在Linux Kernel中有些constant需要被C code 跟 assembler共同使用
在用constant的時候，不能單方面給0x1000UL因為assembler無法看這東西。
但是C compiler卻可以。所以使用這個gcc feature.
讓C compiler看到的constant是0x1000UL.
而assembler看起來是0x1000.
| 6 | /* Some constant macros are used in both assembler and |
|---|---|
| 7 | * C code. Therefore we cannot annotate them always with |
| 8 | * 'UL' and other type specifiers unilaterally. We |
| 9 | * use the following macros to deal with this. |
| 10 | * |
| 11 | * Similarly, _AT() will cast an expression with a type in C, but |
| 12 | * leave it unchanged in asm. |
| 13 | */ |
ref.
[1] http://code.woboq.org/linux/linux/include/uapi/linux/const.h.html