指针访问与数组访问的效率分析

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作者:小马

 

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
}

 

//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.