gcc 疯狂优化级别（-O3）还不够疯狂吗？

发布于 2024-10-18 13:57:48 字数 975 浏览 1 评论 0原文

作为回答另一个问题的一部分，我想表明 gcc (-O3) 的疯狂优化级别基本上会删除 main 中未使用的任何变量。代码是：

#include <stdio.h>
int main (void) {
   char bing[71];
   int x = 7;
   bing[0] = 11;
   return 0;
}

并且 gcc -O3 输出是：

    .file "qq.c"
    .text
    .p2align 4,,15
.globl main
    .type main, @function
main:
    pushl %ebp
    xorl %eax, %eax
    movl %esp, %ebp
    popl %ebp
    ret
    .size main, .-main
    .ident "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"
    .section .note.GNU-stack,"",@progbits

现在我可以看到它已经删除了局部变量，但其中仍然存在相当多的浪费。在我看来，整个：

    pushl %ebp
    xorl %eax, %eax
    movl %esp, %ebp
    popl %ebp
    ret

部分可以替换为更简单的：

    xorl %eax, %eax
    ret

有谁知道为什么gcc不执行此优化？我知道这对于 main 本身来说节省的很少，但是，如果这也用普通函数完成，那么在大规模循环中不必要地调整堆栈指针的影响将是相当大的。

用于生成程序集的命令是：

gcc -O3 -std=c99 -S qq.c

原文

As part of answering another question, I wanted to show that the insane level of optimisation of gcc (-O3) would basically strip out any variables that weren't used in main. The code was:

#include <stdio.h>
int main (void) {
   char bing[71];
   int x = 7;
   bing[0] = 11;
   return 0;
}

and the gcc -O3 output was:

    .file "qq.c"
    .text
    .p2align 4,,15
.globl main
    .type main, @function
main:
    pushl %ebp
    xorl %eax, %eax
    movl %esp, %ebp
    popl %ebp
    ret
    .size main, .-main
    .ident "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"
    .section .note.GNU-stack,"",@progbits

Now I can see it's removed the local variables but there's still quite a bit of wastage in there. It seems to me that the entire:

    pushl %ebp
    xorl %eax, %eax
    movl %esp, %ebp
    popl %ebp
    ret

section could be replaced with the simpler:

    xorl %eax, %eax
    ret

Does anyone have any idea why gcc does not perform this optimisation? I know that would save very little for main itself but, if this were done with normal functions as well, the effect of unnecessarily adjusting the stack pointer in a massive loop would be considerable.

The command used to generate the assembly was: