-cast
Suppress warnings from casts to pointer type of an integer of a different size. In C++, casting to a pointer type of
smaller size is an error. Wint-to-pointer-cast is enabled by default.
-Wno-pointer-to-int-cast (C and Objective-C only)
Suppress warnings from casts from a pointer to an integer type of a different size.
-Winvalid-pch
Warn if a precompiled header is found in the search path but can't be used.
-Wlong-long
Warn if long long type is used. This is enabled by either -pedantic or -Wtraditional in ISO C90 and C++98 modes. To
inhibit the warning messages, use -Wno-long-long.
-Wvariadic-macros
Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU alternate syntax when in pedantic ISO C99 mode.
This is default. To inhibit the warning messages, use -Wno-variadic-macros.
-Wvector-operation-performance
Warn if vector operation is not implemented via SIMD capabilities of the architecture. Mainly useful for the performance
tuning. Vector operation can be implemented "piecewise", which means that the scalar operation is performed on every
vector element; "in parallel", which means that the vector operation is implemented using scalars of wider type, which
normally is more performance efficient; and "as a single scalar", which means that vector fits into a scalar type.
-Wvla
Warn if variable length array is used in the code. -Wno-vla will prevent the -pedantic warning of the variable length
array.
-Wvolatile-register-var
Warn if a register variable is declared volatile. The volatile modifier does not inhibit all optimizations that may
eliminate reads and/or writes to register variables. This warning is enabled by -Wall.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning does not generally indicate that there is anything wrong
with your code; it merely indicates that GCC's optimizers were unable to handle the code effectively. Often, the problem
is that your code is too big or too complex; GCC will refuse to optimize programs when the optimization itself is likely
to take inordinate amounts of time.
-Wpointer-sign (C and Objective-C only)
Warn for pointer argument passing or assignment with different signedness. This option is only supported for C and
Objective-C. It is implied by -Wall and by -pedantic, which can be disabled with -Wno-pointer-sign.
-Wstack-protector
This option is only active when -fstack-protector is active. It warns about functions that will not be protected against
stack smashing.
-Wno-mudflap
Suppress warnings about constructs that cannot be instrumented by -fmudflap.
-Woverlength-strings
Warn about string constants that are longer than the "minimum maximum" length specified in the C standard. Modern
compilers generally allow string constants that are much longer than the standard's minimum limit, but very portable
programs should avoid using longer strings.
The limit applies after string constant concatenation, and does not count the trailing NUL. In C90, the limit was 509
characters; in C99, it was raised to 4095. C++98 does not specify a normative minimum maximum, so we do not diagnose
overlength strings in C++.
This option is implied by -pedantic, and can be disabled with -Wno-overlength-strings.
-Wunsuffixed-float-constants (C and Objective-C only)
GCC will issue a warning for any floating constant that does not have a suffix. When used together with -Wsystem-headers
it will warn about such constants in system header files. This can be useful when preparing code to use with the
"FLOAT_CONST_DECIMAL64" pragma from the decimal floating-point extension to C99.
Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging either your program or GCC:
-g Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF 2). GDB can work
with this debugging information.
On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra
information makes debugging work better in GDB but will probably make other debuggers crash or refuse to read the
program. If you want to control for certain whether to generate the extra information, use -gstabs+, -gstabs, -gxcoff+,
-gxcoff, or -gvms (see below).
GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally produce surprising results:
some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some
statements may not be executed because they compute constant results or their values were already at hand; some
statements may execute in different places because they were moved out of loops.
Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer for programs
that might have bugs.
The following options are useful when GCC is generated with the capability for more than one debugging format.
-ggdb
Produce debugging information for use by GDB. This means to use the most expressive format available (DWARF 2, stabs, or
the native format if neither of those are supported), including GDB extensions if at all possible.
-gstabs
Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by
DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output that
is not understood by DBX or SDB. On System V Release 4 systems this option requires the GNU assembler.
-feliminate-unused-debug-symbols
Produce debugging information in stabs format (if that is supported), for only symbols that are actually used.
-femit-class-debug-always
Instead of emitting debugging information for a C++ class in only one object file, emit it in all object files using the
class. This option should be used only with debuggers that are unable to handle the way GCC normally emits debugging
information for classes because using this option will increase the size of debugging information by as much as a factor
of two.
-fno-debug-types-section
By default when using DWARF v4 or higher type DIEs will be put into their own .debug_types section instead of making them
part of the .debug_info section. It is more efficient to put them in a separate comdat sections since the linker will
then be able to remove duplicates. But not all DWARF consumers support .debug_types sections yet.
-gstabs+
Produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU
debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program.
-gcoff
Produce debugging information in COFF format (if that is supported). This is the format used by SDB on most System V
systems prior to System V Release 4.
-gxcoff
Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM
RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only by the GNU
debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program, and
may cause assemblers other than the GNU assembler (GAS) to fail with an error.
-gdwarf-version
Produce debugging information in DWARF format (if that is supported). This is the format used by DBX on IRIX 6. The
value of version may be either 2, 3 or 4; the default version is 2.
Note that with DWARF version 2 some ports require, and will always use, some non-conflicting DWARF 3 extensions in the
unwind tables.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit.
-grecord-gcc-switches
This switch causes the command-line options used to invoke the compiler that may affect code generation to be appended to
the DW_AT_producer attribute in DWARF debugging information. The options are concatenated with spaces separating them
from each other and from the compiler version. See also -frecord-gcc-switches for another way of storing compiler
options into the object file.
-gno-record-gcc-switches
Disallow appending command-line options to the DW_AT_producer attribute in DWARF debugging information. This is the
default.
-gstrict-dwarf
Disallow using extensions of later DWARF standard version than selected with -gdwarf-version. On most targets using non-
conflicting DWARF extensions from later standard versions is allowed.
-gno-strict-dwarf
Allow using extensions of later DWARF standard version than selected with -gdwarf-version.
-gvms
Produce debugging information in VMS debug format (if that is supported). This is the format used by DEBUG on VMS
systems.
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gvmslevel
Request debugging information and also use level to specify how much information. The default level is 2.
Level 0 produces no debug information at all. Thus, -g0 negates -g.
Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug.
This includes descriptions of functions and external variables, but no information about local variables and no line
numbers.
Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support
macro expansion when you use -g3.
-gdwarf-2 does not accept a concatenated debug level, because GCC used to support an option -gdwarf that meant to
generate debug information in version 1 of the DWARF format (which is very different from version 2), and it would have
been too confusing. That debug format is long obsolete, but the option cannot be changed now. Instead use an additional
-glevel option to change the debug level for DWARF.
-gtoggle
Turn off generation of debug info, if leaving out this option would have generated it, or turn it on at level 2
otherwise. The position of this argument in the command line does not matter, it takes effect after all other options
are processed, and it does so only once, no matter how many times it is given. This is mainly intended to be used with
-fcompare-debug.
-fdump-final-insns[=file]
Dump the final internal representation (RTL) to file. If the optional argument is omitted (or if file is "."), the name
of the dump file will be determined by appending ".gkd" to the compilation output file name.
-fcompare-debug[=opts]
If no error occurs during compilation, run the compiler a second time, adding opts and -fcompare-debug-second to the
arguments passed to the second compilation. Dump the final internal representation in both compilations, and print an
error if they differ.
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero, implicitly enables -fcompare-debug. If
GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it is used for opts, otherwise the default -gtoggle
is used.
-fcompare-debug=, with the equal sign but without opts, is equivalent to -fno-compare-debug, which disables the dumping
of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect.
To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden,
which GCC will reject as an invalid option in any actual compilation (rather than preprocessing, assembly or linking).
To get just a warning, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do.
-fcompare-debug-second
This option is implicitly passed to the compiler for the second compilation requested by -fcompare-debug, along with
options to silence warnings, and omitting other options that would cause side-effect compiler outputs to files or to the
standard output. Dump files and preserved temporary files are renamed so as to contain the ".gk" additional extension
during the second compilation, to avoid overwriting those generated by the first.
When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it useful
for little other than debugging the compiler proper.
-feliminate-dwarf2-dups
Compress DWARF2 debugging information by eliminating duplicated information about each symbol. This option only makes
sense when generating DWARF2 debugging information with -gdwarf-2.
-femit-struct-debug-baseonly
Emit debug information for struct-like types only when the base name of the compilation source file matches the base name
of file in which the struct was defined.
This option substantially reduces the size of debugging information, but at significant potential loss in type
information to the debugger. See -femit-struct-debug-reduced for a less aggressive option. See
-femit-struct-debug-detailed for more detailed control.
This option works only with DWARF 2.
-femit-struct-debug-reduced
Emit debug information for struct-like types only when the base name of the compilation source file matches the base name
of file in which the type was defined, unless the struct is a template or defined in a system header.
This option significantly reduces the size of debugging information, with some potential loss in type information to the
debugger. See -femit-struct-debug-baseonly for a more aggressive option. See -femit-struct-debug-detailed for more
detailed control.
This option works only with DWARF 2.
-femit-struct-debug-detailed[=spec-list]
Specify the struct-like types for which the compiler will generate debug information. The intent is to reduce duplicate
struct debug information between different object files within the same program.
This option is a detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, which will serve for
most needs.
A specification has the syntax[dir:|ind:][ord:|gen:](any|sys|base|none)
The optional first word limits the specification to structs that are used directly (dir:) or used indirectly (ind:). A
struct type is used directly when it is the type of a variable, member. Indirect uses arise through pointers to structs.
That is, when use of an incomplete struct would be legal, the use is indirect. An example is struct one direct; struct
two * indirect;.
The optional second word limits the specification to ordinary structs (ord:) or generic structs (gen:). Generic structs
are a bit complicated to explain. For C++, these are non-explicit specializations of template classes, or non-template
classes within the above. Other programming languages have generics, but -femit-struct-debug-detailed does not yet
implement them.
The third word specifies the source files for those structs for which the compiler will emit debug information. The
values none and any have the normal meaning. The value base means that the base of name of the file in which the type
declaration appears must match the base of the name of the main compilation file. In practice, this means that types
declared in foo.c and foo.h will have debug information, but types declared in other header will not. The value sys
means those types satisfying base or declared in system or compiler headers.
You may need to experiment to determine the best settings for your application.
The default is -femit-struct-debug-detailed=all.
This option works only with DWARF 2.
-fno-merge-debug-strings
Direct the linker to not merge together strings in the debugging information that are identical in different object
files. Merging is not supported by all assemblers or linkers. Merging decreases the size of the debug information in
the output file at the cost of increasing link processing time. Merging is enabled by default.
-fdebug-prefix-map=old=new
When compiling files in directory old, record debugging information describing them as in new instead.
-fno-dwarf2-cfi-asm
Emit DWARF 2 unwind info as compiler generated ".eh_frame" section instead of using GAS ".cfi_*" directives.
-p Generate extra code to write profile information suitable for the analysis program prof. You must use this option when
compiling the source files you want data about, and you must also use it when linking.
-pg Generate extra code to write profile information suitable for the analysis program gprof. You must use this option when
compiling the source files you want data about, and you must also use it when linking.
-Q Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it
finishes.
-ftime-report
Makes the compiler print some statistics about the time consumed by each pass when it finishes.
-fmem-report
Makes the compiler print some statistics about permanent memory allocation when it finishes.
-fpre-ipa-mem-report
-fpost-ipa-mem-report
Makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization.
-fstack-usage
Makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is
made by appending .su to the auxname. auxname is generated from the name of the output file, if explicitly specified and
it is not an executable, otherwise it is the basename of the source file. An entry is made up of three fields:
· The name of the function.
· A number of bytes.
· One or more qualifiers: "static", "dynamic", "bounded".
The qualifier "static" means that the function manipulates the stack statically: a fixed number of bytes are allocated
for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function.
The second field is this fixed number of bytes.
The qualifier "dynamic" means that the function manipulates the stack dynamically: in addition to the static allocation
described above, stack adjustments are made in the body of the function, for example to push/pop arguments around
function calls. If the qualifier "bounded" is also present, the amount of these adjustments is bounded at compile time
and the second field is an upper bound of the total amount of stack used by the function. If it is not present, the
amount of these adjustments is not bounded at compile time and the second field only represents the bounded part.
-fprofile-arcs
Add code so that program flow arcs are instrumented. During execution the program records how many times each branch and
call is executed and how many times it is taken or returns. When the compiled program exits it saves this data to a file
called auxname.gcda for each source file. The data may be used for profile-directed optimizations
(-fbranch-probabilities), or for test coverage analysis (-ftest-coverage). Each object file's auxname is generated from
the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of
the source file. In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output
file specified as -o dir/foo.o).
--coverage
This option is used to compile and link code instrumented for coverage analysis. The option is a synonym for
-fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the documentation for those options for
more details.
· Compile the source files with -fprofile-arcs plus optimization and code generation options. For test coverage
analysis, use the additional -ftest-coverage option. You do not need to profile every source file in a program.
· Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
· Run the program on a representative workload to generate the arc profile information. This may be repeated any
number of times. You can run concurrent instances of your program, and provided that the file system supports
locking, the data files will be correctly updated. Also "fork" calls are detected and correctly handled (double
counting will not happen).
· For profile-directed optimizations, compile the source files again with the same optimization and code generation
options plus -fbranch-probabilities.
· For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda files. Refer to
the gcov documentation for further information.
With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for
the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the
number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the
instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation
code.
-ftest-coverage
Produce a notes file that the gcov code-coverage utility can use to show program coverage. Each source file's note file
is called auxname.gcno. Refer to the -fprofile-arcs option above for a description of auxname and instructions on how to
generate test coverage data. Coverage data will match the source files more closely, if you do not optimize.
-fdbg-cnt-list
Print the name and the counter upper bound for all debug counters.
-fdbg-cnt=counter-value-list
Set the internal debug counter upper bound. counter-value-list is a comma-separated list of name:value pairs which sets
the upper bound of each debug counter name to value. All debug counters have the initial upper bound of UINT_MAX, thus
dbg_cnt() returns true always unless the upper bound is set by this option. e.g. With -fdbg-cnt=dce:10,tail_call:0
dbg_cnt(dce) will return true only for first 10 invocations
-fenable-kind-pass
-fdisable-kind-pass=range-list
This is a set of debugging options that are used to explicitly disable/enable optimization passes. For compiler users,
regular options for enabling/disabling passes should be used instead.
*<-fdisable-ipa-pass>
Disable ipa pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times,
the pass name should be appended with a sequential number starting from 1.
*<-fdisable-rtl-pass>
*<-fdisable-rtl-pass=range-list>
Disable rtl pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple
times, the pass name should be appended with a sequential number starting from 1. range-list is a comma seperated
list of function ranges or assembler names. Each range is a number pair seperated by a colon. The range is
inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the
function's cgraph node's uid is falling within one of the specified ranges, the pass is disabled for that function.
The uid is shown in the function header of a dump file, and the pass names can be dumped by using option
-fdump-passes.
*<-fdisable-tree-pass>
*<-fdisable-tree-pass=range-list>
Disable tree pass pass. See -fdisable-rtl for the description of option arguments.
*<-fenable-ipa-pass>
Enable ipa pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times,
the pass name should be appended with a sequential number starting from 1.
*<-fenable-rtl-pass>
*<-fenable-rtl-pass=range-list>
Enable rtl pass pass. See -fdisable-rtl for option argument description and examples.
*<-fenable-tree-pass>
*<-fenable-tree-pass=range-list>
Enable tree pass pass. See -fdisable-rtl for the description of option arguments.
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll
-dletters
-fdump-rtl-pass
Says to make debugging dumps during compilation at times specified by letters. This is used for debugging the RTL-based
passes of the compiler. The file names for most of the dumps are made by appending a pass number and a word to the
dumpname, and the files are created in the directory of the output file. Note that the pass number is computed
statically as passes get registered into the pass manager. Thus the numbering is not related to the dynamic order of
execution of passes. In particular, a pass installed by a plugin could have a number over 200 even if it executed quite
early. dumpname is generated from the name of the output file, if explicitly specified and it is not an executable,
otherwise it is the basename of the source file. These switches may have different effects when -E is used for
preprocessing.
Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters. Here are the possible letters for use in
pass and letters, and their meanings:
-fdump-rtl-alignments
Dump after branch alignments have been computed.
-fdump-rtl-asmcons
Dump after fixing rtl statements that have unsatisfied in/out constraints.
-fdump-rtl-auto_inc_dec
Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec
instructions.
-fdump-rtl-barriers
Dump after cleaning up the barrier instructions.
-fdump-rtl-bbpart
Dump after partitioning hot and cold basic blocks.
-fdump-rtl-bbro
Dump after block reordering.
-fdump-rtl-btl1
-fdump-rtl-btl2
-fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization passes.
-fdump-rtl-bypass
Dump after jump bypassing and control flow optimizations.
-fdump-rtl-combine
Dump after the RTL instruction combination pass.
-fdump-rtl-compgotos
Dump after duplicating the computed gotos.
-fdump-rtl-ce1
-fdump-rtl-ce2
-fdump-rtl-ce3
-fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion passes.
-fdump-rtl-cprop_hardreg
Dump after hard register copy propagation.
-fdump-rtl-csa
Dump after combining stack adjustments.
-fdump-rtl-cse1
-fdump-rtl-cse2
-fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common sub-expression elimination passes.
-fdump-rtl-dce
Dump after the standalone dead code elimination passes.
-fdump-rtl-dbr
Dump after delayed branch scheduling.
-fdump-rtl-dce1
-fdump-rtl-dce2
-fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.
-fdump-rtl-eh
Dump after finalization of EH handling code.
-fdump-rtl-eh_ranges
Dump after conversion of EH handling range regions.
-fdump-rtl-expand
Dump after RTL generation.
-fdump-rtl-fwprop1
-fdump-rtl-fwprop2
-fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation passes.
-fdump-rtl-gcse1
-fdump-rtl-gcse2
-fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression elimination.
-fdump-rtl-init-regs
Dump after the initialization of the registers.
-fdump-rtl-initvals
Dump after the computation of the initial value sets.
-fdump-rtl-into_cfglayout
Dump after converting to cfglayout mode.
-fdump-rtl-ira
Dump after iterated register allocation.
-fdump-rtl-jump
Dump after the second jump optimization.
-fdump-rtl-loop2
-fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.
-fdump-rtl-mach
Dump after performing the machine dependent reorganization pass, if that pass exists.
-fdump-rtl-mode_sw
Dump after removing redundant mode switches.
-fdump-rtl-rnreg
Dump after register renumbering.
-fdump-rtl-outof_cfglayout
Dump after converting from cfglayout mode.
-fdump-rtl-peephole2
Dump after the peephole pass.
-fdump-rtl-postreload
Dump after post-reload optimizations.
-fdump-rtl-pro_and_epilogue
Dump after generating the function prologues and epilogues.
-fdump-rtl-regmove
Dump after the register move pass.
-fdump-rtl-sched1
-fdump-rtl-sched2
-fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block scheduling passes.
-fdump-rtl-see
Dump after sign extension elimination.
-fdump-rtl-seqabstr
Dump after common sequence discovery.
-fdump-rtl-shorten
Dump after shortening branches.
-fdump-rtl-sibling
Dump after sibling call optimizations.
-fdump-rtl-split1
-fdump-rtl-split2
-fdump-rtl-split3
-fdump-rtl-split4
-fdump-rtl-split5
-fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3, -fdump-rtl-split4 and -fdump-rtl-split5 enable dumping after
five rounds of instruction splitting.
-fdump-rtl-sms
Dump after modulo scheduling. This pass is only run on some architectures.
-fdump-rtl-stack
Dump after conversion from GCC's "flat register file" registers to the x87's stack-like registers. This pass is only
run on x86 variants.
-fdump-rtl-subreg1
-fdump-rtl-subreg2
-fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after the two subreg expansion passes.
-fdump-rtl-unshare
Dump after all rtl has been unshared.
-fdump-rtl-vartrack
Dump after variable tracking.
-fdump-rtl-vregs
Dump after converting virtual registers to hard registers.
-fdump-rtl-web
Dump after live range splitting.
-fdump-rtl-regclass
-fdump-rtl-subregs_of_mode_init
-fdump-rtl-subregs_of_mode_finish
-fdump-rtl-dfinit
-fdump-rtl-dfinish
These dumps are defined but always produce empty files.
-da
-fdump-rtl-all
Produce all the dumps listed above.
-dA Annotate the assembler output with miscellaneous debugging information.
-dD Dump all macro definitions, at the end of preprocessing, in addition to normal output.
-dH Produce a core dump whenever an error occurs.
-dp Annotate the assembler output with a comment indicating which pattern and alternative was used. The length of each
instruction is also printed.
-dP Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annotation.
-dv For each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph
suitable for viewing with VCG to file.pass.vcg.
-dx Just generate RTL for a function instead of compiling it. Usually used with -fdump-rtl-expand.
-fdump-noaddr
When doing debugging dumps, suppress address output. This makes it more feasible to use diff on debugging dumps for
compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start
locations.
-fdump-unnumbered
When doing debugging dumps, suppress instruction numbers and address output. This makes it more feasible to use diff on
debugging dumps for compiler invocations with different options, in particular with and without -g.
-fdump-unnumbered-links
When doing debugging dumps (see -d option above), suppress instruction numbers for the links to the previous and next
instructions in a sequence.
-fdump-translation-unit (C++ only)
-fdump-translation-unit-options (C++ only)
Dump a representation of the tree structure for the entire translation unit to a file. The file name is made by
appending .tu to the source file name, and the file is created in the same directory as the output file. If the -options
form is used, options controls the details of the dump as described for the -fdump-tree options.
-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-options (C++ only)
Dump a representation of each class's hierarchy and virtual function table layout to a file. The file name is made by
appending .class to the source file name, and the file is created in the same directory as the output file. If the
-options form is used, options controls the details of the dump as described for the -fdump-tree options.
-fdump-ipa-switch
Control the dumping at various stages of inter-procedural analysis language tree to a file. The file name is generated
by appending a switch specific suffix to the source file name, and the file is created in the same directory as the
output file. The following dumps are possible:
all Enables all inter-procedural analysis dumps.
cgraph
Dumps information about call-graph optimization, unused function removal, and inlining decisions.
inline
Dump after function inlining.
-fdump-passes
Dump the list of optimization passes that are turned on and off by the current command-line options.
-fdump-statistics-option
Enable and control dumping of pass statistics in a separate file. The file name is generated by appending a suffix
ending in .statistics to the source file name, and the file is created in the same directory as the output file. If the
-option form is used, -stats will cause counters to be summed over the whole compilation unit while -details will dump
every event as the passes generate them. The default with no option is to sum counters for each function compiled.
-fdump-tree-switch
-fdump-tree-switch-options
Control the dumping at various stages of processing the intermediate language tree to a file. The file name is generated
by appending a switch specific suffix to the source file name, and the file is created in the same directory as the
output file. If the -options form is used, options is a list of - separated options which control the details of the
dump. Not all options are applicable to all dumps; those that are not meaningful will be ignored. The following options
are available
address
Print the address of each node. Usually this is not meaningful as it changes according to the environment and source
file. Its primary use is for tying up a dump file with a debug environment.
asmname
If "DECL_ASSEMBLER_NAME" has been set for a given decl, use that in the dump instead of "DECL_NAME". Its primary use
is ease of use working backward from mangled names in the assembly file.
slim
Inhibit dumping of members of a scope or body of a function merely because that scope has been reached. Only dump
such items when they are directly reachable by some other path. When dumping pretty-printed trees, this option
inhibits dumping the bodies of control structures.
raw Print a raw representation of the tree. By default, trees are pretty-printed into a C-like representation.
details
Enable more detailed dumps (not honored by every dump option).
stats
Enable dumping various statistics about the pass (not honored by every dump option).
blocks
Enable showing basic block boundaries (disabled in raw dumps).
vops
Enable showing virtual operands for every statement.
lineno
Enable showing line numbers for statements.
uid Enable showing the unique ID ("DECL_UID") for each variable.
verbose
Enable showing the tree dump for each statement.
eh Enable showing the EH region number holding each statement.
scev
Enable showing scalar evolution analysis details.
all Turn on all options, except raw, slim, verbose and lineno.
The following tree dumps are possible:
original
Dump before any tree based optimization, to file.original.
optimized
Dump after all tree based optimization, to file.optimized.
gimple
Dump each function before and after the gimplification pass to a file. The file name is made by appending .gimple to
the source file name.
cfg Dump the control flow graph of each function to a file. The file name is made by appending .cfg to the source file
name.
vcg Dump the control flow graph of each function to a file in VCG format. The file name is made by appending .vcg to the
source file name. Note that if the file contains more than one function, the generated file cannot be used directly
by VCG. You will need to cut and paste each function's graph into its own separate file first.
ch Dump each function after copying loop headers. The file name is made by appending .ch to the source file name.
ssa Dump SSA related information to a file. The file name is made by appending .ssa to the source file name.
alias
Dump aliasing information for each function. The file name is made by appending .alias to the source file name.
ccp Dump each function after CCP. The file name is made by appending .ccp to the source file name.
storeccp
Dump each function after STORE-CCP. The file name is made by appending .storeccp to the source file name.
pre Dump trees after partial redundancy elimination. The file name is made by appending .pre to the source file name.
fre Dump trees after full redundancy elimination. The file name is made by appending .fre to the source file name.
copyprop
Dump trees after copy propagation. The file name is made by appending .copyprop to the source file name.
store_copyprop
Dump trees after store copy-propagation. The file name is made by appending .store_copyprop to the source file name.
dce Dump each function after dead code elimination. The file name is made by appending .dce to the source file name.
mudflap
Dump each function after adding mudflap instrumentation. The file name is made by appending .mudflap to the source
file name.
sra Dump each function after performing scalar replacement of aggregates. The file name is made by appending .sra to the
source file name.
sink
Dump each function after performing code sinking. The file name is made by appending .sink to the source file name.
dom Dump each function after applying dominator tree optimizations. The file name is made by appending .dom to the
source file name.
dse Dump each function after applying dead store elimination. The file name is made by appending .dse to the source file
name.
phiopt
Dump each function after optimizing PHI nodes into straightline code. The file name is made by appending .phiopt to
the source file name.
forwprop
Dump each function after forward propagating single use variables. The file name is made by appending .forwprop to
the source file name.
copyrename
Dump each function after applying the copy rename optimization. The file name is made by appending .copyrename to
the source file name.
nrv Dump each function after applying the named return value optimization on generic trees. The file name is made by
appending .nrv to the source file name.
vect
Dump each function after applying vectorization of loops. The file name is made by appending .vect to the source
file name.
slp Dump each function after applying vectorization of basic blocks. The file name is made by appending .slp to the
source file name.
vrp Dump each function after Value Range Propagation (VRP). The file name is made by appending .vrp to the source file
name.
all Enable all the available tree dumps with the flags provided in this option.
-ftree-vectorizer-verbose=n
This option controls the amount of debugging output the vectorizer prints. This information is written to standard
error, unless -fdump-tree-all or -fdump-tree-vect is specified, in which case it is output to the usual dump listing
file, .vect. For n=0 no diagnostic information is reported. If n=1 the vectorizer reports each loop that got
vectorized, and the total number of loops that got vectorized. If n=2 the vectorizer also reports non-vectorized loops
that passed the first analysis phase (vect_analyze_loop_form) - i.e. countable, inner-most, single-bb, single-entry/exit
loops. This is the same verbosity level that -fdump-tree-vect-stats uses. Higher verbosity levels mean either more
information dumped for each reported loop, or same amount of information reported for more loops: if n=3, vectorizer cost
model information is reported. If n=4, alignment related information is added to the reports. If n=5, data-references
related information (e.g. memory dependences, memory access-patterns) is added to the reports. If n=6, the vectorizer
reports also non-vectorized inner-most loops that did not pass the first analysis phase (i.e., may not be countable, or
may have complicated control-flow). If n=7, the vectorizer reports also non-vectorized nested loops. If n=8, SLP
related information is added to the reports. For n=9, all the information the vectorizer generates during its analysis
and transformation is reported. This is the same verbosity level that -fdump-tree-vect-details uses.
-frandom-seed=string
This option provides a seed that GCC uses when it would otherwise use random numbers. It is used to generate certain
symbol names that have to be different in every compiled file. It is also used to place unique stamps in coverage data
files and the object files that produce them. You can use the -frandom-seed option to produce reproducibly identical
object files.
The string should be different for every file you compile.
-fsched-verbose=n
On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints.
This information is written to standard error, unless -fdump-rtl-sched1 or -fdump-rtl-sched2 is specified, in which case
it is output to the usual dump listing file, .sched1 or .sched2 respectively. However for n greater than nine, the
output is always printed to standard error.
For n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n
greater than one, it also output basic block probabilities, detailed ready list information and unit/insn info. For n
greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, -fsched-verbose
also includes dependence info.
-save-temps
-save-temps=cwd
Store the usual "temporary" intermediate files permanently; place them in the current directory and name them based on
the source file. Thus, compiling foo.c with -c -save-temps would produce files foo.i and foo.s, as well as foo.o. This
creates a preprocessed foo.i output file even though the compiler now normally uses an integrated preprocessor.
When used in combination with the -x command-line option, -save-temps is sensible enough to avoid over writing an input
source file with the same extension as an intermediate file. The corresponding intermediate file may be obtained by
renaming the source file before using -save-temps.
If you invoke GCC in parallel, compiling several different source files that share a common base name in different
subdirectories or the same source file compiled for multiple output destinations, it is likely that the different
parallel compilers will interfere with each other, and overwrite the temporary files. For instance:
gcc -save-temps -o outdir1/foo.o indir1/foo.c&
gcc -save-temps -o outdir2/foo.o indir2/foo.c&
may result in foo.i and foo.o being written to simultaneously by both compilers.
-save-temps=obj
Store the usual "temporary" intermediate files permanently. If the -o option is used, the temporary files are based on
the object file. If the -o option is not used, the -save-temps=obj switch behaves like -save-temps.
For example:
gcc -save-temps=obj -c foo.c
gcc -save-temps=obj -c bar.c -o dir/xbar.o
gcc -save-temps=obj foobar.c -o dir2/yfoobar
would create foo.i, foo.s, dir/xbar.i, dir/xbar.s, dir2/yfoobar.i, dir2/yfoobar.s, and dir2/yfoobar.o.
-time[=file]
Report the CPU time taken by each subprocess in the compilation sequence. For C source files, this is the compiler
proper and assembler (plus the linker if linking is done).
Without the specification of an output file, the output looks like this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the "user time", that is time spent executing the program itself. The second number is
"system time", time spent executing operating system routines on behalf of the program. Both numbers are in seconds.
With the specification of an output file, the output is appended to the named file, and it looks like this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>
The "user time" and the "system time" are moved before the program name, and the options passed to the program are
displayed, so that one can later tell what file was being compiled, and with which options.
-fvar-tracking
Run variable tracking pass. It computes where variables are stored at each position in code. Better debugging
information is then generated (if the debugging information format supports this information).
It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the debug
info format supports it.
-fvar-tracking-assignments
Annotate assignments to user variables early in the compilation and attempt to carry the annotations over throughout the
compilation all the way to the end, in an attempt to improve debug information while optimizing. Use of -gdwarf-4 is
recommended along with it.
It can be enabled even if var-tracking is disabled, in which case annotations will be created and maintained, but
discarded at the end.
-fvar-tracking-assignments-toggle
Toggle -fvar-tracking-assignments, in the same way that -gtoggle toggles -g.
-print-file-name=library
Print the full absolute name of the library file library that would be used when linking---and don't do anything else.
With this option, GCC does not compile or link anything; it just prints the file name.
-print-multi-directory
Print the directory name corresponding to the multilib selected by any other switches present in the command line. This
directory is supposed to exist in GCC_EXEC_PREFIX.
-print-multi-lib
Print the mapping from multilib directory names to compiler switches that enable them. The directory name is separated
from the switches by ;, and each switch starts with an @ instead of the -, without spaces between multiple switches.
This is supposed to ease shell-processing.
-print-multi-os-directory
Print the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS libraries are present
in the lib subdirectory and no multilibs are used, this is usually just ., if OS libraries are present in libsuffix
sibling directories this prints e.g. ../lib64, ../lib or ../lib32, or if OS libraries are present in lib/subdir
subdirectories it prints e.g. amd64, sparcv9 or ev6.
-print-multiarch
Print the path to OS libraries for the selected multiarch, relative to some lib subdirectory.
-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can do
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
-print-search-dirs
Print the name of the configured installation directory and a list of program and library directories gcc will
search---and don't do anything else.
This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file or directory. To
resolve this you either need to put cpp0 and the other compiler components where gcc expects to find them, or you can set
the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don't forget the trailing /.
-print-sysroot
Print the target sysroot directory that will be used during compilation. This is the target sysroot specified either at
configure time or using the --sysroot option, possibly with an extra suffix that depends on compilation options. If no
target sysroot is specified, the option prints nothing.
-print-sysroot-headers-suffix
Print the suffix added to the target sysroot when searching for headers, or give an error if the compiler is not
configured with such a suffix---and don't do anything else.
-dumpmachine
Print the compiler's target machine (for example, i686-pc-linux-gnu)---and don't do anything else.
-dumpversion
Print the compiler version (for example, 3.0)---and don't do anything else.
-dumpspecs
Print the compiler's built-in specs---and don't do anything else. (This is used when GCC itself is being built.)
-feliminate-unused-debug-types
Normally, when producing DWARF2 output, GCC will emit debugging information for all types declared in a compilation unit,
regardless of whether or not they are actually used in that compilation unit. Sometimes this is useful, such as if, in
the debugger, you want to cast a value to a type that is not actually used in your program (but is declared). More
often, however, this results in a significant amount of wasted space. With this option, GCC will avoid producing debug
symbol output for types that are nowhere used in the source file being compiled.
Options That Control Optimization
These options control various sorts of optimizations.
Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the
expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then
assign a new value to any variable or change the program counter to any other statement in the function and get exactly the
results you would expect from the source code.
Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of
compilation time and possibly the ability to debug the program.
The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a
single output file mode allows the compiler to use information gained from all of the files when compiling each of them.
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section.
Most optimizations are only enabled if an -O level is set on the command line. Otherwise they are disabled, even if
individual optimization flags are specified.
Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level
than those listed here. You can invoke GCC with -Q --help=optimizers to find out the exact set of optimizations that are
enabled at each level.
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.
With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a
great deal of compilation time.
-O turns on the following optimization flags:
-fauto-inc-dec -fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch -fdse -fguess-branch-probability
-fif-conversion2 -fif-conversion -fipa-pure-const -fipa-profile -fipa-reference -fmerge-constants -fsplit-wide-types
-ftree-bit-ccp -ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse
-ftree-forwprop -ftree-fre -ftree-phiprop -ftree-sra -ftree-pta -ftree-ter -funit-at-a-time
-O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.
-O2 Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. As
compared to -O, this option increases both compilation time and the performance of the generated code.
-O2 turns on all optimization flags specified by -O. It also turns on the following optimization flags: -fthread-jumps
-falign-functions -falign-jumps -falign-loops -falign-labels -fcaller-saves -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fdelete-null-pointer-checks -fdevirtualize -fexpensive-optimizations -fgcse -fgcse-lm
-finline-small-functions -findirect-inlining -fipa-sra -foptimize-sibling-calls -fpartial-inlining -fpeephole2 -fregmove
-freorder-blocks -freorder-functions -frerun-cse-after-loop -fsched-interblock -fsched-spec -fschedule-insns
-fschedule-insns2 -fstrict-aliasing -fstrict-overflow -ftree-switch-conversion -ftree-tail-merge -ftree-pre -ftree-vrp
Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos.
NOTE: In Ubuntu 8.10 and later versions, -D_FORTIFY_SOURCE=2 is set by default, and is activated when -O is set to 2 or
higher. This enables additional compile-time and run-time checks for several libc functions. To disable, specify either
-U_FORTIFY_SOURCE or -D_FORTIFY_SOURCE=0.
-O3 Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions,
-funswitch-loops, -fpredictive-commoning, -fgcse-after-reload, -ftree-vectorize, -ftree-partial-pre and -fipa-cp-clone
options.
-O0 Reduce compilation time and make debugging produce the expected results. This is the default.
-Os Optimize for size. -Os enables all -O2 optimizations that do not typically increase code size. It also performs further
optimizations designed to reduce code size.
-Os disables the following optimization flags: -falign-functions -falign-jumps -falign-loops -falign-labels
-freorder-blocks -freorder-blocks-and-partition -fprefetch-loop-arrays -ftree-vect-loop-version
-Ofast
Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also enables optimizations that are not
valid for all standard compliant programs. It turns on -ffast-math and the Fortran-specific -fno-protect-parens and
-fstack-arrays.
If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.
Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative forms; the negative
form of -ffoo would be -fno-foo. In the table below, only one of the forms is listed---the one you typically will use. You
can figure out the other form by either removing no- or adding it.
The following options control specific optimizations. They are either activated by -O options or are related to ones that
are. You can use the following flags in the rare cases when "fine-tuning" of optimizations to be performed is desired.
-fno-default-inline
Do not make member functions inline by default merely because they are defined inside the class scope (C++ only).
Otherwise, when you specify -O, member functions defined inside class scope are compiled inline by default; i.e., you
don't need to add inline in front of the member function name.
-fno-defer-pop
Always pop the arguments to each function call as soon as that function returns. For machines that must pop arguments
after a function call, the compiler normally lets arguments accumulate on the stack for several function calls and pops
them all at once.
Disabled at levels -O, -O2, -O3, -Os.
-fforward-propagate
Perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks if the result can be
simplified. If loop unrolling is active, two passes are performed and the second is scheduled after loop unrolling.
This option is enabled by default at optimization levels -O, -O2, -O3, -Os.
-ffp-contract=style
-ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-point expression
contraction such as forming of fused multiply-add operations if the target has native support for them. -ffp-contract=on
enables floating-point expression contraction if allowed by the language standard. This is currently not implemented and
treated equal to -ffp-contract=off.
The default is -ffp-contract=fast.
-fomit-frame-pointer
Don't keep the frame pointer in a register for functions that don't need one. This avoids the instructions to save, set
up and restore frame pointers; it also makes an extra register available in many functions. It also makes debugging
impossible on some machines.
On some machines, such as the VAX, this flag has no effect, because the standard calling sequence automatically handles
the frame pointer and nothing is saved by pretending it doesn't exist. The machine-description macro
"FRAME_POINTER_REQUIRED" controls whether a target machine supports this flag.
Starting with GCC version 4.6, the default setting (when not optimizing for size) for 32-bit Linux x86 and 32-bit Darwin
x86 targets has been changed to -fomit-frame-pointer. The default can be reverted to -fno-omit-frame-pointer by
configuring GCC with the --enable-frame-pointer configure option.
Enabled at levels -O, -O2, -O3, -Os.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-fno-inline
Do not expand any functions inline apart from those marked with the "always_inline" attribute. This is the default when
not optimizing.
Single functions can be exempted from inlining by marking them with the "noinline" attribute.
-finline-small-functions
Integrate functions into their callers when their body is smaller than expected function call code (so overall size of
program gets smaller). The compiler heuristically decides which functions are simple enough to be worth integrating in
this way. This inlining applies to all functions, even those not declared inline.
Enabled at level -O2.
-findirect-inlining
Inline also indirect calls that are discovered to be known at compile time thanks to previous inlining. This option has
any effect only when inlining itself is turned on by the -finline-functions or -finline-small-functions options.
Enabled at level -O2.
-finline-functions
Consider all functions for inlining, even if they are not declared inline. The compiler heuristically decides which
functions are worth integrating in this way.
If all calls to a given function are integrated, and the function is declared "static", then the function is normally not
output as assembler code in its own right.
Enabled at level -O3.
-finline-functions-called-once
Consider all "static" functions called once for inlining into their caller even if they are not marked "inline". If a
call to a given function is integrated, then the function is not output as assembler code in its own right.
Enabled at levels -O1, -O2, -O3 and -Os.
-fearly-inlining
Inline functions marked by "always_inline" and functions whose body seems smaller than the function call overhead early
before doing -fprofile-generate instrumentation and real inlining pass. Doing so makes profiling significantly cheaper
and usually inlining faster on programs having large chains of nested wrapper functions.
Enabled by default.
-fipa-sra
Perform interprocedural scalar replacement of aggregates, removal of unused parameters and replacement of parameters
passed by reference by parameters passed by value.
Enabled at levels -O2, -O3 and -Os.
-finline-limit=n
By default, GCC limits the size of functions that can be inlined. This flag allows coarse control of this limit. n is
the size of functions that can be inlined in number of pseudo instructions.
Inlining is actually controlled by a number of parameters, which may be specified individually by using --param
name=value. The -finline-limit=n option sets some of these parameters as follows:
max-inline-insns-single
is set to n/2.
max-inline-insns-auto
is set to n/2.
See below for a documentation of the individual parameters controlling inlining and for the defaults of these parameters.
Note: there may be no value to -finline-limit that results in default behavior.
Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way
does it represent a count of assembly instructions and as such its exact meaning might change from one release to an
another.
-fno-keep-inline-dllexport
This is a more fine-grained version of -fkeep-inline-functions, which applies only to functions that are declared using
the "dllexport" attribute or declspec
-fkeep-inline-functions
In C, emit "static" functions that are declared "inline" into the object file, even if the function has been inlined into
all of its callers. This switch does not affect functions using the "extern inline" extension in GNU C90. In C++, emit
any and all inline functions into the object file.
-fkeep-static-consts
Emit variables declared "static const" when optimization isn't turned on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the compiler to check if the variable was referenced,
regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.
-fmerge-constants
Attempt to merge identical constants (string constants and floating-point constants) across compilation units.
This option is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to
inhibit this behavior.
Enabled at levels -O, -O2, -O3, -Os.
-fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to -fmerge-constants this considers e.g. even constant initialized
arrays or initialized constant variables with integral or floating-point types. Languages like C or C++ require each
variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this
option will result in non-conforming behavior.
-fmodulo-sched
Perform swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops and
reorders their instructions by overlapping different iterations.
-fmodulo-sched-allow-regmoves
Perform more aggressive SMS based modulo scheduling with register moves allowed. By setting this flag certain anti-
dependences edges will be deleted which will trigger the generation of reg-moves based on the life-range analysis. This
option is effective only with -fmodulo-sched enabled.
-fno-branch-count-reg
Do not use "decrement and branch" instructions on a count register, but instead generate a sequence of instructions that
decrement a register, compare it against zero, then branch based upon the result. This option is only meaningful on
architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390.
The default is -fbranch-count-reg.
-fno-function-cse
Do not put function addresses in registers; make each instruction that calls a constant function contain the function's
address explicitly.
This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the
optimizations performed when this option is not used.
The default is -ffunction-cse
-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS. This can save
space in the resulting code.
This option turns off this behavior because some programs explicitly rely on variables going to the data section. E.g.,
so that the resulting executable can find the beginning of that section and/or make assumptions based on that.
The default is -fzero-initialized-in-bss.
-fmudflap -fmudflapth -fmudflapir
For front-ends that support it (C and C++), instrument all risky pointer/array dereferencing operations, some standard
library string/heap functions, and some other associated constructs with range/validity tests. Modules so instrumented
should be immune to buffer overflows, invalid heap use, and some other classes of C/C++ programming errors. The
instrumentation relies on a separate runtime library (libmudflap), which will be linked into a program if -fmudflap is
given at link time. Run-time behavior of the instrumented program is controlled by the MUDFLAP_OPTIONS environment
variable. See "env MUDFLAP_OPTIONS=-help a.out" for its options.
Use -fmudflapth instead of -fmudflap to compile and to link if your program is multi-threaded. Use -fmudflapir, in
addition to -fmudflap or -fmudflapth, if instrumentation should ignore pointer reads. This produces less instrumentation
(and therefore faster execution) and still provides some protection against outright memory corrupting writes, but allows
erroneously read data to propagate within a program.
-fthread-jumps
Perform optimizations where we check to see if a jump branches to a location where another comparison subsumed by the
first is found. If so, the first branch is redirected to either the destination of the second branch or a point
immediately following it, depending on whether the condition is known to be true or false.
Enabled at levels -O2, -O3, -Os.
-fsplit-wide-types
When using a type that occupies multiple registers, such as "long long" on a 32-bit system, split the registers apart and
allocate them independently. This normally generates better code for those types, but may make debugging more difficult.
