为什么GCC输出机code有NOP指令 [英] Why does gcc output machine code have nop instructions

问题描述

每次我做一个objdump的我-d总是看到ASM code。与NOP指令（什么也不做说明）批次

Everytime I do an objdump -d I always see the asm code with batches of nop instructions (instructions that do nothing)

例如借此同一个程序：

#include <stdio.h>
#include <math.h>

int main()
{
    printf("Hello World!\n");
    printf("cos:  %f\n", cos(1));
    return 1;
}

有关exampe的objdump的有2个空指令在入口点的结束

The objdump for exampe has 2 nops at the end of the entry point

0000000000400450 <_start>:
400450: 31 ed                   xor    %ebp,%ebp
400452: 49 89 d1                mov    %rdx,%r9
400455: 5e                      pop    %rsi
400456: 48 89 e2                mov    %rsp,%rdx
400459: 48 83 e4 f0             and    $0xfffffffffffffff0,%rsp
40045d: 50                      push   %rax
40045e: 54                      push   %rsp
40045f: 49 c7 c0 00 06 40 00    mov    $0x400600,%r8
400466: 48 c7 c1 70 05 40 00    mov    $0x400570,%rcx
40046d: 48 c7 c7 34 05 40 00    mov    $0x400534,%rdi
400474: e8 bf ff ff ff          callq  400438 <__libc_start_main@plt>
400479: f4                      hlt    
40047a: 90                      nop
40047b: 90                      nop 

而这只是众多例子中的一个，但你的想法。为什么在C code编译这种方式？先谢谢了。

And that is just one of many examples but you get the idea. Why is the C code compiled this way? Thanks in Advance.

推荐答案

很多时候，那些只是用来做填充，这样后续的东西开始就一个字或边界再次，因为获得任意code未对齐字边界是CPU昂贵得多。

Very often those are just used to do padding so that subsequent stuff starts on a word or boundary again, as access to arbitrary code that is not aligned on word boundaries is much more expensive for the cpu.

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