内在的 memcmp [英] intrinsic memcmp

问题描述

根据 gcc 文档，memcmp 不是 GCC 的内在函数.如果您想在 gcc 下加速 glibc 的 memcmp，则需要使用文档中定义的较低级别的内在函数.但是，在网上搜索时，似乎很多人都有一个印象，memcmp 是一个内置函数.它适用于某些编译器而不适用于其他编译器吗?

According to the gcc docs, memcmp is not an intrinsic function of GCC. If you wanted to speed up glibc's memcmp under gcc, you would need to use the lower level intrinsics defined in the docs. However, when searching around the internet, it seems that many people have the impression that memcmp is a builtin function. Is it for some compilers and not for others?

推荐答案

根据 这个 memcmp 由 gcc 实现为与架构无关的内置.

Your link appears to be for the x86 architecture-specific built-in functions, according to this memcmp is implemented as an architecture-independent built-in by gcc.

使用 Cygwin gcc version 3.3.1 for i686, -O2 编译以下代码:

Compiling the following code with Cygwin gcc version 3.3.1 for i686, -O2:

#include <stdlib.h>

struct foo {
    int a;
    int b;
} ;

int func(struct foo *x, struct foo *y)
{
    return memcmp(x, y, sizeof (struct foo));
}

产生以下输出(注意对 memcmp() 的调用被转换为 8 字节的repz cmpsb"):

Produces the following output (note that the call to memcmp() is converted to an 8-byte "repz cmpsb"):

   0:   55                      push   %ebp
   1:   b9 08 00 00 00          mov    $0x8,%ecx
   6:   89 e5                   mov    %esp,%ebp
   8:   fc                      cld    
   9:   83 ec 08                sub    $0x8,%esp
   c:   89 34 24                mov    %esi,(%esp)
   f:   8b 75 08                mov    0x8(%ebp),%esi
  12:   89 7c 24 04             mov    %edi,0x4(%esp)
  16:   8b 7d 0c                mov    0xc(%ebp),%edi
  19:   f3 a6                   repz cmpsb %es:(%edi),%ds:(%esi)
  1b:   0f 92 c0                setb   %al
  1e:   8b 34 24                mov    (%esp),%esi
  21:   8b 7c 24 04             mov    0x4(%esp),%edi
  25:   0f 97 c2                seta   %dl
  28:   89 ec                   mov    %ebp,%esp
  2a:   5d                      pop    %ebp
  2b:   28 c2                   sub    %al,%dl
  2d:   0f be c2                movsbl %dl,%eax
  30:   c3                      ret    
  31:   90                      nop    

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