Enabled at levels -O, -O2, -O3, -Os.
-fcse-follow-jumps
In common subexpression elimination (CSE), scan through jump instructions when the target of the jump is not reached by
any other path. For example, when CSE encounters an "if" statement with an "else" clause, CSE will follow the jump when
the condition tested is false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to follow jumps that conditionally skip over blocks. When CSE
encounters a simple "if" statement with no else clause, -fcse-skip-blocks causes CSE to follow the jump around the body
of the "if".
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations has been performed.
Enabled at levels -O2, -O3, -Os.
-fgcse
Perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation.
Note: When compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you
disable the global common subexpression elimination pass by adding -fno-gcse to the command line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lm
When -fgcse-lm is enabled, global common subexpression elimination will attempt to move loads that are only killed by
stores into themselves. This allows a loop containing a load/store sequence to be changed to a load outside the loop,
and a copy/store within the loop.
Enabled by default when gcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination. This pass will
attempt to move stores out of loops. When used in conjunction with -fgcse-lm, loops containing a load/store sequence can
be changed to a load before the loop and a store after the loop.
Not enabled at any optimization level.
-fgcse-las
When -fgcse-las is enabled, the global common subexpression elimination pass eliminates redundant loads that come after
stores to the same memory location (both partial and full redundancies).
Not enabled at any optimization level.
-fgcse-after-reload
When -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload. The purpose of this
pass is to cleanup redundant spilling.
-funsafe-loop-optimizations
If given, the loop optimizer will assume that loop indices do not overflow, and that the loops with nontrivial exit
condition are not infinite. This enables a wider range of loop optimizations even if the loop optimizer itself cannot
prove that these assumptions are valid. Using -Wunsafe-loop-optimizations, the compiler will warn you if it finds this
kind of loop.
-fcrossjumping
Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The resulting
code may or may not perform better than without cross-jumping.
Enabled at levels -O2, -O3, -Os.
-fauto-inc-dec
Combine increments or decrements of addresses with memory accesses. This pass is always skipped on architectures that do
not have instructions to support this. Enabled by default at -O and higher on architectures that support this.
-fdce
Perform dead code elimination (DCE) on RTL. Enabled by default at -O and higher.
-fdse
Perform dead store elimination (DSE) on RTL. Enabled by default at -O and higher.
-fif-conversion
Attempt to transform conditional jumps into branch-less equivalents. This include use of conditional moves, min, max,
set flags and abs instructions, and some tricks doable by standard arithmetics. The use of conditional execution on
chips where it is available is controlled by "if-conversion2".
Enabled at levels -O, -O2, -O3, -Os.
-fif-conversion2
Use conditional execution (where available) to transform conditional jumps into branch-less equivalents.
Enabled at levels -O, -O2, -O3, -Os.
-fdelete-null-pointer-checks
Assume that programs cannot safely dereference null pointers, and that no code or data element resides there. This
enables simple constant folding optimizations at all optimization levels. In addition, other optimization passes in GCC
use this flag to control global dataflow analyses that eliminate useless checks for null pointers; these assume that if a
pointer is checked after it has already been dereferenced, it cannot be null.
Note however that in some environments this assumption is not true. Use -fno-delete-null-pointer-checks to disable this
optimization for programs that depend on that behavior.
Some targets, especially embedded ones, disable this option at all levels. Otherwise it is enabled at all levels: -O0,
-O1, -O2, -O3, -Os. Passes that use the information are enabled independently at different optimization levels.
-fdevirtualize
Attempt to convert calls to virtual functions to direct calls. This is done both within a procedure and
interprocedurally as part of indirect inlining ("-findirect-inlining") and interprocedural constant propagation
(-fipa-cp). Enabled at levels -O2, -O3, -Os.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively expensive.
Enabled at levels -O2, -O3, -Os.
-free
Attempt to remove redundant extension instructions. This is especially helpful for the x86-64 architecture which
implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half.
Enabled for x86 at levels -O2, -O3.
-foptimize-register-move
-fregmove
Attempt to reassign register numbers in move instructions and as operands of other simple instructions in order to
maximize the amount of register tying. This is especially helpful on machines with two-operand instructions.
Note -fregmove and -foptimize-register-move are the same optimization.
Enabled at levels -O2, -O3, -Os.
-fira-algorithm=algorithm
Use the specified coloring algorithm for the integrated register allocator. The algorithm argument can be priority,
which specifies Chow's priority coloring, or CB, which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring is not
implemented for all architectures, but for those targets that do support it, it is the default because it generates
better code.
-fira-region=region
Use specified regions for the integrated register allocator. The region argument should be one of the following:
all Use all loops as register allocation regions. This can give the best results for machines with a small and/or
irregular register set.
mixed
Use all loops except for loops with small register pressure as the regions. This value usually gives the best
results in most cases and for most architectures, and is enabled by default when compiling with optimization for
speed (-O, -O2, ...).
one Use all functions as a single region. This typically results in the smallest code size, and is enabled by default
for -Os or -O0.
-fira-loop-pressure
Use IRA to evaluate register pressure in loops for decisions to move loop invariants. This option usually results in
generation of faster and smaller code on machines with large register files (>= 32 registers), but it can slow the
compiler down.
This option is enabled at level -O3 for some targets.
-fno-ira-share-save-slots
Disable sharing of stack slots used for saving call-used hard registers living through a call. Each hard register gets a
separate stack slot, and as a result function stack frames are larger.
-fno-ira-share-spill-slots
Disable sharing of stack slots allocated for pseudo-registers. Each pseudo-register that does not get a hard register
gets a separate stack slot, and as a result function stack frames are larger.
-fira-verbose=n
Control the verbosity of the dump file for the integrated register allocator. The default value is 5. If the value n is
greater or equal to 10, the dump output is sent to stderr using the same format as n minus 10.
-fdelayed-branch
If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed
branch instructions.
Enabled at levels -O, -O2, -O3, -Os.
-fschedule-insns
If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to required data
being unavailable. This helps machines that have slow floating point or memory load instructions by allowing other
instructions to be issued until the result of the load or floating-point instruction is required.
Enabled at levels -O2, -O3.
-fschedule-insns2
Similar to -fschedule-insns, but requests an additional pass of instruction scheduling after register allocation has been
done. This is especially useful on machines with a relatively small number of registers and where memory load
instructions take more than one cycle.
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock
Don't schedule instructions across basic blocks. This is normally enabled by default when scheduling before register
allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fno-sched-spec
Don't allow speculative motion of non-load instructions. This is normally enabled by default when scheduling before
register allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-pressure
Enable register pressure sensitive insn scheduling before the register allocation. This only makes sense when scheduling
before register allocation is enabled, i.e. with -fschedule-insns or at -O2 or higher. Usage of this option can improve
the generated code and decrease its size by preventing register pressure increase above the number of available hard
registers and as a consequence register spills in the register allocation.
-fsched-spec-load
Allow speculative motion of some load instructions. This only makes sense when scheduling before register allocation,
i.e. with -fschedule-insns or at -O2 or higher.
-fsched-spec-load-dangerous
Allow speculative motion of more load instructions. This only makes sense when scheduling before register allocation,
i.e. with -fschedule-insns or at -O2 or higher.
-fsched-stalled-insns
-fsched-stalled-insns=n
Define how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list, during the
second scheduling pass. -fno-sched-stalled-insns means that no insns will be moved prematurely, -fsched-stalled-insns=0
means there is no limit on how many queued insns can be moved prematurely. -fsched-stalled-insns without a value is
equivalent to -fsched-stalled-insns=1.
-fsched-stalled-insns-dep
-fsched-stalled-insns-dep=n
Define how many insn groups (cycles) will be examined for a dependency on a stalled insn that is candidate for premature
removal from the queue of stalled insns. This has an effect only during the second scheduling pass, and only if
-fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0.
-fsched-stalled-insns-dep without a value is equivalent to -fsched-stalled-insns-dep=1.
-fsched2-use-superblocks
When scheduling after register allocation, do use superblock scheduling algorithm. Superblock scheduling allows motion
across basic block boundaries resulting on faster schedules. This option is experimental, as not all machine
descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm.
This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.
-fsched-group-heuristic
Enable the group heuristic in the scheduler. This heuristic favors the instruction that belongs to a schedule group.
This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or
higher.
-fsched-critical-path-heuristic
Enable the critical-path heuristic in the scheduler. This heuristic favors instructions on the critical path. This is
enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-fsched-spec-insn-heuristic
Enable the speculative instruction heuristic in the scheduler. This heuristic favors speculative instructions with
greater dependency weakness. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
-fsched-rank-heuristic
Enable the rank heuristic in the scheduler. This heuristic favors the instruction belonging to a basic block with
greater size or frequency. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
-fsched-last-insn-heuristic
Enable the last-instruction heuristic in the scheduler. This heuristic favors the instruction that is less dependent on
the last instruction scheduled. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
-fsched-dep-count-heuristic
Enable the dependent-count heuristic in the scheduler. This heuristic favors the instruction that has more instructions
depending on it. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2
or at -O2 or higher.
-freschedule-modulo-scheduled-loops
The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled we may want to prevent the
later scheduling passes from changing its schedule, we use this option to control that.
-fselective-scheduling
Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the first scheduler
pass.
-fselective-scheduling2
Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the second scheduler
pass.
-fsel-sched-pipelining
Enable software pipelining of innermost loops during selective scheduling. This option has no effect until one of
-fselective-scheduling or -fselective-scheduling2 is turned on.
-fsel-sched-pipelining-outer-loops
When pipelining loops during selective scheduling, also pipeline outer loops. This option has no effect until
-fsel-sched-pipelining is turned on.
-fshrink-wrap
Emit function prologues only before parts of the function that need it, rather than at the top of the function. This
flag is enabled by default at -O and higher.
-fcaller-saves
Enable values to be allocated in registers that will be clobbered by function calls, by emitting extra instructions to
save and restore the registers around such calls. Such allocation is done only when it seems to result in better code
than would otherwise be produced.
This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use
instead.
Enabled at levels -O2, -O3, -Os.
-fcombine-stack-adjustments
Tracks stack adjustments (pushes and pops) and stack memory references and then tries to find ways to combine them.
Enabled by default at -O1 and higher.
-fconserve-stack
Attempt to minimize stack usage. The compiler will attempt to use less stack space, even if that makes the program
slower. This option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to
400.
-ftree-reassoc
Perform reassociation on trees. This flag is enabled by default at -O and higher.
-ftree-pre
Perform partial redundancy elimination (PRE) on trees. This flag is enabled by default at -O2 and -O3.
-ftree-partial-pre
Make partial redundancy elimination (PRE) more aggressive. This flag is enabled by default at -O3.
-ftree-forwprop
Perform forward propagation on trees. This flag is enabled by default at -O and higher.
-ftree-fre
Perform full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE only considers
expressions that are computed on all paths leading to the redundant computation. This analysis is faster than PRE,
though it exposes fewer redundancies. This flag is enabled by default at -O and higher.
-ftree-phiprop
Perform hoisting of loads from conditional pointers on trees. This pass is enabled by default at -O and higher.
-ftree-copy-prop
Perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is enabled by default at
-O and higher.
-fipa-pure-const
Discover which functions are pure or constant. Enabled by default at -O and higher.
-fipa-reference
Discover which static variables do not escape cannot escape the compilation unit. Enabled by default at -O and higher.
-fipa-pta
Perform interprocedural pointer analysis and interprocedural modification and reference analysis. This option can cause
excessive memory and compile-time usage on large compilation units. It is not enabled by default at any optimization
level.
-fipa-profile
Perform interprocedural profile propagation. The functions called only from cold functions are marked as cold. Also
functions executed once (such as "cold", "noreturn", static constructors or destructors) are identified. Cold functions
and loop less parts of functions executed once are then optimized for size. Enabled by default at -O and higher.
-fipa-cp
Perform interprocedural constant propagation. This optimization analyzes the program to determine when values passed to
functions are constants and then optimizes accordingly. This optimization can substantially increase performance if the
application has constants passed to functions. This flag is enabled by default at -O2, -Os and -O3.
-fipa-cp-clone
Perform function cloning to make interprocedural constant propagation stronger. When enabled, interprocedural constant
propagation will perform function cloning when externally visible function can be called with constant arguments.
Because this optimization can create multiple copies of functions, it may significantly increase code size (see --param
ipcp-unit-growth=value). This flag is enabled by default at -O3.
-fipa-matrix-reorg
Perform matrix flattening and transposing. Matrix flattening tries to replace an m-dimensional matrix with its
equivalent n-dimensional matrix, where n < m. This reduces the level of indirection needed for accessing the elements of
the matrix. The second optimization is matrix transposing, which attempts to change the order of the matrix's dimensions
in order to improve cache locality. Both optimizations need the -fwhole-program flag. Transposing is enabled only if
profiling information is available.
-ftree-sink
Perform forward store motion on trees. This flag is enabled by default at -O and higher.
-ftree-bit-ccp
Perform sparse conditional bit constant propagation on trees and propagate pointer alignment information. This pass only
operates on local scalar variables and is enabled by default at -O and higher. It requires that -ftree-ccp is enabled.
-ftree-ccp
Perform sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar variables and is
enabled by default at -O and higher.
-ftree-switch-conversion
Perform conversion of simple initializations in a switch to initializations from a scalar array. This flag is enabled by
default at -O2 and higher.
-ftree-tail-merge
Look for identical code sequences. When found, replace one with a jump to the other. This optimization is known as tail
merging or cross jumping. This flag is enabled by default at -O2 and higher. The compilation time in this pass can be
limited using max-tail-merge-comparisons parameter and max-tail-merge-iterations parameter.
-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is enabled by default at -O and higher.
-ftree-builtin-call-dce
Perform conditional dead code elimination (DCE) for calls to builtin functions that may set "errno" but are otherwise
side-effect free. This flag is enabled by default at -O2 and higher if -Os is not also specified.
-ftree-dominator-opts
Perform a variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation and
expression simplification) based on a dominator tree traversal. This also performs jump threading (to reduce jumps to
jumps). This flag is enabled by default at -O and higher.
-ftree-dse
Perform dead store elimination (DSE) on trees. A dead store is a store into a memory location that is later overwritten
by another store without any intervening loads. In this case the earlier store can be deleted. This flag is enabled by
default at -O and higher.
-ftree-ch
Perform loop header copying on trees. This is beneficial since it increases effectiveness of code motion optimizations.
It also saves one jump. This flag is enabled by default at -O and higher. It is not enabled for -Os, since it usually
increases code size.
-ftree-loop-optimize
Perform loop optimizations on trees. This flag is enabled by default at -O and higher.
-ftree-loop-linear
Perform loop interchange transformations on tree. Same as -floop-interchange. To use this code transformation, GCC has
to be configured with --with-ppl and --with-cloog to enable the Graphite loop transformation infrastructure.
-floop-interchange
Perform loop interchange transformations on loops. Interchanging two nested loops switches the inner and outer loops.
For example, given a loop like:
DO J = 1, M
DO I = 1, N
A(J, I) = A(J, I) * C
ENDDO
ENDDO
loop interchange will transform the loop as if the user had written:
DO I = 1, N
DO J = 1, M
A(J, I) = A(J, I) * C
ENDDO
ENDDO
which can be beneficial when "N" is larger than the caches, because in Fortran, the elements of an array are stored in
memory contiguously by column, and the original loop iterates over rows, potentially creating at each access a cache
miss. This optimization applies to all the languages supported by GCC and is not limited to Fortran. To use this code
transformation, GCC has to be configured with --with-ppl and --with-cloog to enable the Graphite loop transformation
infrastructure.
-floop-strip-mine
Perform loop strip mining transformations on loops. Strip mining splits a loop into two nested loops. The outer loop
has strides equal to the strip size and the inner loop has strides of the original loop within a strip. The strip length
can be changed using the loop-block-tile-size parameter. For example, given a loop like:
DO I = 1, N
A(I) = A(I) + C
ENDDO
loop strip mining will transform the loop as if the user had written:
DO II = 1, N, 51
DO I = II, min (II + 50, N)
A(I) = A(I) + C
ENDDO
ENDDO
This optimization applies to all the languages supported by GCC and is not limited to Fortran. To use this code
transformation, GCC has to be configured with --with-ppl and --with-cloog to enable the Graphite loop transformation
infrastructure.
-floop-block
Perform loop blocking transformations on loops. Blocking strip mines each loop in the loop nest such that the memory
accesses of the element loops fit inside caches. The strip length can be changed using the loop-block-tile-size
parameter. For example, given a loop like:
DO I = 1, N
DO J = 1, M
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
loop blocking will transform the loop as if the user had written:
DO II = 1, N, 51
DO JJ = 1, M, 51
DO I = II, min (II + 50, N)
DO J = JJ, min (JJ + 50, M)
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
ENDDO
ENDDO
which can be beneficial when "M" is larger than the caches, because the innermost loop will iterate over a smaller amount
of data which can be kept in the caches. This optimization applies to all the languages supported by GCC and is not
limited to Fortran. To use this code transformation, GCC has to be configured with --with-ppl and --with-cloog to enable
the Graphite loop transformation infrastructure.
-fgraphite-identity
Enable the identity transformation for graphite. For every SCoP we generate the polyhedral representation and transform
it back to gimple. Using -fgraphite-identity we can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE
transformation. Some minimal optimizations are also performed by the code generator CLooG, like index splitting and dead
code elimination in loops.
-floop-flatten
Removes the loop nesting structure: transforms the loop nest into a single loop. This transformation can be useful as an
enablement transform for vectorization and parallelization. This feature is experimental. To use this code
transformation, GCC has to be configured with --with-ppl and --with-cloog to enable the Graphite loop transformation
infrastructure.
-floop-parallelize-all
Use the Graphite data dependence analysis to identify loops that can be parallelized. Parallelize all the loops that can
be analyzed to not contain loop carried dependences without checking that it is profitable to parallelize the loops.
-fcheck-data-deps
Compare the results of several data dependence analyzers. This option is used for debugging the data dependence
analyzers.
-ftree-loop-if-convert
Attempt to transform conditional jumps in the innermost loops to branch-less equivalents. The intent is to remove
control-flow from the innermost loops in order to improve the ability of the vectorization pass to handle these loops.
This is enabled by default if vectorization is enabled.
-ftree-loop-if-convert-stores
Attempt to also if-convert conditional jumps containing memory writes. This transformation can be unsafe for multi-
threaded programs as it transforms conditional memory writes into unconditional memory writes. For example,
for (i = 0; i < N; i++)
if (cond)
A[i] = expr;
would be transformed to
for (i = 0; i < N; i++)
A[i] = cond ? expr : A[i];
potentially producing data races.
-ftree-loop-distribution
Perform loop distribution. This flag can improve cache performance on big loop bodies and allow further loop
optimizations, like parallelization or vectorization, to take place. For example, the loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDO
is transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDO
-ftree-loop-distribute-patterns
Perform loop distribution of patterns that can be code generated with calls to a library. This flag is enabled by
default at -O3.
This pass distributes the initialization loops and generates a call to memset zero. For example, the loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO
is transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO
and the initialization loop is transformed into a call to memset zero.
-ftree-loop-im
Perform loop invariant motion on trees. This pass moves only invariants that would be hard to handle at RTL level
(function calls, operations that expand to nontrivial sequences of insns). With -funswitch-loops it also moves operands
of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop
unswitching. The pass also includes store motion.
-ftree-loop-ivcanon
Create a canonical counter for number of iterations in loops for which determining number of iterations requires
complicated analysis. Later optimizations then may determine the number easily. Useful especially in connection with
unrolling.
-fivopts
Perform induction variable optimizations (strength reduction, induction variable merging and induction variable
elimination) on trees.
-ftree-parallelize-loops=n
Parallelize loops, i.e., split their iteration space to run in n threads. This is only possible for loops whose
iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor
machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth. This option implies
-pthread, and thus is only supported on targets that have support for -pthread.
-ftree-pta
Perform function-local points-to analysis on trees. This flag is enabled by default at -O and higher.
-ftree-sra
Perform scalar replacement of aggregates. This pass replaces structure references with scalars to prevent committing
structures to memory too early. This flag is enabled by default at -O and higher.
-ftree-copyrename
Perform copy renaming on trees. This pass attempts to rename compiler temporaries to other variables at copy locations,
usually resulting in variable names which more closely resemble the original variables. This flag is enabled by default
at -O and higher.
-ftree-ter
Perform temporary expression replacement during the SSA->normal phase. Single use/single def temporaries are replaced at
their use location with their defining expression. This results in non-GIMPLE code, but gives the expanders much more
complex trees to work on resulting in better RTL generation. This is enabled by default at -O and higher.
-ftree-vectorize
Perform loop vectorization on trees. This flag is enabled by default at -O3.
-ftree-slp-vectorize
Perform basic block vectorization on trees. This flag is enabled by default at -O3 and when -ftree-vectorize is enabled.
-ftree-vect-loop-version
Perform loop versioning when doing loop vectorization on trees. When a loop appears to be vectorizable except that data
alignment or data dependence cannot be determined at compile time, then vectorized and non-vectorized versions of the
loop are generated along with run-time checks for alignment or dependence to control which version is executed. This
option is enabled by default except at level -Os where it is disabled.
-fvect-cost-model
Enable cost model for vectorization.
-ftree-vrp
Perform Value Range Propagation on trees. This is similar to the constant propagation pass, but instead of values,
ranges of values are propagated. This allows the optimizers to remove unnecessary range checks like array bound checks
and null pointer checks. This is enabled by default at -O2 and higher. Null pointer check elimination is only done if
-fdelete-null-pointer-checks is enabled.
-ftracer
Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function
allowing other optimizations to do better job.
-funroll-loops
Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. -funroll-loops
implies -frerun-cse-after-loop. This option makes code larger, and may or may not make it run faster.
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs
run more slowly. -funroll-all-loops implies the same options as -funroll-loops,
-fsplit-ivs-in-unroller
Enables expressing of values of induction variables in later iterations of the unrolled loop using the value in the first
iteration. This breaks long dependency chains, thus improving efficiency of the scheduling passes.
Combination of -fweb and CSE is often sufficient to obtain the same effect. However in cases the loop body is more
complicated than a single basic block, this is not reliable. It also does not work at all on some of the architectures
due to restrictions in the CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller
With this option, the compiler will create multiple copies of some local variables when unrolling a loop which can result
in superior code.
-fpartial-inlining
Inline parts of functions. This option has any effect only when inlining itself is turned on by the -finline-functions
or -finline-small-functions options.
Enabled at level -O2.
-fpredictive-commoning
Perform predictive commoning optimization, i.e., reusing computations (especially memory loads and stores) performed in
previous iterations of loops.
This option is enabled at level -O3.
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that
access large arrays.
This option may generate better or worse code; results are highly dependent on the structure of loops within the source
code.
Disabled at level -Os.
-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations. The difference between -fno-peephole and -fno-peephole2 is in how
they are implemented in the compiler; some targets use one, some use the other, a few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.
-fno-guess-branch-probability
Do not guess branch probabilities using heuristics.
GCC will use heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs).
These heuristics are based on the control flow graph. If some branch probabilities are specified by __builtin_expect,
then the heuristics will be used to guess branch probabilities for the rest of the control flow graph, taking the
__builtin_expect info into account. The interactions between the heuristics and __builtin_expect can be complex, and in
some cases, it may be useful to disable the heuristics so that the effects of __builtin_expect are easier to understand.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.
-freorder-blocks
Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality.
Enabled at levels -O2, -O3.
-freorder-blocks-and-partition
In addition to reordering basic blocks in the compiled function, in order to reduce number of taken branches, partitions
hot and cold basic blocks into separate sections of the assembly and .o files, to improve paging and cache locality
performance.
This optimization is automatically turned off in the presence of exception handling, for linkonce sections, for functions
with a user-defined section attribute and on any architecture that does not support named sections.
-freorder-functions
Reorder functions in the object file in order to improve code locality. This is implemented by using special subsections
".text.hot" for most frequently executed functions and ".text.unlikely" for unlikely executed functions. Reordering is
done by the linker so object file format must support named sections and linker must place them in a reasonable way.
Also profile feedback must be available in to make this option effective. See -fprofile-arcs for details.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing
Allow the compiler to assume the strictest aliasing rules applicable to the language being compiled. For C (and C++),
this activates optimizations based on the type of expressions. In particular, an object of one type is assumed never to
reside at the same address as an object of a different type, unless the types are almost the same. For example, an
"unsigned int" can alias an "int", but not a "void*" or a "double". A character type may alias any other type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one most recently written to (called "type-punning") is
common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type.
So, the code above will work as expected. However, this code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined
behavior, even if the cast uses a union type, e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.
-fstrict-overflow
Allow the compiler to assume strict signed overflow rules, depending on the language being compiled. For C (and C++)
this means that overflow when doing arithmetic with signed numbers is undefined, which means that the compiler may assume
that it will not happen. This permits various optimizations. For example, the compiler will assume that an expression
like "i + 10 > i" will always be true for signed "i". This assumption is only valid if signed overflow is undefined, as
the expression is false if "i + 10" overflows when using twos complement arithmetic. When this option is in effect any
attempt to determine whether an operation on signed numbers will overflow must be written carefully to not actually
involve overflow.
This option also allows the compiler to assume strict pointer semantics: given a pointer to an object, if adding an
offset to that pointer does not produce a pointer to the same object, the addition is undefined. This permits the
compiler to conclude that "p + u > p" is always true for a pointer "p" and unsigned integer "u". This assumption is only
valid because pointer wraparound is undefined, as the expression is false if "p + u" overflows using twos complement
arithmetic.
See also the -fwrapv option. Using -fwrapv means that integer signed overflow is fully defined: it wraps. When -fwrapv
is used, there is no difference between -fstrict-overflow and -fno-strict-overflow for integers. With -fwrapv certain
types of overflow are permitted. For example, if the compiler gets an overflow when doing arithmetic on constants, the
overflowed value can still be used with -fwrapv, but not otherwise.
The -fstrict-overflow option is enabled at levels -O2, -O3, -Os.
-falign-functions
-falign-functions=n
Align the start of functions to the next power-of-two greater than n, skipping up to n bytes. For instance,
-falign-functions=32 aligns functions to the next 32-byte boundary, but -falign-functions=24 would align to the next
32-byte boundary only if this can be done by skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are equivalent and mean that functions will not be aligned.
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-labels
-falign-labels=n
Align all branch targets to a power-of-two boundary, skipping up to n bytes like -falign-functions. This option can
easily make code slower, because it must insert dummy operations for when the branch target is reached in the usual flow
of the code.
-fno-align-labels and -falign-labels=1 are equivalent and mean that labels will not be aligned.
If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead.
If n is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment.
Enabled at levels -O2, -O3.
-falign-loops
-falign-loops=n
Align loops to a power-of-two boundary, skipping up to n bytes like -falign-functions. The hope is that the loop will be
executed many times, which will make up for any execution of the dummy operations.
-fno-align-loops and -falign-loops=1 are equivalent and mean that loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps
-falign-jumps=n
Align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping,
skipping up to n bytes like -falign-functions. In this case, no dummy operations need be executed.
-fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-funit-at-a-time
This option is left for compatibility reasons. -funit-at-a-time has no effect, while -fno-unit-at-a-time implies
-fno-toplevel-reorder and -fno-section-anchors.
Enabled by default.
-fno-toplevel-reorder
Do not reorder top-level functions, variables, and "asm" statements. Output them in the same order that they appear in
the input file. When this option is used, unreferenced static variables will not be removed. This option is intended to
support existing code that relies on a particular ordering. For new code, it is better to use attributes.
Enabled at level -O0. When disabled explicitly, it also implies -fno-section-anchors, which is otherwise enabled at -O0
on some targets.
-fweb
Constructs webs as commonly used for register allocation purposes and assign each web individual pseudo register. This
allows the register allocation pass to operate on pseudos directly, but also strengthens several other optimization
passes, such as CSE, loop optimizer and trivial dead code remover. It can, however, make debugging impossible, since
variables will no longer stay in a "home register".
Enabled by default with -funroll-loops.
-fwhole-program
Assume that the current compilation unit represents the whole program being compiled. All public functions and variables
with the exception of "main" and those merged by attribute "externally_visible" become static functions and in effect are
optimized more aggressively by interprocedural optimizers. If gold is used as the linker plugin, "externally_visible"
attributes are automatically added to functions (not variable yet due to a current gold issue) that are accessed outside
of LTO objects according to resolution file produced by gold. For other linkers that cannot generate resolution file,
explicit "externally_visible" attributes are still necessary. While this option is equivalent to proper use of the
"static" keyword for programs consisting of a single file, in combination with option -flto this flag can be used to
compile many smaller scale programs since the functions and variables become local for the whole combined compilation
unit, not for the single source file itself.
This option implies -fwhole-file for Fortran programs.
-flto[=n]
This option runs the standard link-time optimizer. When invoked with source code, it generates GIMPLE (one of GCC's
internal representations) and writes it to special ELF sections in the object file. When the object files are linked
together, all the function bodies are read from these ELF sections and instantiated as if they had been part of the same
translation unit.
To use the link-time optimizer, -flto needs to be specified at compile time and during the final link. For example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o and
bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into a single internal
image, and compiles the result as usual. Since both foo.o and bar.o are merged into a single image, this causes all the
interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one. This means,
for example, that the inliner is able to inline functions in bar.o into functions in foo.o and vice-versa.
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.c
The above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation and optimizes
them as usual to produce myprog.
The only important thing to keep in mind is that to enable link-time optimizations the -flto flag needs to be passed to
both the compile and the link commands.
To make whole program optimization effective, it is necessary to make certain whole program assumptions. The compiler
needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized
unit. When supported by the linker, the linker plugin (see -fuse-linker-plugin) passes information to the compiler about
used and externally visible symbols. When the linker plugin is not available, -fwhole-program should be used to allow
the compiler to make these assumptions, which leads to more aggressive optimization decisions.
Note that when a file is compiled with -flto, the generated object file is larger than a regular object file because it
contains GIMPLE bytecodes and the usual final code. This means that object files with LTO information can be linked as
normal object files; if -flto is not passed to the linker, no interprocedural optimizations are applied.
Additionally, the optimization flags used to compile individual files are not necessarily related to those used at link
time. For instance,
gcc -c -O0 -flto foo.c
gcc -c -O0 -flto bar.c
gcc -o myprog -flto -O3 foo.o bar.o
This produces individual object files with unoptimized assembler code, but the resulting binary myprog is optimized at
-O3. If, instead, the final binary is generated without -flto, then myprog is not optimized.
When producing the final binary with -flto, GCC only applies link-time optimizations to those files that contain
bytecode. Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC
automatically selects which files to optimize in LTO mode and which files to link without further processing.
There are some code generation flags preserved by GCC when generating bytecodes, as they need to be used during the final
link stage. Currently, the following options are saved into the GIMPLE bytecode files: -fPIC, -fcommon and all the -m
target flags.
At link time, these options are read in and reapplied. Note that the current implementation makes no attempt to
recognize conflicting values for these options. If different files have conflicting option values (e.g., one file is
compiled with -fPIC and another isn't), the compiler simply uses the last value read from the bytecode files. It is
recommended, then, that you compile all the files participating in the same link with the same options.
If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked
together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still
undefined at run time.
Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in different
languages. This requires support in the language front end. Currently, the C, C++ and Fortran front ends are capable of
emitting GIMPLE bytecodes, so something like this should work:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
Notice that the final link is done with g++ to get the C++ runtime libraries and -lgfortran is added to get the Fortran
runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when
mixing languages in a regular (non-LTO) compilation; all you need to add is -flto to all the compile and link commands.
If object files containing GIMPLE bytecode are stored in a library archive, say libfoo.a, it is possible to extract and
use them in an LTO link if you are using a linker with plugin support. To enable this feature, use the flag
-fuse-linker-plugin at link time:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the
running GCC to make them part of the aggregated GIMPLE image to be optimized.
If you are not using a linker with plugin support and/or do not enable the linker plugin, then the objects inside
libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization process.
Link-time optimizations do not require the presence of the whole program to operate. If the program does not require any
symbols to be exported, it is possible to combine -flto and -fwhole-program to allow the interprocedural optimizers to
use more aggressive assumptions which may lead to improved optimization opportunities. Use of -fwhole-program is not
needed when linker plugin is active (see -fuse-linker-plugin).
The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of
hosts. The bytecode files are versioned and there is a strict version check, so bytecode files generated in one version
of GCC will not work with an older/newer version of GCC.
Link-time optimization does not work well with generation of debugging information. Combining -flto with -g is currently
experimental and expected to produce wrong results.
If you specify the optional n, the optimization and code generation done at link time is executed in parallel using n
parallel jobs by utilizing an installed make program. The environment variable MAKE may be used to override the program
used. The default value for n is 1.
You can also specify -flto=jobserver to use GNU make's job server mode to determine the number of parallel jobs. This is
useful when the Makefile calling GCC is already executing in parallel. You must prepend a + to the command recipe in the
parent Makefile for this to work. This option likely only works if MAKE is GNU make.
This option is disabled by default
-flto-partition=alg
Specify the partitioning algorithm used by the link-time optimizer. The value is either "1to1" to specify a partitioning
mirroring the original source files or "balanced" to specify partitioning into equally sized chunks (whenever possible).
Specifying "none" as an algorithm disables partitioning and streaming completely. The default value is "balanced".
-flto-compression-level=n
This option specifies the level of compression used for intermediate language written to LTO object files, and is only
meaningful in conjunction with LTO mode (-flto). Valid values are 0 (no compression) to 9 (maximum compression). Values
outside this range are clamped to either 0 or 9. If the option is not given, a default balanced compression setting is
used.
-flto-report
Prints a report with internal details on the workings of the link-time optimizer. The contents of this report vary from
version to version. It is meant to be useful to GCC developers when processing object files in LTO mode (via -flto).
Disabled by default.
-fuse-linker-plugin
Enables the use of a linker plugin during link-time optimization. This option relies on plugin support in the linker,
which is available in gold or in GNU ld 2.21 or newer.
This option enables the extraction of object files with GIMPLE bytecode out of library archives. This improves the
quality of optimization by exposing more code to the link-time optimizer. This information specifies what symbols can be
accessed externally (by non-LTO object or during dynamic linking). Resulting code quality improvements on binaries (and
shared libraries that use hidden visibility) are similar to "-fwhole-program". See -flto for a description of the effect
of this flag and how to use it.
This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker
supporting plugins (GNU ld 2.21 or newer or gold).
-ffat-lto-objects
Fat LTO objects are object files that contain both the intermediate language and the object code. This makes them usable
for both LTO linking and normal linking. This option is effective only when compiling with -flto and is ignored at link
time.
-fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to be aware of LTO. It
requires a linker with linker plugin support for basic functionality. Additionally, nm, ar and ranlib need to support
linker plugins to allow a full-featured build environment (capable of building static libraries etc).
The default is -ffat-lto-objects but this default is intended to change in future releases when linker plugin enabled
environments become more common.
-fcompare-elim
After register allocation and post-register allocation instruction splitting, identify arithmetic instructions that
compute processor flags similar to a comparison operation based on that arithmetic. If possible, eliminate the explicit
comparison operation.
This pass only applies to certain targets that cannot explicitly represent the comparison operation before register
allocation is complete.
Enabled at levels -O, -O2, -O3, -Os.
-fcprop-registers
After register allocation and post-register allocation instruction splitting, we perform a copy-propagation pass to try
to reduce scheduling dependencies and occasionally eliminate the copy.
Enabled at levels -O, -O2, -O3, -Os.
-fprofile-correction
Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter
updates. When this option is specified, GCC will use heuristics to correct or smooth out such inconsistencies. By
default, GCC will emit an error message when an inconsistent profile is detected.
-fprofile-dir=path
Set the directory to search for the profile data files in to path. This option affects only the profile data generated
by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by -fprofile-use and -fbranch-probabilities and its
related options. Both absolute and relative paths can be used. By default, GCC will use the current directory as path,
thus the profile data file will appear in the same directory as the object file.
-fprofile-generate
-fprofile-generate=path
Enable options usually used for instrumenting application to produce profile useful for later recompilation with profile
feedback based optimization. You must use -fprofile-generate both when compiling and when linking your program.
The following options are enabled: "-fprofile-arcs", "-fprofile-values", "-fvpt".
If path is specified, GCC will look at the path to find the profile feedback data files. See -fprofile-dir.
-fprofile-use
-fprofile-use=path
Enable profile feedback directed optimizations, and optimizations generally profitable only with profile feedback
available.
The following options are enabled: "-fbranch-probabilities", "-fvpt", "-funroll-loops", "-fpeel-loops", "-ftracer"
By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be turned
into a warning by using -Wcoverage-mismatch. Note this may result in poorly optimized code.
If path is specified, GCC will look at the path to find the profile feedback data files. See -fprofile-dir.
The following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed
and correctness. All must be specifically enabled.
-ffloat-store
Do not store floating-point variables in registers, and inhibit other options that might change whether a floating-point
value is taken from a register or memory.
This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the
68881) keep more precision than a "double" is supposed to have. Similarly for the x86 architecture. For most programs,
the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use
-ffloat-store for such programs, after modifying them to store all pertinent intermediate computations into variables.
-fexcess-precision=style
This option allows further control over excess precision on machines where floating-point registers have more precision
than the IEEE "float" and "double" types and the processor does not support operations rounding to those types. By
default, -fexcess-precision=fast is in effect; this means that operations are carried out in the precision of the
registers and that it is unpredictable when rounding to the types specified in the source code takes place. When
compiling C, if -fexcess-precision=standard is specified then excess precision will follow the rules specified in ISO
C99; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas -ffloat-store
only affects assignments). This option is enabled by default for C if a strict conformance option such as -std=c99 is
used.
-fexcess-precision=standard is not implemented for languages other than C, and has no effect if
-funsafe-math-optimizations or -ffast-math is specified. On the x86, it also has no effect if -mfpmath=sse or
-mfpmath=sse+387 is specified; in the former case, IEEE semantics apply without excess precision, and in the latter,
rounding is unpredictable.
-ffast-math
Sets -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans and
-fcx-limited-range.
This option causes the preprocessor macro "__FAST_MATH__" to be defined.
This option is not turned on by any -O option besides -Ofast since it can result in incorrect output for programs that
depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster
code for programs that do not require the guarantees of these specifications.
-fno-math-errno
Do not set ERRNO after calling math functions that are executed with a single instruction, e.g., sqrt. A program that
relies on IEEE exceptions for math error handling may want to use this flag for speed while maintaining IEEE arithmetic
compatibility.
This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an
exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for
programs that do not require the guarantees of these specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno". There is therefore no reason for the compiler to consider the
possibility that it might, and -fno-math-errno is the default.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may
violate IEEE or ANSI standards. When used at link-time, it may include libraries or startup files that change the
default FPU control word or other similar optimizations.
This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an
exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for
programs that do not require the guarantees of these specifications. Enables -fno-signed-zeros, -fno-trapping-math,
-fassociative-math and -freciprocal-math.
The default is -fno-unsafe-math-optimizations.
-fassociative-math
Allow re-association of operands in series of floating-point operations. This violates the ISO C and C++ language
standard by possibly changing computation result. NOTE: re-ordering may change the sign of zero as well as ignore NaNs
and inhibit or create underflow or overflow (and thus cannot be used on code that relies on rounding behavior like "(x +
2**52) - 2**52". May also reorder floating-point comparisons and thus may not be used when ordered comparisons are
required. This option requires that both -fno-signed-zeros and -fno-trapping-math be in effect. Moreover, it doesn't
make much sense with -frounding-math. For Fortran the option is automatically enabled when both -fno-signed-zeros and
-fno-trapping-math are in effect.
The default is -fno-associative-math.
-freciprocal-math
Allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations. For example
"x / y" can be replaced with "x * (1/y)", which is useful if "(1/y)" is subject to common subexpression elimination.
Note that this loses precision and increases the number of flops operating on the value.
The default is -fno-reciprocal-math.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs.
This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an
exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for
programs that do not require the guarantees of these specifications.
The default is -fno-finite-math-only.
-fno-signed-zeros
Allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE arithmetic specifies the
behavior of distinct +0.0 and -0.0 values, which then prohibits simplification of expressions such as x+0.0 or 0.0*x
(even with -ffinite-math-only). This option implies that the sign of a zero result isn't significant.
The default is -fsigned-zeros.
-fno-trapping-math
Compile code assuming that floating-point operations cannot generate user-visible traps. These traps include division by
zero, overflow, underflow, inexact result and invalid operation. This option requires that -fno-signaling-nans be in
effect. Setting this option may allow faster code if one relies on "non-stop" IEEE arithmetic, for example.
This option should never be turned on by any -O option since it can result in incorrect output for programs that depend
on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -ftrapping-math.
-frounding-math
Disable transformations and optimizations that assume default floating-point rounding behavior. This is round-to-zero
for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations. This option
should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default
rounding mode. This option disables constant folding of floating-point expressions at compile time (which may be
affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding
modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by
rounding mode. Future versions of GCC may provide finer control of this setting using C99's "FENV_ACCESS" pragma. This
command-line option will be used to specify the default state for "FENV_ACCESS".
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. Setting
this option disables optimizations that may change the number of exceptions visible with signaling NaNs. This option
implies -ftrapping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__" to be defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN
behavior.
-fsingle-precision-constant
Treat floating-point constants as single precision instead of implicitly converting them to double-precision constants.
-fcx-limited-range
When enabled, this option states that a range reduction step is not needed when performing complex division. Also, there
is no checking whether the result of a complex multiplication or division is "NaN + I*NaN", with an attempt to rescue the
situation in that case. The default is -fno-cx-limited-range, but is enabled by -ffast-math.
This option controls the default setting of the ISO C99 "CX_LIMITED_RANGE" pragma. Nevertheless, the option applies to
all languages.
-fcx-fortran-rules
Complex multiplication and division follow Fortran rules. Range reduction is done as part of complex division, but there
is no checking whether the result of a complex multiplication or division is "NaN + I*NaN", with an attempt to rescue the
situation in that case.
The default is -fno-cx-fortran-rules.
The following options control optimizations that may improve performance, but are not enabled by any -O options. This
section includes experimental options that may produce broken code.
-fbranch-probabilities
After running a program compiled with -fprofile-arcs, you can compile it a second time using -fbranch-probabilities, to
improve optimizations based on the number of times each branch was taken. When the program compiled with -fprofile-arcs
exits it saves arc execution counts to a file called sourcename.gcda for each source file. The information in this data
file is very dependent on the structure of the generated code, so you must use the same source code and the same
optimization options for both compilations.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to improve
optimization. Currently, they are only used in one place: in reorg.c, instead of guessing which path a branch is most
likely to take, the REG_BR_PROB values are used to exactly determine which path is taken more often.
-fprofile-values
If combined with -fprofile-arcs, it adds code so that some data about values of expressions in the program is gathered.
With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions for usage in
optimizations.
Enabled with -fprofile-generate and -fprofile-use.
-fvpt
If combined with -fprofile-arcs, it instructs the compiler to add a code to gather information about values of
expressions.
With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based on them.
Currently the optimizations include specialization of division operation using the knowledge about the value of the
denominator.
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation.
This optimization will most benefit processors with lots of registers. Depending on the debug information format adopted
by the target, however, it can make debugging impossible, since variables will no longer stay in a "home register".
Enabled by default with -funroll-loops and -fpeel-loops.
-ftracer
Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function
allowing other optimizations to do better job.
Enabled with -fprofile-use.
-funroll-loops
Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. -funroll-loops
implies -frerun-cse-after-loop, -fweb and -frename-registers. It also turns on complete loop peeling (i.e. complete
removal of loops with small constant number of iterations). This option makes code larger, and may or may not make it
run faster.
Enabled with -fprofile-use.
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs
run more slowly. -funroll-all-loops implies the same options as -funroll-loops.
-fpeel-loops
Peels loops for which there is enough information that they do not roll much (from profile feedback). It also turns on
complete loop peeling (i.e. complete removal of loops with small constant number of iterations).
Enabled with -fprofile-use.
-fmove-loop-invariants
Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1
-funswitch-loops
Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified
according to result of the condition).
