出处：http://blog.csdn.net/pony_maggie/article/details/6748662
很多人都知道指针访问比数组访问效率高, 但很少人明白其中的原理, 这篇文章从汇编级别上分析一下原因.
我用一个"reverse"函数来作为例子, 分别用指针和数组的形式来实现这个函数.
//指针访问 void reverse(char *pStr,int nLen) { char *pStart=pStr;//指向字符串开始 char *pEnd=pStr+nLen-1;//指向字符串结尾 char chTmp; while(pEnd > pStart) { chTmp = *pStart; *pStart = *pEnd; *pEnd = chTmp; pStart++; pEnd--; } }
上面的函数在vc6下的反汇编代码,我加了一些注释方便理解
19: void reverse(char *pStr,int nLen) 20: { 00401070 push ebp 00401071 mov ebp,esp 00401073 sub esp,4Ch 00401076 push ebx 00401077 push esi 00401078 push edi 00401079 lea edi,[ebp-4Ch] 0040107C mov ecx,13h 00401081 mov eax,0CCCCCCCCh 00401086 rep stos dword ptr [edi]//调用子函数时进堆栈保存信息. 00401088 mov eax,dword ptr [ebp+8] 0040108B mov dword ptr [ebp-4],eax//char *pStart=pStr; 0040108E mov ecx,dword ptr [ebp+0Ch] 00401091 mov edx,dword ptr [ebp+8] 00401094 lea eax,[edx+ecx-1] 00401098 mov dword ptr [ebp-8],eax//char *pEnd=pStr+nLen-1; 0040109B mov ecx,dword ptr [ebp-8] 0040109E cmp ecx,dword ptr [ebp-4]//while(pEnd > pStart) 004010A1 jbe reverse+61h (004010d1) 004010A3 mov edx,dword ptr [ebp-4]//chTmp = *pStart; 004010A6 mov al,byte ptr [edx]//取一个字节 004010A8 mov byte ptr [ebp-0Ch],al 004010AB mov ecx,dword ptr [ebp-4]//*pStart = *pEnd; 004010AE mov edx,dword ptr [ebp-8] 004010B1 mov al,byte ptr [edx]//从pEnd中取一个字节 004010B3 mov byte ptr [ecx],al 004010B5 mov ecx,dword ptr [ebp-8]// *pEnd = chTmp; 004010B8 mov dl,byte ptr [ebp-0Ch] 004010BB mov byte ptr [ecx],dl 004010BD mov eax,dword ptr [ebp-4]//pStart++; 004010C0 add eax,1 004010C3 mov dword ptr [ebp-4],eax 004010C6 mov ecx,dword ptr [ebp-8]//pEnd--; 004010C9 sub ecx,1 004010CC mov dword ptr [ebp-8],ecx }
下面是以数组访问的方式：
//数组访问 void reverse(char *pStr,int nLen) { int i = 0; int j = nLen-1; char chTmp; while(j>i) { chTmp=pStr[i]; pStr[i]=pStr[j]; pStr[j]=chTmp; i++; j--; } }
这是上面代码的反汇编
19: void reverse(char *pStr,int nLen) 20: { 004010D0 push ebp 004010D1 mov ebp,esp 004010D3 sub esp,4Ch 004010D6 push ebx 004010D7 push esi 004010D8 push edi 004010D9 lea edi,[ebp-4Ch] 004010DC mov ecx,13h 004010E1 mov eax,0CCCCCCCCh 004010E6 rep stos dword ptr [edi] 004010E8 mov dword ptr [ebp-4],0 004010EF mov eax,dword ptr [ebp+0Ch] 004010F2 sub eax,1 004010F5 mov dword ptr [ebp-8],eax 004010F8 mov ecx,dword ptr [ebp-8] 004010FB cmp ecx,dword ptr [ebp-4] 004010FE jle reverse+6Ah (0040113a) 00401100 mov edx,dword ptr [ebp+8] <span style="color:#ff0000;">//注意这里</span> 00401103 add edx,dword ptr [ebp-4] 00401106 mov al,byte ptr [edx] 00401108 mov byte ptr [ebp-0Ch],al 0040110B mov ecx,dword ptr [ebp+8] <span style="color:#ff0000;">//注意这里</span> 0040110E add ecx,dword ptr [ebp-4] 00401111 mov edx,dword ptr [ebp+8] <span style="color:#ff0000;">//注意这里</span> 00401114 add edx,dword ptr [ebp-8] 00401117 mov al,byte ptr [edx] 00401119 mov byte ptr [ecx],al 0040111B mov ecx,dword ptr [ebp+8] <span style="color:#ff0000;">//注意这里</span> 0040111E add ecx,dword ptr [ebp-8] 00401121 mov dl,byte ptr [ebp-0Ch] 00401124 mov byte ptr [ecx],dl 00401126 mov eax,dword ptr [ebp-4] 00401129 add eax,1 0040112C mov dword ptr [ebp-4],eax 0040112F mov ecx,dword ptr [ebp-8] 00401132 sub ecx,1 00401135 mov dword ptr [ebp-8],ecx }
两个函数反汇编后的代码基本是一致的,但第二段汇编中多了几个add语句, 这是因为在用数组下标的形式访问字符串时,总要对数组的首地址做加减操作才能定位到想要的值, 而指针的实现则一开始就copy了一个首地址，不需要再用加减来定位.显然在这个例子中，用指针来实现reverse函数效率要高.