为什么 GCC 先将整数加载到 eax，然后再加载到其他一些相关寄存器?

问题描述

使用这个站点，我为一个简单程序生成了汇编代码.

Using this site, I generated assembly code for a simple program.

main:
        push    rbp
        mov     rbp, rsp
        sub     rsp, 16
        mov     DWORD PTR [rbp-4], 5
        mov     eax, DWORD PTR [rbp-4]
        mov     edi, eax
        mov     eax, 0
        call    ifunc
        mov     eax, 0
        leave
        ret

这里 a 的值从内存加载到 eax 中，然后移动到 edi 传递给函数.但是我写了一个类似的汇编代码，直接将a的值加载到edi中.两者都工作正常.GCC 是否遵循此进行某种优化?先加载eax有什么好处?

Here value of a is loaded from memory into eax and then it is moved to edi to pass to function. But I wrote a similar assembly code and directly loaded value of a into edi. Both works fine. Does GCC follows this for some kind of optimization? What is the benefit of loading eax first?

推荐答案

恰恰相反.您正在编译没有优化，因此 gcc 不会进行优化，该优化会删除多余的 mov 和不必要的 a 作为堆栈内存中的变量.

Just the opposite. You're compiling without optimizations so gcc doesn't do the optimization that would have removed the redundant movs and the unnecessary allocation of a as a variable in stack memory.

先加载 eax 有什么好处?

它没有，只是多余的.未经优化的编译器代码包含这样看起来很愚蠢的东西是很常见的.

It has none and is just redundant. It is very common for unoptimized compiler code to contain stupid-looking stuff like this.

这篇关于为什么 GCC 先将整数加载到 eax，然后再加载到其他一些相关寄存器?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！