-ffunction-sections
-fdata-sections
Place each function or data item into its own section in the output file if the target supports arbitrary sections. The
name of the function or the name of the data item determines the section's name in the output file.
Use these options on systems where the linker can perform optimizations to improve locality of reference in the
instruction space. Most systems using the ELF object format and SPARC processors running Solaris 2 have linkers with
such optimizations. AIX may have these optimizations in the future.
Only use these options when there are significant benefits from doing so. When you specify these options, the assembler
and linker will create larger object and executable files and will also be slower. You will not be able to use "gprof"
on all systems if you specify this option and you may have problems with debugging if you specify both this option and
-g.
-fbranch-target-load-optimize
Perform branch target register load optimization before prologue / epilogue threading. The use of target registers can
typically be exposed only during reload, thus hoisting loads out of loops and doing inter-block scheduling needs a
separate optimization pass.
-fbranch-target-load-optimize2
Perform branch target register load optimization after prologue / epilogue threading.
-fbtr-bb-exclusive
When performing branch target register load optimization, don't reuse branch target registers in within any basic block.
-fstack-protector
Emit extra code to check for buffer overflows, such as stack smashing attacks. This is done by adding a guard variable
to functions with vulnerable objects. This includes functions that call alloca, and functions with buffers larger than 8
bytes. The guards are initialized when a function is entered and then checked when the function exits. If a guard check
fails, an error message is printed and the program exits.
NOTE: In Ubuntu 6.10 and later versions this option is enabled by default for C, C++, ObjC, ObjC++, if none of
-fno-stack-protector, -nostdlib, nor -ffreestanding are found.
-fstack-protector-all
Like -fstack-protector except that all functions are protected.
-fsection-anchors
Try to reduce the number of symbolic address calculations by using shared "anchor" symbols to address nearby objects.
This transformation can help to reduce the number of GOT entries and GOT accesses on some targets.
For example, the implementation of the following function "foo":
static int a, b, c;
int foo (void) { return a + b + c; }
would usually calculate the addresses of all three variables, but if you compile it with -fsection-anchors, it will
access the variables from a common anchor point instead. The effect is similar to the following pseudocode (which isn't
valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
Not all targets support this option.
--param name=value
In some places, GCC uses various constants to control the amount of optimization that is done. For example, GCC will not
inline functions that contain more than a certain number of instructions. You can control some of these constants on the
command line using the --param option.
The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are
subject to change without notice in future releases.
In each case, the value is an integer. The allowable choices for name are given in the following table:
predictable-branch-outcome
When branch is predicted to be taken with probability lower than this threshold (in percent), then it is considered
well predictable. The default is 10.
max-crossjump-edges
The maximum number of incoming edges to consider for crossjumping. The algorithm used by -fcrossjumping is O(N^2) in
the number of edges incoming to each block. Increasing values mean more aggressive optimization, making the
compilation time increase with probably small improvement in executable size.
min-crossjump-insns
The minimum number of instructions that must be matched at the end of two blocks before crossjumping will be
performed on them. This value is ignored in the case where all instructions in the block being crossjumped from are
matched. The default value is 5.
max-grow-copy-bb-insns
The maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a
jump instruction. The default value is 8.
max-goto-duplication-insns
The maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoid O(N^2) behavior
in a number of passes, GCC factors computed gotos early in the compilation process, and unfactors them as late as
possible. Only computed jumps at the end of a basic blocks with no more than max-goto-duplication-insns are
unfactored. The default value is 8.
max-delay-slot-insn-search
The maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than
this arbitrary number of instructions is searched, the time savings from filling the delay slot will be minimal so
stop searching. Increasing values mean more aggressive optimization, making the compilation time increase with
probably small improvement in execution time.
max-delay-slot-live-search
When trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid
live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing
the compilation time. This parameter should be removed when the delay slot code is rewritten to maintain the
control-flow graph.
max-gcse-memory
The approximate maximum amount of memory that will be allocated in order to perform the global common subexpression
elimination optimization. If more memory than specified is required, the optimization will not be done.
max-gcse-insertion-ratio
If the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE will
insert or remove the expression and thus leave partially redundant computations in the instruction stream. The
default value is 20.
max-pending-list-length
The maximum number of pending dependencies scheduling will allow before flushing the current state and starting over.
Large functions with few branches or calls can create excessively large lists which needlessly consume memory and
resources.
max-modulo-backtrack-attempts
The maximum number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can
exponentially increase compilation time.
max-inline-insns-single
Several parameters control the tree inliner used in gcc. This number sets the maximum number of instructions
(counted in GCC's internal representation) in a single function that the tree inliner will consider for inlining.
This only affects functions declared inline and methods implemented in a class declaration (C++). The default value
is 400.
max-inline-insns-auto
When you use -finline-functions (included in -O3), a lot of functions that would otherwise not be considered for
inlining by the compiler will be investigated. To those functions, a different (more restrictive) limit compared to
functions declared inline can be applied. The default value is 40.
large-function-insns
The limit specifying really large functions. For functions larger than this limit after inlining, inlining is
constrained by --param large-function-growth. This parameter is useful primarily to avoid extreme compilation time
caused by non-linear algorithms used by the back end. The default value is 2700.
large-function-growth
Specifies maximal growth of large function caused by inlining in percents. The default value is 100 which limits
large function growth to 2.0 times the original size.
large-unit-insns
The limit specifying large translation unit. Growth caused by inlining of units larger than this limit is limited by
--param inline-unit-growth. For small units this might be too tight (consider unit consisting of function A that is
inline and B that just calls A three time. If B is small relative to A, the growth of unit is 300\% and yet such
inlining is very sane. For very large units consisting of small inlineable functions however the overall unit growth
limit is needed to avoid exponential explosion of code size. Thus for smaller units, the size is increased to
--param large-unit-insns before applying --param inline-unit-growth. The default is 10000
inline-unit-growth
Specifies maximal overall growth of the compilation unit caused by inlining. The default value is 30 which limits
unit growth to 1.3 times the original size.
ipcp-unit-growth
Specifies maximal overall growth of the compilation unit caused by interprocedural constant propagation. The default
value is 10 which limits unit growth to 1.1 times the original size.
large-stack-frame
The limit specifying large stack frames. While inlining the algorithm is trying to not grow past this limit too
much. Default value is 256 bytes.
large-stack-frame-growth
Specifies maximal growth of large stack frames caused by inlining in percents. The default value is 1000 which
limits large stack frame growth to 11 times the original size.
max-inline-insns-recursive
max-inline-insns-recursive-auto
Specifies maximum number of instructions out-of-line copy of self recursive inline function can grow into by
performing recursive inlining.
For functions declared inline --param max-inline-insns-recursive is taken into account. For function not declared
inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled and --param max-inline-
insns-recursive-auto is used. The default value is 450.
max-inline-recursive-depth
max-inline-recursive-depth-auto
Specifies maximum recursion depth used by the recursive inlining.
For functions declared inline --param max-inline-recursive-depth is taken into account. For function not declared
inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled and --param max-inline-
recursive-depth-auto is used. The default value is 8.
min-inline-recursive-probability
Recursive inlining is profitable only for function having deep recursion in average and can hurt for function having
little recursion depth by increasing the prologue size or complexity of function body to other optimizers.
When profile feedback is available (see -fprofile-generate) the actual recursion depth can be guessed from
probability that function will recurse via given call expression. This parameter limits inlining only to call
expression whose probability exceeds given threshold (in percents). The default value is 10.
early-inlining-insns
Specify growth that early inliner can make. In effect it increases amount of inlining for code having large
abstraction penalty. The default value is 10.
max-early-inliner-iterations
max-early-inliner-iterations
Limit of iterations of early inliner. This basically bounds number of nested indirect calls early inliner can
resolve. Deeper chains are still handled by late inlining.
comdat-sharing-probability
comdat-sharing-probability
Probability (in percent) that C++ inline function with comdat visibility will be shared across multiple compilation
units. The default value is 20.
min-vect-loop-bound
The minimum number of iterations under which a loop will not get vectorized when -ftree-vectorize is used. The
number of iterations after vectorization needs to be greater than the value specified by this option to allow
vectorization. The default value is 0.
gcse-cost-distance-ratio
Scaling factor in calculation of maximum distance an expression can be moved by GCSE optimizations. This is
currently supported only in the code hoisting pass. The bigger the ratio, the more aggressive code hoisting will be
with simple expressions, i.e., the expressions that have cost less than gcse-unrestricted-cost. Specifying 0 will
disable hoisting of simple expressions. The default value is 10.
gcse-unrestricted-cost
Cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations will not
constrain the distance an expression can travel. This is currently supported only in the code hoisting pass. The
lesser the cost, the more aggressive code hoisting will be. Specifying 0 will allow all expressions to travel
unrestricted distances. The default value is 3.
max-hoist-depth
The depth of search in the dominator tree for expressions to hoist. This is used to avoid quadratic behavior in
hoisting algorithm. The value of 0 will avoid limiting the search, but may slow down compilation of huge functions.
The default value is 30.
max-tail-merge-comparisons
The maximum amount of similar bbs to compare a bb with. This is used to avoid quadratic behavior in tree tail
merging. The default value is 10.
max-tail-merge-iterations
The maximum amount of iterations of the pass over the function. This is used to limit compilation time in tree tail
merging. The default value is 2.
max-unrolled-insns
The maximum number of instructions that a loop should have if that loop is unrolled, and if the loop is unrolled, it
determines how many times the loop code is unrolled.
max-average-unrolled-insns
The maximum number of instructions biased by probabilities of their execution that a loop should have if that loop is
unrolled, and if the loop is unrolled, it determines how many times the loop code is unrolled.
max-unroll-times
The maximum number of unrollings of a single loop.
max-peeled-insns
The maximum number of instructions that a loop should have if that loop is peeled, and if the loop is peeled, it
determines how many times the loop code is peeled.
max-peel-times
The maximum number of peelings of a single loop.
max-completely-peeled-insns
The maximum number of insns of a completely peeled loop.
max-completely-peel-times
The maximum number of iterations of a loop to be suitable for complete peeling.
max-completely-peel-loop-nest-depth
The maximum depth of a loop nest suitable for complete peeling.
max-unswitch-insns
The maximum number of insns of an unswitched loop.
max-unswitch-level
The maximum number of branches unswitched in a single loop.
lim-expensive
The minimum cost of an expensive expression in the loop invariant motion.
iv-consider-all-candidates-bound
Bound on number of candidates for induction variables below that all candidates are considered for each use in
induction variable optimizations. Only the most relevant candidates are considered if there are more candidates, to
avoid quadratic time complexity.
iv-max-considered-uses
The induction variable optimizations give up on loops that contain more induction variable uses.
iv-always-prune-cand-set-bound
If number of candidates in the set is smaller than this value, we always try to remove unnecessary ivs from the set
during its optimization when a new iv is added to the set.
scev-max-expr-size
Bound on size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer.
scev-max-expr-complexity
Bound on the complexity of the expressions in the scalar evolutions analyzer. Complex expressions slow the analyzer.
omega-max-vars
The maximum number of variables in an Omega constraint system. The default value is 128.
omega-max-geqs
The maximum number of inequalities in an Omega constraint system. The default value is 256.
omega-max-eqs
The maximum number of equalities in an Omega constraint system. The default value is 128.
omega-max-wild-cards
The maximum number of wildcard variables that the Omega solver will be able to insert. The default value is 18.
omega-hash-table-size
The size of the hash table in the Omega solver. The default value is 550.
omega-max-keys
The maximal number of keys used by the Omega solver. The default value is 500.
omega-eliminate-redundant-constraints
When set to 1, use expensive methods to eliminate all redundant constraints. The default value is 0.
vect-max-version-for-alignment-checks
The maximum number of run-time checks that can be performed when doing loop versioning for alignment in the
vectorizer. See option ftree-vect-loop-version for more information.
vect-max-version-for-alias-checks
The maximum number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer.
See option ftree-vect-loop-version for more information.
max-iterations-to-track
The maximum number of iterations of a loop the brute force algorithm for analysis of # of iterations of the loop
tries to evaluate.
hot-bb-count-fraction
Select fraction of the maximal count of repetitions of basic block in program given basic block needs to have to be
considered hot.
hot-bb-frequency-fraction
Select fraction of the entry block frequency of executions of basic block in function given basic block needs to have
to be considered hot.
max-predicted-iterations
The maximum number of loop iterations we predict statically. This is useful in cases where function contain single
loop with known bound and other loop with unknown. We predict the known number of iterations correctly, while the
unknown number of iterations average to roughly 10. This means that the loop without bounds would appear
artificially cold relative to the other one.
align-threshold
Select fraction of the maximal frequency of executions of basic block in function given basic block will get aligned.
align-loop-iterations
A loop expected to iterate at lest the selected number of iterations will get aligned.
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
This value is used to limit superblock formation once the given percentage of executed instructions is covered. This
limits unnecessary code size expansion.
The tracer-dynamic-coverage-feedback is used only when profile feedback is available. The real profiles (as opposed
to statically estimated ones) are much less balanced allowing the threshold to be larger value.
tracer-max-code-growth
Stop tail duplication once code growth has reached given percentage. This is rather hokey argument, as most of the
duplicates will be eliminated later in cross jumping, so it may be set to much higher values than is the desired code
growth.
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent).
tracer-min-branch-ratio
tracer-min-branch-ratio-feedback
Stop forward growth if the best edge do have probability lower than this threshold.
Similarly to tracer-dynamic-coverage two values are present, one for compilation for profile feedback and one for
compilation without. The value for compilation with profile feedback needs to be more conservative (higher) in order
to make tracer effective.
max-cse-path-length
Maximum number of basic blocks on path that cse considers. The default is 10.
max-cse-insns
The maximum instructions CSE process before flushing. The default is 1000.
ggc-min-expand
GCC uses a garbage collector to manage its own memory allocation. This parameter specifies the minimum percentage by
which the garbage collector's heap should be allowed to expand between collections. Tuning this may improve
compilation speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If "getrlimit" is available, the
notion of "RAM" is the smallest of actual RAM and "RLIMIT_DATA" or "RLIMIT_AS". If GCC is not able to calculate RAM
on a particular platform, the lower bound of 30% is used. Setting this parameter and ggc-min-heapsize to zero causes
a full collection to occur at every opportunity. This is extremely slow, but can be useful for debugging.
ggc-min-heapsize
Minimum size of the garbage collector's heap before it begins bothering to collect garbage. The first collection
occurs after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning this may improve compilation
speed, and has no effect on code generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to ensure that RLIMIT_DATA or RLIMIT_AS are
not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If GCC
is not able to calculate RAM on a particular platform, the lower bound is used. Setting this parameter very large
effectively disables garbage collection. Setting this parameter and ggc-min-expand to zero causes a full collection
to occur at every opportunity.
max-reload-search-insns
The maximum number of instruction reload should look backward for equivalent register. Increasing values mean more
aggressive optimization, making the compilation time increase with probably slightly better performance. The default
value is 100.
max-cselib-memory-locations
The maximum number of memory locations cselib should take into account. Increasing values mean more aggressive
optimization, making the compilation time increase with probably slightly better performance. The default value is
500.
reorder-blocks-duplicate
reorder-blocks-duplicate-feedback
Used by basic block reordering pass to decide whether to use unconditional branch or duplicate the code on its
destination. Code is duplicated when its estimated size is smaller than this value multiplied by the estimated size
of unconditional jump in the hot spots of the program.
The reorder-block-duplicate-feedback is used only when profile feedback is available and may be set to higher values
than reorder-block-duplicate since information about the hot spots is more accurate.
max-sched-ready-insns
The maximum number of instructions ready to be issued the scheduler should consider at any given time during the
first scheduling pass. Increasing values mean more thorough searches, making the compilation time increase with
probably little benefit. The default value is 100.
max-sched-region-blocks
The maximum number of blocks in a region to be considered for interblock scheduling. The default value is 10.
max-pipeline-region-blocks
The maximum number of blocks in a region to be considered for pipelining in the selective scheduler. The default
value is 15.
max-sched-region-insns
The maximum number of insns in a region to be considered for interblock scheduling. The default value is 100.
max-pipeline-region-insns
The maximum number of insns in a region to be considered for pipelining in the selective scheduler. The default
value is 200.
min-spec-prob
The minimum probability (in percents) of reaching a source block for interblock speculative scheduling. The default
value is 40.
max-sched-extend-regions-iters
The maximum number of iterations through CFG to extend regions. 0 - disable region extension, N - do at most N
iterations. The default value is 0.
max-sched-insn-conflict-delay
The maximum conflict delay for an insn to be considered for speculative motion. The default value is 3.
sched-spec-prob-cutoff
The minimal probability of speculation success (in percents), so that speculative insn will be scheduled. The
default value is 40.
sched-mem-true-dep-cost
Minimal distance (in CPU cycles) between store and load targeting same memory locations. The default value is 1.
selsched-max-lookahead
The maximum size of the lookahead window of selective scheduling. It is a depth of search for available
instructions. The default value is 50.
selsched-max-sched-times
The maximum number of times that an instruction will be scheduled during selective scheduling. This is the limit on
the number of iterations through which the instruction may be pipelined. The default value is 2.
selsched-max-insns-to-rename
The maximum number of best instructions in the ready list that are considered for renaming in the selective
scheduler. The default value is 2.
sms-min-sc
The minimum value of stage count that swing modulo scheduler will generate. The default value is 2.
max-last-value-rtl
The maximum size measured as number of RTLs that can be recorded in an expression in combiner for a pseudo register
as last known value of that register. The default is 10000.
integer-share-limit
Small integer constants can use a shared data structure, reducing the compiler's memory usage and increasing its
speed. This sets the maximum value of a shared integer constant. The default value is 256.
min-virtual-mappings
Specifies the minimum number of virtual mappings in the incremental SSA updater that should be registered to trigger
the virtual mappings heuristic defined by virtual-mappings-ratio. The default value is 100.
virtual-mappings-ratio
If the number of virtual mappings is virtual-mappings-ratio bigger than the number of virtual symbols to be updated,
then the incremental SSA updater switches to a full update for those symbols. The default ratio is 3.
ssp-buffer-size
The minimum size of buffers (i.e. arrays) that will receive stack smashing protection when -fstack-protection is
used.
This default before Ubuntu 10.10 was "8". Currently it is "4", to increase the number of functions protected by the
stack protector.
max-jump-thread-duplication-stmts
Maximum number of statements allowed in a block that needs to be duplicated when threading jumps.
max-fields-for-field-sensitive
Maximum number of fields in a structure we will treat in a field sensitive manner during pointer analysis. The
default is zero for -O0, and -O1 and 100 for -Os, -O2, and -O3.
prefetch-latency
Estimate on average number of instructions that are executed before prefetch finishes. The distance we prefetch
ahead is proportional to this constant. Increasing this number may also lead to less streams being prefetched (see
simultaneous-prefetches).
simultaneous-prefetches
Maximum number of prefetches that can run at the same time.
l1-cache-line-size
The size of cache line in L1 cache, in bytes.
l1-cache-size
The size of L1 cache, in kilobytes.
l2-cache-size
The size of L2 cache, in kilobytes.
min-insn-to-prefetch-ratio
The minimum ratio between the number of instructions and the number of prefetches to enable prefetching in a loop.
prefetch-min-insn-to-mem-ratio
The minimum ratio between the number of instructions and the number of memory references to enable prefetching in a
loop.
use-canonical-types
Whether the compiler should use the "canonical" type system. By default, this should always be 1, which uses a more
efficient internal mechanism for comparing types in C++ and Objective-C++. However, if bugs in the canonical type
system are causing compilation failures, set this value to 0 to disable canonical types.
switch-conversion-max-branch-ratio
Switch initialization conversion will refuse to create arrays that are bigger than switch-conversion-max-branch-ratio
times the number of branches in the switch.
max-partial-antic-length
Maximum length of the partial antic set computed during the tree partial redundancy elimination optimization
(-ftree-pre) when optimizing at -O3 and above. For some sorts of source code the enhanced partial redundancy
elimination optimization can run away, consuming all of the memory available on the host machine. This parameter
sets a limit on the length of the sets that are computed, which prevents the runaway behavior. Setting a value of 0
for this parameter will allow an unlimited set length.
sccvn-max-scc-size
Maximum size of a strongly connected component (SCC) during SCCVN processing. If this limit is hit, SCCVN processing
for the whole function will not be done and optimizations depending on it will be disabled. The default maximum SCC
size is 10000.
ira-max-loops-num
IRA uses regional register allocation by default. If a function contains more loops than the number given by this
parameter, only at most the given number of the most frequently-executed loops form regions for regional register
allocation. The default value of the parameter is 100.
ira-max-conflict-table-size
Although IRA uses a sophisticated algorithm to compress the conflict table, the table can still require excessive
amounts of memory for huge functions. If the conflict table for a function could be more than the size in MB given
by this parameter, the register allocator instead uses a faster, simpler, and lower-quality algorithm that does not
require building a pseudo-register conflict table. The default value of the parameter is 2000.
ira-loop-reserved-regs
IRA can be used to evaluate more accurate register pressure in loops for decisions to move loop invariants (see -O3).
The number of available registers reserved for some other purposes is given by this parameter. The default value of
the parameter is 2, which is the minimal number of registers needed by typical instructions. This value is the best
found from numerous experiments.
loop-invariant-max-bbs-in-loop
Loop invariant motion can be very expensive, both in compilation time and in amount of needed compile-time memory,
with very large loops. Loops with more basic blocks than this parameter won't have loop invariant motion
optimization performed on them. The default value of the parameter is 1000 for -O1 and 10000 for -O2 and above.
loop-max-datarefs-for-datadeps
Building data dapendencies is expensive for very large loops. This parameter limits the number of data references in
loops that are considered for data dependence analysis. These large loops will not be handled then by the
optimizations using loop data dependencies. The default value is 1000.
max-vartrack-size
Sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function. If this
limit is exceeded with variable tracking at assignments enabled, analysis for that function is retried without it,
after removing all debug insns from the function. If the limit is exceeded even without debug insns, var tracking
analysis is completely disabled for the function. Setting the parameter to zero makes it unlimited.
max-vartrack-expr-depth
Sets a maximum number of recursion levels when attempting to map variable names or debug temporaries to value
expressions. This trades compilation time for more complete debug information. If this is set too low, value
expressions that are available and could be represented in debug information may end up not being used; setting this
higher may enable the compiler to find more complex debug expressions, but compile time and memory use may grow. The
default is 12.
min-nondebug-insn-uid
Use uids starting at this parameter for nondebug insns. The range below the parameter is reserved exclusively for
debug insns created by -fvar-tracking-assignments, but debug insns may get (non-overlapping) uids above it if the
reserved range is exhausted.
ipa-sra-ptr-growth-factor
IPA-SRA will replace a pointer to an aggregate with one or more new parameters only when their cumulative size is
less or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter.
tm-max-aggregate-size
When making copies of thread-local variables in a transaction, this parameter specifies the size in bytes after which
variables will be saved with the logging functions as opposed to save/restore code sequence pairs. This option only
applies when using -fgnu-tm.
graphite-max-nb-scop-params
To avoid exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part
(SCoP) is bounded. The default value is 10 parameters. A variable whose value is unknown at compilation time and
defined outside a SCoP is a parameter of the SCoP.
graphite-max-bbs-per-function
To avoid exponential effects in the detection of SCoPs, the size of the functions analyzed by Graphite is bounded.
The default value is 100 basic blocks.
loop-block-tile-size
Loop blocking or strip mining transforms, enabled with -floop-block or -floop-strip-mine, strip mine each loop in the
loop nest by a given number of iterations. The strip length can be changed using the loop-block-tile-size parameter.
The default value is 51 iterations.
ipa-cp-value-list-size
IPA-CP attempts to track all possible values and types passed to a function's parameter in order to propagate them
and perform devirtualization. ipa-cp-value-list-size is the maximum number of values and types it stores per one
formal parameter of a function.
lto-partitions
Specify desired number of partitions produced during WHOPR compilation. The number of partitions should exceed the
number of CPUs used for compilation. The default value is 32.
lto-minpartition
Size of minimal partition for WHOPR (in estimated instructions). This prevents expenses of splitting very small
programs into too many partitions.
cxx-max-namespaces-for-diagnostic-help
The maximum number of namespaces to consult for suggestions when C++ name lookup fails for an identifier. The
default is 1000.
sink-frequency-threshold
The maximum relative execution frequency (in percents) of the target block relative to a statement's original block
to allow statement sinking of a statement. Larger numbers result in more aggressive statement sinking. The default
value is 75. A small positive adjustment is applied for statements with memory operands as those are even more
profitable so sink.
max-stores-to-sink
The maximum number of conditional stores paires that can be sunk. Set to 0 if either vectorization
(-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled. The default is 2.
allow-load-data-races
Allow optimizers to introduce new data races on loads. Set to 1 to allow, otherwise to 0. This option is enabled by
default unless implicitly set by the -fmemory-model= option.
allow-store-data-races
Allow optimizers to introduce new data races on stores. Set to 1 to allow, otherwise to 0. This option is enabled
by default unless implicitly set by the -fmemory-model= option.
allow-packed-load-data-races
Allow optimizers to introduce new data races on packed data loads. Set to 1 to allow, otherwise to 0. This option
is enabled by default unless implicitly set by the -fmemory-model= option.
allow-packed-store-data-races
Allow optimizers to introduce new data races on packed data stores. Set to 1 to allow, otherwise to 0. This option
is enabled by default unless implicitly set by the -fmemory-model= option.
case-values-threshold
The smallest number of different values for which it is best to use a jump-table instead of a tree of conditional
branches. If the value is 0, use the default for the machine. The default is 0.
tree-reassoc-width
Set the maximum number of instructions executed in parallel in reassociated tree. This parameter overrides target
dependent heuristics used by default if has non zero value.
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source file before actual compilation.
If you use the -E option, nothing is done except preprocessing. Some of these options make sense only together with -E
because they cause the preprocessor output to be unsuitable for actual compilation.
-Wp,option
You can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor. If option
contains commas, it is split into multiple options at the commas. However, many options are modified, translated or
interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase. The
preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp
and let the driver handle the options instead.
-Xpreprocessor option
Pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor options that GCC
does not know how to recognize.
If you want to pass an option that takes an argument, you must use -Xpreprocessor twice, once for the option and once for
the argument.
-D name
Predefine name as a macro, with definition 1.
-D name=definition
The contents of definition are tokenized and processed as if they appeared during translation phase three in a #define
directive. In particular, the definition will be truncated by embedded newline characters.
If you are invoking the preprocessor from a shell or shell-like program you may need to use the shell's quoting syntax to
protect characters such as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write its argument list with surrounding parentheses
before the equals sign (if any). Parentheses are meaningful to most shells, so you will need to quote the option. With
sh and csh, -D'name(args...)=definition' works.
-D and -U options are processed in the order they are given on the command line. All -imacros file and -include file
options are processed after all -D and -U options.
-U name
Cancel any previous definition of name, either built in or provided with a -D option.
-undef
Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined.
-I dir
Add the directory dir to the list of directories to be searched for header files. Directories named by -I are searched
before the standard system include directories. If the directory dir is a standard system include directory, the option
is ignored to ensure that the default search order for system directories and the special treatment of system headers are
not defeated . If dir begins with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.
-o file
Write output to file. This is the same as specifying file as the second non-option argument to cpp. gcc has a different
interpretation of a second non-option argument, so you must use -o to specify the output file.
-Wall
Turns on all optional warnings which are desirable for normal code. At present this is -Wcomment, -Wtrigraphs,
-Wmultichar and a warning about integer promotion causing a change of sign in "#if" expressions. Note that many of the
preprocessor's warnings are on by default and have no options to control them.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears in a //
comment. (Both forms have the same effect.)
-Wtrigraphs
Most trigraphs in comments cannot affect the meaning of the program. However, a trigraph that would form an escaped
newline (??/ at the end of a line) can, by changing where the comment begins or ends. Therefore, only trigraphs that
would form escaped newlines produce warnings inside a comment.
This option is implied by -Wall. If -Wall is not given, this option is still enabled unless trigraphs are enabled. To
get trigraph conversion without warnings, but get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs.
-Wtraditional
Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that
have no traditional C equivalent, and problematic constructs which should be avoided.
-Wundef
Warn whenever an identifier which is not a macro is encountered in an #if directive, outside of defined. Such
identifiers are replaced with zero.
-Wunused-macros
Warn about macros defined in the main file that are unused. A macro is used if it is expanded or tested for existence at
least once. The preprocessor will also warn if the macro has not been used at the time it is redefined or undefined.
Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then CPP will report it as unused. To
avoid the warning in such a case, you might improve the scope of the macro's definition by, for example, moving it into
the first skipped block. Alternatively, you could provide a dummy use with something like:
#if defined the_macro_causing_the_warning
#endif
-Wendif-labels
Warn whenever an #else or an #endif are followed by text. This usually happens in code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments, but often are not in older programs. This warning is on by default.
-Werror
Make all warnings into hard errors. Source code which triggers warnings will be rejected.
-Wsystem-headers
Issue warnings for code in system headers. These are normally unhelpful in finding bugs in your own code, therefore
suppressed. If you are responsible for the system library, you may want to see them.
-w Suppress all warnings, including those which GNU CPP issues by default.
-pedantic
Issue all the mandatory diagnostics listed in the C standard. Some of them are left out by default, since they trigger
frequently on harmless code.
-pedantic-errors
Issue all the mandatory diagnostics, and make all mandatory diagnostics into errors. This includes mandatory diagnostics
that GCC issues without -pedantic but treats as warnings.
-M Instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of the
main source file. The preprocessor outputs one make rule containing the object file name for that source file, a colon,
and the names of all the included files, including those coming from -include or -imacros command line options.
Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any
suffix replaced with object file suffix and with any leading directory parts removed. If there are many included files
then the rule is split into several lines using \-newline. The rule has no commands.
This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug output with the
dependency rules you should explicitly specify the dependency output file with -MF, or use an environment variable like
DEPENDENCIES_OUTPUT. Debug output will still be sent to the regular output stream as normal.
Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.
-MM Like -M but do not mention header files that are found in system header directories, nor header files that are included,
directly or indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in an #include directive does not in itself determine
whether that header will appear in -MM dependency output. This is a slight change in semantics from GCC versions 3.0 and
earlier.
-MF file
When used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given the preprocessor
sends the rules to the same place it would have sent preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.
-MG In conjunction with an option such as -M requesting dependency generation, -MG assumes missing header files are generated
files and adds them to the dependency list without raising an error. The dependency filename is taken directly from the
"#include" directive without prepending any path. -MG also suppresses preprocessed output, as a missing header file
renders this useless.
This feature is used in automatic updating of makefiles.
-MP This option instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on
nothing. These dummy rules work around errors make gives if you remove header files without updating the Makefile to
match.
This is typical output:
test.o: test.c test.h
test.h:
-MT target
Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file,
deletes any directory components and any file suffix such as .c, and appends the platform's usual object suffix. The
result is the target.
An -MT option will set the target to be exactly the string you specify. If you want multiple targets, you can specify
them as a single argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special to Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with -MQ.
-MD -MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file based on whether an -o
option is given. If it is, the driver uses its argument but with a suffix of .d, otherwise it takes the name of the
input file, removes any directory components and suffix, and applies a .d suffix.
If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file, but if used
without -E, each -o is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate a dependency output file as a side-effect of the compilation
process.
-MMD
Like -MD except mention only user header files, not system header files.
-fpch-deps
When using precompiled headers, this flag will cause the dependency-output flags to also list the files from the
precompiled header's dependencies. If not specified only the precompiled header would be listed and not the files that
were used to create it because those files are not consulted when a precompiled header is used.
-fpch-preprocess
This option allows use of a precompiled header together with -E. It inserts a special "#pragma", "#pragma GCC
pch_preprocess "filename"" in the output to mark the place where the precompiled header was found, and its filename.
When -fpreprocessed is in use, GCC recognizes this "#pragma" and loads the PCH.
This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. It is
switched on by -save-temps.
You should not write this "#pragma" in your own code, but it is safe to edit the filename if the PCH file is available in
a different location. The filename may be absolute or it may be relative to GCC's current directory.
-x c
-x c++
-x objective-c
-x assembler-with-cpp
Specify the source language: C, C++, Objective-C, or assembly. This has nothing to do with standards conformance or
extensions; it merely selects which base syntax to expect. If you give none of these options, cpp will deduce the
language from the extension of the source file: .c, .cc, .m, or .S. Some other common extensions for C++ and assembly
are also recognized. If cpp does not recognize the extension, it will treat the file as C; this is the most generic
mode.
Note: Previous versions of cpp accepted a -lang option which selected both the language and the standards conformance
level. This option has been removed, because it conflicts with the -l option.
-std=standard
-ansi
Specify the standard to which the code should conform. Currently CPP knows about C and C++ standards; others may be
added in the future.
standard may be one of:
"c90"
"c89"
"iso9899:1990"
The ISO C standard from 1990. c90 is the customary shorthand for this version of the standard.
The -ansi option is equivalent to -std=c90.
"iso9899:199409"
The 1990 C standard, as amended in 1994.
"iso9899:1999"
"c99"
"iso9899:199x"
"c9x"
The revised ISO C standard, published in December 1999. Before publication, this was known as C9X.
"iso9899:2011"
"c11"
"c1x"
The revised ISO C standard, published in December 2011. Before publication, this was known as C1X.
"gnu90"
"gnu89"
The 1990 C standard plus GNU extensions. This is the default.
"gnu99"
"gnu9x"
The 1999 C standard plus GNU extensions.
"gnu11"
"gnu1x"
The 2011 C standard plus GNU extensions.
"c++98"
The 1998 ISO C++ standard plus amendments.
"gnu++98"
The same as -std=c++98 plus GNU extensions. This is the default for C++ code.
-I- Split the include path. Any directories specified with -I options before -I- are searched only for headers requested
with "#include "file""; they are not searched for "#include <file>". If additional directories are specified with -I
options after the -I-, those directories are searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current file directory as the first search directory for
"#include "file"". This option has been deprecated.
-nostdinc
Do not search the standard system directories for header files. Only the directories you have specified with -I options
(and the directory of the current file, if appropriate) are searched.
-nostdinc++
Do not search for header files in the C++-specific standard directories, but do still search the other standard
directories. (This option is used when building the C++ library.)
-include file
Process file as if "#include "file"" appeared as the first line of the primary source file. However, the first directory
searched for file is the preprocessor's working directory instead of the directory containing the main source file. If
not found there, it is searched for in the remainder of the "#include "..."" search chain as normal.
If multiple -include options are given, the files are included in the order they appear on the command line.
-imacros file
Exactly like -include, except that any output produced by scanning file is thrown away. Macros it defines remain
defined. This allows you to acquire all the macros from a header without also processing its declarations.
All files specified by -imacros are processed before all files specified by -include.
-idirafter dir
Search dir for header files, but do it after all directories specified with -I and the standard system directories have
been exhausted. dir is treated as a system include directory. If dir begins with "=", then the "=" will be replaced by
the sysroot prefix; see --sysroot and -isysroot.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should
include the final /.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path.
-iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where -idirafter would.
-isysroot dir
This option is like the --sysroot option, but applies only to header files (except for Darwin targets, where it applies
to both header files and libraries). See the --sysroot option for more information.
-imultilib dir
Use dir as a subdirectory of the directory containing target-specific C++ headers.
-isystem dir
Search dir for header files, after all directories specified by -I but before the standard system directories. Mark it
as a system directory, so that it gets the same special treatment as is applied to the standard system directories. If
dir begins with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.
-iquote dir
Search dir only for header files requested with "#include "file""; they are not searched for "#include <file>", before
all directories specified by -I and before the standard system directories. If dir begins with "=", then the "=" will be
replaced by the sysroot prefix; see --sysroot and -isysroot.
-fdirectives-only
When preprocessing, handle directives, but do not expand macros.
The option's behavior depends on the -E and -fpreprocessed options.
With -E, preprocessing is limited to the handling of directives such as "#define", "#ifdef", and "#error". Other
preprocessor operations, such as macro expansion and trigraph conversion are not performed. In addition, the -dD option
is implicitly enabled.
With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros such as "__LINE__", which
are contextually dependent, are handled normally. This enables compilation of files previously preprocessed with "-E
-fdirectives-only".
With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. This enables full preprocessing of files
previously preprocessed with "-E -fdirectives-only".
-fdollars-in-identifiers
Accept $ in identifiers.
-fextended-identifiers
Accept universal character names in identifiers. This option is experimental; in a future version of GCC, it will be
enabled by default for C99 and C++.
-fpreprocessed
Indicate to the preprocessor that the input file has already been preprocessed. This suppresses things like macro
expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still
recognizes and removes comments, so that you can pass a file preprocessed with -C to the compiler without problems. In
this mode the integrated preprocessor is little more than a tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These are the extensions that GCC
uses for preprocessed files created by -save-temps.
-ftabstop=width
Set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors,
even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored. The default is
8.
-fdebug-cpp
This option is only useful for debugging GCC. When used with -E, dumps debugging information about location maps. Every
token in the output is preceded by the dump of the map its location belongs to. The dump of the map holding the location
of a token would be:
{"P":F</file/path>;"F":F</includer/path>;"L":<line_num>;"C":<col_num>;"S":<system_header_p>;"M":<map_address>;"E":<macro_expansion_p>,"loc":<location>}
When used without -E, this option has no effect.
-ftrack-macro-expansion[=level]
Track locations of tokens across macro expansions. This allows the compiler to emit diagnostic about the current macro
expansion stack when a compilation error occurs in a macro expansion. Using this option makes the preprocessor and the
compiler consume more memory. The level parameter can be used to choose the level of precision of token location tracking
thus decreasing the memory consumption if necessary. Value 0 of level de-activates this option just as if no
-ftrack-macro-expansion was present on the command line. Value 1 tracks tokens locations in a degraded mode for the sake
of minimal memory overhead. In this mode all tokens resulting from the expansion of an argument of a function-like macro
have the same location. Value 2 tracks tokens locations completely. This value is the most memory hungry. When this
option is given no argument, the default parameter value is 2.
-fexec-charset=charset
Set the execution character set, used for string and character constants. The default is UTF-8. charset can be any
encoding supported by the system's "iconv" library routine.
-fwide-exec-charset=charset
Set the wide execution character set, used for wide string and character constants. The default is UTF-32 or UTF-16,
whichever corresponds to the width of "wchar_t". As with -fexec-charset, charset can be any encoding supported by the
system's "iconv" library routine; however, you will have problems with encodings that do not fit exactly in "wchar_t".
-finput-charset=charset
Set the input character set, used for translation from the character set of the input file to the source character set
used by GCC. If the locale does not specify, or GCC cannot get this information from the locale, the default is UTF-8.
This can be overridden by either the locale or this command line option. Currently the command line option takes
precedence if there's a conflict. charset can be any encoding supported by the system's "iconv" library routine.
-fworking-directory
Enable generation of linemarkers in the preprocessor output that will let the compiler know the current working directory
at the time of preprocessing. When this option is enabled, the preprocessor will emit, after the initial linemarker, a
second linemarker with the current working directory followed by two slashes. GCC will use this directory, when it's
present in the preprocessed input, as the directory emitted as the current working directory in some debugging
information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited
with the negated form -fno-working-directory. If the -P flag is present in the command line, this option has no effect,
since no "#line" directives are emitted whatsoever.
-fno-show-column
Do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that
does not understand the column numbers, such as dejagnu.
-A predicate=answer
Make an assertion with the predicate predicate and answer answer. This form is preferred to the older form -A
predicate(answer), which is still supported, because it does not use shell special characters.
-A -predicate=answer
Cancel an assertion with the predicate predicate and answer answer.
-dCHARS
CHARS is a sequence of one or more of the following characters, and must not be preceded by a space. Other characters
are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you
specify characters whose behavior conflicts, the result is undefined.
M Instead of the normal output, generate a list of #define directives for all the macros defined during the execution
of the preprocessor, including predefined macros. This gives you a way of finding out what is predefined in your
version of the preprocessor. Assuming you have no file foo.h, the command
touch foo.h; cpp -dM foo.h
will show all the predefined macros.
If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach.
D Like M except in two respects: it does not include the predefined macros, and it outputs both the #define directives
and the result of preprocessing. Both kinds of output go to the standard output file.
N Like D, but emit only the macro names, not their expansions.
I Output #include directives in addition to the result of preprocessing.
U Like D except that only macros that are expanded, or whose definedness is tested in preprocessor directives, are
output; the output is delayed until the use or test of the macro; and #undef directives are also output for macros
tested but undefined at the time.
-P Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the
preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers.
-C Do not discard comments. All comments are passed through to the output file, except for comments in processed
directives, which are deleted along with the directive.
You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their
own right. For example, comments appearing at the start of what would be a directive line have the effect of turning
that line into an ordinary source line, since the first token on the line is no longer a #.
-CC Do not discard comments, including during macro expansion. This is like -C, except that comments contained within macros
are also passed through to the output file where the macro is expanded.
In addition to the side-effects of the -C option, the -CC option causes all C++-style comments inside a macro to be
converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the
remainder of the source line.
The -CC option is generally used to support lint comments.
-traditional-cpp
Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C preprocessors.
-trigraphs
Process trigraph sequences. These are three-character sequences, all starting with ??, that are defined by ISO C to
stand for single characters. For example, ??/ stands for \, so '??/n' is a character constant for a newline. By
default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi options.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
-remap
Enable special code to work around file systems which only permit very short file names, such as MS-DOS.
--help
--target-help
Print text describing all the command line options instead of preprocessing anything.
-v Verbose mode. Print out GNU CPP's version number at the beginning of execution, and report the final form of the include
path.
-H Print the name of each header file used, in addition to other normal activities. Each name is indented to show how deep
in the #include stack it is. Precompiled header files are also printed, even if they are found to be invalid; an invalid
precompiled header file is printed with ...x and a valid one with ...! .
-version
--version
Print out GNU CPP's version number. With one dash, proceed to preprocess as normal. With two dashes, exit immediately.
Passing Options to the Assembler
You can pass options to the assembler.
-Wa,option
Pass option as an option to the assembler. If option contains commas, it is split into multiple options at the commas.
-Xassembler option
Pass option as an option to the assembler. You can use this to supply system-specific assembler options that GCC does
not know how to recognize.
If you want to pass an option that takes an argument, you must use -Xassembler twice, once for the option and once for
the argument.
Options for Linking
These options come into play when the compiler links object files into an executable output file. They are meaningless if
the compiler is not doing a link step.
object-file-name
A file name that does not end in a special recognized suffix is considered to name an object file or library. (Object
files are distinguished from libraries by the linker according to the file contents.) If linking is done, these object
files are used as input to the linker.
-c
-S
-E If any of these options is used, then the linker is not run, and object file names should not be used as arguments.
-llibrary
-l library
Search the library named library when linking. (The second alternative with the library as a separate argument is only
for POSIX compliance and is not recommended.)
It makes a difference where in the command you write this option; the linker searches and processes libraries and object
files in the order they are specified. Thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o. If
bar.o refers to functions in z, those functions may not be loaded.
The linker searches a standard list of directories for the library, which is actually a file named liblibrary.a. The
linker then uses this file as if it had been specified precisely by name.
The directories searched include several standard system directories plus any that you specify with -L.
Normally the files found this way are library files---archive files whose members are object files. The linker handles
an archive file by scanning through it for members which define symbols that have so far been referenced but not defined.
But if the file that is found is an ordinary object file, it is linked in the usual fashion. The only difference between
using an -l option and specifying a file name is that -l surrounds library with lib and .a and searches several
directories.
-lobjc
You need this special case of the -l option in order to link an Objective-C or Objective-C++ program.
-nostartfiles
Do not use the standard system startup files when linking. The standard system libraries are used normally, unless
-nostdlib or -nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking. Only the libraries you specify will be passed to the linker,
options specifying linkage of the system libraries, such as "-static-libgcc" or "-shared-libgcc", will be ignored. The
standard startup files are used normally, unless -nostartfiles is used. The compiler may generate calls to "memcmp",
"memset", "memcpy" and "memmove". These entries are usually resolved by entries in libc. These entry points should be
supplied through some other mechanism when this option is specified.
-nostdlib
Do not use the standard system startup files or libraries when linking. No startup files and only the libraries you
specify will be passed to the linker, options specifying linkage of the system libraries, such as "-static-libgcc" or
"-shared-libgcc", will be ignored. The compiler may generate calls to "memcmp", "memset", "memcpy" and "memmove". These
entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when
this option is specified.
One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines
which GCC uses to overcome shortcomings of particular machines, or special needs for some languages.
In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words, when you specify
-nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no unresolved
references to internal GCC library subroutines. (For example, __main, used to ensure C++ constructors will be called.)
-pie
Produce a position independent executable on targets that support it. For predictable results, you must also specify the
same set of options that were used to generate code (-fpie, -fPIE, or model suboptions) when you specify this option.
-rdynamic
Pass the flag -export-dynamic to the ELF linker, on targets that support it. This instructs the linker to add all
symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses of "dlopen" or to allow
obtaining backtraces from within a program.
-s Remove all symbol table and relocation information from the executable.
-static
On systems that support dynamic linking, this prevents linking with the shared libraries. On other systems, this option
has no effect.
-shared
Produce a shared object which can then be linked with other objects to form an executable. Not all systems support this
option. For predictable results, you must also specify the same set of options that were used to generate code (-fpic,
-fPIC, or model suboptions) when you specify this option.[1]
-shared-libgcc
-static-libgcc
On systems that provide libgcc as a shared library, these options force the use of either the shared or static version
respectively. If no shared version of libgcc was built when the compiler was configured, these options have no effect.
There are several situations in which an application should use the shared libgcc instead of the static version. The
most common of these is when the application wishes to throw and catch exceptions across different shared libraries. In
that case, each of the libraries as well as the application itself should use the shared libgcc.
Therefore, the G++ and GCJ drivers automatically add -shared-libgcc whenever you build a shared library or a main
executable, because C++ and Java programs typically use exceptions, so this is the right thing to do.
If, instead, you use the GCC driver to create shared libraries, you may find that they will not always be linked with the
shared libgcc. If GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that does not
support option --eh-frame-hdr, it will link the shared version of libgcc into shared libraries by default. Otherwise, it
will take advantage of the linker and optimize away the linking with the shared version of libgcc, linking with the
static version of libgcc by default. This allows exceptions to propagate through such shared libraries, without
incurring relocation costs at library load time.
However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ or GCJ
driver, as appropriate for the languages used in the program, or using the option -shared-libgcc, such that it is linked
with the shared libgcc.
-static-libstdc++
When the g++ program is used to link a C++ program, it will normally automatically link against libstdc++. If libstdc++
is available as a shared library, and the -static option is not used, then this will link against the shared version of
libstdc++. That is normally fine. However, it is sometimes useful to freeze the version of libstdc++ used by the
program without going all the way to a fully static link. The -static-libstdc++ option directs the g++ driver to link
libstdc++ statically, without necessarily linking other libraries statically.
-symbolic
Bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden
by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this option.
-T script
Use script as the linker script. This option is supported by most systems using the GNU linker. On some targets, such
as bare-board targets without an operating system, the -T option may be required when linking to avoid references to
undefined symbols.
-Xlinker option
Pass option as an option to the linker. You can use this to supply system-specific linker options that GCC does not
recognize.
If you want to pass an option that takes a separate argument, you must use -Xlinker twice, once for the option and once
for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions. It
does not work to write -Xlinker "-assert definitions", because this passes the entire string as a single argument, which
is not what the linker expects.
When using the GNU linker, it is usually more convenient to pass arguments to linker options using the option=value
syntax than as separate arguments. For example, you can specify -Xlinker -Map=output.map rather than -Xlinker -Map
-Xlinker output.map. Other linkers may not support this syntax for command-line options.
-Wl,option
Pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas. You
can use this syntax to pass an argument to the option. For example, -Wl,-Map,output.map passes -Map output.map to the
linker. When using the GNU linker, you can also get the same effect with -Wl,-Map=output.map.
NOTE: In Ubuntu 8.10 and later versions, for LDFLAGS, the option -Wl,-z,relro is used. To disable, use -Wl,-z,norelro.
-u symbol
Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use -u multiple times
with different symbols to force loading of additional library modules.
Options for Directory Search
These options specify directories to search for header files, for libraries and for parts of the compiler:
-Idir
Add the directory dir to the head of the list of directories to be searched for header files. This can be used to
override a system header file, substituting your own version, since these directories are searched before the system
header file directories. However, you should not use this option to add directories that contain vendor-supplied system
header files (use -isystem for that). If you use more than one -I option, the directories are scanned in left-to-right
order; the standard system directories come after.
If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option
will be ignored. The directory will still be searched but as a system directory at its normal position in the system
include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the include_next
directive are not inadvertently changed. If you really need to change the search order for system directories, use the
-nostdinc and/or -isystem options.
-iplugindir=dir
Set the directory to search for plugins that are passed by -fplugin=name instead of -fplugin=path/name.so. This option
is not meant to be used by the user, but only passed by the driver.
-iquotedir
Add the directory dir to the head of the list of directories to be searched for header files only for the case of
#include "file"; they are not searched for #include <file>, otherwise just like -I.
-Ldir
Add directory dir to the list of directories to be searched for -l.
-Bprefix
This option specifies where to find the executables, libraries, include files, and data files of the compiler itself.
The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix for
each program it tries to run, both with and without machine/version/.
For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not found, or if
-B was not specified, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those
results in a file name that is found, the unmodified program name is searched for using the directories specified in your
PATH environment variable.
The compiler will check to see if the path provided by the -B refers to a directory, and if necessary it will add a
directory separator character at the end of the path.
-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler
translates these options into -L options for the linker. They also apply to includes files in the preprocessor, because
the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends
include to the prefix.
The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not found there, the
two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those
means.
Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it will
be replaced by [dir/]include. This is to help with boot-strapping the compiler.
-specs=file
Process file after the compiler reads in the standard specs file, in order to override the defaults which the gcc driver
program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. More than one -specs=file can be
specified on the command line, and they are processed in order, from left to right.
--sysroot=dir
Use dir as the logical root directory for headers and libraries. For example, if the compiler would normally search for
headers in /usr/include and libraries in /usr/lib, it will instead search dir/usr/include and dir/usr/lib.
If you use both this option and the -isysroot option, then the --sysroot option will apply to libraries, but the
-isysroot option will apply to header files.
The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support
this option, the header file aspect of --sysroot will still work, but the library aspect will not.
-I- This option has been deprecated. Please use -iquote instead for -I directories before the -I- and remove the -I-. Any
directories you specify with -I options before the -I- option are searched only for the case of #include "file"; they are
not searched for #include <file>.
If additional directories are specified with -I options after the -I-, these directories are searched for all #include
directives. (Ordinarily all -I directories are used this way.)
In addition, the -I- option inhibits the use of the current directory (where the current input file came from) as the
first search directory for #include "file". There is no way to override this effect of -I-. With -I. you can specify
searching the directory that was current when the compiler was invoked. That is not exactly the same as what the
preprocessor does by default, but it is often satisfactory.
-I- does not inhibit the use of the standard system directories for header files. Thus, -I- and -nostdinc are
independent.
Specifying Target Machine and Compiler Version
The usual way to run GCC is to run the executable called gcc, or machine-gcc when cross-compiling, or machine-gcc-version to
run a version other than the one that was installed last.
Hardware Models and Configurations
Each target machine types can have its own special options, starting with -m, to choose among various hardware models or
configurations---for example, 68010 vs 68020, floating coprocessor or none. A single installed version of the compiler can
compile for any model or configuration, according to the options specified.
Some configurations of the compiler also support additional special options, usually for compatibility with other compilers
on the same platform.
Adapteva Epiphany Options
These -m options are defined for Adapteva Epiphany:
-mhalf-reg-file
Don't allocate any register in the range "r32"..."r63". That allows code to run on hardware variants that lack these
registers.
-mprefer-short-insn-regs
Preferrentially allocate registers that allow short instruction generation. This can result in increasesd instruction
count, so if this reduces or increases code size might vary from case to case.
-mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not guaranteed to
produce consistent results across releases.
-mcmove
Enable the generation of conditional moves.
-mnops=num
Emit num nops before every other generated instruction.
-mno-soft-cmpsf
For single-precision floating-point comparisons, emit an fsub instruction and test the flags. This is faster than a
software comparison, but can get incorrect results in the presence of NaNs, or when two different small numbers are
compared such that their difference is calculated as zero. The default is -msoft-cmpsf, which uses slower, but IEEE-
compliant, software comparisons.
-mstack-offset=num
Set the offset between the top of the stack and the stack pointer. E.g., a value of 8 means that the eight bytes in the
range sp+0...sp+7 can be used by leaf functions without stack allocation. Values other than 8 or 16 are untested and
unlikely to work. Note also that this option changes the ABI, compiling a program with a different stack offset than the
libraries have been compiled with will generally not work. This option can be useful if you want to evaluate if a
different stack offset would give you better code, but to actually use a different stack offset to build working
programs, it is recommended to configure the toolchain with the appropriate --with-stack-offset=num option.
-mno-round-nearest
Make the scheduler assume that the rounding mode has been set to truncating. The default is -mround-nearest.
-mlong-calls
If not otherwise specified by an attribute, assume all calls might be beyond the offset range of the b / bl instructions,
and therefore load the function address into a register before performing a (otherwise direct) call. This is the
default.
-mshort-calls
If not otherwise specified by an attribute, assume all direct calls are in the range of the b / bl instructions, so use
these instructions for direct calls. The default is -mlong-calls.
-msmall16
Assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses for which
-mlong-calls semantics are in effect.
-mfp-mode=mode
Set the prevailing mode of the floating-point unit. This determines the floating-point mode that is provided and
expected at function call and return time. Making this mode match the mode you predominantly need at function start can
make your programs smaller and faster by avoiding unnecessary mode switches.
mode can be set to one the following values:
caller
Any mode at function entry is valid, and retained or restored when the function returns, and when it calls other
functions. This mode is useful for compiling libraries or other compilation units you might want to incorporate into
different programs with different prevailing FPU modes, and the convenience of being able to use a single object file
outweighs the size and speed overhead for any extra mode switching that might be needed, compared with what would be
needed with a more specific choice of prevailing FPU mode.
truncate
This is the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. That
includes conversion from floating point to integer.
round-nearest
This is the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.
int This is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-
accumulate.
The default is -mfp-mode=caller
-mnosplit-lohi
-mno-postinc
-mno-postmodify
Code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-increment addresses, and
generation of post-modify addresses. The defaults are msplit-lohi, -mpost-inc, and -mpost-modify.
-mnovect-double
Change the preferred SIMD mode to SImode. The default is -mvect-double, which uses DImode as preferred SIMD mode.
-max-vect-align=num
The maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8. Note that this is an ABI
change, even though many library function interfaces will be unaffected, if they don't use SIMD vector modes in places
where they affect size and/or alignment of relevant types.
-msplit-vecmove-early
Split vector moves into single word moves before reload. In theory this could give better register allocation, but so
far the reverse seems to be generally the case.
-m1reg-reg
Specify a register to hold the constant -1, which makes loading small negative constants and certain bitmasks faster.
Allowable values for reg are r43 and r63, which specify to use that register as a fixed register, and none, which means
that no register is used for this purpose. The default is -m1reg-none.
ARM Options
These -m options are defined for Advanced RISC Machines (ARM) architectures:
-mabi=name
Generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux and iwmmxt.
-mapcs-frame
Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is not
strictly necessary for correct execution of the code. Specifying -fomit-frame-pointer with this option will cause the
stack frames not to be generated for leaf functions. The default is -mno-apcs-frame.
-mapcs
This is a synonym for -mapcs-frame.
-mthumb-interwork
Generate code that supports calling between the ARM and Thumb instruction sets. Without this option, on pre-v5
architectures, the two instruction sets cannot be reliably used inside one program. The default is -mno-thumb-interwork,
since slightly larger code is generated when -mthumb-interwork is specified. In AAPCS configurations this option is
meaningless.
-mno-sched-prolog
Prevent the reordering of instructions in the function prologue, or the merging of those instruction with the
instructions in the function's body. This means that all functions will start with a recognizable set of instructions
(or in fact one of a choice from a small set of different function prologues), and this information can be used to locate
the start if functions inside an executable piece of code. The default is -msched-prolog.
-mfloat-abi=name
Specifies which floating-point ABI to use. Permissible values are: soft, softfp and hard.
Specifying soft causes GCC to generate output containing library calls for floating-point operations. softfp allows the
generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions. hard
allows generation of floating-point instructions and uses FPU-specific calling conventions.
The default depends on the specific target configuration. Note that the hard-float and soft-float ABIs are not link-
compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries.
-mlittle-endian
Generate code for a processor running in little-endian mode. This is the default for all standard configurations.
-mbig-endian
Generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor.
-mwords-little-endian
This option only applies when generating code for big-endian processors. Generate code for a little-endian word order
but a big-endian byte order. That is, a byte order of the form 32107654. Note: this option should only be used if you
require compatibility with code for big-endian ARM processors generated by versions of the compiler prior to 2.8. This
option is now deprecated.
-mcpu=name
This specifies the name of the target ARM processor. GCC uses this name to determine what kind of instructions it can
emit when generating assembly code. Permissible names are: arm2, arm250, arm3, arm6, arm60, arm600, arm610, arm620,
arm7, arm7m, arm7d, arm7dm, arm7di, arm7dmi, arm70, arm700, arm700i, arm710, arm710c, arm7100, arm720, arm7500,
arm7500fe, arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm, strongarm110, strongarm1100, strongarm1110, arm8,
arm810, arm9, arm9e, arm920, arm920t, arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t, arm9tdmi, arm10tdmi,
arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp, arm1156t2-s,
arm1156t2f-s, arm1176jz-s, arm1176jzf-s, cortex-a5, cortex-a7, cortex-a8, cortex-a9, cortex-a15, cortex-r4, cortex-r4f,
cortex-r5, cortex-m4, cortex-m3, cortex-m1, cortex-m0, xscale, iwmmxt, iwmmxt2, ep9312, fa526, fa626, fa606te, fa626te,
fmp626, fa726te.
-mcpu=generic-arch is also permissible, and is equivalent to -march=arch -mtune=generic-arch. See -mtune for more
information.
-mcpu=native causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only
supported on Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no
effect.
-mtune=name
This option is very similar to the -mcpu= option, except that instead of specifying the actual target processor type, and
hence restricting which instructions can be used, it specifies that GCC should tune the performance of the code as if the
target were of the type specified in this option, but still choosing the instructions that it will generate based on the
CPU specified by a -mcpu= option. For some ARM implementations better performance can be obtained by using this option.
-mtune=generic-arch specifies that GCC should tune the performance for a blend of processors within architecture arch.
The aim is to generate code that run well on the current most popular processors, balancing between optimizations that
benefit some CPUs in the range, and avoiding performance pitfalls of other CPUs. The effects of this option may change
in future GCC versions as CPU models come and go.
-mtune=native causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only
supported on Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no
effect.
-march=name
This specifies the name of the target ARM architecture. GCC uses this name to determine what kind of instructions it can
emit when generating assembly code. This option can be used in conjunction with or instead of the -mcpu= option.
Permissible names are: armv2, armv2a, armv3, armv3m, armv4, armv4t, armv5, armv5t, armv5e, armv5te, armv6, armv6j,
armv6t2, armv6z, armv6zk, armv6-m, armv7, armv7-a, armv7-r, armv7-m, iwmmxt, iwmmxt2, ep9312.
-march=native causes the compiler to auto-detect the architecture of the build computer. At present, this feature is
only supported on Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no
effect.
-mfpu=name
-mfpe=number
-mfp=number
This specifies what floating-point hardware (or hardware emulation) is available on the target. Permissible names are:
fpa, fpe2, fpe3, maverick, vfp, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16, neon, neon-fp16,
vfpv4, vfpv4-d16, fpv4-sp-d16 and neon-vfpv4. -mfp and -mfpe are synonyms for -mfpu=fpenumber, for compatibility with
older versions of GCC.
If -msoft-float is specified this specifies the format of floating-point values.
If the selected floating-point hardware includes the NEON extension (e.g. -mfpu=neon), note that floating-point
operations will not be used by GCC's auto-vectorization pass unless -funsafe-math-optimizations is also specified. This
is because NEON hardware does not fully implement the IEEE 754 standard for floating-point arithmetic (in particular
denormal values are treated as zero), so the use of NEON instructions may lead to a loss of precision.
-mfp16-format=name
Specify the format of the "__fp16" half-precision floating-point type. Permissible names are none, ieee, and
alternative; the default is none, in which case the "__fp16" type is not defined.
-mstructure-size-boundary=n
The size of all structures and unions will be rounded up to a multiple of the number of bits set by this option.
Permissible values are 8, 32 and 64. The default value varies for different toolchains. For the COFF targeted toolchain
the default value is 8. A value of 64 is only allowed if the underlying ABI supports it.
Specifying the larger number can produce faster, more efficient code, but can also increase the size of the program.
Different values are potentially incompatible. Code compiled with one value cannot necessarily expect to work with code
or libraries compiled with another value, if they exchange information using structures or unions.
-mabort-on-noreturn
Generate a call to the function "abort" at the end of a "noreturn" function. It will be executed if the function tries
to return.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of the function into a register and then
performing a subroutine call on this register. This switch is needed if the target function will lie outside of the 64
megabyte addressing range of the offset based version of subroutine call instruction.
Even if this switch is enabled, not all function calls will be turned into long calls. The heuristic is that static
functions, functions that have the short-call attribute, functions that are inside the scope of a #pragma no_long_calls
directive and functions whose definitions have already been compiled within the current compilation unit, will not be
turned into long calls. The exception to this rule is that weak function definitions, functions with the long-call
attribute or the section attribute, and functions that are within the scope of a #pragma long_calls directive, will
always be turned into long calls.
This feature is not enabled by default. Specifying -mno-long-calls will restore the default behavior, as will placing
the function calls within the scope of a #pragma long_calls_off directive. Note these switches have no effect on how the
compiler generates code to handle function calls via function pointers.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The
runtime system is responsible for initializing this register with an appropriate value before execution begins.
-mpic-register=reg
Specify the register to be used for PIC addressing. The default is R10 unless stack-checking is enabled, when R9 is
used.
-mcirrus-fix-invalid-insns
Insert NOPs into the instruction stream to in order to work around problems with invalid Maverick instruction
combinations. This option is only valid if the -mcpu=ep9312 option has been used to enable generation of instructions
for the Cirrus Maverick floating-point co-processor. This option is not enabled by default, since the problem is only
present in older Maverick implementations. The default can be re-enabled by use of the -mno-cirrus-fix-invalid-insns
switch.
-mpoke-function-name
Write the name of each function into the text section, directly preceding the function prologue. The generated code is
similar to this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When performing a stack backtrace, code can inspect the value of "pc" stored at "fp + 0". If the trace function then
looks at location "pc - 12" and the top 8 bits are set, then we know that there is a function name embedded immediately
preceding this location and has length "((pc[-3]) & 0xff000000)".
-mthumb
-marm
Select between generating code that executes in ARM and Thumb states. The default for most configurations is to generate
code that executes in ARM state, but the default can be changed by configuring GCC with the --with-mode=state configure
option.
-mtpcs-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf
function is one that does not call any other functions.) The default is -mno-tpcs-frame.
-mtpcs-leaf-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf function
is one that does not call any other functions.) The default is -mno-apcs-leaf-frame.
-mcallee-super-interworking
Gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb
mode before executing the rest of the function. This allows these functions to be called from non-interworking code.
This option is not valid in AAPCS configurations because interworking is enabled by default.
-mcaller-super-interworking
Allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target
code has been compiled for interworking or not. There is a small overhead in the cost of executing a function pointer if
this option is enabled. This option is not valid in AAPCS configurations because interworking is enabled by default.
-mtp=name
Specify the access model for the thread local storage pointer. The valid models are soft, which generates calls to
"__aeabi_read_tp", cp15, which fetches the thread pointer from "cp15" directly (supported in the arm6k architecture), and
auto, which uses the best available method for the selected processor. The default setting is auto.
-mtls-dialect=dialect
Specify the dialect to use for accessing thread local storage. Two dialects are supported --- gnu and gnu2. The gnu
dialect selects the original GNU scheme for supporting local and global dynamic TLS models. The gnu2 dialect selects the
GNU descriptor scheme, which provides better performance for shared libraries. The GNU descriptor scheme is compatible
with the original scheme, but does require new assembler, linker and library support. Initial and local exec TLS models
are unaffected by this option and always use the original scheme.
-mword-relocations
Only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is enabled by default on targets
(uClinux, SymbianOS) where the runtime loader imposes this restriction, and when -fpic or -fPIC is specified.
-mfix-cortex-m3-ldrd
Some Cortex-M3 cores can cause data corruption when "ldrd" instructions with overlapping destination and base registers
are used. This option avoids generating these instructions. This option is enabled by default when -mcpu=cortex-m3 is
specified.
-munaligned-access
-mno-unaligned-access
Enables (or disables) reading and writing of 16- and 32- bit values from addresses that are not 16- or 32- bit aligned.
By default unaligned access is disabled for all pre-ARMv6 and all ARMv6-M architectures, and enabled for all other
architectures. If unaligned access is not enabled then words in packed data structures will be accessed a byte at a
time.
The ARM attribute "Tag_CPU_unaligned_access" will be set in the generated object file to either true or false, depending
upon the setting of this option. If unaligned access is enabled then the preprocessor symbol "__ARM_FEATURE_UNALIGNED"
will also be defined.
AVR Options
-mmcu=mcu
Specify Atmel AVR instruction set architectures (ISA) or MCU type.
For a complete list of mcu values that are supported by avr-gcc, see the compiler output when called with the
--help=target command line option. The default for this option is@tie{}"avr2".
GCC supports the following AVR devices and ISAs:
"avr2"
"Classic" devices with up to 8@tie{}KiB of program memory. mcu@tie{}= "at90c8534", "at90s2313", "at90s2323",
"at90s2333", "at90s2343", "at90s4414", "at90s4433", "at90s4434", "at90s8515", "at90s8535", "attiny22", "attiny26".
"avr25"
"Classic" devices with up to 8@tie{}KiB of program memory and with the "MOVW" instruction. mcu@tie{}= "at86rf401",
"ata6289", "attiny13", "attiny13a", "attiny2313", "attiny2313a", "attiny24", "attiny24a", "attiny25", "attiny261",
"attiny261a", "attiny4313", "attiny43u", "attiny44", "attiny44a", "attiny45", "attiny461", "attiny461a", "attiny48",
"attiny84", "attiny84a", "attiny85", "attiny861", "attiny861a", "attiny87", "attiny88".
"avr3"
"Classic" devices with 16@tie{}KiB up to 64@tie{}KiB of program memory. mcu@tie{}= "at43usb355", "at76c711".
"avr31"
"Classic" devices with 128@tie{}KiB of program memory. mcu@tie{}= "at43usb320", "atmega103".
"avr35"
"Classic" devices with 16@tie{}KiB up to 64@tie{}KiB of program memory and with the "MOVW" instruction. mcu@tie{}=
"at90usb162", "at90usb82", "atmega16u2", "atmega32u2", "atmega8u2", "attiny167".
"avr4"
"Enhanced" devices with up to 8@tie{}KiB of program memory. mcu@tie{}= "at90pwm1", "at90pwm2", "at90pwm2b",
"at90pwm3", "at90pwm3b", "at90pwm81", "atmega48", "atmega48a", "atmega48p", "atmega8", "atmega8515", "atmega8535",
"atmega88", "atmega88a", "atmega88p", "atmega88pa", "atmega8hva".
"avr5"
"Enhanced" devices with 16@tie{}KiB up to 64@tie{}KiB of program memory. mcu@tie{}= "at90can32", "at90can64",
"at90pwm216", "at90pwm316", "at90scr100", "at90usb646", "at90usb647", "at94k", "atmega16", "atmega161", "atmega162",
"atmega163", "atmega164a", "atmega164p", "atmega165", "atmega165a", "atmega165p", "atmega168", "atmega168a",
"atmega168p", "atmega169", "atmega169a", "atmega169p", "atmega169pa", "atmega16a", "atmega16hva", "atmega16hva2",
"atmega16hvb", "atmega16m1", "atmega16u4", "atmega32", "atmega323", "atmega324a", "atmega324p", "atmega324pa",
"atmega325", "atmega3250", "atmega3250a", "atmega3250p", "atmega325a", "atmega325p", "atmega328", "atmega328p",
"atmega329", "atmega3290", "atmega3290a", "atmega3290p", "atmega329a", "atmega329p", "atmega329pa", "atmega32c1",
"atmega32hvb", "atmega32m1", "atmega32u4", "atmega32u6", "atmega406", "atmega64", "atmega640", "atmega644",
"atmega644a", "atmega644p", "atmega644pa", "atmega645", "atmega6450", "atmega6450a", "atmega6450p", "atmega645a",
"atmega645p", "atmega649", "atmega6490", "atmega649a", "atmega649p", "atmega64c1", "atmega64hve", "atmega64m1",
"m3000".
"avr51"
"Enhanced" devices with 128@tie{}KiB of program memory. mcu@tie{}= "at90can128", "at90usb1286", "at90usb1287",
"atmega128", "atmega1280", "atmega1281", "atmega1284p", "atmega128rfa1".
"avr6"
"Enhanced" devices with 3-byte PC, i.e. with more than 128@tie{}KiB of program memory. mcu@tie{}= "atmega2560",
"atmega2561".
"avrxmega2"
"XMEGA" devices with more than 8@tie{}KiB and up to 64@tie{}KiB of program memory. mcu@tie{}= "atxmega16a4",
"atxmega16d4", "atxmega16x1", "atxmega32a4", "atxmega32d4", "atxmega32x1".
"avrxmega4"
"XMEGA" devices with more than 64@tie{}KiB and up to 128@tie{}KiB of program memory. mcu@tie{}= "atxmega64a3",
"atxmega64d3".
"avrxmega5"
"XMEGA" devices with more than 64@tie{}KiB and up to 128@tie{}KiB of program memory and more than 64@tie{}KiB of RAM.
mcu@tie{}= "atxmega64a1", "atxmega64a1u".
"avrxmega6"
"XMEGA" devices with more than 128@tie{}KiB of program memory. mcu@tie{}= "atxmega128a3", "atxmega128d3",
"atxmega192a3", "atxmega192d3", "atxmega256a3", "atxmega256a3b", "atxmega256a3bu", "atxmega256d3".
"avrxmega7"
"XMEGA" devices with more than 128@tie{}KiB of program memory and more than 64@tie{}KiB of RAM. mcu@tie{}=
"atxmega128a1", "atxmega128a1u".
"avr1"
This ISA is implemented by the minimal AVR core and supported for assembler only. mcu@tie{}= "at90s1200",
"attiny11", "attiny12", "attiny15", "attiny28".
-maccumulate-args
Accumulate outgoing function arguments and acquire/release the needed stack space for outgoing function arguments once in
function prologue/epilogue. Without this option, outgoing arguments are pushed before calling a function and popped
afterwards.
Popping the arguments after the function call can be expensive on AVR so that accumulating the stack space might lead to
smaller executables because arguments need not to be removed from the stack after such a function call.
This option can lead to reduced code size for functions that perform several calls to functions that get their arguments
on the stack like calls to printf-like functions.
-mbranch-cost=cost
Set the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-negative
integers. The default branch cost is 0.
-mcall-prologues
Functions prologues/epilogues are expanded as calls to appropriate subroutines. Code size is smaller.
-mint8
Assume "int" to be 8-bit integer. This affects the sizes of all types: a "char" is 1 byte, an "int" is 1 byte, a "long"
is 2 bytes, and "long long" is 4 bytes. Please note that this option does not conform to the C standards, but it results
in smaller code size.
-mno-interrupts
Generated code is not compatible with hardware interrupts. Code size is smaller.
-mrelax
Try to replace "CALL" resp. "JMP" instruction by the shorter "RCALL" resp. "RJMP" instruction if applicable. Setting
"-mrelax" just adds the "--relax" option to the linker command line when the linker is called.
Jump relaxing is performed by the linker because jump offsets are not known before code is located. Therefore, the
assembler code generated by the compiler is the same, but the instructions in the executable may differ from instructions
in the assembler code.
Relaxing must be turned on if linker stubs are needed, see the section on "EIND" and linker stubs below.
-mshort-calls
Use "RCALL"/"RJMP" instructions even on devices with 16@tie{}KiB or more of program memory, i.e. on devices that have the
"CALL" and "JMP" instructions. See also the "-mrelax" command line option.
-msp8
Treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. In
general, you don't need to set this option by hand.
This option is used internally by the compiler to select and build multilibs for architectures "avr2" and "avr25". These
architectures mix devices with and without "SPH". For any setting other than "-mmcu=avr2" or "-mmcu=avr25" the compiler
driver will add or remove this option from the compiler proper's command line, because the compiler then knows if the
device or architecture has an 8-bit stack pointer and thus no "SPH" register or not.
-mstrict-X
Use address register "X" in a way proposed by the hardware. This means that "X" is only used in indirect, post-increment
or pre-decrement addressing.
Without this option, the "X" register may be used in the same way as "Y" or "Z" which then is emulated by additional
instructions. For example, loading a value with "X+const" addressing with a small non-negative "const < 64" to a
register Rn is performed as
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const
-mtiny-stack
Only change the lower 8@tie{}bits of the stack pointer.
"EIND" and Devices with more than 128 Ki Bytes of Flash
Pointers in the implementation are 16@tie{}bits wide. The address of a function or label is represented as word address so
that indirect jumps and calls can target any code address in the range of 64@tie{}Ki words.
In order to facilitate indirect jump on devices with more than 128@tie{}Ki bytes of program memory space, there is a special
function register called "EIND" that serves as most significant part of the target address when "EICALL" or "EIJMP"
instructions are used.
Indirect jumps and calls on these devices are handled as follows by the compiler and are subject to some limitations:
· The compiler never sets "EIND".
· The compiler uses "EIND" implicitely in "EICALL"/"EIJMP" instructions or might read "EIND" directly in order to emulate
an indirect call/jump by means of a "RET" instruction.
· The compiler assumes that "EIND" never changes during the startup code or during the application. In particular, "EIND"
is not saved/restored in function or interrupt service routine prologue/epilogue.
· For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads sometimes also called
trampolines. Thus, the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address.
· Linker relaxation must be turned on so that the linker will generate the stubs correctly an all situaltion. See the
compiler option "-mrelax" and the linler option "--relax". There are corner cases where the linker is supposed to
generate stubs but aborts without relaxation and without a helpful error message.
· The default linker script is arranged for code with "EIND = 0". If code is supposed to work for a setup with "EIND !=
0", a custom linker script has to be used in order to place the sections whose name start with ".trampolines" into the
segment where "EIND" points to.
· The startup code from libgcc never sets "EIND". Notice that startup code is a blend of code from libgcc and AVR-LibC.
For the impact of AVR-LibC on "EIND", see the AVR-LibC user manual ("http://nongnu.org/avr-libc/user-manual").
· It is legitimate for user-specific startup code to set up "EIND" early, for example by means of initialization code
located in section ".init3". Such code runs prior to general startup code that initializes RAM and calls constructors,
but after the bit of startup code from AVR-LibC that sets "EIND" to the segment where the vector table is located.
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}
The "__trampolines_start" symbol is defined in the linker script.
· Stubs are generated automatically by the linker if the following two conditions are met:
-<The address of a label is taken by means of the "gs" modifier>
(short for generate stubs) like so:
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))
-<The final location of that label is in a code segment>
outside the segment where the stubs are located.
· The compiler emits such "gs" modifiers for code labels in the following situations:
-<Taking address of a function or code label.>
-<Computed goto.>
-<If prologue-save function is used, see -mcall-prologues>
command-line option.
-<Switch/case dispatch tables. If you do not want such dispatch>
tables you can specify the -fno-jump-tables command-line option.
-<C and C++ constructors/destructors called during startup/shutdown.>
-<If the tools hit a "gs()" modifier explained above.>
· Jumping to non-symbolic addresses like so is not supported:
int main (void)
{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}
Instead, a stub has to be set up, i.e. the function has to be called through a symbol ("func_4" in the example):
int main (void)
{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}
and the application be linked with "-Wl,--defsym,func_4=0x4". Alternatively, "func_4" can be defined in the linker
script.
Handling of the "RAMPD", "RAMPX", "RAMPY" and "RAMPZ" Special Function Registers
Some AVR devices support memories larger than the 64@tie{}KiB range that can be accessed with 16-bit pointers. To access
memory locations outside this 64@tie{}KiB range, the contentent of a "RAMP" register is used as high part of the address: The
"X", "Y", "Z" address register is concatenated with the "RAMPX", "RAMPY", "RAMPZ" special function register, respectively, to
get a wide address. Similarly, "RAMPD" is used together with direct addressing.
· The startup code initializes the "RAMP" special function registers with zero.
· If a AVR Named Address Spaces,named address space other than generic or "__flash" is used, then "RAMPZ" is set as needed
before the operation.
· If the device supports RAM larger than 64@tie{KiB} and the compiler needs to change "RAMPZ" to accomplish an operation,
"RAMPZ" is reset to zero after the operation.
· If the device comes with a specific "RAMP" register, the ISR prologue/epilogue saves/restores that SFR and initializes it
with zero in case the ISR code might (implicitly) use it.
· RAM larger than 64@tie{KiB} is not supported by GCC for AVR targets. If you use inline assembler to read from locations
outside the 16-bit address range and change one of the "RAMP" registers, you must reset it to zero after the access.
AVR Built-in Macros
GCC defines several built-in macros so that the user code can test for the presence or absence of features. Almost any of
the following built-in macros are deduced from device capabilities and thus triggered by the "-mmcu=" command-line option.
For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions.
"__AVR_Device__"
Setting "-mmcu=device" defines this built-in macro which reflects the device's name. For example, "-mmcu=atmega8" defines
the built-in macro "__AVR_ATmega8__", "-mmcu=attiny261a" defines "__AVR_ATtiny261A__", etc.
The built-in macros' names follow the scheme "__AVR_Device__" where Device is the device name as from the AVR user
manual. The difference between Device in the built-in macro and device in "-mmcu=device" is that the latter is always
lowercase.
"__AVR_HAVE_ELPM__"
The device has the the "ELPM" instruction.
"__AVR_HAVE_ELPMX__"
The device has the "ELPM Rn,Z" and "ELPM Rn,Z+" instructions.
"__AVR_HAVE_MOVW__"
The device has the "MOVW" instruction to perform 16-bit register-register moves.
"__AVR_HAVE_LPMX__"
The device has the "LPM Rn,Z" and "LPM Rn,Z+" instructions.
"__AVR_HAVE_MUL__"
The device has a hardware multiplier.
"__AVR_HAVE_JMP_CALL__"
The device has the "JMP" and "CALL" instructions. This is the case for devices with at least 16@tie{}KiB of program
memory and if "-mshort-calls" is not set.
"__AVR_HAVE_EIJMP_EICALL__"
"__AVR_3_BYTE_PC__"
The device has the "EIJMP" and "EICALL" instructions. This is the case for devices with more than 128@tie{}KiB of
program memory. This also means that the program counter (PC) is 3@tie{}bytes wide.
"__AVR_2_BYTE_PC__"
The program counter (PC) is 2@tie{}bytes wide. This is the case for devices with up to 128@tie{}KiB of program memory.
"__AVR_HAVE_8BIT_SP__"
"__AVR_HAVE_16BIT_SP__"
The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The definition of
these macros is affected by "-mtiny-stack".
"__AVR_HAVE_SPH__"
"__AVR_SP8__"
The device has the SPH (high part of stack pointer) special function register or has an 8-bit stack pointer,
respectively. The definition of these macros is affected by "-mmcu=" and in the cases of "-mmcu=avr2" and "-mmcu=avr25"
also by "-msp8".
"__AVR_HAVE_RAMPD__"
"__AVR_HAVE_RAMPX__"
"__AVR_HAVE_RAMPY__"
"__AVR_HAVE_RAMPZ__"
The device has the "RAMPD", "RAMPX", "RAMPY", "RAMPZ" special function register, respectively.
"__NO_INTERRUPTS__"
This macro reflects the "-mno-interrupts" command line option.
"__AVR_ERRATA_SKIP__"
"__AVR_ERRATA_SKIP_JMP_CALL__"
Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum. Skip
instructions are "SBRS", "SBRC", "SBIS", "SBIC" and "CPSE". The second macro is only defined if "__AVR_HAVE_JMP_CALL__"
is also set.
"__AVR_SFR_OFFSET__=offset"
Instructions that can address I/O special function registers directly like "IN", "OUT", "SBI", etc. may use a different
address as if addressed by an instruction to access RAM like "LD" or "STS". This offset depends on the device
architecture and has to be subtracted from the RAM address in order to get the respective I/O@tie{}address.
"__WITH_AVRLIBC__"
The compiler is configured to be used together with AVR-Libc. See the "--with-avrlibc" configure option.
Blackfin Options
-mcpu=cpu[-sirevision]
Specifies the name of the target Blackfin processor. Currently, cpu can be one of bf512, bf514, bf516, bf518, bf522,
bf523, bf524, bf525, bf526, bf527, bf531, bf532, bf533, bf534, bf536, bf537, bf538, bf539, bf542, bf544, bf547, bf548,
bf549, bf542m, bf544m, bf547m, bf548m, bf549m, bf561, bf592. The optional sirevision specifies the silicon revision of
the target Blackfin processor. Any workarounds available for the targeted silicon revision will be enabled. If
sirevision is none, no workarounds are enabled. If sirevision is any, all workarounds for the targeted processor will be
enabled. The "__SILICON_REVISION__" macro is defined to two hexadecimal digits representing the major and minor numbers
in the silicon revision. If sirevision is none, the "__SILICON_REVISION__" is not defined. If sirevision is any, the
"__SILICON_REVISION__" is defined to be 0xffff. If this optional sirevision is not used, GCC assumes the latest known
silicon revision of the targeted Blackfin processor.
Support for bf561 is incomplete. For bf561, Only the processor macro is defined. Without this option, bf532 is used as
the processor by default. The corresponding predefined processor macros for cpu is to be defined. And for bfin-elf
toolchain, this causes the hardware BSP provided by libgloss to be linked in if -msim is not given.
-msim
Specifies that the program will be run on the simulator. This causes the simulator BSP provided by libgloss to be linked
in. This option has effect only for bfin-elf toolchain. Certain other options, such as -mid-shared-library and -mfdpic,
imply -msim.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore
frame pointers and makes an extra register available in leaf functions. The option -fomit-frame-pointer removes the
frame pointer for all functions, which might make debugging harder.
-mspecld-anomaly
When enabled, the compiler will ensure that the generated code does not contain speculative loads after jump
instructions. If this option is used, "__WORKAROUND_SPECULATIVE_LOADS" is defined.
-mno-specld-anomaly
Don't generate extra code to prevent speculative loads from occurring.
-mcsync-anomaly
When enabled, the compiler will ensure that the generated code does not contain CSYNC or SSYNC instructions too soon
after conditional branches. If this option is used, "__WORKAROUND_SPECULATIVE_SYNCS" is defined.
-mno-csync-anomaly
Don't generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a conditional branch.
-mlow-64k
When enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of
memory.
-mno-low-64k
Assume that the program is arbitrarily large. This is the default.
-mstack-check-l1
Do stack checking using information placed into L1 scratchpad memory by the uClinux kernel.
-mid-shared-library
Generate code that supports shared libraries via the library ID method. This allows for execute in place and shared
libraries in an environment without virtual memory management. This option implies -fPIC. With a bfin-elf target, this
option implies -msim.
-mno-id-shared-library
Generate code that doesn't assume ID based shared libraries are being used. This is the default.
-mleaf-id-shared-library
Generate code that supports shared libraries via the library ID method, but assumes that this library or executable won't
link against any other ID shared libraries. That allows the compiler to use faster code for jumps and calls.
-mno-leaf-id-shared-library
Do not assume that the code being compiled won't link against any ID shared libraries. Slower code will be generated for
jump and call insns.
-mshared-library-id=n
Specified the identification number of the ID based shared library being compiled. Specifying a value of 0 will generate
more compact code, specifying other values will force the allocation of that number to the current library but is no more
space or time efficient than omitting this option.
-msep-data
Generate code that allows the data segment to be located in a different area of memory from the text segment. This
allows for execute in place in an environment without virtual memory management by eliminating relocations against the
text section.
-mno-sep-data
Generate code that assumes that the data segment follows the text segment. This is the default.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of the function into a register and then
performing a subroutine call on this register. This switch is needed if the target function lies outside of the 24-bit
addressing range of the offset-based version of subroutine call instruction.
This feature is not enabled by default. Specifying -mno-long-calls will restore the default behavior. Note these
switches have no effect on how the compiler generates code to handle function calls via function pointers.
-mfast-fp
Link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's rules for
checking inputs against Not-a-Number (NAN), in the interest of performance.
-minline-plt
Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect
without -mfdpic.
-mmulticore
Build standalone application for multicore Blackfin processor. Proper start files and link scripts will be used to
support multicore. This option defines "__BFIN_MULTICORE". It can only be used with -mcpu=bf561[-sirevision]. It can be
used with -mcorea or -mcoreb. If it's used without -mcorea or -mcoreb, single application/dual core programming model is
used. In this model, the main function of Core B should be named as coreb_main. If it's used with -mcorea or -mcoreb, one
application per core programming model is used. If this option is not used, single core application programming model is
used.
-mcorea
Build standalone application for Core A of BF561 when using one application per core programming model. Proper start
files and link scripts will be used to support Core A. This option defines "__BFIN_COREA". It must be used with
-mmulticore.
-mcoreb
Build standalone application for Core B of BF561 when using one application per core programming model. Proper start
files and link scripts will be used to support Core B. This option defines "__BFIN_COREB". When this option is used,
coreb_main should be used instead of main. It must be used with -mmulticore.
-msdram
Build standalone application for SDRAM. Proper start files and link scripts will be used to put the application into
SDRAM. Loader should initialize SDRAM before loading the application into SDRAM. This option defines "__BFIN_SDRAM".
-micplb
Assume that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux targets, the
default is to assume ICPLBs are enabled; for standalone applications the default is off.
C6X Options
-march=name
This specifies the name of the target architecture. GCC uses this name to determine what kind of instructions it can
emit when generating assembly code. Permissible names are: c62x, c64x, c64x+, c67x, c67x+, c674x.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target. This is the default.
-msim
Choose startup files and linker script suitable for the simulator.
-msdata=default
Put small global and static data in the .neardata section, which is pointed to by register "B14". Put small
uninitialized global and static data in the .bss section, which is adjacent to the .neardata section. Put small read-
only data into the .rodata section. The corresponding sections used for large pieces of data are .fardata, .far and
.const.
-msdata=all
Put all data, not just small objets, into the sections reserved for small data, and use addressing relative to the "B14"
register to access them.
-msdata=none
Make no use of the sections reserved for small data, and use absolute addresses to access all data. Put all initialized
global and static data in the .fardata section, and all uninitialized data in the .far section. Put all constant data
into the .const section.
CRIS Options
These options are defined specifically for the CRIS ports.
-march=architecture-type
-mcpu=architecture-type
Generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10 for respectively
ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0 except for cris-axis-linux-gnu, where the default is v10.
-mtune=architecture-type
Tune to architecture-type everything applicable about the generated code, except for the ABI and the set of available
instructions. The choices for architecture-type are the same as for -march=architecture-type.
-mmax-stack-frame=n
Warn when the stack frame of a function exceeds n bytes.
-metrax4
-metrax100
The options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively.
-mmul-bug-workaround
-mno-mul-bug-workaround
Work around a bug in the "muls" and "mulu" instructions for CPU models where it applies. This option is active by
default.
-mpdebug
Enable CRIS-specific verbose debug-related information in the assembly code. This option also has the effect to turn off
the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly file.
-mcc-init
Do not use condition-code results from previous instruction; always emit compare and test instructions before use of
condition codes.
-mno-side-effects
Do not emit instructions with side-effects in addressing modes other than post-increment.
-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align
These options (no-options) arranges (eliminate arrangements) for the stack-frame, individual data and constants to be
aligned for the maximum single data access size for the chosen CPU model. The default is to arrange for 32-bit
alignment. ABI details such as structure layout are not affected by these options.
-m32-bit
-m16-bit
-m8-bit
Similar to the stack- data- and const-align options above, these options arrange for stack-frame, writable data and
constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment.
-mno-prologue-epilogue
-mprologue-epilogue
With -mno-prologue-epilogue, the normal function prologue and epilogue which set up the stack frame are omitted and no
return instructions or return sequences are generated in the code. Use this option only together with visual inspection
of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or storage for local
variable needs to be allocated.
-mno-gotplt
-mgotplt
With -fpic and -fPIC, don't generate (do generate) instruction sequences that load addresses for functions from the PLT
part of the GOT rather than (traditional on other architectures) calls to the PLT. The default is -mgotplt.
-melf
Legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-gnu targets.
-mlinux
Legacy no-op option only recognized with the cris-axis-linux-gnu target.
-sim
This option, recognized for the cris-axis-elf arranges to link with input-output functions from a simulator library.
Code, initialized data and zero-initialized data are allocated consecutively.
-sim2
Like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000.
CR16 Options
These options are defined specifically for the CR16 ports.
-mmac
Enable the use of multiply-accumulate instructions. Disabled by default.
-mcr16cplus
-mcr16c
Generate code for CR16C or CR16C+ architecture. CR16C+ architecture is default.
-msim
Links the library libsim.a which is in compatible with simulator. Applicable to elf compiler only.
-mint32
Choose integer type as 32-bit wide.
-mbit-ops
Generates sbit/cbit instructions for bit manipulations.
-mdata-model=model
Choose a data model. The choices for model are near, far or medium. medium is default. However, far is not valid when
-mcr16c option is chosen as CR16C architecture does not support far data model.
Darwin Options
These options are defined for all architectures running the Darwin operating system.
FSF GCC on Darwin does not create "fat" object files; it will create an object file for the single architecture that it was
built to target. Apple's GCC on Darwin does create "fat" files if multiple -arch options are used; it does so by running the
compiler or linker multiple times and joining the results together with lipo.
The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the ISA that GCC is
targetting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to override this.
The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, as, will only permit
instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in
a ppc750 object file. The linker for shared libraries, /usr/bin/libtool, will fail and print an error if asked to create a
shared library with a less restrictive subtype than its input files (for instance, trying to put a ppc970 object file in a
ppc7400 library). The linker for executables, ld, will quietly give the executable the most restrictive subtype of any of
its input files.
-Fdir
Add the framework directory dir to the head of the list of directories to be searched for header files. These
directories are interleaved with those specified by -I options and are scanned in a left-to-right order.
A framework directory is a directory with frameworks in it. A framework is a directory with a "Headers" and/or
"PrivateHeaders" directory contained directly in it that ends in ".framework". The name of a framework is the name of
this directory excluding the ".framework". Headers associated with the framework are found in one of those two
directories, with "Headers" being searched first. A subframework is a framework directory that is in a framework's
"Frameworks" directory. Includes of subframework headers can only appear in a header of a framework that contains the
subframework, or in a sibling subframework header. Two subframeworks are siblings if they occur in the same framework.
A subframework should not have the same name as a framework, a warning will be issued if this is violated. Currently a
subframework cannot have subframeworks, in the future, the mechanism may be extended to support this. The standard
frameworks can be found in "/System/Library/Frameworks" and "/Library/Frameworks". An example include looks like
"#include <Framework/header.h>", where Framework denotes the name of the framework and header.h is found in the
"PrivateHeaders" or "Headers" directory.
-iframeworkdir
Like -F except the directory is a treated as a system directory. The main difference between this -iframework and -F is
that with -iframework the compiler does not warn about constructs contained within header files found via dir. This
option is valid only for the C family of languages.
-gused
Emit debugging information for symbols that are used. For STABS debugging format, this enables
-feliminate-unused-debug-symbols. This is by default ON.
-gfull
Emit debugging information for all symbols and types.
-mmacosx-version-min=version
The earliest version of MacOS X that this executable will run on is version. Typical values of version include 10.1,
10.2, and 10.3.9.
If the compiler was built to use the system's headers by default, then the default for this option is the system version
on which the compiler is running, otherwise the default is to make choices that are compatible with as many systems and
code bases as possible.
-mkernel
Enable kernel development mode. The -mkernel option sets -static, -fno-common, -fno-cxa-atexit, -fno-exceptions,
-fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti where applicable. This mode also sets -mno-altivec,
-msoft-float, -fno-builtin and -mlong-branch for PowerPC targets.
-mone-byte-bool
Override the defaults for bool so that sizeof(bool)==1. By default sizeof(bool) is 4 when compiling for Darwin/PowerPC
and 1 when compiling for Darwin/x86, so this option has no effect on x86.
Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary compatible with code generated without
that switch. Using this switch may require recompiling all other modules in a program, including system libraries. Use
this switch to conform to a non-default data model.
-mfix-and-continue
-ffix-and-continue
-findirect-data
Generate code suitable for fast turn around development. Needed to enable gdb to dynamically load ".o" files into
already running programs. -findirect-data and -ffix-and-continue are provided for backwards compatibility.
-all_load
Loads all members of static archive libraries. See man ld(1) for more information.
-arch_errors_fatal
Cause the errors having to do with files that have the wrong architecture to be fatal.
-bind_at_load
Causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is
loaded or launched.
-bundle
Produce a Mach-o bundle format file. See man ld(1) for more information.
-bundle_loader executable
This option specifies the executable that will be loading the build output file being linked. See man ld(1) for more
information.
-dynamiclib
When passed this option, GCC will produce a dynamic library instead of an executable when linking, using the Darwin
libtool command.
-force_cpusubtype_ALL
This causes GCC's output file to have the ALL subtype, instead of one controlled by the -mcpu or -march option.
-allowable_client client_name
-client_name
-compatibility_version
-current_version
-dead_strip
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-no_dead_strip_inits_and_terms
-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-segaddr
-segs_read_only_addr
-segs_read_write_addr
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
These options are passed to the Darwin linker. The Darwin linker man page describes them in detail.
DEC Alpha Options
These -m options are defined for the DEC Alpha implementations:
-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point instructions for floating-point operations. When -msoft-float is specified,
functions in libgcc.a will be used to perform floating-point operations. Unless they are replaced by routines that
emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines will
issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure
that the library is built so as not to call them.
Note that Alpha implementations without floating-point operations are required to have floating-point registers.
-mfp-reg
-mno-fp-regs
Generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float. If the
floating-point register set is not used, floating-point operands are passed in integer registers as if they were integers
and floating-point results are passed in $0 instead of $f0. This is a non-standard calling sequence, so any function
with a floating-point argument or return value called by code compiled with -mno-fp-regs must also be compiled with that
option.
A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-
point registers.
-mieee
The Alpha architecture implements floating-point hardware optimized for maximum performance. It is mostly compliant with
the IEEE floating-point standard. However, for full compliance, software assistance is required. This option generates
code fully IEEE-compliant code except that the inexact-flag is not maintained (see below). If this option is turned on,
the preprocessor macro "_IEEE_FP" is defined during compilation. The resulting code is less efficient but is able to
correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity. Other
Alpha compilers call this option -ieee_with_no_inexact.
-mieee-with-inexact
This is like -mieee except the generated code also maintains the IEEE inexact-flag. Turning on this option causes the
generated code to implement fully-compliant IEEE math. In addition to "_IEEE_FP", "_IEEE_FP_EXACT" is defined as a
preprocessor macro. On some Alpha implementations the resulting code may execute significantly slower than the code
generated by default. Since there is very little code that depends on the inexact-flag, you should normally not specify
this option. Other Alpha compilers call this option -ieee_with_inexact.
-mfp-trap-mode=trap-mode
This option controls what floating-point related traps are enabled. Other Alpha compilers call this option -fptm trap-
mode. The trap mode can be set to one of four values:
n This is the default (normal) setting. The only traps that are enabled are the ones that cannot be disabled in
software (e.g., division by zero trap).
u In addition to the traps enabled by n, underflow traps are enabled as well.
su Like u, but the instructions are marked to be safe for software completion (see Alpha architecture manual for
details).
sui Like su, but inexact traps are enabled as well.
-mfp-rounding-mode=rounding-mode
Selects the IEEE rounding mode. Other Alpha compilers call this option -fprm rounding-mode. The rounding-mode can be
one of:
n Normal IEEE rounding mode. Floating-point numbers are rounded towards the nearest machine number or towards the even
machine number in case of a tie.
m Round towards minus infinity.
c Chopped rounding mode. Floating-point numbers are rounded towards zero.
d Dynamic rounding mode. A field in the floating-point control register (fpcr, see Alpha architecture reference
manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus
infinity. Thus, unless your program modifies the fpcr, d corresponds to round towards plus infinity.
-mtrap-precision=trap-precision
In the Alpha architecture, floating-point traps are imprecise. This means without software assistance it is impossible
to recover from a floating trap and program execution normally needs to be terminated. GCC can generate code that can
assist operating system trap handlers in determining the exact location that caused a floating-point trap. Depending on
the requirements of an application, different levels of precisions can be selected:
p Program precision. This option is the default and means a trap handler can only identify which program caused a
floating-point exception.
f Function precision. The trap handler can determine the function that caused a floating-point exception.
i Instruction precision. The trap handler can determine the exact instruction that caused a floating-point exception.
Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.
-mieee-conformant
This option marks the generated code as IEEE conformant. You must not use this option unless you also specify
-mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect is to emit the line .eflag 48 in
the function prologue of the generated assembly file. Under DEC Unix, this has the effect that IEEE-conformant math
library routines will be linked in.
-mbuild-constants
Normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or
three instructions. If it cannot, it will output the constant as a literal and generate code to load it from the data
segment at run time.
Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the
maximum is six).
You would typically use this option to build a shared library dynamic loader. Itself a shared library, it must relocate
itself in memory before it can find the variables and constants in its own data segment.
-malpha-as
-mgas
Select whether to generate code to be assembled by the vendor-supplied assembler (-malpha-as) or by the GNU assembler
-mgas.
-mbwx
-mno-bwx
-mcix
-mno-cix
-mfix
-mno-fix
-mmax
-mno-max
Indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets. The default is to
use the instruction sets supported by the CPU type specified via -mcpu= option or that of the CPU on which GCC was built
if none was specified.
-mfloat-vax
-mfloat-ieee
Generate code that uses (does not use) VAX F and G floating-point arithmetic instead of IEEE single and double precision.
-mexplicit-relocs
-mno-explicit-relocs
Older Alpha assemblers provided no way to generate symbol relocations except via assembler macros. Use of these macros
does not allow optimal instruction scheduling. GNU binutils as of version 2.12 supports a new syntax that allows the
compiler to explicitly mark which relocations should apply to which instructions. This option is mostly useful for
debugging, as GCC detects the capabilities of the assembler when it is built and sets the default accordingly.
-msmall-data
-mlarge-data
When -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations. When -msmall-data is used,
objects 8 bytes long or smaller are placed in a small data area (the ".sdata" and ".sbss" sections) and are accessed via
16-bit relocations off of the $gp register. This limits the size of the small data area to 64KB, but allows the
variables to be directly accessed via a single instruction.
The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs that require more
than 2GB of data must use "malloc" or "mmap" to allocate the data in the heap instead of in the program's data segment.
When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.
-msmall-text
-mlarge-text
When -msmall-text is used, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, and
is thus reachable with a branch instruction. When -msmall-data is used, the compiler can assume that all local symbols
share the same $gp value, and thus reduce the number of instructions required for a function call from 4 to 1.
The default is -mlarge-text.
-mcpu=cpu_type
Set the instruction set and instruction scheduling parameters for machine type cpu_type. You can specify either the EV
style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of
processors and will choose the default values for the instruction set from the processor you specify. If you do not
specify a processor type, GCC will default to the processor on which the compiler was built.
Supported values for cpu_type are
ev4
ev45
21064
Schedules as an EV4 and has no instruction set extensions.
ev5
21164
Schedules as an EV5 and has no instruction set extensions.
ev56
21164a
Schedules as an EV5 and supports the BWX extension.
pca56
21164pc
21164PC
Schedules as an EV5 and supports the BWX and MAX extensions.
ev6
21264
Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
ev67
21264a
Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
Native toolchains also support the value native, which selects the best architecture option for the host processor.
-mcpu=native has no effect if GCC does not recognize the processor.
-mtune=cpu_type
Set only the instruction scheduling parameters for machine type cpu_type. The instruction set is not changed.
Native toolchains also support the value native, which selects the best architecture option for the host processor.
-mtune=native has no effect if GCC does not recognize the processor.
-mmemory-latency=time
Sets the latency the scheduler should assume for typical memory references as seen by the application. This number is
highly dependent on the memory access patterns used by the application and the size of the external cache on the machine.
Valid options for time are
number
A decimal number representing clock cycles.
L1
L2
L3
main
The compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3
caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that L3 is only valid for EV5.
DEC Alpha/VMS Options
These -m options are defined for the DEC Alpha/VMS implementations:
-mvms-return-codes
Return VMS condition codes from main. The default is to return POSIX style condition (e.g. error) codes.
-mdebug-main=prefix
Flag the first routine whose name starts with prefix as the main routine for the debugger.
-mmalloc64
Default to 64-bit memory allocation routines.
FR30 Options
These options are defined specifically for the FR30 port.
-msmall-model
Use the small address space model. This can produce smaller code, but it does assume that all symbolic values and
addresses will fit into a 20-bit range.
-mno-lsim
Assume that runtime support has been provided and so there is no need to include the simulator library (libsim.a) on the
linker command line.
FRV Options
-mgpr-32
Only use the first 32 general-purpose registers.
-mgpr-64
Use all 64 general-purpose registers.
-mfpr-32
Use only the first 32 floating-point registers.
-mfpr-64
Use all 64 floating-point registers.
-mhard-float
Use hardware instructions for floating-point operations.
-msoft-float
Use library routines for floating-point operations.
-malloc-cc
Dynamically allocate condition code registers.
-mfixed-cc
Do not try to dynamically allocate condition code registers, only use "icc0" and "fcc0".
-mdword
Change ABI to use double word insns.
-mno-dword
Do not use double word instructions.
-mdouble
Use floating-point double instructions.
-mno-double
Do not use floating-point double instructions.
-mmedia
Use media instructions.
-mno-media
Do not use media instructions.
-mmuladd
Use multiply and add/subtract instructions.
-mno-muladd
Do not use multiply and add/subtract instructions.
-mfdpic
Select the FDPIC ABI, which uses function descriptors to represent pointers to functions. Without any PIC/PIE-related
options, it implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small data are within a 12-bit range from the
GOT base address; with -fPIC or -fPIE, GOT offsets are computed with 32 bits. With a bfin-elf target, this option
implies -msim.
-minline-plt
Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect
without -mfdpic. It's enabled by default if optimizing for speed and compiling for shared libraries (i.e., -fPIC or
-fpic), or when an optimization option such as -O3 or above is present in the command line.
-mTLS
Assume a large TLS segment when generating thread-local code.
-mtls
Do not assume a large TLS segment when generating thread-local code.
-mgprel-ro
Enable the use of "GPREL" relocations in the FDPIC ABI for data that is known to be in read-only sections. It's enabled
by default, except for -fpic or -fpie: even though it may help make the global offset table smaller, it trades 1
instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols,
and it avoids the need for a GOT entry for the referenced symbol, so it's more likely to be a win. If it is not,
-mno-gprel-ro can be used to disable it.
-multilib-library-pic
Link with the (library, not FD) pic libraries. It's implied by -mlibrary-pic, as well as by -fPIC and -fpic without
-mfdpic. You should never have to use it explicitly.
-mlinked-fp
Follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This option is
enabled by default and can be disabled with -mno-linked-fp.
-mlong-calls
Use indirect addressing to call functions outside the current compilation unit. This allows the functions to be placed
anywhere within the 32-bit address space.
-malign-labels
Try to align labels to an 8-byte boundary by inserting nops into the previous packet. This option only has an effect
when VLIW packing is enabled. It doesn't create new packets; it merely adds nops to existing ones.
-mlibrary-pic
Generate position-independent EABI code.
-macc-4
Use only the first four media accumulator registers.
-macc-8
Use all eight media accumulator registers.
-mpack
Pack VLIW instructions.
-mno-pack
Do not pack VLIW instructions.
-mno-eflags
Do not mark ABI switches in e_flags.
-mcond-move
Enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-cond-move
Disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mscc
Enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-scc
Disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mcond-exec
Enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-cond-exec
Disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mvliw-branch
Run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-vliw-branch
Do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mmulti-cond-exec
Enable optimization of "&&" and "||" in conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-multi-cond-exec
Disable optimization of "&&" and "||" in conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mnested-cond-exec
Enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-nested-cond-exec
Disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-moptimize-membar
This switch removes redundant "membar" instructions from the compiler generated code. It is enabled by default.
-mno-optimize-membar
This switch disables the automatic removal of redundant "membar" instructions from the generated code.
-mtomcat-stats
Cause gas to print out tomcat statistics.
-mcpu=cpu
Select the processor type for which to generate code. Possible values are frv, fr550, tomcat, fr500, fr450, fr405,
fr400, fr300 and simple.
GNU/Linux Options
These -m options are defined for GNU/Linux targets:
-mglibc
Use the GNU C library. This is the default except on *-*-linux-*uclibc* and *-*-linux-*android* targets.
-muclibc
Use uClibc C library. This is the default on *-*-linux-*uclibc* targets.
-mbionic
Use Bionic C library. This is the default on *-*-linux-*android* targets.
-mandroid
Compile code compatible with Android platform. This is the default on *-*-linux-*android* targets.
When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When linking, this option
makes the GCC driver pass Android-specific options to the linker. Finally, this option causes the preprocessor macro
"__ANDROID__" to be defined.
-tno-android-cc
Disable compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.
-tno-android-ld
Disable linking effects of -mandroid, i.e., pass standard Linux linking options to the linker.
H8/300 Options
These -m options are defined for the H8/300 implementations:
-mrelax
Shorten some address references at link time, when possible; uses the linker option -relax.
-mh Generate code for the H8/300H.
-ms Generate code for the H8S.
-mn Generate code for the H8S and H8/300H in the normal mode. This switch must be used either with -mh or -ms.
-ms2600
Generate code for the H8S/2600. This switch must be used with -ms.
-mint32
Make "int" data 32 bits by default.
-malign-300
On the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8S is to align
longs and floats on 4-byte boundaries. -malign-300 causes them to be aligned on 2-byte boundaries. This option has no
effect on the H8/300.
HPPA Options
These -m options are defined for the HPPA family of computers:
-march=architecture-type
Generate code for the specified architecture. The choices for architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and
2.0 for PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper architecture option
for your machine. Code compiled for lower numbered architectures will run on higher numbered architectures, but not the
other way around.
-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
-mbig-switch
Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range
branches within a switch table.
-mjump-in-delay
Fill delay slots of function calls with unconditional jump instructions by modifying the return pointer for the function
call to be the target of the conditional jump.
-mdisable-fpregs
Prevent floating-point registers from being used in any manner. This is necessary for compiling kernels that perform
lazy context switching of floating-point registers. If you use this option and attempt to perform floating-point
operations, the compiler aborts.
-mdisable-indexing
Prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG
generated code under MACH.
-mno-space-regs
Generate code that assumes the target has no space registers. This allows GCC to generate faster indirect calls and use
unscaled index address modes.
Such code is suitable for level 0 PA systems and kernels.
-mfast-indirect-calls
Generate code that assumes calls never cross space boundaries. This allows GCC to emit code that performs faster
indirect calls.
This option will not work in the presence of shared libraries or nested functions.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator
can not use. This is useful when compiling kernel code. A register range is specified as two registers separated by a
dash. Multiple register ranges can be specified separated by a comma.
-mlong-load-store
Generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the
+k option to the HP compilers.
-mportable-runtime
Use the portable calling conventions proposed by HP for ELF systems.
-mgas
Enable the use of assembler directives only GAS understands.
-mschedule=cpu-type
Schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are 700 7100, 7100LC,
7200, 7300 and 8000. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper scheduling option for
your machine. The default scheduling is 8000.
-mlinker-opt
Enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging impossible. It also triggers a bug
in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking some programs.
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all
HPPA targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in
cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a
program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with
-msoft-float in order for this to work.
-msio
Generate the predefine, "_SIO", for server IO. The default is -mwsio. This generates the predefines, "__hp9000s700",
"__hp9000s700__" and "_WSIO", for workstation IO. These options are available under HP-UX and HI-UX.
-mgnu-ld
Use GNU ld specific options. This passes -shared to ld when building a shared library. It is the default when GCC is
configured, explicitly or implicitly, with the GNU linker. This option does not have any affect on which ld is called,
it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure
option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which
`gcc -print-prog-name=ld`. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
-mhp-ld
Use HP ld specific options. This passes -b to ld when building a shared library and passes +Accept TypeMismatch to ld on
all links. It is the default when GCC is configured, explicitly or implicitly, with the HP linker. This option does not
have any affect on which ld is called, it only changes what parameters are passed to that ld. The ld that is called is
determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used
by GCC can be printed using which `gcc -print-prog-name=ld`. This option is only available on the 64-bit HP-UX GCC, i.e.
configured with hppa*64*-*-hpux*.
-mlong-calls
Generate code that uses long call sequences. This ensures that a call is always able to reach linker generated stubs.
The default is to generate long calls only when the distance from the call site to the beginning of the function or
translation unit, as the case may be, exceeds a predefined limit set by the branch type being used. The limits for
normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always
limited at 240,000 bytes.
Distances are measured from the beginning of functions when using the -ffunction-sections option, or when using the -mgas
and -mno-portable-runtime options together under HP-UX with the SOM linker.
It is normally not desirable to use this option as it will degrade performance. However, it may be useful in large
applications, particularly when partial linking is used to build the application.
The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being
generated. The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative calls
should be relatively small. However, an indirect call is used on 32-bit ELF systems in pic code and it is quite long.
-munix=unix-std
Generate compiler predefines and select a startfile for the specified UNIX standard. The choices for unix-std are 93, 95
and 98. 93 is supported on all HP-UX versions. 95 is available on HP-UX 10.10 and later. 98 is available on HP-UX
11.11 and later. The default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10 though to 11.00, and 98 for HP-UX 11.11
and later.
-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional predefines for "XOPEN_UNIX" and
"_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o. -munix=98 provides additional predefines for "_XOPEN_UNIX",
"_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and "_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.
It is important to note that this option changes the interfaces for various library routines. It also affects the
operational behavior of the C library. Thus, extreme care is needed in using this option.
Library code that is intended to operate with more than one UNIX standard must test, set and restore the variable
__xpg4_extended_mask as appropriate. Most GNU software doesn't provide this capability.
-nolibdld
Suppress the generation of link options to search libdld.sl when the -static option is specified on HP-UX 10 and later.
-static
The HP-UX implementation of setlocale in libc has a dependency on libdld.sl. There isn't an archive version of
libdld.sl. Thus, when the -static option is specified, special link options are needed to resolve this dependency.
On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the -static option is
specified. This causes the resulting binary to be dynamic. On the 64-bit port, the linkers generate dynamic binaries by
default in any case. The -nolibdld option can be used to prevent the GCC driver from adding these link options.
-threads
Add support for multithreading with the dce thread library under HP-UX. This option sets flags for both the preprocessor
and linker.
Intel 386 and AMD x86-64 Options
These -m options are defined for the i386 and x86-64 family of computers:
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available
instructions. The choices for cpu-type are:
generic
Produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which your code will
run, then you should use the corresponding -mtune option instead of -mtune=generic. But, if you do not know exactly
what CPU users of your application will have, then you should use this option.
As new processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you
upgrade to a newer version of GCC, the code generated option will change to reflect the processors that were most
common when that version of GCC was released.
There is no -march=generic option because -march indicates the instruction set the compiler can use, and there is no
generic instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case,
collection of processors) for which the code is optimized.
native
This selects the CPU to tune for at compilation time by determining the processor type of the compiling machine.
Using -mtune=native will produce code optimized for the local machine under the constraints of the selected
instruction set. Using -march=native will enable all instruction subsets supported by the local machine (hence the
result might not run on different machines).
i386
Original Intel's i386 CPU.
i486
Intel's i486 CPU. (No scheduling is implemented for this chip.)
i586, pentium
Intel Pentium CPU with no MMX support.
pentium-mmx
Intel PentiumMMX CPU based on Pentium core with MMX instruction set support.
pentiumpro
Intel PentiumPro CPU.
i686
Same as "generic", but when used as "march" option, PentiumPro instruction set will be used, so the code will run on
all i686 family chips.
pentium2
Intel Pentium2 CPU based on PentiumPro core with MMX instruction set support.
pentium3, pentium3m
Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction set support.
pentium-m
Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction set support. Used by Centrino notebooks.
pentium4, pentium4m
Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set support.
prescott
Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support.
nocona
Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 instruction set support.
core2
Intel Core2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
corei7
Intel Core i7 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1 and SSE4.2 instruction set support.
corei7-avx
Intel Core i7 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AES and PCLMUL
instruction set support.
core-avx-i
Intel Core CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AES, PCLMUL, FSGSBASE, RDRND
and F16C instruction set support.
atom
Intel Atom CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
k6 AMD K6 CPU with MMX instruction set support.
k6-2, k6-3
Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support.
athlon, athlon-tbird
AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions support.
athlon-4, athlon-xp, athlon-mp
Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE instruction set support.
k8, opteron, athlon64, athlon-fx
AMD K8 core based CPUs with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow!
and 64-bit instruction set extensions.)
k8-sse3, opteron-sse3, athlon64-sse3
Improved versions of k8, opteron and athlon64 with SSE3 instruction set support.
amdfam10, barcelona
AMD Family 10h core based CPUs with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSE4A,
3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.)
bdver1
AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets FMA4, AVX, XOP, LWP, AES,
PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
bdver2
AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, AVX, XOP,
LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set
extensions.)
btver1
AMD Family 14h core based CPUs with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSSE3,
SSE4A, CX16, ABM and 64-bit instruction set extensions.)
winchip-c6
IDT Winchip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.
winchip2
IDT Winchip2 CPU, dealt in same way as i486 with additional MMX and 3DNow! instruction set support.
c3 Via C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)
c3-2
Via C3-2 CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
geode
Embedded AMD CPU with MMX and 3DNow! instruction set support.
While picking a specific cpu-type will schedule things appropriately for that particular chip, the compiler will not
generate any code that does not run on the default machine type without the -march=cpu-type option being used. For
example, if GCC is configured for i686-pc-linux-gnu then -mtune=pentium4 will generate code that is tuned for Pentium4
but will still run on i686 machines.
-march=cpu-type
Generate instructions for the machine type cpu-type. The choices for cpu-type are the same as for -mtune. Moreover,
specifying -march=cpu-type implies -mtune=cpu-type.
-mcpu=cpu-type
A deprecated synonym for -mtune.
-mfpmath=unit
Generate floating-point arithmetic for selected unit unit. The choices for unit are:
387 Use the standard 387 floating-point coprocessor present on the majority of chips and emulated otherwise. Code
compiled with this option runs almost everywhere. The temporary results are computed in 80-bit precision instead of
the precision specified by the type, resulting in slightly different results compared to most of other chips. See
-ffloat-store for more detailed description.
This is the default choice for i386 compiler.
sse Use scalar floating-point instructions present in the SSE instruction set. This instruction set is supported by
Pentium3 and newer chips, in the AMD line by Athlon-4, Athlon-xp and Athlon-mp chips. The earlier version of SSE
instruction set supports only single-precision arithmetic, thus the double and extended-precision arithmetic are
still done using 387. A later version, present only in Pentium4 and the future AMD x86-64 chips, supports double-
precision arithmetic too.
For the i386 compiler, you need to use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make
this option effective. For the x86-64 compiler, these extensions are enabled by default.
The resulting code should be considerably faster in the majority of cases and avoid the numerical instability
problems of 387 code, but may break some existing code that expects temporaries to be 80 bits.
This is the default choice for the x86-64 compiler.
sse,387
sse+387
both
Attempt to utilize both instruction sets at once. This effectively double the amount of available registers and on
chips with separate execution units for 387 and SSE the execution resources too. Use this option with care, as it is
still experimental, because the GCC register allocator does not model separate functional units well resulting in
instable performance.
-masm=dialect
Output asm instructions using selected dialect. Supported choices are intel or att (the default one). Darwin does not
support intel.
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating-point comparisons. These handle correctly the case where the
result of a comparison is unordered.
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GCC.
Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation.
You must make your own arrangements to provide suitable library functions for cross-compilation.
On machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may
be emitted even if -msoft-float is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types "float" and "double" in an FPU register, even if there
is no FPU. The idea is that the operating system should emulate an FPU.
The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead.
-mno-fancy-math-387
Some 387 emulators do not support the "sin", "cos" and "sqrt" instructions for the 387. Specify this option to avoid
generating those instructions. This option is the default on FreeBSD, OpenBSD and NetBSD. This option is overridden
when -march indicates that the target CPU will always have an FPU and so the instruction will not need emulation. As of
revision 2.6.1, these instructions are not generated unless you also use the -funsafe-math-optimizations switch.
-malign-double
-mno-align-double
Control whether GCC aligns "double", "long double", and "long long" variables on a two-word boundary or a one-word
boundary. Aligning "double" variables on a two-word boundary produces code that runs somewhat faster on a Pentium at the
expense of more memory.
On x86-64, -malign-double is enabled by default.
Warning: if you use the -malign-double switch, structures containing the above types will be aligned differently than the
published application binary interface specifications for the 386 and will not be binary compatible with structures in
code compiled without that switch.
-m96bit-long-double
-m128bit-long-double
These switches control the size of "long double" type. The i386 application binary interface specifies the size to be 96
bits, so -m96bit-long-double is the default in 32-bit mode.
Modern architectures (Pentium and newer) prefer "long double" to be aligned to an 8- or 16-byte boundary. In arrays or
structures conforming to the ABI, this is not possible. So specifying -m128bit-long-double aligns "long double" to a
16-byte boundary by padding the "long double" with an additional 32-bit zero.
In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that "long double" is to be
aligned on 16-byte boundary.
Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a "long double".
Warning: if you override the default value for your target ABI, the structures and arrays containing "long double"
variables will change their size as well as function calling convention for function taking "long double" will be
modified. Hence they will not be binary compatible with arrays or structures in code compiled without that switch.
-mlarge-data-threshold=number
When -mcmodel=medium is specified, the data greater than threshold are placed in large data section. This value must be
the same across all object linked into the binary and defaults to 65535.
-mrtd
Use a different function-calling convention, in which functions that take a fixed number of arguments return with the
"ret" num instruction, which pops their arguments while returning. This saves one instruction in the caller since there
is no need to pop the arguments there.
You can specify that an individual function is called with this calling sequence with the function attribute stdcall.
You can also override the -mrtd option by using the function attribute cdecl.
Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to
call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments (including
"printf"); otherwise incorrect code will be generated for calls to those functions.
In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra
arguments are harmlessly ignored.)
-mregparm=num
Control how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and
at most 3 registers can be used. You can control this behavior for a specific function by using the function attribute
regparm.
Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any
libraries. This includes the system libraries and startup modules.
-msseregparm
Use SSE register passing conventions for float and double arguments and return values. You can control this behavior for
a specific function by using the function attribute sseregparm.
Warning: if you use this switch then you must build all modules with the same value, including any libraries. This
includes the system libraries and startup modules.
-mvect8-ret-in-mem
Return 8-byte vectors in memory instead of MMX registers. This is the default on Solaris@tie{}8 and 9 and VxWorks to
match the ABI of the Sun Studio compilers until version 12. Later compiler versions (starting with Studio 12
Update@tie{}1) follow the ABI used by other x86 targets, which is the default on Solaris@tie{}10 and later. Only use
this option if you need to remain compatible with existing code produced by those previous compiler versions or older
versions of GCC.
-mpc32
-mpc64
-mpc80
Set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32 is specified, the significands of results of
floating-point operations are rounded to 24 bits (single precision); -mpc64 rounds the significands of results of
floating-point operations to 53 bits (double precision) and -mpc80 rounds the significands of results of floating-point
operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point
operations in higher precisions are not available to the programmer without setting the FPU control word explicitly.
Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more.
Note that some mathematical libraries assume that extended-precision (80-bit) floating-point operations are enabled by
default; routines in such libraries could suffer significant loss of accuracy, typically through so-called "catastrophic
cancellation", when this option is used to set the precision to less than extended precision.
-mstackrealign
Realign the stack at entry. On the Intel x86, the -mstackrealign option will generate an alternate prologue and epilogue
that realigns the run-time stack if necessary. This supports mixing legacy codes that keep a 4-byte aligned stack with
modern codes that keep a 16-byte stack for SSE compatibility. See also the attribute "force_align_arg_pointer",
applicable to individual functions.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is not
specified, the default is 4 (16 bytes or 128 bits).
-mincoming-stack-boundary=num
Assume the incoming stack is aligned to a 2 raised to num byte boundary. If -mincoming-stack-boundary is not specified,
the one specified by -mpreferred-stack-boundary will be used.
On Pentium and PentiumPro, "double" and "long double" values should be aligned to an 8-byte boundary (see -malign-double)
or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type
"__m128" may not work properly if it is not 16-byte aligned.
To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any
value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling
a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary
will most likely misalign the stack. It is recommended that libraries that use callbacks always use the default setting.
This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack
space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to
-mpreferred-stack-boundary=2.
-mmmx
-mno-mmx
-msse
-mno-sse
-msse2
-mno-sse2
-msse3
-mno-sse3
-mssse3
-mno-ssse3
-msse4.1
-mno-sse4.1
-msse4.2
-mno-sse4.2
-msse4
-mno-sse4
-mavx
-mno-avx
-mavx2
-mno-avx2
-maes
-mno-aes
-mpclmul
-mno-pclmul
-mfsgsbase
-mno-fsgsbase
-mrdrnd
-mno-rdrnd
-mf16c
-mno-f16c
-mfma
-mno-fma
-msse4a
-mno-sse4a
-mfma4
-mno-fma4
-mxop
-mno-xop
-mlwp
-mno-lwp
-m3dnow
-mno-3dnow
-mpopcnt
-mno-popcnt
-mabm
-mno-abm
-mbmi
-mbmi2
-mno-bmi
-mno-bmi2
-mlzcnt
-mno-lzcnt
-mtbm
-mno-tbm
These switches enable or disable the use of instructions in the MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, AVX, AVX2, AES,
PCLMUL, FSGSBASE, RDRND, F16C, FMA, SSE4A, FMA4, XOP, LWP, ABM, BMI, BMI2, LZCNT or 3DNow!
extended instruction sets. These extensions are also available as built-in functions: see X86 Built-in Functions, for
details of the functions enabled and disabled by these switches.
To have SSE/SSE2 instructions generated automatically from floating-point code (as opposed to 387 instructions), see
-mfpmath=sse.
GCC depresses SSEx instructions when -mavx is used. Instead, it generates new AVX instructions or AVX equivalence for all
SSEx instructions when needed.
These options will enable GCC to use these extended instructions in generated code, even without -mfpmath=sse.
Applications that perform run-time CPU detection must compile separate files for each supported architecture, using the
appropriate flags. In particular, the file containing the CPU detection code should be compiled without these options.
-mcld
This option instructs GCC to emit a "cld" instruction in the prologue of functions that use string instructions. String
instructions depend on the DF flag to select between autoincrement or autodecrement mode. While the ABI specifies the DF
flag to be cleared on function entry, some operating systems violate this specification by not clearing the DF flag in
their exception dispatchers. The exception handler can be invoked with the DF flag set, which leads to wrong direction
mode when string instructions are used. This option can be enabled by default on 32-bit x86 targets by configuring GCC
with the --enable-cld configure option. Generation of "cld" instructions can be suppressed with the -mno-cld compiler
option in this case.
-mvzeroupper
This option instructs GCC to emit a "vzeroupper" instruction before a transfer of control flow out of the function to
minimize AVX to SSE transition penalty as well as remove unnecessary zeroupper intrinsics.
-mcx16
This option will enable GCC to use CMPXCHG16B instruction in generated code. CMPXCHG16B allows for atomic operations on
128-bit double quadword (or oword) data types. This is useful for high resolution counters that could be updated by
multiple processors (or cores). This instruction is generated as part of atomic built-in functions: see __sync Builtins
or __atomic Builtins for details.
-msahf
This option will enable GCC to use SAHF instruction in generated 64-bit code. Early Intel CPUs with Intel 64 lacked LAHF
and SAHF instructions supported by AMD64 until introduction of Pentium 4 G1 step in December 2005. LAHF and SAHF are
load and store instructions, respectively, for certain status flags. In 64-bit mode, SAHF instruction is used to
optimize "fmod", "drem" or "remainder" built-in functions: see Other Builtins for details.
-mmovbe
This option will enable GCC to use movbe instruction to implement "__builtin_bswap32" and "__builtin_bswap64".
-mcrc32
This option will enable built-in functions, "__builtin_ia32_crc32qi", "__builtin_ia32_crc32hi". "__builtin_ia32_crc32si"
and "__builtin_ia32_crc32di" to generate the crc32 machine instruction.
-mrecip
This option will enable GCC to use RCPSS and RSQRTSS instructions (and their vectorized variants RCPPS and RSQRTPS) with
an additional Newton-Raphson step to increase precision instead of DIVSS and SQRTSS (and their vectorized variants) for
single-precision floating-point arguments. These instructions are generated only when -funsafe-math-optimizations is
enabled together with -finite-math-only and -fno-trapping-math. Note that while the throughput of the sequence is higher
than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp
(i.e. the inverse of 1.0 equals 0.99999994).
Note that GCC implements "1.0f/sqrtf(x)" in terms of RSQRTSS (or RSQRTPS) already with -ffast-math (or the above option
combination), and doesn't need -mrecip.
Also note that GCC emits the above sequence with additional Newton-Raphson step for vectorized single-float division and
vectorized "sqrtf(x)" already with -ffast-math (or the above option combination), and doesn't need -mrecip.
-mrecip=opt
This option allows to control which reciprocal estimate instructions may be used. opt is a comma separated list of
options, which may be preceded by a "!" to invert the option: "all": enable all estimate instructions, "default": enable
the default instructions, equivalent to -mrecip, "none": disable all estimate instructions, equivalent to -mno-recip,
"div": enable the approximation for scalar division, "vec-div": enable the approximation for vectorized division, "sqrt":
enable the approximation for scalar square root, "vec-sqrt": enable the approximation for vectorized square root.
So for example, -mrecip=all,!sqrt would enable all of the reciprocal approximations, except for square root.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an external library. Supported types are "svml" for the
Intel short vector math library and "acml" for the AMD math core library style of interfacing. GCC will currently emit
calls to "vmldExp2", "vmldLn2", "vmldLog102", "vmldLog102", "vmldPow2", "vmldTanh2", "vmldTan2", "vmldAtan2",
"vmldAtanh2", "vmldCbrt2", "vmldSinh2", "vmldSin2", "vmldAsinh2", "vmldAsin2", "vmldCosh2", "vmldCos2", "vmldAcosh2",
"vmldAcos2", "vmlsExp4", "vmlsLn4", "vmlsLog104", "vmlsLog104", "vmlsPow4", "vmlsTanh4", "vmlsTan4", "vmlsAtan4",
"vmlsAtanh4", "vmlsCbrt4", "vmlsSinh4", "vmlsSin4", "vmlsAsinh4", "vmlsAsin4", "vmlsCosh4", "vmlsCos4", "vmlsAcosh4" and
"vmlsAcos4" for corresponding function type when -mveclibabi=svml is used and "__vrd2_sin", "__vrd2_cos", "__vrd2_exp",
"__vrd2_log", "__vrd2_log2", "__vrd2_log10", "__vrs4_sinf", "__vrs4_cosf", "__vrs4_expf", "__vrs4_logf", "__vrs4_log2f",
"__vrs4_log10f" and "__vrs4_powf" for corresponding function type when -mveclibabi=acml is used. Both -ftree-vectorize
and -funsafe-math-optimizations have to be enabled. A SVML or ACML ABI compatible library will have to be specified at
link time.
-mabi=name
Generate code for the specified calling convention. Permissible values are: sysv for the ABI used on GNU/Linux and other
systems and ms for the Microsoft ABI. The default is to use the Microsoft ABI when targeting Windows. On all other
systems, the default is the SYSV ABI. You can control this behavior for a specific function by using the function
attribute ms_abi/sysv_abi.
-mtls-dialect=type
Generate code to access thread-local storage using the gnu or gnu2 conventions. gnu is the conservative default; gnu2 is
more efficient, but it may add compile- and run-time requirements that cannot be satisfied on all systems.
-mpush-args
-mno-push-args
Use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as method using
SUB/MOV operations and is enabled by default. In some cases disabling it may improve performance because of improved
scheduling and reduced dependencies.
-maccumulate-outgoing-args
If enabled, the maximum amount of space required for outgoing arguments will be computed in the function prologue. This
is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when preferred
stack boundary is not equal to 2. The drawback is a notable increase in code size. This switch implies -mno-push-args.
-mthreads
Support thread-safe exception handling on Mingw32. Code that relies on thread-safe exception handling must compile and
link all code with the -mthreads option. When compiling, -mthreads defines -D_MT; when linking, it links in a special
thread helper library -lmingwthrd which cleans up per thread exception handling data.
-mno-align-stringops
Do not align destination of inlined string operations. This switch reduces code size and improves performance in case
the destination is already aligned, but GCC doesn't know about it.
-minline-all-stringops
By default GCC inlines string operations only when the destination is known to be aligned to least a 4-byte boundary.
This enables more inlining, increase code size, but may improve performance of code that depends on fast memcpy, strlen
and memset for short lengths.
-minline-stringops-dynamically
For string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large
blocks.
-mstringop-strategy=alg
Overwrite internal decision heuristic about particular algorithm to inline string operation with. The allowed values are
"rep_byte", "rep_4byte", "rep_8byte" for expanding using i386 "rep" prefix of specified size, "byte_loop", "loop",
"unrolled_loop" for expanding inline loop, "libcall" for always expanding library call.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore
frame pointers and makes an extra register available in leaf functions. The option -fomit-frame-pointer removes the
frame pointer for all functions, which might make debugging harder.
-mtls-direct-seg-refs
-mno-tls-direct-seg-refs
Controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs for
64-bit), or whether the thread base pointer must be added. Whether or not this is legal depends on the operating system,
and whether it maps the segment to cover the entire TLS area.
For systems that use GNU libc, the default is on.
-msse2avx
-mno-sse2avx
Specify that the assembler should encode SSE instructions with VEX prefix. The option -mavx turns this on by default.
-mfentry
-mno-fentry
If profiling is active -pg put the profiling counter call before prologue. Note: On x86 architectures the attribute
"ms_hook_prologue" isn't possible at the moment for -mfentry and -pg.
-m8bit-idiv
-mno-8bit-idiv
On some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit integer divide.
This option generates a run-time check. If both dividend and divisor are within range of 0 to 255, 8-bit unsigned
integer divide is used instead of 32-bit/64-bit integer divide.
-mavx256-split-unaligned-load
-mavx256-split-unaligned-store
Split 32-byte AVX unaligned load and store.
These -m switches are supported in addition to the above on AMD x86-64 processors in 64-bit environments.
-m32
-m64
-mx32
Generate code for a 32-bit or 64-bit environment. The -m32 option sets int, long and pointer to 32 bits and generates
code that runs on any i386 system. The -m64 option sets int to 32 bits and long and pointer to 64 bits and generates
code for AMD's x86-64 architecture. The -mx32 option sets int, long and pointer to 32 bits and generates code for AMD's
x86-64 architecture. For darwin only the -m64 option turns off the -fno-pic and -mdynamic-no-pic options.
-mno-red-zone
Do not use a so called red zone for x86-64 code. The red zone is mandated by the x86-64 ABI, it is a 128-byte area
beyond the location of the stack pointer that will not be modified by signal or interrupt handlers and therefore can be
used for temporary data without adjusting the stack pointer. The flag -mno-red-zone disables this red zone.
-mcmodel=small
Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address
space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the default code model.
-mcmodel=kernel
Generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space. This model has to
be used for Linux kernel code.
-mcmodel=medium
Generate code for the medium model: The program is linked in the lower 2 GB of the address space. Small symbols are also
placed there. Symbols with sizes larger than -mlarge-data-threshold are put into large data or bss sections and can be
located above 2GB. Programs can be statically or dynamically linked.
-mcmodel=large
Generate code for the large model: This model makes no assumptions about addresses and sizes of sections.
i386 and x86-64 Windows Options
These additional options are available for Windows targets:
-mconsole
This option is available for Cygwin and MinGW targets. It specifies that a console application is to be generated, by
instructing the linker to set the PE header subsystem type required for console applications. This is the default
behavior for Cygwin and MinGW targets.
-mdll
This option is available for Cygwin and MinGW targets. It specifies that a DLL - a dynamic link library - is to be
generated, enabling the selection of the required runtime startup object and entry point.
-mnop-fun-dllimport
This option is available for Cygwin and MinGW targets. It specifies that the dllimport attribute should be ignored.
-mthread
This option is available for MinGW targets. It specifies that MinGW-specific thread support is to be used.
-municode
This option is available for mingw-w64 targets. It specifies that the UNICODE macro is getting pre-defined and that the
unicode capable runtime startup code is chosen.
-mwin32
This option is available for Cygwin and MinGW targets. It specifies that the typical Windows pre-defined macros are to
be set in the pre-processor, but does not influence the choice of runtime library/startup code.
-mwindows
This option is available for Cygwin and MinGW targets. It specifies that a GUI application is to be generated by
instructing the linker to set the PE header subsystem type appropriately.
-fno-set-stack-executable
This option is available for MinGW targets. It specifies that the executable flag for stack used by nested functions
isn't set. This is necessary for binaries running in kernel mode of Windows, as there the user32 API, which is used to
set executable privileges, isn't available.
-mpe-aligned-commons
This option is available for Cygwin and MinGW targets. It specifies that the GNU extension to the PE file format that
permits the correct alignment of COMMON variables should be used when generating code. It will be enabled by default if
GCC detects that the target assembler found during configuration supports the feature.
See also under i386 and x86-64 Options for standard options.
IA-64 Options
These are the -m options defined for the Intel IA-64 architecture.
-mbig-endian
Generate code for a big-endian target. This is the default for HP-UX.
-mlittle-endian
Generate code for a little-endian target. This is the default for AIX5 and GNU/Linux.
-mgnu-as
-mno-gnu-as
Generate (or don't) code for the GNU assembler. This is the default.
-mgnu-ld
-mno-gnu-ld
Generate (or don't) code for the GNU linker. This is the default.
-mno-pic
Generate code that does not use a global pointer register. The result is not position independent code, and violates the
IA-64 ABI.
-mvolatile-asm-stop
-mno-volatile-asm-stop
Generate (or don't) a stop bit immediately before and after volatile asm statements.
-mregister-names
-mno-register-names
Generate (or don't) in, loc, and out register names for the stacked registers. This may make assembler output more
readable.
-mno-sdata
-msdata
Disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs.
-mconstant-gp
Generate code that uses a single constant global pointer value. This is useful when compiling kernel code.
-mauto-pic
Generate code that is self-relocatable. This implies -mconstant-gp. This is useful when compiling firmware code.
-minline-float-divide-min-latency
Generate code for inline divides of floating-point values using the minimum latency algorithm.
-minline-float-divide-max-throughput
Generate code for inline divides of floating-point values using the maximum throughput algorithm.
-mno-inline-float-divide
Do not generate inline code for divides of floating-point values.
-minline-int-divide-min-latency
Generate code for inline divides of integer values using the minimum latency algorithm.
-minline-int-divide-max-throughput
Generate code for inline divides of integer values using the maximum throughput algorithm.
-mno-inline-int-divide
Do not generate inline code for divides of integer values.
-minline-sqrt-min-latency
Generate code for inline square roots using the minimum latency algorithm.
-minline-sqrt-max-throughput
Generate code for inline square roots using the maximum throughput algorithm.
-mno-inline-sqrt
Do not generate inline code for sqrt.
-mfused-madd
-mno-fused-madd
Do (don't) generate code that uses the fused multiply/add or multiply/subtract instructions. The default is to use these
instructions.
-mno-dwarf2-asm
-mdwarf2-asm
Don't (or do) generate assembler code for the DWARF2 line number debugging info. This may be useful when not using the
GNU assembler.
-mearly-stop-bits
-mno-early-stop-bits
Allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit. This can
improve instruction scheduling, but does not always do so.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator
can not use. This is useful when compiling kernel code. A register range is specified as two registers separated by a
dash. Multiple register ranges can be specified separated by a comma.
-mtls-size=tls-size
Specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64.
-mtune=cpu-type
Tune the instruction scheduling for a particular CPU, Valid values are itanium, itanium1, merced, itanium2, and mckinley.
-milp32
-mlp64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The
64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are HP-UX specific flags.
-mno-sched-br-data-spec
-msched-br-data-spec
(Dis/En)able data speculative scheduling before reload. This will result in generation of the ld.a instructions and the
corresponding check instructions (ld.c / chk.a). The default is 'disable'.
-msched-ar-data-spec
-mno-sched-ar-data-spec
(En/Dis)able data speculative scheduling after reload. This will result in generation of the ld.a instructions and the
corresponding check instructions (ld.c / chk.a). The default is 'enable'.
-mno-sched-control-spec
-msched-control-spec
(Dis/En)able control speculative scheduling. This feature is available only during region scheduling (i.e. before
reload). This will result in generation of the ld.s instructions and the corresponding check instructions chk.s . The
default is 'disable'.
-msched-br-in-data-spec
-mno-sched-br-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative loads before reload.
This is effective only with -msched-br-data-spec enabled. The default is 'enable'.
-msched-ar-in-data-spec
-mno-sched-ar-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative loads after reload.
This is effective only with -msched-ar-data-spec enabled. The default is 'enable'.
-msched-in-control-spec
-mno-sched-in-control-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the control speculative loads. This is
effective only with -msched-control-spec enabled. The default is 'enable'.
-mno-sched-prefer-non-data-spec-insns
-msched-prefer-non-data-spec-insns
If enabled, data speculative instructions will be chosen for schedule only if there are no other choices at the moment.
This will make the use of the data speculation much more conservative. The default is 'disable'.
-mno-sched-prefer-non-control-spec-insns
-msched-prefer-non-control-spec-insns
If enabled, control speculative instructions will be chosen for schedule only if there are no other choices at the
moment. This will make the use of the control speculation much more conservative. The default is 'disable'.
-mno-sched-count-spec-in-critical-path
-msched-count-spec-in-critical-path
If enabled, speculative dependencies will be considered during computation of the instructions priorities. This will
make the use of the speculation a bit more conservative. The default is 'disable'.
-msched-spec-ldc
Use a simple data speculation check. This option is on by default.
-msched-control-spec-ldc
Use a simple check for control speculation. This option is on by default.
-msched-stop-bits-after-every-cycle
Place a stop bit after every cycle when scheduling. This option is on by default.
-msched-fp-mem-deps-zero-cost
Assume that floating-point stores and loads are not likely to cause a conflict when placed into the same instruction
group. This option is disabled by default.
-msel-sched-dont-check-control-spec
Generate checks for control speculation in selective scheduling. This flag is disabled by default.
-msched-max-memory-insns=max-insns
Limit on the number of memory insns per instruction group, giving lower priority to subsequent memory insns attempting to
schedule in the same instruction group. Frequently useful to prevent cache bank conflicts. The default value is 1.
-msched-max-memory-insns-hard-limit
Disallow more than `msched-max-memory-insns' in instruction group. Otherwise, limit is `soft' meaning that we would
prefer non-memory operations when limit is reached but may still schedule memory operations.
IA-64/VMS Options
These -m options are defined for the IA-64/VMS implementations:
-mvms-return-codes
Return VMS condition codes from main. The default is to return POSIX style condition (e.g. error) codes.
-mdebug-main=prefix
Flag the first routine whose name starts with prefix as the main routine for the debugger.
-mmalloc64
Default to 64-bit memory allocation routines.
LM32 Options
These -m options are defined for the Lattice Mico32 architecture:
-mbarrel-shift-enabled
Enable barrel-shift instructions.
-mdivide-enabled
Enable divide and modulus instructions.
-mmultiply-enabled
Enable multiply instructions.
-msign-extend-enabled
Enable sign extend instructions.
-muser-enabled
Enable user-defined instructions.
M32C Options
-mcpu=name
Select the CPU for which code is generated. name may be one of r8c for the R8C/Tiny series, m16c for the M16C (up to
/60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series.
-msim
Specifies that the program will be run on the simulator. This causes an alternate runtime library to be linked in which
supports, for example, file I/O. You must not use this option when generating programs that will run on real hardware;
you must provide your own runtime library for whatever I/O functions are needed.
-memregs=number
Specifies the number of memory-based pseudo-registers GCC will use during code generation. These pseudo-registers will
be used like real registers, so there is a tradeoff between GCC's ability to fit the code into available registers, and
the performance penalty of using memory instead of registers. Note that all modules in a program must be compiled with
the same value for this option. Because of that, you must not use this option with the default runtime libraries gcc
builds.
M32R/D Options
These -m options are defined for Renesas M32R/D architectures:
-m32r2
Generate code for the M32R/2.
-m32rx
Generate code for the M32R/X.
-m32r
Generate code for the M32R. This is the default.
-mmodel=small
Assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with the "ld24" instruction),
and assume all subroutines are reachable with the "bl" instruction. This is the default.
The addressability of a particular object can be set with the "model" attribute.
-mmodel=medium
Assume objects may be anywhere in the 32-bit address space (the compiler will generate "seth/add3" instructions to load
their addresses), and assume all subroutines are reachable with the "bl" instruction.
-mmodel=large
Assume objects may be anywhere in the 32-bit address space (the compiler will generate "seth/add3" instructions to load
their addresses), and assume subroutines may not be reachable with the "bl" instruction (the compiler will generate the
much slower "seth/add3/jl" instruction sequence).
-msdata=none
Disable use of the small data area. Variables will be put into one of .data, bss, or .rodata (unless the "section"
attribute has been specified). This is the default.
The small data area consists of sections .sdata and .sbss. Objects may be explicitly put in the small data area with the
"section" attribute using one of these sections.
-msdata=sdata
Put small global and static data in the small data area, but do not generate special code to reference them.
-msdata=use
Put small global and static data in the small data area, and generate special instructions to reference them.
-G num
Put global and static objects less than or equal to num bytes into the small data or bss sections instead of the normal
data or bss sections. The default value of num is 8. The -msdata option must be set to one of sdata or use for this
option to have any effect.
All modules should be compiled with the same -G num value. Compiling with different values of num may or may not work;
if it doesn't the linker will give an error message---incorrect code will not be generated.
-mdebug
Makes the M32R specific code in the compiler display some statistics that might help in debugging programs.
-malign-loops
Align all loops to a 32-byte boundary.
-mno-align-loops
Do not enforce a 32-byte alignment for loops. This is the default.
-missue-rate=number
Issue number instructions per cycle. number can only be 1 or 2.
-mbranch-cost=number
number can only be 1 or 2. If it is 1 then branches will be preferred over conditional code, if it is 2, then the
opposite will apply.
-mflush-trap=number
Specifies the trap number to use to flush the cache. The default is 12. Valid numbers are between 0 and 15 inclusive.
-mno-flush-trap
Specifies that the cache cannot be flushed by using a trap.
-mflush-func=name
Specifies the name of the operating system function to call to flush the cache. The default is _flush_cache, but a
function call will only be used if a trap is not available.
-mno-flush-func
Indicates that there is no OS function for flushing the cache.
M680x0 Options
These are the -m options defined for M680x0 and ColdFire processors. The default settings depend on which architecture was
selected when the compiler was configured; the defaults for the most common choices are given below.
-march=arch
Generate code for a specific M680x0 or ColdFire instruction set architecture. Permissible values of arch for M680x0
architectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. ColdFire architectures are selected according to
Freescale's ISA classification and the permissible values are: isaa, isaaplus, isab and isac.
gcc defines a macro __mcfarch__ whenever it is generating code for a ColdFire target. The arch in this macro is one of
the -march arguments given above.
When used together, -march and -mtune select code that runs on a family of similar processors but that is optimized for a
particular microarchitecture.
-mcpu=cpu
Generate code for a specific M680x0 or ColdFire processor. The M680x0 cpus are: 68000, 68010, 68020, 68030, 68040,
68060, 68302, 68332 and cpu32. The ColdFire cpus are given by the table below, which also classifies the CPUs into
families:
Family : -mcpu arguments
51 : 51 51ac 51cn 51em 51qe
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485
-mcpu=cpu overrides -march=arch if arch is compatible with cpu. Other combinations of -mcpu and -march are rejected.
gcc defines the macro __mcf_cpu_cpu when ColdFire target cpu is selected. It also defines __mcf_family_family, where the
value of family is given by the table above.
-mtune=tune
Tune the code for a particular microarchitecture, within the constraints set by -march and -mcpu. The M680x0
microarchitectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. The ColdFire microarchitectures are: cfv1,
cfv2, cfv3, cfv4 and cfv4e.
You can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040 targets.
-mtune=68020-60 is similar but includes 68060 targets as well. These two options select the same tuning decisions as
-m68020-40 and -m68020-60 respectively.
gcc defines the macros __mcarch and __mcarch__ when tuning for 680x0 architecture arch. It also defines mcarch unless
either -ansi or a non-GNU -std option is used. If gcc is tuning for a range of architectures, as selected by
-mtune=68020-40 or -mtune=68020-60, it defines the macros for every architecture in the range.
gcc also defines the macro __muarch__ when tuning for ColdFire microarchitecture uarch, where uarch is one of the
arguments given above.
-m68000
-mc68000
Generate output for a 68000. This is the default when the compiler is configured for 68000-based systems. It is
equivalent to -march=68000.
Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328
and 68356.
-m68010
Generate output for a 68010. This is the default when the compiler is configured for 68010-based systems. It is
equivalent to -march=68010.
-m68020
-mc68020
Generate output for a 68020. This is the default when the compiler is configured for 68020-based systems. It is
equivalent to -march=68020.
-m68030
Generate output for a 68030. This is the default when the compiler is configured for 68030-based systems. It is
equivalent to -march=68030.
-m68040
Generate output for a 68040. This is the default when the compiler is configured for 68040-based systems. It is
equivalent to -march=68040.
This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Use this
option if your 68040 does not have code to emulate those instructions.
-m68060
Generate output for a 68060. This is the default when the compiler is configured for 68060-based systems. It is
equivalent to -march=68060.
This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060.
Use this option if your 68060 does not have code to emulate those instructions.
-mcpu32
Generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems. It is
equivalent to -march=cpu32.
Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336,
68340, 68341, 68349 and 68360.
-m5200
Generate output for a 520X ColdFire CPU. This is the default when the compiler is configured for 520X-based systems. It
is equivalent to -mcpu=5206, and is now deprecated in favor of that option.
Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206.
-m5206e
Generate output for a 5206e ColdFire CPU. The option is now deprecated in favor of the equivalent -mcpu=5206e.
-m528x
Generate output for a member of the ColdFire 528X family. The option is now deprecated in favor of the equivalent
-mcpu=528x.
-m5307
Generate output for a ColdFire 5307 CPU. The option is now deprecated in favor of the equivalent -mcpu=5307.
-m5407
Generate output for a ColdFire 5407 CPU. The option is now deprecated in favor of the equivalent -mcpu=5407.
-mcfv4e
Generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware floating-point
instructions. The option is equivalent to -mcpu=547x, and is now deprecated in favor of that option.
-m68020-40
Generate output for a 68040, without using any of the new instructions. This results in code that can run relatively
efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are
emulated on the 68040.
The option is equivalent to -march=68020 -mtune=68020-40.
-m68020-60
Generate output for a 68060, without using any of the new instructions. This results in code that can run relatively
efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are
emulated on the 68060.
The option is equivalent to -march=68020 -mtune=68020-60.
-mhard-float
-m68881
Generate floating-point instructions. This is the default for 68020 and above, and for ColdFire devices that have an
FPU. It defines the macro __HAVE_68881__ on M680x0 targets and __mcffpu__ on ColdFire targets.
-msoft-float
Do not generate floating-point instructions; use library calls instead. This is the default for 68000, 68010, and 68832
targets. It is also the default for ColdFire devices that have no FPU.
-mdiv
-mno-div
Generate (do not generate) ColdFire hardware divide and remainder instructions. If -march is used without -mcpu, the
default is "on" for ColdFire architectures and "off" for M680x0 architectures. Otherwise, the default is taken from the
target CPU (either the default CPU, or the one specified by -mcpu). For example, the default is "off" for -mcpu=5206 and
"on" for -mcpu=5206e.
gcc defines the macro __mcfhwdiv__ when this option is enabled.
-mshort
Consider type "int" to be 16 bits wide, like "short int". Additionally, parameters passed on the stack are also aligned
to a 16-bit boundary even on targets whose API mandates promotion to 32-bit.
-mno-short
Do not consider type "int" to be 16 bits wide. This is the default.
-mnobitfield
-mno-bitfield
Do not use the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield.
-mbitfield
Do use the bit-field instructions. The -m68020 option implies -mbitfield. This is the default if you use a
configuration designed for a 68020.
-mrtd
Use a different function-calling convention, in which functions that take a fixed number of arguments return with the
"rtd" instruction, which pops their arguments while returning. This saves one instruction in the caller since there is
no need to pop the arguments there.
This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call
libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments (including
"printf"); otherwise incorrect code will be generated for calls to those functions.
In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra
arguments are harmlessly ignored.)
The "rtd" instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the 68000 or
5200.
-mno-rtd
Do not use the calling conventions selected by -mrtd. This is the default.
-malign-int
-mno-align-int
Control whether GCC aligns "int", "long", "long long", "float", "double", and "long double" variables on a 32-bit
boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on 32-bit boundaries produces code that
runs somewhat faster on processors with 32-bit busses at the expense of more memory.
Warning: if you use the -malign-int switch, GCC will align structures containing the above types differently than most
published application binary interface specifications for the m68k.
-mpcrel
Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present, this
option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is not presently supported with
-mpcrel, though this could be supported for 68020 and higher processors.
-mno-strict-align
-mstrict-align
Do not (do) assume that unaligned memory references will be handled by the system.
-msep-data
Generate code that allows the data segment to be located in a different area of memory from the text segment. This
allows for execute in place in an environment without virtual memory management. This option implies -fPIC.
-mno-sep-data
Generate code that assumes that the data segment follows the text segment. This is the default.
-mid-shared-library
Generate code that supports shared libraries via the library ID method. This allows for execute in place and shared
libraries in an environment without virtual memory management. This option implies -fPIC.
-mno-id-shared-library
Generate code that doesn't assume ID based shared libraries are being used. This is the default.
-mshared-library-id=n
Specified the identification number of the ID based shared library being compiled. Specifying a value of 0 will generate
more compact code, specifying other values will force the allocation of that number to the current library but is no more
space or time efficient than omitting this option.
-mxgot
-mno-xgot
When generating position-independent code for ColdFire, generate code that works if the GOT has more than 8192 entries.
This code is larger and slower than code generated without this option. On M680x0 processors, this option is not needed;
-fPIC suffices.
GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if
the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:
relocation truncated to fit: R_68K_GOT16O foobar
If this happens, you should recompile your code with -mxgot. It should then work with very large GOTs. However, code
generated with -mxgot is less efficient, since it takes 4 instructions to fetch the value of a global symbol.
Note that some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort GOT entries. If
you have such a linker, you should only need to use -mxgot when compiling a single object file that accesses more than
8192 GOT entries. Very few do.
These options have no effect unless GCC is generating position-independent code.
MCore Options
These are the -m options defined for the Motorola M*Core processors.
-mhardlit
-mno-hardlit
Inline constants into the code stream if it can be done in two instructions or less.
-mdiv
-mno-div
Use the divide instruction. (Enabled by default).
-mrelax-immediate
-mno-relax-immediate
Allow arbitrary sized immediates in bit operations.
-mwide-bitfields
-mno-wide-bitfields
Always treat bit-fields as int-sized.
-m4byte-functions
-mno-4byte-functions
Force all functions to be aligned to a 4-byte boundary.
-mcallgraph-data
-mno-callgraph-data
Emit callgraph information.
-mslow-bytes
-mno-slow-bytes
Prefer word access when reading byte quantities.
-mlittle-endian
-mbig-endian
Generate code for a little-endian target.
-m210
-m340
Generate code for the 210 processor.
-mno-lsim
Assume that runtime support has been provided and so omit the simulator library (libsim.a) from the linker command line.
-mstack-increment=size
Set the maximum amount for a single stack increment operation. Large values can increase the speed of programs that
contain functions that need a large amount of stack space, but they can also trigger a segmentation fault if the stack is
extended too much. The default value is 0x1000.
MeP Options
-mabsdiff
Enables the "abs" instruction, which is the absolute difference between two registers.
-mall-opts
Enables all the optional instructions - average, multiply, divide, bit operations, leading zero, absolute difference,
min/max, clip, and saturation.
-maverage
Enables the "ave" instruction, which computes the average of two registers.
-mbased=n
Variables of size n bytes or smaller will be placed in the ".based" section by default. Based variables use the $tp
register as a base register, and there is a 128-byte limit to the ".based" section.
-mbitops
Enables the bit operation instructions - bit test ("btstm"), set ("bsetm"), clear ("bclrm"), invert ("bnotm"), and test-
and-set ("tas").
-mc=name
Selects which section constant data will be placed in. name may be "tiny", "near", or "far".
-mclip
Enables the "clip" instruction. Note that "-mclip" is not useful unless you also provide "-mminmax".
-mconfig=name
Selects one of the build-in core configurations. Each MeP chip has one or more modules in it; each module has a core CPU
and a variety of coprocessors, optional instructions, and peripherals. The "MeP-Integrator" tool, not part of GCC,
provides these configurations through this option; using this option is the same as using all the corresponding command-
line options. The default configuration is "default".
-mcop
Enables the coprocessor instructions. By default, this is a 32-bit coprocessor. Note that the coprocessor is normally
enabled via the "-mconfig=" option.
-mcop32
Enables the 32-bit coprocessor's instructions.
-mcop64
Enables the 64-bit coprocessor's instructions.
-mivc2
Enables IVC2 scheduling. IVC2 is a 64-bit VLIW coprocessor.
-mdc
Causes constant variables to be placed in the ".near" section.
-mdiv
Enables the "div" and "divu" instructions.
-meb
Generate big-endian code.
-mel
Generate little-endian code.
-mio-volatile
Tells the compiler that any variable marked with the "io" attribute is to be considered volatile.
-ml Causes variables to be assigned to the ".far" section by default.
-mleadz
Enables the "leadz" (leading zero) instruction.
-mm Causes variables to be assigned to the ".near" section by default.
-mminmax
Enables the "min" and "max" instructions.
-mmult
Enables the multiplication and multiply-accumulate instructions.
-mno-opts
Disables all the optional instructions enabled by "-mall-opts".
-mrepeat
Enables the "repeat" and "erepeat" instructions, used for low-overhead looping.
-ms Causes all variables to default to the ".tiny" section. Note that there is a 65536-byte limit to this section. Accesses
to these variables use the %gp base register.
-msatur
Enables the saturation instructions. Note that the compiler does not currently generate these itself, but this option is
included for compatibility with other tools, like "as".
-msdram
Link the SDRAM-based runtime instead of the default ROM-based runtime.
-msim
Link the simulator runtime libraries.
-msimnovec
Link the simulator runtime libraries, excluding built-in support for reset and exception vectors and tables.
-mtf
Causes all functions to default to the ".far" section. Without this option, functions default to the ".near" section.
-mtiny=n
Variables that are n bytes or smaller will be allocated to the ".tiny" section. These variables use the $gp base
register. The default for this option is 4, but note that there's a 65536-byte limit to the ".tiny" section.
MicroBlaze Options
-msoft-float
Use software emulation for floating point (default).
-mhard-float
Use hardware floating-point instructions.
-mmemcpy
Do not optimize block moves, use "memcpy".
-mno-clearbss
This option is deprecated. Use -fno-zero-initialized-in-bss instead.
-mcpu=cpu-type
Use features of and schedule code for given CPU. Supported values are in the format vX.YY.Z, where X is a major version,
YY is the minor version, and Z is compatibility code. Example values are v3.00.a, v4.00.b, v5.00.a, v5.00.b, v5.00.b,
v6.00.a.
-mxl-soft-mul
Use software multiply emulation (default).
-mxl-soft-div
Use software emulation for divides (default).
-mxl-barrel-shift
Use the hardware barrel shifter.
-mxl-pattern-compare
Use pattern compare instructions.
-msmall-divides
Use table lookup optimization for small signed integer divisions.
-mxl-stack-check
This option is deprecated. Use -fstack-check instead.
-mxl-gp-opt
Use GP relative sdata/sbss sections.
-mxl-multiply-high
Use multiply high instructions for high part of 32x32 multiply.
-mxl-float-convert
Use hardware floating-point conversion instructions.
-mxl-float-sqrt
Use hardware floating-point square root instruction.
-mxl-mode-app-model
Select application model app-model. Valid models are
executable
normal executable (default), uses startup code crt0.o.
xmdstub
for use with Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub. This uses
startup file crt1.o and sets the start address of the program to be 0x800.
bootstrap
for applications that are loaded using a bootloader. This model uses startup file crt2.o which does not contain a
processor reset vector handler. This is suitable for transferring control on a processor reset to the bootloader
rather than the application.
novectors
for applications that do not require any of the MicroBlaze vectors. This option may be useful for applications
running within a monitoring application. This model uses crt3.o as a startup file.
Option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model.
MIPS Options
-EB Generate big-endian code.
-EL Generate little-endian code. This is the default for mips*el-*-* configurations.
-march=arch
Generate code that will run on arch, which can be the name of a generic MIPS ISA, or the name of a particular processor.
The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, mips64 and mips64r2. The processor names are: 4kc, 4km,
4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1,
34kf1_1, 74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1, loongson2e, loongson2f, loongson3a, m4k, octeon,
octeon+, octeon2, orion, r2000, r3000, r3900, r4000, r4400, r4600, r4650, r6000, r8000, rm7000, rm9000, r10000, r12000,
r14000, r16000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130, vr4300, vr5000, vr5400, vr5500 and xlr. The special value
from-abi selects the most compatible architecture for the selected ABI (that is, mips1 for 32-bit ABIs and mips3 for
64-bit ABIs).
Native Linux/GNU and IRIX toolchains also support the value native, which selects the best architecture option for the
host processor. -march=native has no effect if GCC does not recognize the processor.
In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional, and vr may be
written r.
Names of the form nf2_1 refer to processors with FPUs clocked at half the rate of the core, names of the form nf1_1 refer
to processors with FPUs clocked at the same rate as the core, and names of the form nf3_2 refer to processors with FPUs
clocked a ratio of 3:2 with respect to the core. For compatibility reasons, nf is accepted as a synonym for nf2_1 while
nx and bfx are accepted as synonyms for nf1_1.
GCC defines two macros based on the value of this option. The first is _MIPS_ARCH, which gives the name of target
architecture, as a string. The second has the form _MIPS_ARCH_foo, where foo is the capitalized value of _MIPS_ARCH.
For example, -march=r2000 will set _MIPS_ARCH to "r2000" and define the macro _MIPS_ARCH_R2000.
Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it will have the full prefix and
will not abbreviate 000 as k. In the case of from-abi, the macro names the resolved architecture (either "mips1" or
"mips3"). It names the default architecture when no -march option is given.
-mtune=arch
Optimize for arch. Among other things, this option controls the way instructions are scheduled, and the perceived cost
of arithmetic operations. The list of arch values is the same as for -march.
When this option is not used, GCC will optimize for the processor specified by -march. By using -march and -mtune
together, it is possible to generate code that will run on a family of processors, but optimize the code for one
particular member of that family.
-mtune defines the macros _MIPS_TUNE and _MIPS_TUNE_foo, which work in the same way as the -march ones described above.
-mips1
Equivalent to -march=mips1.
-mips2
Equivalent to -march=mips2.
-mips3
Equivalent to -march=mips3.
-mips4
Equivalent to -march=mips4.
-mips32
Equivalent to -march=mips32.
-mips32r2
Equivalent to -march=mips32r2.
-mips64
Equivalent to -march=mips64.
-mips64r2
Equivalent to -march=mips64r2.
-mips16
-mno-mips16
Generate (do not generate) MIPS16 code. If GCC is targetting a MIPS32 or MIPS64 architecture, it will make use of the
MIPS16e ASE.
MIPS16 code generation can also be controlled on a per-function basis by means of "mips16" and "nomips16" attributes.
-mflip-mips16
Generate MIPS16 code on alternating functions. This option is provided for regression testing of mixed MIPS16/non-MIPS16
code generation, and is not intended for ordinary use in compiling user code.
-minterlink-mips16
-mno-interlink-mips16
Require (do not require) that non-MIPS16 code be link-compatible with MIPS16 code.
For example, non-MIPS16 code cannot jump directly to MIPS16 code; it must either use a call or an indirect jump.
-minterlink-mips16 therefore disables direct jumps unless GCC knows that the target of the jump is not MIPS16.
-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
Generate code for the given ABI.
Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit
architecture, but you can use -mgp32 to get 32-bit code instead.
For information about the O64 ABI, see <http://gcc.gnu.org/projects/mipso64-abi.html>.
GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits wide. You can select
this combination with -mabi=32 -mfp64. This ABI relies on the mthc1 and mfhc1 instructions and is therefore only
supported for MIPS32R2 processors.
The register assignments for arguments and return values remain the same, but each scalar value is passed in a single
64-bit register rather than a pair of 32-bit registers. For example, scalar floating-point values are returned in $f0
only, not a $f0/$f1 pair. The set of call-saved registers also remains the same, but all 64 bits are saved.
-mabicalls
-mno-abicalls
Generate (do not generate) code that is suitable for SVR4-style dynamic objects. -mabicalls is the default for
SVR4-based systems.
-mshared
-mno-shared
Generate (do not generate) code that is fully position-independent, and that can therefore be linked into shared
libraries. This option only affects -mabicalls.
All -mabicalls code has traditionally been position-independent, regardless of options like -fPIC and -fpic. However, as
an extension, the GNU toolchain allows executables to use absolute accesses for locally-binding symbols. It can also use
shorter GP initialization sequences and generate direct calls to locally-defined functions. This mode is selected by
-mno-shared.
-mno-shared depends on binutils 2.16 or higher and generates objects that can only be linked by the GNU linker. However,
the option does not affect the ABI of the final executable; it only affects the ABI of relocatable objects. Using
-mno-shared will generally make executables both smaller and quicker.
-mshared is the default.
-mplt
-mno-plt
Assume (do not assume) that the static and dynamic linkers support PLTs and copy relocations. This option only affects
-mno-shared -mabicalls. For the n64 ABI, this option has no effect without -msym32.
You can make -mplt the default by configuring GCC with --with-mips-plt. The default is -mno-plt otherwise.
-mxgot
-mno-xgot
Lift (do not lift) the usual restrictions on the size of the global offset table.
GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it will only
work if the GOT is smaller than about 64k. Anything larger will cause the linker to report an error such as:
relocation truncated to fit: R_MIPS_GOT16 foobar
If this happens, you should recompile your code with -mxgot. It should then work with very large GOTs, although it will
also be less efficient, since it will take three instructions to fetch the value of a global symbol.
Note that some linkers can create multiple GOTs. If you have such a linker, you should only need to use -mxgot when a
single object file accesses more than 64k's worth of GOT entries. Very few do.
These options have no effect unless GCC is generating position independent code.
-mgp32
Assume that general-purpose registers are 32 bits wide.
-mgp64
Assume that general-purpose registers are 64 bits wide.
-mfp32
Assume that floating-point registers are 32 bits wide.
-mfp64
Assume that floating-point registers are 64 bits wide.
-mhard-float
Use floating-point coprocessor instructions.
-msoft-float
Do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead.
-msingle-float
Assume that the floating-point coprocessor only supports single-precision operations.
-mdouble-float
Assume that the floating-point coprocessor supports double-precision operations. This is the default.
-mllsc
-mno-llsc
Use (do not use) ll, sc, and sync instructions to implement atomic memory built-in functions. When neither option is
specified, GCC will use the instructions if the target architecture supports them.
-mllsc is useful if the runtime environment can emulate the instructions and -mno-llsc can be useful when compiling for
nonstandard ISAs. You can make either option the default by configuring GCC with --with-llsc and --without-llsc
respectively. --with-llsc is the default for some configurations; see the installation documentation for details.
-mdsp
-mno-dsp
Use (do not use) revision 1 of the MIPS DSP ASE.
This option defines the preprocessor macro __mips_dsp. It also defines __mips_dsp_rev to 1.
-mdspr2
-mno-dspr2
Use (do not use) revision 2 of the MIPS DSP ASE.
This option defines the preprocessor macros __mips_dsp and __mips_dspr2. It also defines __mips_dsp_rev to 2.
-msmartmips
-mno-smartmips
Use (do not use) the MIPS SmartMIPS ASE.
-mpaired-single
-mno-paired-single
Use (do not use) paired-single floating-point instructions.
This option requires hardware floating-point support to be enabled.
-mdmx
-mno-mdmx
Use (do not use) MIPS Digital Media Extension instructions. This option can only be used when generating 64-bit code and
requires hardware floating-point support to be enabled.
-mips3d
-mno-mips3d
Use (do not use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single.
-mmt
-mno-mt
Use (do not use) MT Multithreading instructions.
-mlong64
Force "long" types to be 64 bits wide. See -mlong32 for an explanation of the default and the way that the pointer size
is determined.
-mlong32
Force "long", "int", and pointer types to be 32 bits wide.
The default size of "int"s, "long"s and pointers depends on the ABI. All the supported ABIs use 32-bit "int"s. The n64
ABI uses 64-bit "long"s, as does the 64-bit EABI; the others use 32-bit "long"s. Pointers are the same size as "long"s,
or the same size as integer registers, whichever is smaller.
-msym32
-mno-sym32
Assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is useful in
combination with -mabi=64 and -mno-abicalls because it allows GCC to generate shorter and faster references to symbolic
addresses.
-G num
Put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. GCC can
then access the data more efficiently; see -mgpopt for details.
The default -G option depends on the configuration.
-mlocal-sdata
-mno-local-sdata
Extend (do not extend) the -G behavior to local data too, such as to static variables in C. -mlocal-sdata is the default
for all configurations.
If the linker complains that an application is using too much small data, you might want to try rebuilding the less
performance-critical parts with -mno-local-sdata. You might also want to build large libraries with -mno-local-sdata, so
that the libraries leave more room for the main program.
-mextern-sdata
-mno-extern-sdata
Assume (do not assume) that externally-defined data will be in a small data section if that data is within the -G limit.
-mextern-sdata is the default for all configurations.
If you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that is no bigger than
num bytes, you must make sure that Var is placed in a small data section. If Var is defined by another module, you must
either compile that module with a high-enough -G setting or attach a "section" attribute to Var's definition. If Var is
common, you must link the application with a high-enough -G setting.
The easiest way of satisfying these restrictions is to compile and link every module with the same -G option. However,
you may wish to build a library that supports several different small data limits. You can do this by compiling the
library with the highest supported -G setting and additionally using -mno-extern-sdata to stop the library from making
assumptions about externally-defined data.
-mgpopt
-mno-gpopt
Use (do not use) GP-relative accesses for symbols that are known to be in a small data section; see -G, -mlocal-sdata and
-mextern-sdata. -mgpopt is the default for all configurations.
-mno-gpopt is useful for cases where the $gp register might not hold the value of "_gp". For example, if the code is
part of a library that might be used in a boot monitor, programs that call boot monitor routines will pass an unknown
value in $gp. (In such situations, the boot monitor itself would usually be compiled with -G0.)
-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata.
-membedded-data
-mno-embedded-data
Allocate variables to the read-only data section first if possible, then next in the small data section if possible,
otherwise in data. This gives slightly slower code than the default, but reduces the amount of RAM required when
executing, and thus may be preferred for some embedded systems.
-muninit-const-in-rodata
-mno-uninit-const-in-rodata
Put uninitialized "const" variables in the read-only data section. This option is only meaningful in conjunction with
-membedded-data.
-mcode-readable=setting
Specify whether GCC may generate code that reads from executable sections. There are three possible settings:
-mcode-readable=yes
Instructions may freely access executable sections. This is the default setting.
-mcode-readable=pcrel
MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. This
option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It is also useful on
processors that can be configured to have a dual instruction/data SRAM interface and that, like the M4K,
automatically redirect PC-relative loads to the instruction RAM.
-mcode-readable=no
Instructions must not access executable sections. This option can be useful on targets that are configured to have a
dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the
instruction RAM.
-msplit-addresses
-mno-split-addresses
Enable (disable) use of the "%hi()" and "%lo()" assembler relocation operators. This option has been superseded by
-mexplicit-relocs but is retained for backwards compatibility.
-mexplicit-relocs
-mno-explicit-relocs
Use (do not use) assembler relocation operators when dealing with symbolic addresses. The alternative, selected by
-mno-explicit-relocs, is to use assembler macros instead.
-mexplicit-relocs is the default if GCC was configured to use an assembler that supports relocation operators.
-mcheck-zero-division
-mno-check-zero-division
Trap (do not trap) on integer division by zero.
The default is -mcheck-zero-division.
-mdivide-traps
-mdivide-breaks
MIPS systems check for division by zero by generating either a conditional trap or a break instruction. Using traps
results in smaller code, but is only supported on MIPS II and later. Also, some versions of the Linux kernel have a bug
that prevents trap from generating the proper signal ("SIGFPE"). Use -mdivide-traps to allow conditional traps on
architectures that support them and -mdivide-breaks to force the use of breaks.
The default is usually -mdivide-traps, but this can be overridden at configure time using --with-divide=breaks. Divide-
by-zero checks can be completely disabled using -mno-check-zero-division.
-mmemcpy
-mno-memcpy
Force (do not force) the use of "memcpy()" for non-trivial block moves. The default is -mno-memcpy, which allows GCC to
inline most constant-sized copies.
-mlong-calls
-mno-long-calls
Disable (do not disable) use of the "jal" instruction. Calling functions using "jal" is more efficient but requires the
caller and callee to be in the same 256 megabyte segment.
This option has no effect on abicalls code. The default is -mno-long-calls.
-mmad
-mno-mad
Enable (disable) use of the "mad", "madu" and "mul" instructions, as provided by the R4650 ISA.
-mfused-madd
-mno-fused-madd
Enable (disable) use of the floating-point multiply-accumulate instructions, when they are available. The default is
-mfused-madd.
When multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not
subject to the FCSR Flush to Zero bit. This may be undesirable in some circumstances.
-nocpp
Tell the MIPS assembler to not run its preprocessor over user assembler files (with a .s suffix) when assembling them.
-mfix-24k
-mno-fix-24k
Work around the 24K E48 (lost data on stores during refill) errata. The workarounds are implemented by the assembler
rather than by GCC.
-mfix-r4000
-mno-fix-r4000
Work around certain R4000 CPU errata:
- A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer
division.
- A double-word or a variable shift may give an incorrect result if executed while an integer multiplication is in
progress.
- An integer division may give an incorrect result if started in a delay slot of a taken branch or a jump.
-mfix-r4400
-mno-fix-r4400
Work around certain R4400 CPU errata:
- A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer
division.
-mfix-r10000
-mno-fix-r10000
Work around certain R10000 errata:
- "ll"/"sc" sequences may not behave atomically on revisions prior to 3.0. They may deadlock on revisions 2.6 and
earlier.
This option can only be used if the target architecture supports branch-likely instructions. -mfix-r10000 is the default
when -march=r10000 is used; -mno-fix-r10000 is the default otherwise.
-mfix-vr4120
-mno-fix-vr4120
Work around certain VR4120 errata:
- "dmultu" does not always produce the correct result.
- "div" and "ddiv" do not always produce the correct result if one of the operands is negative.
The workarounds for the division errata rely on special functions in libgcc.a. At present, these functions are only
provided by the "mips64vr*-elf" configurations.
Other VR4120 errata require a nop to be inserted between certain pairs of instructions. These errata are handled by the
assembler, not by GCC itself.
-mfix-vr4130
Work around the VR4130 "mflo"/"mfhi" errata. The workarounds are implemented by the assembler rather than by GCC,
although GCC will avoid using "mflo" and "mfhi" if the VR4130 "macc", "macchi", "dmacc" and "dmacchi" instructions are
available instead.
-mfix-sb1
-mno-fix-sb1
Work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2 "F1" and "F2" floating-
point errata.)
-mr10k-cache-barrier=setting
Specify whether GCC should insert cache barriers to avoid the side-effects of speculation on R10K processors.
In common with many processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes
instructions from the "taken" branch. It later aborts these instructions if the predicted outcome was wrong. However,
on the R10K, even aborted instructions can have side effects.
This problem only affects kernel stores and, depending on the system, kernel loads. As an example, a speculatively-
executed store may load the target memory into cache and mark the cache line as dirty, even if the store itself is later
aborted. If a DMA operation writes to the same area of memory before the "dirty" line is flushed, the cached data will
overwrite the DMA-ed data. See the R10K processor manual for a full description, including other potential problems.
One workaround is to insert cache barrier instructions before every memory access that might be speculatively executed
and that might have side effects even if aborted. -mr10k-cache-barrier=setting controls GCC's implementation of this
workaround. It assumes that aborted accesses to any byte in the following regions will not have side effects:
1. the memory occupied by the current function's stack frame;
2. the memory occupied by an incoming stack argument;
3. the memory occupied by an object with a link-time-constant address.
It is the kernel's responsibility to ensure that speculative accesses to these regions are indeed safe.
If the input program contains a function declaration such as:
void foo (void);
then the implementation of "foo" must allow "j foo" and "jal foo" to be executed speculatively. GCC honors this
restriction for functions it compiles itself. It expects non-GCC functions (such as hand-written assembly code) to do
the same.
The option has three forms:
-mr10k-cache-barrier=load-store
Insert a cache barrier before a load or store that might be speculatively executed and that might have side effects
even if aborted.
-mr10k-cache-barrier=store
Insert a cache barrier before a store that might be speculatively executed and that might have side effects even if
aborted.
-mr10k-cache-barrier=none
Disable the insertion of cache barriers. This is the default setting.
-mflush-func=func
-mno-flush-func
Specifies the function to call to flush the I and D caches, or to not call any such function. If called, the function
must take the same arguments as the common "_flush_func()", that is, the address of the memory range for which the cache
is being flushed, the size of the memory range, and the number 3 (to flush both caches). The default depends on the
target GCC was configured for, but commonly is either _flush_func or __cpu_flush.
mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not guaranteed to
produce consistent results across releases. A zero cost redundantly selects the default, which is based on the -mtune
setting.
-mbranch-likely
-mno-branch-likely
Enable or disable use of Branch Likely instructions, regardless of the default for the selected architecture. By
default, Branch Likely instructions may be generated if they are supported by the selected architecture. An exception is
for the MIPS32 and MIPS64 architectures and processors that implement those architectures; for those, Branch Likely
instructions will not be generated by default because the MIPS32 and MIPS64 architectures specifically deprecate their
use.
-mfp-exceptions
-mno-fp-exceptions
Specifies whether FP exceptions are enabled. This affects how we schedule FP instructions for some processors. The
default is that FP exceptions are enabled.
For instance, on the SB-1, if FP exceptions are disabled, and we are emitting 64-bit code, then we can use both FP pipes.
Otherwise, we can only use one FP pipe.
-mvr4130-align
-mno-vr4130-align
The VR4130 pipeline is two-way superscalar, but can only issue two instructions together if the first one is 8-byte
aligned. When this option is enabled, GCC will align pairs of instructions that it thinks should execute in parallel.
This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at the expense of
making it bigger. It is enabled by default at optimization level -O3.
-msynci
-mno-synci
Enable (disable) generation of "synci" instructions on architectures that support it. The "synci" instructions (if
enabled) will be generated when "__builtin___clear_cache()" is compiled.
This option defaults to "-mno-synci", but the default can be overridden by configuring with "--with-synci".
When compiling code for single processor systems, it is generally safe to use "synci". However, on many multi-core (SMP)
systems, it will not invalidate the instruction caches on all cores and may lead to undefined behavior.
-mrelax-pic-calls
-mno-relax-pic-calls
Try to turn PIC calls that are normally dispatched via register $25 into direct calls. This is only possible if the
linker can resolve the destination at link-time and if the destination is within range for a direct call.
-mrelax-pic-calls is the default if GCC was configured to use an assembler and a linker that supports the ".reloc"
assembly directive and "-mexplicit-relocs" is in effect. With "-mno-explicit-relocs", this optimization can be performed
by the assembler and the linker alone without help from the compiler.
-mmcount-ra-address
-mno-mcount-ra-address
Emit (do not emit) code that allows "_mcount" to modify the calling function's return address. When enabled, this option
extends the usual "_mcount" interface with a new ra-address parameter, which has type "intptr_t *" and is passed in
register $12. "_mcount" can then modify the return address by doing both of the following:
· Returning the new address in register $31.
· Storing the new address in "*ra-address", if ra-address is nonnull.
The default is -mno-mcount-ra-address.
MMIX Options
These options are defined for the MMIX:
-mlibfuncs
-mno-libfuncs
Specify that intrinsic library functions are being compiled, passing all values in registers, no matter the size.
-mepsilon
-mno-epsilon
Generate floating-point comparison instructions that compare with respect to the "rE" epsilon register.
-mabi=mmixware
-mabi=gnu
Generate code that passes function parameters and return values that (in the called function) are seen as registers $0
and up, as opposed to the GNU ABI which uses global registers $231 and up.
-mzero-extend
-mno-zero-extend
When reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load instructions by
default, rather than sign-extending ones.
-mknuthdiv
-mno-knuthdiv
Make the result of a division yielding a remainder have the same sign as the divisor. With the default, -mno-knuthdiv,
the sign of the remainder follows the sign of the dividend. Both methods are arithmetically valid, the latter being
almost exclusively used.
-mtoplevel-symbols
-mno-toplevel-symbols
Prepend (do not prepend) a : to all global symbols, so the assembly code can be used with the "PREFIX" assembly
directive.
-melf
Generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator.
-mbranch-predict
-mno-branch-predict
Use (do not use) the probable-branch instructions, when static branch prediction indicates a probable branch.
-mbase-addresses
-mno-base-addresses
Generate (do not generate) code that uses base addresses. Using a base address automatically generates a request
(handled by the assembler and the linker) for a constant to be set up in a global register. The register is used for one
or more base address requests within the range 0 to 255 from the value held in the register. The generally leads to
short and fast code, but the number of different data items that can be addressed is limited. This means that a program
that uses lots of static data may require -mno-base-addresses.
-msingle-exit
-mno-single-exit
Force (do not force) generated code to have a single exit point in each function.
MN10300 Options
These -m options are defined for Matsushita MN10300 architectures:
-mmult-bug
Generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.
-mno-mult-bug
Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.
-mam33
Generate code using features specific to the AM33 processor.
-mno-am33
Do not generate code using features specific to the AM33 processor. This is the default.
-mam33-2
Generate code using features specific to the AM33/2.0 processor.
-mam34
Generate code using features specific to the AM34 processor.
-mtune=cpu-type
Use the timing characteristics of the indicated CPU type when scheduling instructions. This does not change the targeted
processor type. The CPU type must be one of mn10300, am33, am33-2 or am34.
-mreturn-pointer-on-d0
When generating a function that returns a pointer, return the pointer in both "a0" and "d0". Otherwise, the pointer is
returned only in a0, and attempts to call such functions without a prototype would result in errors. Note that this
option is on by default; use -mno-return-pointer-on-d0 to disable it.
-mno-crt0
Do not link in the C run-time initialization object file.
-mrelax
Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and absolute
memory addresses. This option only has an effect when used on the command line for the final link step.
This option makes symbolic debugging impossible.
-mliw
Allow the compiler to generate Long Instruction Word instructions if the target is the AM33 or later. This is the
default. This option defines the preprocessor macro __LIW__.
-mnoliw
Do not allow the compiler to generate Long Instruction Word instructions. This option defines the preprocessor macro
__NO_LIW__.
-msetlb
Allow the compiler to generate the SETLB and Lcc instructions if the target is the AM33 or later. This is the default.
This option defines the preprocessor macro __SETLB__.
-mnosetlb
Do not allow the compiler to generate SETLB or Lcc instructions. This option defines the preprocessor macro
__NO_SETLB__.
PDP-11 Options
These options are defined for the PDP-11:
-mfpu
Use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported.)
-msoft-float
Do not use hardware floating point.
-mac0
Return floating-point results in ac0 (fr0 in Unix assembler syntax).
-mno-ac0
Return floating-point results in memory. This is the default.
-m40
Generate code for a PDP-11/40.
-m45
Generate code for a PDP-11/45. This is the default.
-m10
Generate code for a PDP-11/10.
-mbcopy-builtin
Use inline "movmemhi" patterns for copying memory. This is the default.
-mbcopy
Do not use inline "movmemhi" patterns for copying memory.
-mint16
-mno-int32
Use 16-bit "int". This is the default.
-mint32
-mno-int16
Use 32-bit "int".
-mfloat64
-mno-float32
Use 64-bit "float". This is the default.
-mfloat32
-mno-float64
Use 32-bit "float".
-mabshi
Use "abshi2" pattern. This is the default.
-mno-abshi
Do not use "abshi2" pattern.
-mbranch-expensive
Pretend that branches are expensive. This is for experimenting with code generation only.
-mbranch-cheap
Do not pretend that branches are expensive. This is the default.
-munix-asm
Use Unix assembler syntax. This is the default when configured for pdp11-*-bsd.
-mdec-asm
Use DEC assembler syntax. This is the default when configured for any PDP-11 target other than pdp11-*-bsd.
picoChip Options
These -m options are defined for picoChip implementations:
-mae=ae_type
Set the instruction set, register set, and instruction scheduling parameters for array element type ae_type. Supported
values for ae_type are ANY, MUL, and MAC.
-mae=ANY selects a completely generic AE type. Code generated with this option will run on any of the other AE types.
The code will not be as efficient as it would be if compiled for a specific AE type, and some types of operation (e.g.,
multiplication) will not work properly on all types of AE.
-mae=MUL selects a MUL AE type. This is the most useful AE type for compiled code, and is the default.
-mae=MAC selects a DSP-style MAC AE. Code compiled with this option may suffer from poor performance of byte (char)
manipulation, since the DSP AE does not provide hardware support for byte load/stores.
-msymbol-as-address
Enable the compiler to directly use a symbol name as an address in a load/store instruction, without first loading it
into a register. Typically, the use of this option will generate larger programs, which run faster than when the option
isn't used. However, the results vary from program to program, so it is left as a user option, rather than being
permanently enabled.
-mno-inefficient-warnings
Disables warnings about the generation of inefficient code. These warnings can be generated, for example, when compiling
code that performs byte-level memory operations on the MAC AE type. The MAC AE has no hardware support for byte-level
memory operations, so all byte load/stores must be synthesized from word load/store operations. This is inefficient and
a warning will be generated indicating to the programmer that they should rewrite the code to avoid byte operations, or
to target an AE type that has the necessary hardware support. This option enables the warning to be turned off.
PowerPC Options
These are listed under
RL78 Options
-msim
Links in additional target libraries to support operation within a simulator.
-mmul=none
-mmul=g13
-mmul=rl78
Specifies the type of hardware multiplication support to be used. The default is "none", which uses software
multiplication functions. The "g13" option is for the hardware multiply/divide peripheral only on the RL78/G13 targets.
The "rl78" option is for the standard hardware multiplication defined in the RL78 software manual.
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpower
-mno-power
-mpower2
-mno-power2
-mpowerpc
-mno-powerpc
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
-mpopcntd
-mno-popcntd
-mfprnd
-mno-fprnd
-mcmpb
-mno-cmpb
-mmfpgpr
-mno-mfpgpr
-mhard-dfp
-mno-hard-dfp
GCC supports two related instruction set architectures for the RS/6000 and PowerPC. The POWER instruction set are those
instructions supported by the rios chip set used in the original RS/6000 systems and the PowerPC instruction set is the
architecture of the Freescale MPC5xx, MPC6xx, MPC8xx microprocessors, and the IBM 4xx, 6xx, and follow-on
microprocessors.
Neither architecture is a subset of the other. However there is a large common subset of instructions supported by both.
An MQ register is included in processors supporting the POWER architecture.
You use these options to specify which instructions are available on the processor you are using. The default value of
these options is determined when configuring GCC. Specifying the -mcpu=cpu_type overrides the specification of these
options. We recommend you use the -mcpu=cpu_type option rather than the options listed above.
The -mpower option allows GCC to generate instructions that are found only in the POWER architecture and to use the MQ
register. Specifying -mpower2 implies -power and also allows GCC to generate instructions that are present in the POWER2
architecture but not the original POWER architecture.
The -mpowerpc option allows GCC to generate instructions that are found only in the 32-bit subset of the PowerPC
architecture. Specifying -mpowerpc-gpopt implies -mpowerpc and also allows GCC to use the optional PowerPC architecture
instructions in the General Purpose group, including floating-point square root. Specifying -mpowerpc-gfxopt implies
-mpowerpc and also allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including
floating-point select.
The -mmfcrf option allows GCC to generate the move from condition register field instruction implemented on the POWER4
processor and other processors that support the PowerPC V2.01 architecture. The -mpopcntb option allows GCC to generate
the popcount and double-precision FP reciprocal estimate instruction implemented on the POWER5 processor and other
processors that support the PowerPC V2.02 architecture. The -mpopcntd option allows GCC to generate the popcount
instruction implemented on the POWER7 processor and other processors that support the PowerPC V2.06 architecture. The
-mfprnd option allows GCC to generate the FP round to integer instructions implemented on the POWER5+ processor and other
processors that support the PowerPC V2.03 architecture. The -mcmpb option allows GCC to generate the compare bytes
instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture. The
-mmfpgpr option allows GCC to generate the FP move to/from general-purpose register instructions implemented on the
POWER6X processor and other processors that support the extended PowerPC V2.05 architecture. The -mhard-dfp option
allows GCC to generate the decimal floating-point instructions implemented on some POWER processors.
The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are found in the full PowerPC64
architecture and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64.
If you specify both -mno-power and -mno-powerpc, GCC will use only the instructions in the common subset of both
architectures plus some special AIX common-mode calls, and will not use the MQ register. Specifying both -mpower and
-mpowerpc permits GCC to use any instruction from either architecture and to allow use of the MQ register; specify this
for the Motorola MPC601.
-mnew-mnemonics
-mold-mnemonics
Select which mnemonics to use in the generated assembler code. With -mnew-mnemonics, GCC uses the assembler mnemonics
defined for the PowerPC architecture. With -mold-mnemonics it uses the assembler mnemonics defined for the POWER
architecture. Instructions defined in only one architecture have only one mnemonic; GCC uses that mnemonic irrespective
of which of these options is specified.
GCC defaults to the mnemonics appropriate for the architecture in use. Specifying -mcpu=cpu_type sometimes overrides the
value of these option. Unless you are building a cross-compiler, you should normally not specify either -mnew-mnemonics
or -mold-mnemonics, but should instead accept the default.
-mcpu=cpu_type
Set architecture type, register usage, choice of mnemonics, and instruction scheduling parameters for machine type
cpu_type. Supported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505, 601, 602,
603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc,
e500mc64, ec603e, G3, G4, G5, titan, power, power2, power3, power4, power5, power5+, power6, power6x, power7, common,
powerpc, powerpc64, rios, rios1, rios2, rsc, and rs64.
-mcpu=common selects a completely generic processor. Code generated under this option will run on any POWER or PowerPC
processor. GCC will use only the instructions in the common subset of both architectures, and will not use the MQ
register. GCC assumes a generic processor model for scheduling purposes.
-mcpu=power, -mcpu=power2, -mcpu=powerpc, and -mcpu=powerpc64 specify generic POWER, POWER2, pure 32-bit PowerPC (i.e.,
not MPC601), and 64-bit PowerPC architecture machine types, with an appropriate, generic processor model assumed for
scheduling purposes.
The other options specify a specific processor. Code generated under those options will run best on that processor, and
may not run at all on others.
The -mcpu options automatically enable or disable the following options:
-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple -mnew-mnemonics -mpopcntb -mpopcntd -mpower -mpower2
-mpowerpc64 -mpowerpc-gpopt -mpowerpc-gfxopt -msingle-float -mdouble-float -msimple-fpu -mstring -mmulhw -mdlmzb
-mmfpgpr -mvsx
The particular options set for any particular CPU will vary between compiler versions, depending on what setting seems to
produce optimal code for that CPU; it doesn't necessarily reflect the actual hardware's capabilities. If you wish to set
an individual option to a particular value, you may specify it after the -mcpu option, like -mcpu=970 -mno-altivec.
On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at present because AIX does
not have full support for these options. You may still enable or disable them individually if you're sure it'll work in
your environment.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the architecture type, register
usage, or choice of mnemonics, as -mcpu=cpu_type would. The same values for cpu_type are used for -mtune as for -mcpu.
If both are specified, the code generated will use the architecture, registers, and mnemonics set by -mcpu, but the
scheduling parameters set by -mtune.
-mcmodel=small
Generate PowerPC64 code for the small model: The TOC is limited to 64k.
-mcmodel=medium
Generate PowerPC64 code for the medium model: The TOC and other static data may be up to a total of 4G in size.
-mcmodel=large
Generate PowerPC64 code for the large model: The TOC may be up to 4G in size. Other data and code is only limited by the
64-bit address space.
-maltivec
-mno-altivec
Generate code that uses (does not use) AltiVec instructions, and also enable the use of built-in functions that allow
more direct access to the AltiVec instruction set. You may also need to set -mabi=altivec to adjust the current ABI with
AltiVec ABI enhancements.
-mvrsave
-mno-vrsave
Generate VRSAVE instructions when generating AltiVec code.
-mgen-cell-microcode
Generate Cell microcode instructions
-mwarn-cell-microcode
Warning when a Cell microcode instruction is going to emitted. An example of a Cell microcode instruction is a variable
shift.
-msecure-plt
Generate code that allows ld and ld.so to build executables and shared libraries with non-exec .plt and .got sections.
This is a PowerPC 32-bit SYSV ABI option.
-mbss-plt
Generate code that uses a BSS .plt section that ld.so fills in, and requires .plt and .got sections that are both
writable and executable. This is a PowerPC 32-bit SYSV ABI option.
-misel
-mno-isel
This switch enables or disables the generation of ISEL instructions.
-misel=yes/no
This switch has been deprecated. Use -misel and -mno-isel instead.
-mspe
-mno-spe
This switch enables or disables the generation of SPE simd instructions.
-mpaired
-mno-paired
This switch enables or disables the generation of PAIRED simd instructions.
-mspe=yes/no
This option has been deprecated. Use -mspe and -mno-spe instead.
-mvsx
-mno-vsx
Generate code that uses (does not use) vector/scalar (VSX) instructions, and also enable the use of built-in functions
that allow more direct access to the VSX instruction set.
-mfloat-gprs=yes/single/double/no
-mfloat-gprs
This switch enables or disables the generation of floating-point operations on the general-purpose registers for
architectures that support it.
The argument yes or single enables the use of single-precision floating-point operations.
The argument double enables the use of single and double-precision floating-point operations.
The argument no disables floating-point operations on the general-purpose registers.
This option is currently only available on the MPC854x.
-m32
-m64
Generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux). The 32-bit environment
sets int, long and pointer to 32 bits and generates code that runs on any PowerPC variant. The 64-bit environment sets
int to 32 bits and long and pointer to 64 bits, and generates code for PowerPC64, as for -mpowerpc64.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created for every executable file. The -mfull-toc option is
selected by default. In that case, GCC will allocate at least one TOC entry for each unique non-automatic variable
reference in your program. GCC will also place floating-point constants in the TOC. However, only 16,384 entries are
available in the TOC.
If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount
of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting
floating-point constants in the TOC and -mno-sum-in-toc forces GCC to generate code to calculate the sum of an address
and a constant at run time instead of putting that sum into the TOC. You may specify one or both of these options. Each
causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead.
This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC will produce code
that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that
contain less frequently executed code.
-maix64
-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit "long" type, and the infrastructure needed to
support them. Specifying -maix64 implies -mpowerpc64 and -mpowerpc, while -maix32 disables the 64-bit ABI and implies
-mno-powerpc64. GCC defaults to -maix32.
-mxl-compat
-mno-xl-compat
Produce code that conforms more closely to IBM XL compiler semantics when using AIX-compatible ABI. Pass floating-point
arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. Do not
assume that most significant double in 128-bit long double value is properly rounded when comparing values and converting
to double. Use XL symbol names for long double support routines.
The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a
function that takes the address of its arguments with fewer arguments than declared. IBM XL compilers access floating-
point arguments that do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because
always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by
default and only is necessary when calling subroutines compiled by IBM XL compilers without optimization.
-mpe
Support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message passing with special
startup code to enable the application to run. The system must have PE installed in the standard location
(/usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs= option to specify the appropriate directory
location. The Parallel Environment does not support threads, so the -mpe option and the -pthread option are
incompatible.
-malign-natural
-malign-power
On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural overrides the ABI-defined alignment of
larger types, such as floating-point doubles, on their natural size-based boundary. The option -malign-power instructs
GCC to follow the ABI-specified alignment rules. GCC defaults to the standard alignment defined in the ABI.
On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported.
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point register set. Software floating-point emulation is provided if
you use the -msoft-float option, and pass the option to GCC when linking.
-msingle-float
-mdouble-float
Generate code for single- or double-precision floating-point operations. -mdouble-float implies -msingle-float.
-msimple-fpu
Do not generate sqrt and div instructions for hardware floating-point unit.
-mfpu
Specify type of floating-point unit. Valid values are sp_lite (equivalent to -msingle-float -msimple-fpu), dp_lite
(equivalent to -mdouble-float -msimple-fpu), sp_full (equivalent to -msingle-float), and dp_full (equivalent to
-mdouble-float).
-mxilinx-fpu
Perform optimizations for the floating-point unit on Xilinx PPC 405/440.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions.
These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use
-mmultiple on little-endian PowerPC systems, since those instructions do not work when the processor is in little-endian
mode. The exceptions are PPC740 and PPC750 which permit these instructions in little-endian mode.
-mstring
-mno-string
Generate code that uses (does not use) the load string instructions and the store string word instructions to save
multiple registers and do small block moves. These instructions are generated by default on POWER systems, and not
generated on PowerPC systems. Do not use -mstring on little-endian PowerPC systems, since those instructions do not work
when the processor is in little-endian mode. The exceptions are PPC740 and PPC750 which permit these instructions in
little-endian mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store instructions that update the base register to the address of the
calculated memory location. These instructions are generated by default. If you use -mno-update, there is a small
window between the time that the stack pointer is updated and the address of the previous frame is stored, which means
code that walks the stack frame across interrupts or signals may get corrupted data.
-mavoid-indexed-addresses
-mno-avoid-indexed-addresses
Generate code that tries to avoid (not avoid) the use of indexed load or store instructions. These instructions can incur
a performance penalty on Power6 processors in certain situations, such as when stepping through large arrays that cross a
16M boundary. This option is enabled by default when targetting Power6 and disabled otherwise.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are
generated by default if hardware floating point is used. The machine-dependent -mfused-madd option is now mapped to the
machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off.
-mmulhw
-mno-mulhw
Generate code that uses (does not use) the half-word multiply and multiply-accumulate instructions on the IBM 405, 440,
464 and 476 processors. These instructions are generated by default when targetting those processors.
-mdlmzb
-mno-dlmzb
Generate code that uses (does not use) the string-search dlmzb instruction on the IBM 405, 440, 464 and 476 processors.
This instruction is generated by default when targetting those processors.
-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit-fields to be aligned
to the base type of the bit-field.
For example, by default a structure containing nothing but 8 "unsigned" bit-fields of length 1 is aligned to a 4-byte
boundary and has a size of 4 bytes. By using -mno-bit-align, the structure is aligned to a 1-byte boundary and is 1 byte
in size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references will be handled by the
system.
-mrelocatable
-mno-relocatable
Generate code that allows (does not allow) a static executable to be relocated to a different address at run time. A
simple embedded PowerPC system loader should relocate the entire contents of ".got2" and 4-byte locations listed in the
".fixup" section, a table of 32-bit addresses generated by this option. For this to work, all objects linked together
must be compiled with -mrelocatable or -mrelocatable-lib. -mrelocatable code aligns the stack to an 8-byte boundary.
-mrelocatable-lib
-mno-relocatable-lib
Like -mrelocatable, -mrelocatable-lib generates a ".fixup" section to allow static executables to be relocated at run
time, but -mrelocatable-lib does not use the smaller stack alignment of -mrelocatable. Objects compiled with
-mrelocatable-lib may be linked with objects compiled with any combination of the -mrelocatable options.
-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a global area
pointing to the addresses used in the program.
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the processor in little-endian mode. The -mlittle-endian
option is the same as -mlittle.
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the processor in big-endian mode. The -mbig-endian option is
the same as -mbig.
-mdynamic-no-pic
On Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its external references are
relocatable. The resulting code is suitable for applications, but not shared libraries.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The
runtime system is responsible for initializing this register with an appropriate value before execution begins.
-mprioritize-restricted-insns=priority
This option controls the priority that is assigned to dispatch-slot restricted instructions during the second scheduling
pass. The argument priority takes the value 0/1/2 to assign no/highest/second-highest priority to dispatch slot
restricted instructions.
-msched-costly-dep=dependence_type
This option controls which dependences are considered costly by the target during instruction scheduling. The argument
dependence_type takes one of the following values: no: no dependence is costly, all: all dependences are costly,
true_store_to_load: a true dependence from store to load is costly, store_to_load: any dependence from store to load is
costly, number: any dependence for which latency >= number is costly.
-minsert-sched-nops=scheme
This option controls which nop insertion scheme will be used during the second scheduling pass. The argument scheme
takes one of the following values: no: Don't insert nops. pad: Pad with nops any dispatch group that has vacant issue
slots, according to the scheduler's grouping. regroup_exact: Insert nops to force costly dependent insns into separate
groups. Insert exactly as many nops as needed to force an insn to a new group, according to the estimated processor
grouping. number: Insert nops to force costly dependent insns into separate groups. Insert number nops to force an insn
to a new group.
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using calling conventions that adheres to the March 1995 draft of
the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC
using powerpc-*-eabiaix.
-mcall-sysv-eabi
-mcall-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-aixdesc
On System V.4 and embedded PowerPC systems compile code for the AIX operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.
-mcall-freebsd
On System V.4 and embedded PowerPC systems compile code for the FreeBSD operating system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system.
-mcall-openbsd
On System V.4 and embedded PowerPC systems compile code for the OpenBSD operating system.
-maix-struct-return
Return all structures in memory (as specified by the AIX ABI).
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI).
-mabi=abi-type
Extend the current ABI with a particular extension, or remove such extension. Valid values are altivec, no-altivec, spe,
no-spe, ibmlongdouble, ieeelongdouble.
-mabi=spe
Extend the current ABI with SPE ABI extensions. This does not change the default ABI, instead it adds the SPE ABI
extensions to the current ABI.
-mabi=no-spe
Disable Booke SPE ABI extensions for the current ABI.
-mabi=ibmlongdouble
Change the current ABI to use IBM extended-precision long double. This is a PowerPC 32-bit SYSV ABI option.
-mabi=ieeelongdouble
Change the current ABI to use IEEE extended-precision long double. This is a PowerPC 32-bit Linux ABI option.
-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that all calls to variable argument functions are properly prototyped.
Otherwise, the compiler must insert an instruction before every non prototyped call to set or clear bit 6 of the
condition code register (CR) to indicate whether floating-point values were passed in the floating-point registers in
case the function takes variable arguments. With -mprototype, only calls to prototyped variable argument functions will
set or clear the bit.
-msim
On embedded PowerPC systems, assume that the startup module is called sim-crt0.o and that the standard C libraries are
libsim.a and libc.a. This is the default for powerpc-*-eabisim configurations.
-mmvme
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libmvme.a
and libc.a.
-mads
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libads.a
and libc.a.
-myellowknife
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libyk.a and
libc.a.
-mvxworks
On System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system.
-memb
On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags header to indicate that eabi extended relocations are
used.
-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded Applications Binary Interface (eabi) which
is a set of modifications to the System V.4 specifications. Selecting -meabi means that the stack is aligned to an
8-byte boundary, a function "__eabi" is called to from "main" to set up the eabi environment, and the -msdata option can
use both "r2" and "r13" to point to two separate small data areas. Selecting -mno-eabi means that the stack is aligned
to a 16-byte boundary, do not call an initialization function from "main", and the -msdata option will only use "r13" to
point to a single small data area. The -meabi option is on by default if you configured GCC using one of the
powerpc*-*-eabi* options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized "const" global and static data in the .sdata2 section,
which is pointed to by register "r2". Put small initialized non-"const" global and static data in the .sdata section,
which is pointed to by register "r13". Put small uninitialized global and static data in the .sbss section, which is
adjacent to the .sdata section. The -msdata=eabi option is incompatible with the -mrelocatable option. The -msdata=eabi
option also sets the -memb option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section, which is pointed to
by register "r13". Put small uninitialized global and static data in the .sbss section, which is adjacent to the .sdata
section. The -msdata=sysv option is incompatible with the -mrelocatable option.
-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi, otherwise compile
code the same as -msdata=sysv.
-msdata=data
On System V.4 and embedded PowerPC systems, put small global data in the .sdata section. Put small uninitialized global
data in the .sbss section. Do not use register "r13" to address small data however. This is the default behavior unless
other -msdata options are used.
-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and static data in the .data section, and all uninitialized data
in the .bss section.
-mblock-move-inline-limit=num
Inline all block moves (such as calls to "memcpy" or structure copies) less than or equal to num bytes. The minimum
value for num is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. The default value is target-specific.
-G num
On embedded PowerPC systems, put global and static items less than or equal to num bytes into the small data or bss
sections instead of the normal data or bss section. By default, num is 8. The -G num switch is also passed to the
linker. All modules should be compiled with the same -G num value.
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit register names in the assembly language output using symbolic
forms.
-mlongcall
-mno-longcall
By default assume that all calls are far away so that a longer more expensive calling sequence is required. This is
required for calls further than 32 megabytes (33,554,432 bytes) from the current location. A short call will be
generated if the compiler knows the call cannot be that far away. This setting can be overridden by the "shortcall"
function attribute, or by "#pragma longcall(0)".
Some linkers are capable of detecting out-of-range calls and generating glue code on the fly. On these systems, long
calls are unnecessary and generate slower code. As of this writing, the AIX linker can do this, as can the GNU linker
for PowerPC/64. It is planned to add this feature to the GNU linker for 32-bit PowerPC systems as well.
On Darwin/PPC systems, "#pragma longcall" will generate "jbsr callee, L42", plus a "branch island" (glue code). The two
target addresses represent the callee and the "branch island". The Darwin/PPC linker will prefer the first address and
generate a "bl callee" if the PPC "bl" instruction will reach the callee directly; otherwise, the linker will generate
"bl L42" to call the "branch island". The "branch island" is appended to the body of the calling function; it computes
the full 32-bit address of the callee and jumps to it.
On Mach-O (Darwin) systems, this option directs the compiler emit to the glue for every direct call, and the Darwin
linker decides whether to use or discard it.
In the future, we may cause GCC to ignore all longcall specifications when the linker is known to generate glue.
-mtls-markers
-mno-tls-markers
Mark (do not mark) calls to "__tls_get_addr" with a relocation specifying the function argument. The relocation allows
ld to reliably associate function call with argument setup instructions for TLS optimization, which in turn allows gcc to
better schedule the sequence.
-pthread
Adds support for multithreading with the pthreads library. This option sets flags for both the preprocessor and linker.
-mrecip
-mno-recip
This option will enable GCC to use the reciprocal estimate and reciprocal square root estimate instructions with
additional Newton-Raphson steps to increase precision instead of doing a divide or square root and divide for floating-
point arguments. You should use the -ffast-math option when using -mrecip (or at least -funsafe-math-optimizations,
-finite-math-only, -freciprocal-math and -fno-trapping-math). Note that while the throughput of the sequence is
generally higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by
up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994) for reciprocal square roots.
-mrecip=opt
This option allows to control which reciprocal estimate instructions may be used. opt is a comma separated list of
options, which may be preceded by a "!" to invert the option: "all": enable all estimate instructions, "default": enable
the default instructions, equivalent to -mrecip, "none": disable all estimate instructions, equivalent to -mno-recip;
"div": enable the reciprocal approximation instructions for both single and double precision; "divf": enable the single-
precision reciprocal approximation instructions; "divd": enable the double-precision reciprocal approximation
instructions; "rsqrt": enable the reciprocal square root approximation instructions for both single and double precision;
"rsqrtf": enable the single-precision reciprocal square root approximation instructions; "rsqrtd": enable the double-
precision reciprocal square root approximation instructions;
So for example, -mrecip=all,!rsqrtd would enable the all of the reciprocal estimate instructions, except for the
"FRSQRTE", "XSRSQRTEDP", and "XVRSQRTEDP" instructions which handle the double-precision reciprocal square root
calculations.
-mrecip-precision
-mno-recip-precision
Assume (do not assume) that the reciprocal estimate instructions provide higher-precision estimates than is mandated by
the PowerPC ABI. Selecting -mcpu=power6 or -mcpu=power7 automatically selects -mrecip-precision. The double-precision
square root estimate instructions are not generated by default on low-precision machines, since they do not provide an
estimate that converges after three steps.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an external library. The only type supported at present
is "mass", which specifies to use IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics
using external libraries. GCC will currently emit calls to "acosd2", "acosf4", "acoshd2", "acoshf4", "asind2", "asinf4",
"asinhd2", "asinhf4", "atan2d2", "atan2f4", "atand2", "atanf4", "atanhd2", "atanhf4", "cbrtd2", "cbrtf4", "cosd2",
"cosf4", "coshd2", "coshf4", "erfcd2", "erfcf4", "erfd2", "erff4", "exp2d2", "exp2f4", "expd2", "expf4", "expm1d2",
"expm1f4", "hypotd2", "hypotf4", "lgammad2", "lgammaf4", "log10d2", "log10f4", "log1pd2", "log1pf4", "log2d2", "log2f4",
"logd2", "logf4", "powd2", "powf4", "sind2", "sinf4", "sinhd2", "sinhf4", "sqrtd2", "sqrtf4", "tand2", "tanf4", "tanhd2",
and "tanhf4" when generating code for power7. Both -ftree-vectorize and -funsafe-math-optimizations have to be enabled.
The MASS libraries will have to be specified at link time.
-mfriz
-mno-friz
Generate (do not generate) the "friz" instruction when the -funsafe-math-optimizations option is used to optimize
rounding of floating-point values to 64-bit integer and back to floating point. The "friz" instruction does not return
the same value if the floating-point number is too large to fit in an integer.
-mpointers-to-nested-functions
-mno-pointers-to-nested-functions
Generate (do not generate) code to load up the static chain register (r11) when calling through a pointer on AIX and
64-bit Linux systems where a function pointer points to a 3-word descriptor giving the function address, TOC value to be
loaded in register r2, and static chain value to be loaded in register r11. The -mpointers-to-nested-functions is on by
default. You will not be able to call through pointers to nested functions or pointers to functions compiled in other
languages that use the static chain if you use the -mno-pointers-to-nested-functions.
-msave-toc-indirect
-mno-save-toc-indirect
Generate (do not generate) code to save the TOC value in the reserved stack location in the function prologue if the
function calls through a pointer on AIX and 64-bit Linux systems. If the TOC value is not saved in the prologue, it is
saved just before the call through the pointer. The -mno-save-toc-indirect option is the default.
RX Options
These command-line options are defined for RX targets:
-m64bit-doubles
-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The default is
-m32bit-doubles. Note RX floating-point hardware only works on 32-bit values, which is why the default is
-m32bit-doubles.
-fpu
-nofpu
Enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The default is enabled for the RX600 series
and disabled for the RX200 series.
Floating-point instructions will only be generated for 32-bit floating-point values however, so if the -m64bit-doubles
option is in use then the FPU hardware will not be used for doubles.
Note If the -fpu option is enabled then -funsafe-math-optimizations is also enabled automatically. This is because the
RX FPU instructions are themselves unsafe.
-mcpu=name
Selects the type of RX CPU to be targeted. Currently three types are supported, the generic RX600 and RX200 series
hardware and the specific RX610 CPU. The default is RX600.
The only difference between RX600 and RX610 is that the RX610 does not support the "MVTIPL" instruction.
The RX200 series does not have a hardware floating-point unit and so -nofpu is enabled by default when this type is
selected.
-mbig-endian-data
-mlittle-endian-data
Store data (but not code) in the big-endian format. The default is -mlittle-endian-data, i.e. to store data in the
little-endian format.
-msmall-data-limit=N
Specifies the maximum size in bytes of global and static variables which can be placed into the small data area. Using
the small data area can lead to smaller and faster code, but the size of area is limited and it is up to the programmer
to ensure that the area does not overflow. Also when the small data area is used one of the RX's registers (usually
"r13") is reserved for use pointing to this area, so it is no longer available for use by the compiler. This could
result in slower and/or larger code if variables which once could have been held in the reserved register are now pushed
onto the stack.
Note, common variables (variables that have not been initialized) and constants are not placed into the small data area
as they are assigned to other sections in the output executable.
The default value is zero, which disables this feature. Note, this feature is not enabled by default with higher
optimization levels (-O2 etc) because of the potentially detrimental effects of reserving a register. It is up to the
programmer to experiment and discover whether this feature is of benefit to their program. See the description of the
-mpid option for a description of how the actual register to hold the small data area pointer is chosen.
-msim
-mno-sim
Use the simulator runtime. The default is to use the libgloss board specific runtime.
-mas100-syntax
-mno-as100-syntax
When generating assembler output use a syntax that is compatible with Renesas's AS100 assembler. This syntax can also be
handled by the GAS assembler but it has some restrictions so generating it is not the default option.
-mmax-constant-size=N
Specifies the maximum size, in bytes, of a constant that can be used as an operand in a RX instruction. Although the RX
instruction set does allow constants of up to 4 bytes in length to be used in instructions, a longer value equates to a
longer instruction. Thus in some circumstances it can be beneficial to restrict the size of constants that are used in
instructions. Constants that are too big are instead placed into a constant pool and referenced via register
indirection.
The value N can be between 0 and 4. A value of 0 (the default) or 4 means that constants of any size are allowed.
-mrelax
Enable linker relaxation. Linker relaxation is a process whereby the linker will attempt to reduce the size of a program
by finding shorter versions of various instructions. Disabled by default.
-mint-register=N
Specify the number of registers to reserve for fast interrupt handler functions. The value N can be between 0 and 4. A
value of 1 means that register "r13" will be reserved for the exclusive use of fast interrupt handlers. A value of 2
reserves "r13" and "r12". A value of 3 reserves "r13", "r12" and "r11", and a value of 4 reserves "r13" through "r10".
A value of 0, the default, does not reserve any registers.
-msave-acc-in-interrupts
Specifies that interrupt handler functions should preserve the accumulator register. This is only necessary if normal
code might use the accumulator register, for example because it performs 64-bit multiplications. The default is to
ignore the accumulator as this makes the interrupt handlers faster.
-mpid
-mno-pid
Enables the generation of position independent data. When enabled any access to constant data will done via an offset
from a base address held in a register. This allows the location of constant data to be determined at run time without
requiring the executable to be relocated, which is a benefit to embedded applications with tight memory constraints.
Data that can be modified is not affected by this option.
Note, using this feature reserves a register, usually "r13", for the constant data base address. This can result in
slower and/or larger code, especially in complicated functions.
The actual register chosen to hold the constant data base address depends upon whether the -msmall-data-limit and/or the
-mint-register command-line options are enabled. Starting with register "r13" and proceeding downwards, registers are
allocated first to satisfy the requirements of -mint-register, then -mpid and finally -msmall-data-limit. Thus it is
possible for the small data area register to be "r8" if both -mint-register=4 and -mpid are specified on the command
line.
By default this feature is not enabled. The default can be restored via the -mno-pid command-line option.
Note: The generic GCC command-line option -ffixed-reg has special significance to the RX port when used with the "interrupt"
function attribute. This attribute indicates a function intended to process fast interrupts. GCC will will ensure that it
only uses the registers "r10", "r11", "r12" and/or "r13" and only provided that the normal use of the corresponding registers
have been restricted via the -ffixed-reg or -mint-register command-line options.
S/390 and zSeries Options
These are the -m options defined for the S/390 and zSeries architecture.
-mhard-float
-msoft-float
Use (do not use) the hardware floating-point instructions and registers for floating-point operations. When -msoft-float
is specified, functions in libgcc.a will be used to perform floating-point operations. When -mhard-float is specified,
the compiler generates IEEE floating-point instructions. This is the default.
-mhard-dfp
-mno-hard-dfp
Use (do not use) the hardware decimal-floating-point instructions for decimal-floating-point operations. When
-mno-hard-dfp is specified, functions in libgcc.a will be used to perform decimal-floating-point operations. When
-mhard-dfp is specified, the compiler generates decimal-floating-point hardware instructions. This is the default for
-march=z9-ec or higher.
-mlong-double-64
-mlong-double-128
These switches control the size of "long double" type. A size of 64 bits makes the "long double" type equivalent to the
"double" type. This is the default.
-mbackchain
-mno-backchain
Store (do not store) the address of the caller's frame as backchain pointer into the callee's stack frame. A backchain
may be needed to allow debugging using tools that do not understand DWARF-2 call frame information. When
-mno-packed-stack is in effect, the backchain pointer is stored at the bottom of the stack frame; when -mpacked-stack is
in effect, the backchain is placed into the topmost word of the 96/160 byte register save area.
In general, code compiled with -mbackchain is call-compatible with code compiled with -mmo-backchain; however, use of the
backchain for debugging purposes usually requires that the whole binary is built with -mbackchain. Note that the
combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux kernel use
-msoft-float.
The default is to not maintain the backchain.
-mpacked-stack
-mno-packed-stack
Use (do not use) the packed stack layout. When -mno-packed-stack is specified, the compiler uses the all fields of the
96/160 byte register save area only for their default purpose; unused fields still take up stack space. When
-mpacked-stack is specified, register save slots are densely packed at the top of the register save area; unused space is
reused for other purposes, allowing for more efficient use of the available stack space. However, when -mbackchain is
also in effect, the topmost word of the save area is always used to store the backchain, and the return address register
is always saved two words below the backchain.
As long as the stack frame backchain is not used, code generated with -mpacked-stack is call-compatible with code
generated with -mno-packed-stack. Note that some non-FSF releases of GCC 2.95 for S/390 or zSeries generated code that
uses the stack frame backchain at run time, not just for debugging purposes. Such code is not call-compatible with code
compiled with -mpacked-stack. Also, note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not
supported. In order to build a linux kernel use -msoft-float.
The default is to not use the packed stack layout.
-msmall-exec
-mno-small-exec
Generate (or do not generate) code using the "bras" instruction to do subroutine calls. This only works reliably if the
total executable size does not exceed 64k. The default is to use the "basr" instruction instead, which does not have
this limitation.
-m64
-m31
When -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI. When -m64 is specified, generate code
compliant to the GNU/Linux for zSeries ABI. This allows GCC in particular to generate 64-bit instructions. For the s390
targets, the default is -m31, while the s390x targets default to -m64.
-mzarch
-mesa
When -mzarch is specified, generate code using the instructions available on z/Architecture. When -mesa is specified,
generate code using the instructions available on ESA/390. Note that -mesa is not possible with -m64. When generating
code compliant to the GNU/Linux for S/390 ABI, the default is -mesa. When generating code compliant to the GNU/Linux for
zSeries ABI, the default is -mzarch.
-mmvcle
-mno-mvcle
Generate (or do not generate) code using the "mvcle" instruction to perform block moves. When -mno-mvcle is specified,
use a "mvc" loop instead. This is the default unless optimizing for size.
-mdebug
-mno-debug
Print (or do not print) additional debug information when compiling. The default is to not print debug information.
-march=cpu-type
Generate code that will run on cpu-type, which is the name of a system representing a certain processor type. Possible
values for cpu-type are g5, g6, z900, z990, z9-109, z9-ec and z10. When generating code using the instructions available
on z/Architecture, the default is -march=z900. Otherwise, the default is -march=g5.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available
instructions. The list of cpu-type values is the same as for -march. The default is the value used for -march.
-mtpf-trace
-mno-tpf-trace
Generate code that adds (does not add) in TPF OS specific branches to trace routines in the operating system. This
option is off by default, even when compiling for the TPF OS.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are
generated by default if hardware floating point is used.
-mwarn-framesize=framesize
Emit a warning if the current function exceeds the given frame size. Because this is a compile-time check it doesn't
need to be a real problem when the program runs. It is intended to identify functions that most probably cause a stack
overflow. It is useful to be used in an environment with limited stack size e.g. the linux kernel.
-mwarn-dynamicstack
Emit a warning if the function calls alloca or uses dynamically sized arrays. This is generally a bad idea with a
limited stack size.
-mstack-guard=stack-guard
-mstack-size=stack-size
If these options are provided the s390 back end emits additional instructions in the function prologue which trigger a
trap if the stack size is stack-guard bytes above the stack-size (remember that the stack on s390 grows downward). If
the stack-guard option is omitted the smallest power of 2 larger than the frame size of the compiled function is chosen.
These options are intended to be used to help debugging stack overflow problems. The additionally emitted code causes
only little overhead and hence can also be used in production like systems without greater performance degradation. The
given values have to be exact powers of 2 and stack-size has to be greater than stack-guard without exceeding 64k. In
order to be efficient the extra code makes the assumption that the stack starts at an address aligned to the value given
by stack-size. The stack-guard option can only be used in conjunction with stack-size.
Score Options
These options are defined for Score implementations:
-meb
Compile code for big-endian mode. This is the default.
-mel
Compile code for little-endian mode.
-mnhwloop
Disable generate bcnz instruction.
-muls
Enable generate unaligned load and store instruction.
-mmac
Enable the use of multiply-accumulate instructions. Disabled by default.
-mscore5
Specify the SCORE5 as the target architecture.
-mscore5u
Specify the SCORE5U of the target architecture.
-mscore7
Specify the SCORE7 as the target architecture. This is the default.
-mscore7d
Specify the SCORE7D as the target architecture.
SH Options
These -m options are defined for the SH implementations:
-m1 Generate code for the SH1.
-m2 Generate code for the SH2.
-m2e
Generate code for the SH2e.
-m2a-nofpu
Generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is not used.
-m2a-single-only
Generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are used.
-m2a-single
Generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.
-m2a
Generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default.
-m3 Generate code for the SH3.
-m3e
Generate code for the SH3e.
-m4-nofpu
Generate code for the SH4 without a floating-point unit.
-m4-single-only
Generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
-m4-single
Generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
-m4 Generate code for the SH4.
-m4a-nofpu
Generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.
-m4a-single-only
Generate code for the SH4a, in such a way that no double-precision floating-point operations are used.
-m4a-single
Generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.
-m4a
Generate code for the SH4a.
-m4al
Same as -m4a-nofpu, except that it implicitly passes -dsp to the assembler. GCC doesn't generate any DSP instructions at
the moment.
-mb Compile code for the processor in big-endian mode.
-ml Compile code for the processor in little-endian mode.
-mdalign
Align doubles at 64-bit boundaries. Note that this changes the calling conventions, and thus some functions from the
standard C library will not work unless you recompile it first with -mdalign.
-mrelax
Shorten some address references at link time, when possible; uses the linker option -relax.
-mbigtable
Use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.
-mbitops
Enable the use of bit manipulation instructions on SH2A.
-mfmovd
Enable the use of the instruction "fmovd". Check -mdalign for alignment constraints.
-mhitachi
Comply with the calling conventions defined by Renesas.
-mrenesas
Comply with the calling conventions defined by Renesas.
-mno-renesas
Comply with the calling conventions defined for GCC before the Renesas conventions were available. This option is the
default for all targets of the SH toolchain.
-mnomacsave
Mark the "MAC" register as call-clobbered, even if -mhitachi is given.
-mieee
-mno-ieee
Control the IEEE compliance of floating-point comparisons, which affects the handling of cases where the result of a
comparison is unordered. By default -mieee is implicitly enabled. If -ffinite-math-only is enabled -mno-ieee is
implicitly set, which results in faster floating-point greater-equal and less-equal comparisons. The implcit settings
can be overridden by specifying either -mieee or -mno-ieee.
-minline-ic_invalidate
Inline code to invalidate instruction cache entries after setting up nested function trampolines. This option has no
effect if -musermode is in effect and the selected code generation option (e.g. -m4) does not allow the use of the icbi
instruction. If the selected code generation option does not allow the use of the icbi instruction, and -musermode is
not in effect, the inlined code will manipulate the instruction cache address array directly with an associative write.
This not only requires privileged mode, but it will also fail if the cache line had been mapped via the TLB and has
become unmapped.
-misize
Dump instruction size and location in the assembly code.
-mpadstruct
This option is deprecated. It pads structures to multiple of 4 bytes, which is incompatible with the SH ABI.
-msoft-atomic
Generate GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions. The generated atomic
sequences require support from the interrupt / exception handling code of the system and are only suitable for single-
core systems. They will not perform correctly on multi-core systems. This option is enabled by default when the target
is "sh-*-linux*". For details on the atomic built-in functions see __atomic Builtins.
-mspace
Optimize for space instead of speed. Implied by -Os.
-mprefergot
When generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure
Linkage Table.
-musermode
Don't generate privileged mode only code; implies -mno-inline-ic_invalidate if the inlined code would not work in user
mode. This is the default when the target is "sh-*-linux*".
-multcost=number
Set the cost to assume for a multiply insn.
-mdiv=strategy
Set the division strategy to use for SHmedia code. strategy must be one of: call, call2, fp, inv, inv:minlat, inv20u,
inv20l, inv:call, inv:call2, inv:fp . "fp" performs the operation in floating point. This has a very high latency, but
needs only a few instructions, so it might be a good choice if your code has enough easily-exploitable ILP to allow the
compiler to schedule the floating-point instructions together with other instructions. Division by zero causes a
floating-point exception. "inv" uses integer operations to calculate the inverse of the divisor, and then multiplies the
dividend with the inverse. This strategy allows cse and hoisting of the inverse calculation. Division by zero
calculates an unspecified result, but does not trap. "inv:minlat" is a variant of "inv" where if no cse / hoisting
opportunities have been found, or if the entire operation has been hoisted to the same place, the last stages of the
inverse calculation are intertwined with the final multiply to reduce the overall latency, at the expense of using a few
more instructions, and thus offering fewer scheduling opportunities with other code. "call" calls a library function
that usually implements the inv:minlat strategy. This gives high code density for m5-*media-nofpu compilations. "call2"
uses a different entry point of the same library function, where it assumes that a pointer to a lookup table has already
been set up, which exposes the pointer load to cse / code hoisting optimizations. "inv:call", "inv:call2" and "inv:fp"
all use the "inv" algorithm for initial code generation, but if the code stays unoptimized, revert to the "call",
"call2", or "fp" strategies, respectively. Note that the potentially-trapping side effect of division by zero is carried
by a separate instruction, so it is possible that all the integer instructions are hoisted out, but the marker for the
side effect stays where it is. A recombination to fp operations or a call is not possible in that case. "inv20u" and
"inv20l" are variants of the "inv:minlat" strategy. In the case that the inverse calculation was nor separated from the
multiply, they speed up division where the dividend fits into 20 bits (plus sign where applicable), by inserting a test
to skip a number of operations in this case; this test slows down the case of larger dividends. inv20u assumes the case
of a such a small dividend to be unlikely, and inv20l assumes it to be likely.
-maccumulate-outgoing-args
Reserve space once for outgoing arguments in the function prologue rather than around each call. Generally beneficial
for performance and size. Also needed for unwinding to avoid changing the stack frame around conditional code.
-mdivsi3_libfunc=name
Set the name of the library function used for 32-bit signed division to name. This only affect the name used in the call
and inv:call division strategies, and the compiler will still expect the same sets of input/output/clobbered registers as
if this option was not present.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator
can not use. This is useful when compiling kernel code. A register range is specified as two registers separated by a
dash. Multiple register ranges can be specified separated by a comma.
-madjust-unroll
Throttle unrolling to avoid thrashing target registers. This option only has an effect if the gcc code base supports the
TARGET_ADJUST_UNROLL_MAX target hook.
-mindexed-addressing
Enable the use of the indexed addressing mode for SHmedia32/SHcompact. This is only safe if the hardware and/or OS
implement 32-bit wrap-around semantics for the indexed addressing mode. The architecture allows the implementation of
processors with 64-bit MMU, which the OS could use to get 32-bit addressing, but since no current hardware implementation
supports this or any other way to make the indexed addressing mode safe to use in the 32-bit ABI, the default is
-mno-indexed-addressing.
-mgettrcost=number
Set the cost assumed for the gettr instruction to number. The default is 2 if -mpt-fixed is in effect, 100 otherwise.
-mpt-fixed
Assume pt* instructions won't trap. This will generally generate better scheduled code, but is unsafe on current
hardware. The current architecture definition says that ptabs and ptrel trap when the target anded with 3 is 3. This
has the unintentional effect of making it unsafe to schedule ptabs / ptrel before a branch, or hoist it out of a loop.
For example, __do_global_ctors, a part of libgcc that runs constructors at program startup, calls functions in a list
which is delimited by -1. With the -mpt-fixed option, the ptabs will be done before testing against -1. That means that
all the constructors will be run a bit quicker, but when the loop comes to the end of the list, the program crashes
because ptabs loads -1 into a target register. Since this option is unsafe for any hardware implementing the current
architecture specification, the default is -mno-pt-fixed. Unless the user specifies a specific cost with -mgettrcost,
-mno-pt-fixed also implies -mgettrcost=100; this deters register allocation using target registers for storing ordinary
integers.
-minvalid-symbols
Assume symbols might be invalid. Ordinary function symbols generated by the compiler will always be valid to load with
movi/shori/ptabs or movi/shori/ptrel, but with assembler and/or linker tricks it is possible to generate symbols that
will cause ptabs / ptrel to trap. This option is only meaningful when -mno-pt-fixed is in effect. It will then prevent
cross-basic-block cse, hoisting and most scheduling of symbol loads. The default is -mno-invalid-symbols.
-mbranch-cost=num
Assume num to be the cost for a branch instruction. Higher numbers will make the compiler try to generate more branch-
free code if possible. If not specified the value is selected depending on the processor type that is being compiled
for.
-mcbranchdi
Enable the "cbranchdi4" instruction pattern.
-mcmpeqdi
Emit the "cmpeqdi_t" instruction pattern even when -mcbranchdi is in effect.
-mfused-madd
Allow the usage of the "fmac" instruction (floating-point multiply-accumulate) if the processor type supports it.
Enabling this option might generate code that produces different numeric floating-point results compared to strict IEEE
754 arithmetic.
-mpretend-cmove
Prefer zero-displacement conditional branches for conditional move instruction patterns. This can result in faster code
on the SH4 processor.
Solaris 2 Options
These -m options are supported on Solaris 2:
-mimpure-text
-mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when linking a shared
object. Using this option, you can link position-dependent code into a shared object.
-mimpure-text suppresses the "relocations remain against allocatable but non-writable sections" linker error message.
However, the necessary relocations will trigger copy-on-write, and the shared object is not actually shared across
processes. Instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC.
These switches are supported in addition to the above on Solaris 2:
-pthreads
Add support for multithreading using the POSIX threads library. This option sets flags for both the preprocessor and
linker. This option does not affect the thread safety of object code produced by the compiler or that of libraries
supplied with it.
-pthread
This is a synonym for -pthreads.
SPARC Options
These -m options are supported on the SPARC:
-mno-app-regs
-mapp-regs
Specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for
applications. This is the default.
To be fully SVR4 ABI compliant at the cost of some performance loss, specify -mno-app-regs. You should compile libraries
and system software with this option.
-mflat
-mno-flat
With -mflat, the compiler does not generate save/restore instructions and uses a "flat" or single register window model.
This model is compatible with the regular register window model. The local registers and the input registers (0--5) are
still treated as "call-saved" registers and will be saved on the stack as needed.
With -mno-flat (the default), the compiler generates save/restore instructions (except for leaf functions). This is the
normal operating mode.
-mfpu
-mhard-float
Generate output containing floating-point instructions. This is the default.
-mno-fpu
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all
SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in
cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The
embedded targets sparc-*-aout and sparclite-*-* do provide software floating-point support.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a
program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with
-msoft-float in order for this to work.
-mhard-quad-float
Generate output containing quad-word (long double) floating-point instructions.
-msoft-quad-float
Generate output containing library calls for quad-word (long double) floating-point instructions. The functions called
are those specified in the SPARC ABI. This is the default.
As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating-point
instructions. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the
effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library
routines. Thus the -msoft-quad-float option is the default.
-mno-unaligned-doubles
-munaligned-doubles
Assume that doubles have 8-byte alignment. This is the default.
With -munaligned-doubles, GCC assumes that doubles have 8-byte alignment only if they are contained in another type, or
if they have an absolute address. Otherwise, it assumes they have 4-byte alignment. Specifying this option avoids some
rare compatibility problems with code generated by other compilers. It is not the default because it results in a
performance loss, especially for floating-point code.
-mno-faster-structs
-mfaster-structs
With -mfaster-structs, the compiler assumes that structures should have 8-byte alignment. This enables the use of pairs
of "ldd" and "std" instructions for copies in structure assignment, in place of twice as many "ld" and "st" pairs.
However, the use of this changed alignment directly violates the SPARC ABI. Thus, it's intended only for use on targets
where the developer acknowledges that their resulting code will not be directly in line with the rules of the ABI.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values
for cpu_type are v7, cypress, v8, supersparc, hypersparc, leon, sparclite, f930, f934, sparclite86x, sparclet, tsc701,
v9, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, and niagara4.
Native Solaris and GNU/Linux toolchains also support the value native, which selects the best architecture option for the
host processor. -mcpu=native has no effect if GCC does not recognize the processor.
Default instruction scheduling parameters are used for values that select an architecture and not an implementation.
These are v7, v8, sparclite, sparclet, v9.
Here is a list of each supported architecture and their supported implementations.
v7 cypress
v8 supersparc, hypersparc, leon
sparclite
f930, f934, sparclite86x
sparclet
tsc701
v9 ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4
By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture. With
-mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the
SPARCStation/SPARCServer 3xx series. This is also appropriate for the older SPARCStation 1, 2, IPX etc.
With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture. The only difference from V7 code is that
the compiler emits the integer multiply and integer divide instructions which exist in SPARC-V8 but not in SPARC-V7.
With -mcpu=supersparc, the compiler additionally optimizes it for the SuperSPARC chip, as used in the SPARCStation 10,
1000 and 2000 series.
With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture. This adds the integer
multiply, integer divide step and scan ("ffs") instructions which exist in SPARClite but not in SPARC-V7. With
-mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite, with no
FPU. With -mcpu=f934, the compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the more recent
SPARClite with FPU.
With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture. This adds the integer
multiply, multiply/accumulate, integer divide step and scan ("ffs") instructions which exist in SPARClet but not in
SPARC-V7. With -mcpu=tsc701, the compiler additionally optimizes it for the TEMIC SPARClet chip.
With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture. This adds 64-bit integer and floating-
point move instructions, 3 additional floating-point condition code registers and conditional move instructions. With
-mcpu=ultrasparc, the compiler additionally optimizes it for the Sun UltraSPARC I/II/IIi chips. With -mcpu=ultrasparc3,
the compiler additionally optimizes it for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With -mcpu=niagara, the
compiler additionally optimizes it for Sun UltraSPARC T1 chips. With -mcpu=niagara2, the compiler additionally optimizes
it for Sun UltraSPARC T2 chips. With -mcpu=niagara3, the compiler additionally optimizes it for Sun UltraSPARC T3 chips.
With -mcpu=niagara4, the compiler additionally optimizes it for Sun UltraSPARC T4 chips.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set
that the option -mcpu=cpu_type would.
The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those that select a
particular CPU implementation. Those are cypress, supersparc, hypersparc, leon, f930, f934, sparclite86x, tsc701,
ultrasparc, ultrasparc3, niagara, niagara2, niagara3 and niagara4. With native Solaris and GNU/Linux toolchains, native
can also be used.
-mv8plus
-mno-v8plus
With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI is that the global and out
registers are considered 64 bits wide. This is enabled by default on Solaris in 32-bit mode for all SPARC-V9 processors.
-mvis
-mno-vis
With -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions. The default is
-mno-vis.
-mvis2
-mno-vis2
With -mvis2, GCC generates code that takes advantage of version 2.0 of the UltraSPARC Visual Instruction Set extensions.
The default is -mvis2 when targetting a cpu that supports such instructions, such as UltraSPARC-III and later. Setting
-mvis2 also sets -mvis.
-mvis3
-mno-vis3
With -mvis3, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions.
The default is -mvis3 when targetting a cpu that supports such instructions, such as niagara-3 and later. Setting -mvis3
also sets -mvis2 and -mvis.
-mpopc
-mno-popc
With -mpopc, GCC generates code that takes advantage of the UltraSPARC population count instruction. The default is
-mpopc when targetting a cpu that supports such instructions, such as Niagara-2 and later.
-mfmaf
-mno-fmaf
With -mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-point extensions. The
default is -mfmaf when targetting a cpu that supports such instructions, such as Niagara-3 and later.
-mfix-at697f
Enable the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13
of the AT697E processor).
These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments:
-mlittle-endian
Generate code for a processor running in little-endian mode. It is only available for a few configurations and most
notably not on Solaris and Linux.
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The
64-bit environment sets int to 32 bits and long and pointer to 64 bits.
-mcmodel=which
Set the code model to one of
medlow
The Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be
statically or dynamically linked.
medmid
The Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and
data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
medany
The Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data
segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
embmedany
The Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments must be less than
2GB in size, both starting anywhere in memory (determined at link time). The global register %g4 points to the base
of the data segment. Programs are statically linked and PIC is not supported.
-mmemory-model=mem-model
Set the memory model in force on the processor to one of
default
The default memory model for the processor and operating system.
rmo Relaxed Memory Order
pso Partial Store Order
tso Total Store Order
sc Sequential Consistency
These memory models are formally defined in Appendix D of the Sparc V9 architecture manual, as set in the processor's
"PSTATE.MM" field.
-mstack-bias
-mno-stack-bias
With -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by -2047 which must be
added back when making stack frame references. This is the default in 64-bit mode. Otherwise, assume no such offset is
present.
SPU Options
These -m options are supported on the SPU:
-mwarn-reloc
-merror-reloc
The loader for SPU does not handle dynamic relocations. By default, GCC will give an error when it generates code that
requires a dynamic relocation. -mno-error-reloc disables the error, -mwarn-reloc will generate a warning instead.
-msafe-dma
-munsafe-dma
Instructions that initiate or test completion of DMA must not be reordered with respect to loads and stores of the memory
that is being accessed. Users typically address this problem using the volatile keyword, but that can lead to
inefficient code in places where the memory is known to not change. Rather than mark the memory as volatile we treat the
DMA instructions as potentially effecting all memory. With -munsafe-dma users must use the volatile keyword to protect
memory accesses.
-mbranch-hints
By default, GCC will generate a branch hint instruction to avoid pipeline stalls for always taken or probably taken
branches. A hint will not be generated closer than 8 instructions away from its branch. There is little reason to
disable them, except for debugging purposes, or to make an object a little bit smaller.
-msmall-mem
-mlarge-mem
By default, GCC generates code assuming that addresses are never larger than 18 bits. With -mlarge-mem code is generated
that assumes a full 32-bit address.
-mstdmain
By default, GCC links against startup code that assumes the SPU-style main function interface (which has an
unconventional parameter list). With -mstdmain, GCC will link your program against startup code that assumes a C99-style
interface to "main", including a local copy of "argv" strings.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator
can not use. This is useful when compiling kernel code. A register range is specified as two registers separated by a
dash. Multiple register ranges can be specified separated by a comma.
-mea32
-mea64
Compile code assuming that pointers to the PPU address space accessed via the "__ea" named address space qualifier are
either 32 or 64 bits wide. The default is 32 bits. As this is an ABI changing option, all object code in an executable
must be compiled with the same setting.
-maddress-space-conversion
-mno-address-space-conversion
Allow/disallow treating the "__ea" address space as superset of the generic address space. This enables explicit type
casts between "__ea" and generic pointer as well as implicit conversions of generic pointers to "__ea" pointers. The
default is to allow address space pointer conversions.
-mcache-size=cache-size
This option controls the version of libgcc that the compiler links to an executable and selects a software-managed cache
for accessing variables in the "__ea" address space with a particular cache size. Possible options for cache-size are 8,
16, 32, 64 and 128. The default cache size is 64KB.
-matomic-updates
-mno-atomic-updates
This option controls the version of libgcc that the compiler links to an executable and selects whether atomic updates to
the software-managed cache of PPU-side variables are used. If you use atomic updates, changes to a PPU variable from SPU
code using the "__ea" named address space qualifier will not interfere with changes to other PPU variables residing in
the same cache line from PPU code. If you do not use atomic updates, such interference may occur; however, writing back
cache lines will be more efficient. The default behavior is to use atomic updates.
-mdual-nops
-mdual-nops=n
By default, GCC will insert nops to increase dual issue when it expects it to increase performance. n can be a value
from 0 to 10. A smaller n will insert fewer nops. 10 is the default, 0 is the same as -mno-dual-nops. Disabled with
-Os.
-mhint-max-nops=n
Maximum number of nops to insert for a branch hint. A branch hint must be at least 8 instructions away from the branch
it is effecting. GCC will insert up to n nops to enforce this, otherwise it will not generate the branch hint.
-mhint-max-distance=n
The encoding of the branch hint instruction limits the hint to be within 256 instructions of the branch it is effecting.
By default, GCC makes sure it is within 125.
-msafe-hints
Work around a hardware bug that causes the SPU to stall indefinitely. By default, GCC will insert the "hbrp" instruction
to make sure this stall won't happen.
Options for System V
These additional options are available on System V Release 4 for compatibility with other compilers on those systems:
-G Create a shared object. It is recommended that -symbolic or -shared be used instead.
-Qy Identify the versions of each tool used by the compiler, in a ".ident" assembler directive in the output.
-Qn Refrain from adding ".ident" directives to the output file (this is the default).
-YP,dirs
Search the directories dirs, and no others, for libraries specified with -l.
-Ym,dir
Look in the directory dir to find the M4 preprocessor. The assembler uses this option.
TILE-Gx Options
These -m options are supported on the TILE-Gx:
-mcpu=name
Selects the type of CPU to be targeted. Currently the only supported type is tilegx.
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long, and pointer to 32 bits. The
64-bit environment sets int to 32 bits and long and pointer to 64 bits.
TILEPro Options
These -m options are supported on the TILEPro:
-mcpu=name
Selects the type of CPU to be targeted. Currently the only supported type is tilepro.
-m32
Generate code for a 32-bit environment, which sets int, long, and pointer to 32 bits. This is the only supported
behavior so the flag is essentially ignored.
V850 Options
These -m options are defined for V850 implementations:
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to be far away, the compiler will always load the
functions address up into a register, and call indirect through the pointer.
-mno-ep
-mep
Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into the "ep"
register, and use the shorter "sld" and "sst" instructions. The -mep option is on by default if you optimize.
-mno-prolog-function
-mprolog-function
Do not use (do use) external functions to save and restore registers at the prologue and epilogue of a function. The
external functions are slower, but use less code space if more than one function saves the same number of registers. The
-mprolog-function option is on by default if you optimize.
-mspace
Try to make the code as small as possible. At present, this just turns on the -mep and -mprolog-function options.
-mtda=n
Put static or global variables whose size is n bytes or less into the tiny data area that register "ep" points to. The
tiny data area can hold up to 256 bytes in total (128 bytes for byte references).
-msda=n
Put static or global variables whose size is n bytes or less into the small data area that register "gp" points to. The
small data area can hold up to 64 kilobytes.
-mzda=n
Put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.
-mv850
Specify that the target processor is the V850.
-mbig-switch
Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range
branches within a switch table.
-mapp-regs
This option will cause r2 and r5 to be used in the code generated by the compiler. This setting is the default.
-mno-app-regs
This option will cause r2 and r5 to be treated as fixed registers.
-mv850e2v3
Specify that the target processor is the V850E2V3. The preprocessor constants __v850e2v3__ will be defined if this
option is used.
-mv850e2
Specify that the target processor is the V850E2. The preprocessor constants __v850e2__ will be defined if this option is
used.
-mv850e1
Specify that the target processor is the V850E1. The preprocessor constants __v850e1__ and __v850e__ will be defined if
this option is used.
-mv850es
Specify that the target processor is the V850ES. This is an alias for the -mv850e1 option.
-mv850e
Specify that the target processor is the V850E. The preprocessor constant __v850e__ will be defined if this option is
used.
If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor -mv850e2v3 are defined then a default target processor will
be chosen and the relevant __v850*__ preprocessor constant will be defined.
The preprocessor constants __v850 and __v851__ are always defined, regardless of which processor variant is the target.
-mdisable-callt
This option will suppress generation of the CALLT instruction for the v850e, v850e1, v850e2 and v850e2v3 flavors of the
v850 architecture. The default is -mno-disable-callt which allows the CALLT instruction to be used.
VAX Options
These -m options are defined for the VAX:
-munix
Do not output certain jump instructions ("aobleq" and so on) that the Unix assembler for the VAX cannot handle across
long ranges.
-mgnu
Do output those jump instructions, on the assumption that you will assemble with the GNU assembler.
-mg Output code for G-format floating-point numbers instead of D-format.
VxWorks Options
The options in this section are defined for all VxWorks targets. Options specific to the target hardware are listed with the
other options for that target.
-mrtp
GCC can generate code for both VxWorks kernels and real time processes (RTPs). This option switches from the former to
the latter. It also defines the preprocessor macro "__RTP__".
-non-static
Link an RTP executable against shared libraries rather than static libraries. The options -static and -shared can also
be used for RTPs; -static is the default.
-Bstatic
-Bdynamic
These options are passed down to the linker. They are defined for compatibility with Diab.
-Xbind-lazy
Enable lazy binding of function calls. This option is equivalent to -Wl,-z,now and is defined for compatibility with
Diab.
-Xbind-now
Disable lazy binding of function calls. This option is the default and is defined for compatibility with Diab.
x86-64 Options
These are listed under
Xstormy16 Options
These options are defined for Xstormy16:
-msim
Choose startup files and linker script suitable for the simulator.
Xtensa Options
These options are supported for Xtensa targets:
-mconst16
-mno-const16
Enable or disable use of "CONST16" instructions for loading constant values. The "CONST16" instruction is currently not
a standard option from Tensilica. When enabled, "CONST16" instructions are always used in place of the standard "L32R"
instructions. The use of "CONST16" is enabled by default only if the "L32R" instruction is not available.
-mfused-madd
-mno-fused-madd
Enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option. This has no
effect if the floating-point option is not also enabled. Disabling fused multiply/add and multiply/subtract instructions
forces the compiler to use separate instructions for the multiply and add/subtract operations. This may be desirable in
some cases where strict IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not
round the intermediate result, thereby producing results with more bits of precision than specified by the IEEE standard.
Disabling fused multiply add/subtract instructions also ensures that the program output is not sensitive to the
compiler's ability to combine multiply and add/subtract operations.
-mserialize-volatile
-mno-serialize-volatile
When this option is enabled, GCC inserts "MEMW" instructions before "volatile" memory references to guarantee sequential
consistency. The default is -mserialize-volatile. Use -mno-serialize-volatile to omit the "MEMW" instructions.
-mforce-no-pic
For targets, like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC), this option
disables PIC for compiling kernel code.
-mtext-section-literals
-mno-text-section-literals
Control the treatment of literal pools. The default is -mno-text-section-literals, which places literals in a separate
section in the output file. This allows the literal pool to be placed in a data RAM/ROM, and it also allows the linker
to combine literal pools from separate object files to remove redundant literals and improve code size. With
-mtext-section-literals, the literals are interspersed in the text section in order to keep them as close as possible to
their references. This may be necessary for large assembly files.
-mtarget-align
-mno-target-align
When this option is enabled, GCC instructs the assembler to automatically align instructions to reduce branch penalties
at the expense of some code density. The assembler attempts to widen density instructions to align branch targets and
the instructions following call instructions. If there are not enough preceding safe density instructions to align a
target, no widening will be performed. The default is -mtarget-align. These options do not affect the treatment of
auto-aligned instructions like "LOOP", which the assembler will always align, either by widening density instructions or
by inserting no-op instructions.
-mlongcalls
-mno-longcalls
When this option is enabled, GCC instructs the assembler to translate direct calls to indirect calls unless it can
determine that the target of a direct call is in the range allowed by the call instruction. This translation typically
occurs for calls to functions in other source files. Specifically, the assembler translates a direct "CALL" instruction
into an "L32R" followed by a "CALLX" instruction. The default is -mno-longcalls. This option should be used in programs
where the call target can potentially be out of range. This option is implemented in the assembler, not the compiler, so
the assembly code generated by GCC will still show direct call instructions---look at the disassembled object code to see
the actual instructions. Note that the assembler will use an indirect call for every cross-file call, not just those
that really will be out of range.
zSeries Options
These are listed under
Options for Code Generation Conventions
These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form of -ffoo would be -fno-foo. In the table below, only
one of the forms is listed---the one that is not the default. You can figure out the other form by either removing no- or
adding it.
-fbounds-check
For front ends that support it, generate additional code to check that indices used to access arrays are within the
declared range. This is currently only supported by the Java and Fortran front ends, where this option defaults to true
and false respectively.
-ftrapv
This option generates traps for signed overflow on addition, subtraction, multiplication operations.
-fwrapv
This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication
wraps around using twos-complement representation. This flag enables some optimizations and disables others. This
option is enabled by default for the Java front end, as required by the Java language specification.
-fexceptions
Enable exception handling. Generates extra code needed to propagate exceptions. For some targets, this implies GCC will
generate frame unwind information for all functions, which can produce significant data size overhead, although it does
not affect execution. If you do not specify this option, GCC will enable it by default for languages like C++ that
normally require exception handling, and disable it for languages like C that do not normally require it. However, you
may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written
in C++. You may also wish to disable this option if you are compiling older C++ programs that don't use exception
handling.
-fnon-call-exceptions
Generate code that allows trapping instructions to throw exceptions. Note that this requires platform-specific runtime
support that does not exist everywhere. Moreover, it only allows trapping instructions to throw exceptions, i.e. memory
references or floating-point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such
as "SIGALRM".
-funwind-tables
Similar to -fexceptions, except that it will just generate any needed static data, but will not affect the generated code
in any other way. You will normally not enable this option; instead, a language processor that needs this handling would
enable it on your behalf.
-fasynchronous-unwind-tables
Generate unwind table in dwarf2 format, if supported by target machine. The table is exact at each instruction boundary,
so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector).
-fpcc-struct-return
Return "short" "struct" and "union" values in memory like longer ones, rather than in registers. This convention is less
efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other
compilers, particularly the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends on the target configuration macros.
Short structures and unions are those whose size and alignment match that of some integer type.
Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with the
-freg-struct-return switch. Use it to conform to a non-default application binary interface.
-freg-struct-return
Return "struct" and "union" values in registers when possible. This is more efficient for small structures than
-fpcc-struct-return.
If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for
the target. If there is no standard convention, GCC defaults to -fpcc-struct-return, except on targets where GCC is the
principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return
alternative.
Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with the
-fpcc-struct-return switch. Use it to conform to a non-default application binary interface.
-fshort-enums
Allocate to an "enum" type only as many bytes as it needs for the declared range of possible values. Specifically, the
"enum" type will be equivalent to the smallest integer type that has enough room.
Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code generated without
that switch. Use it to conform to a non-default application binary interface.
-fshort-double
Use the same size for "double" as for "float".
Warning: the -fshort-double switch causes GCC to generate code that is not binary compatible with code generated without
that switch. Use it to conform to a non-default application binary interface.
-fshort-wchar
Override the underlying type for wchar_t to be short unsigned int instead of the default for the target. This option is
useful for building programs to run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code generated without
that switch. Use it to conform to a non-default application binary interface.
-fno-common
In C code, controls the placement of uninitialized global variables. Unix C compilers have traditionally permitted
multiple definitions of such variables in different compilation units by placing the variables in a common block. This
is the behavior specified by -fcommon, and is the default for GCC on most targets. On the other hand, this behavior is
not required by ISO C, and on some targets may carry a speed or code size penalty on variable references. The
-fno-common option specifies that the compiler should place uninitialized global variables in the data section of the
object file, rather than generating them as common blocks. This has the effect that if the same variable is declared
(without "extern") in two different compilations, you will get a multiple-definition error when you link them. In this
case, you must compile with -fcommon instead. Compiling with -fno-common is useful on targets for which it provides
better performance, or if you wish to verify that the program will work on other systems that always treat uninitialized
variable declarations this way.
-fno-ident
Ignore the #ident directive.
-finhibit-size-directive
Don't output a ".size" assembler directive, or anything else that would cause trouble if the function is split in the
middle, and the two halves are placed at locations far apart in memory. This option is used when compiling crtstuff.c;
you should not need to use it for anything else.
-fverbose-asm
Put extra commentary information in the generated assembly code to make it more readable. This option is generally only
of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself).
-fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two assembler
files.
-frecord-gcc-switches
This switch causes the command line that was used to invoke the compiler to be recorded into the object file that is
being created. This switch is only implemented on some targets and the exact format of the recording is target and
binary file format dependent, but it usually takes the form of a section containing ASCII text. This switch is related
to the -fverbose-asm switch, but that switch only records information in the assembler output file as comments, so it
never reaches the object file. See also -grecord-gcc-switches for another way of storing compiler options into the
object file.
-fpic
Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such
code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries
when the program starts (the dynamic loader is not part of GCC; it is part of the operating system). If the GOT size for
the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that
-fpic does not work; in that case, recompile with -fPIC instead. (These maximums are 8k on the SPARC and 32k on the m68k
and RS/6000. The 386 has no such limit.)
Position-independent code requires special support, and therefore works only on certain machines. For the 386, GCC
supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent.
When this flag is set, the macros "__pic__" and "__PIC__" are defined to 1.
-fPIC
If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit
on the size of the global offset table. This option makes a difference on the m68k, PowerPC and SPARC.
Position-independent code requires special support, and therefore works only on certain machines.
When this flag is set, the macros "__pic__" and "__PIC__" are defined to 2.
-fpie
-fPIE
These options are similar to -fpic and -fPIC, but generated position independent code can be only linked into
executables. Usually these options are used when -pie GCC option will be used during linking.
-fpie and -fPIE both define the macros "__pie__" and "__PIE__". The macros have the value 1 for -fpie and 2 for -fPIE.
-fno-jump-tables
Do not use jump tables for switch statements even where it would be more efficient than other code generation strategies.
This option is of use in conjunction with -fpic or -fPIC for building code that forms part of a dynamic linker and cannot
reference the address of a jump table. On some targets, jump tables do not require a GOT and this option is not needed.
-ffixed-reg
Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack
pointer, frame pointer or in some other fixed role).
reg must be the name of a register. The register names accepted are machine-specific and are defined in the
"REGISTER_NAMES" macro in the machine description macro file.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-used-reg
Treat the register named reg as an allocable register that is clobbered by function calls. It may be allocated for
temporaries or variables that do not live across a call. Functions compiled this way will not save and restore the
register reg.
It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have
fixed pervasive roles in the machine's execution model will produce disastrous results.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-saved-reg
Treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or
variables that live across a call. Functions compiled this way will save and restore the register reg if they use it.
It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have
fixed pervasive roles in the machine's execution model will produce disastrous results.
A different sort of disaster will result from the use of this flag for a register in which function values may be
returned.
This flag does not have a negative form, because it specifies a three-way choice.
-fpack-struct[=n]
Without a value specified, pack all structure members together without holes. When a value is specified (which must be a
small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects
with default alignment requirements larger than this will be output potentially unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without
that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary
interface.
-finstrument-functions
Generate instrumentation calls for entry and exit to functions. Just after function entry and just before function exit,
the following profiling functions will be called with the address of the current function and its call site. (On some
platforms, "__builtin_return_address" does not work beyond the current function, so the call site information may not be
available to the profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current function, which may be looked up exactly in the symbol
table.
This instrumentation is also done for functions expanded inline in other functions. The profiling calls will indicate
where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions
must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code
size. If you use extern inline in your C code, an addressable version of such functions must be provided. (This is
normally the case anyways, but if you get lucky and the optimizer always expands the functions inline, you might have
gotten away without providing static copies.)
A function may be given the attribute "no_instrument_function", in which case this instrumentation will not be done.
This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any
functions from which the profiling functions cannot safely be called (perhaps signal handlers, if the profiling routines
generate output or allocate memory).
-finstrument-functions-exclude-file-list=file,file,...
Set the list of functions that are excluded from instrumentation (see the description of "-finstrument-functions"). If
the file that contains a function definition matches with one of file, then that function is not instrumented. The match
is done on substrings: if the file parameter is a substring of the file name, it is considered to be a match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
will exclude any inline function defined in files whose pathnames contain "/bits/stl" or "include/sys".
If, for some reason, you want to include letter ',' in one of sym, write ','. For example,
"-finstrument-functions-exclude-file-list=',,tmp'" (note the single quote surrounding the option).
-finstrument-functions-exclude-function-list=sym,sym,...
This is similar to "-finstrument-functions-exclude-file-list", but this option sets the list of function names to be
excluded from instrumentation. The function name to be matched is its user-visible name, such as "vector<int> blah(const
vector<int> &)", not the internal mangled name (e.g., "_Z4blahRSt6vectorIiSaIiEE"). The match is done on substrings: if
the sym parameter is a substring of the function name, it is considered to be a match. For C99 and C++ extended
identifiers, the function name must be given in UTF-8, not using universal character names.
-fstack-check
Generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag if you are
running in an environment with multiple threads, but only rarely need to specify it in a single-threaded environment
since stack overflow is automatically detected on nearly all systems if there is only one stack.
Note that this switch does not actually cause checking to be done; the operating system or the language runtime must do
that. The switch causes generation of code to ensure that they see the stack being extended.
You can additionally specify a string parameter: "no" means no checking, "generic" means force the use of old-style
checking, "specific" means use the best checking method and is equivalent to bare -fstack-check.
Old-style checking is a generic mechanism that requires no specific target support in the compiler but comes with the
following drawbacks:
1. Modified allocation strategy for large objects: they will always be allocated dynamically if their size exceeds a
fixed threshold.
2. Fixed limit on the size of the static frame of functions: when it is topped by a particular function, stack checking
is not reliable and a warning is issued by the compiler.
3. Inefficiency: because of both the modified allocation strategy and the generic implementation, the performances of
the code are hampered.
Note that old-style stack checking is also the fallback method for "specific" if no target support has been added in the
compiler.
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
Generate code to ensure that the stack does not grow beyond a certain value, either the value of a register or the
address of a symbol. If the stack would grow beyond the value, a signal is raised. For most targets, the signal is
raised before the stack overruns the boundary, so it is possible to catch the signal without taking special precautions.
For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags
-fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Note
that this may only work with the GNU linker.
-fsplit-stack
Generate code to automatically split the stack before it overflows. The resulting program has a discontiguous stack
which can only overflow if the program is unable to allocate any more memory. This is most useful when running threaded
programs, as it is no longer necessary to calculate a good stack size to use for each thread. This is currently only
implemented for the i386 and x86_64 back ends running GNU/Linux.
When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space
available for the latter code to run. If compiling all code, including library code, with -fsplit-stack is not an
option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack.
Support for this is implemented in the gold linker in GNU binutils release 2.21 and later.
-fleading-underscore
This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in the object
file. One use is to help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated
without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete
support for this switch.
-ftls-model=model
Alter the thread-local storage model to be used. The model argument should be one of "global-dynamic", "local-dynamic",
"initial-exec" or "local-exec".
The default without -fpic is "initial-exec"; with -fpic the default is "global-dynamic".
-fvisibility=default|internal|hidden|protected
Set the default ELF image symbol visibility to the specified option---all symbols will be marked with this unless
overridden within the code. Using this feature can very substantially improve linking and load times of shared object
libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is strongly
recommended that you use this in any shared objects you distribute.
Despite the nomenclature, "default" always means public; i.e., available to be linked against from outside the shared
object. "protected" and "internal" are pretty useless in real-world usage so the only other commonly used option will be
"hidden". The default if -fvisibility isn't specified is "default", i.e., make every symbol public---this causes the
same behavior as previous versions of GCC.
A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by "How To Write
Shared Libraries" by Ulrich Drepper (which can be found at <http://people.redhat.com/~drepper/>)---however a superior
solution made possible by this option to marking things hidden when the default is public is to make the default hidden
and mark things public. This is the norm with DLL's on Windows and with -fvisibility=hidden and "__attribute__
((visibility("default")))" instead of "__declspec(dllexport)" you get almost identical semantics with identical syntax.
This is a great boon to those working with cross-platform projects.
For those adding visibility support to existing code, you may find #pragma GCC visibility of use. This works by you
enclosing the declarations you wish to set visibility for with (for example) #pragma GCC visibility push(hidden) and
#pragma GCC visibility pop. Bear in mind that symbol visibility should be viewed as part of the API interface contract
and thus all new code should always specify visibility when it is not the default; i.e., declarations only for use within
the local DSO should always be marked explicitly as hidden as so to avoid PLT indirection overheads---making this
abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C++ specification
requirements, operator new and operator delete must always be of default visibility.
Be aware that headers from outside your project, in particular system headers and headers from any other library you use,
may not be expecting to be compiled with visibility other than the default. You may need to explicitly say #pragma GCC
visibility push(default) before including any such headers.
extern declarations are not affected by -fvisibility, so a lot of code can be recompiled with -fvisibility=hidden with no
modifications. However, this means that calls to extern functions with no explicit visibility will use the PLT, so it is
more effective to use __attribute ((visibility)) and/or #pragma GCC visibility to tell the compiler which extern
declarations should be treated as hidden.
Note that -fvisibility does affect C++ vague linkage entities. This means that, for instance, an exception class that
will be thrown between DSOs must be explicitly marked with default visibility so that the type_info nodes will be unified
between the DSOs.
An overview of these techniques, their benefits and how to use them is at <http://gcc.gnu.org/wiki/Visibility>.
-fstrict-volatile-bitfields
This option should be used if accesses to volatile bit-fields (or other structure fields, although the compiler usually
honors those types anyway) should use a single access of the width of the field's type, aligned to a natural alignment if
possible. For example, targets with memory-mapped peripheral registers might require all such accesses to be 16 bits
wide; with this flag the user could declare all peripheral bit-fields as "unsigned short" (assuming short is 16 bits on
these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access.
If this option is disabled, the compiler will use the most efficient instruction. In the previous example, that might be
a 32-bit load instruction, even though that will access bytes that do not contain any portion of the bit-field, or
memory-mapped registers unrelated to the one being updated.
If the target requires strict alignment, and honoring the field type would require violating this alignment, a warning is
issued. If the field has "packed" attribute, the access is done without honoring the field type. If the field doesn't
have "packed" attribute, the access is done honoring the field type. In both cases, GCC assumes that the user knows
something about the target hardware that it is unaware of.
The default value of this option is determined by the application binary interface for the target processor.
ENVIRONMENT
This section describes several environment variables that affect how GCC operates. Some of them work by specifying
directories or prefixes to use when searching for various kinds of files. Some are used to specify other aspects of the
compilation environment.
Note that you can also specify places to search using options such as -B, -I and -L. These take precedence over places
specified using environment variables, which in turn take precedence over those specified by the configuration of GCC.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
These environment variables control the way that GCC uses localization information which allows GCC to work with
different national conventions. GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has been configured to
do so. These locale categories can be set to any value supported by your installation. A typical value is en_GB.UTF-8
for English in the United Kingdom encoded in UTF-8.
The LC_CTYPE environment variable specifies character classification. GCC uses it to determine the character boundaries
in a string; this is needed for some multibyte encodings that contain quote and escape characters that would otherwise be
interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the language to use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and
LC_MESSAGES default to the value of the LANG environment variable. If none of these variables are set, GCC defaults to
traditional C English behavior.
TMPDIR
If TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temporary files to hold the output of
one stage of compilation which is to be used as input to the next stage: for example, the output of the preprocessor,
which is the input to the compiler proper.
GCC_COMPARE_DEBUG
Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. See the documentation
of this option for more details.
GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No
slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a
slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC will attempt to figure out an appropriate prefix to use based on the pathname it was
invoked with.
If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the prefix to the installed compiler. In many
cases prefix is the value of "prefix" when you ran the configure script.
Other prefixes specified with -B take precedence over this prefix.
This prefix is also used for finding files such as crt0.o that are used for linking.
In addition, the prefix is used in an unusual way in finding the directories to search for header files. For each of the
standard directories whose name normally begins with /usr/local/lib/gcc (more precisely, with the value of
GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name.
Thus, with -Bfoo/, GCC will search foo/bar where it would normally search /usr/local/lib/bar. These alternate
directories are searched first; the standard directories come next. If a standard directory begins with the configured
prefix then the value of prefix is replaced by GCC_EXEC_PREFIX when looking for header files.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories thus
specified when searching for subprograms, if it can't find the subprograms using GCC_EXEC_PREFIX.
LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a native
compiler, GCC tries the directories thus specified when searching for special linker files, if it can't find them using
GCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching for ordinary libraries for the -l option
(but directories specified with -L come first).
LANG
This variable is used to pass locale information to the compiler. One way in which this information is used is to
determine the character set to be used when character literals, string literals and comments are parsed in C and C++.
When the compiler is configured to allow multibyte characters, the following values for LANG are recognized:
C-JIS
Recognize JIS characters.
C-SJIS
Recognize SJIS characters.
C-EUCJP
Recognize EUCJP characters.
If LANG is not defined, or if it has some other value, then the compiler will use mblen and mbtowc as defined by the
default locale to recognize and translate multibyte characters.
Some additional environments variables affect the behavior of the preprocessor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable's value is a list of directories separated by a special character, much like PATH, in which to look for
header files. The special character, "PATH_SEPARATOR", is target-dependent and determined at GCC build time. For
Microsoft Windows-based targets it is a semicolon, and for almost all other targets it is a colon.
CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given with -I options
on the command line. This environment variable is used regardless of which language is being preprocessed.
The remaining environment variables apply only when preprocessing the particular language indicated. Each specifies a
list of directories to be searched as if specified with -isystem, but after any paths given with -isystem options on the
command line.
In all these variables, an empty element instructs the compiler to search its current working directory. Empty elements
can appear at the beginning or end of a path. For instance, if the value of CPATH is ":/special/include", that has the
same effect as -I. -I/special/include.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output dependencies for Make based on the non-system header files
processed by the compiler. System header files are ignored in the dependency output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that file, guessing
the target name from the source file name. Or the value can have the form file target, in which case the rules are
written to file file using target as the target name.
In other words, this environment variable is equivalent to combining the options -MM and -MF, with an optional -MT switch
too.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not ignored, so it
implies -M rather than -MM. However, the dependence on the main input file is omitted.
BUGS
For instructions on reporting bugs, see <file:///usr/share/doc/gcc-4.7/README.Bugs>.
FOOTNOTES
1. On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On multi-libbed systems,
gcc -shared must select the correct support libraries to link against. Failing to supply the correct flags may lead to
subtle defects. Supplying them in cases where they are not necessary is innocuous.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1) and the Info entries for gcc,
cpp, as, ld, binutils and gdb.
AUTHOR
See the Info entry for gcc, or <http://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC.
COPYRIGHT
Copyright (c) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007,
2008, 2009, 2010, 2011, 2012 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License,
Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General
Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being
(b) (see below). A copy of the license is included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
gcc-4.7 2012-09-20 GCC(1)