海湾合作委员会疯狂优化水平（-O3）不够疯狂？ [英] Is the gcc insane optimisation level (-O3) not insane enough?

问题描述

作为回答另一个问题的一部分，我想说明对 gcc （ -O3 >）将基本上去掉任何未在main中使用的变量。代码是：



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;
}

和 gcc -O3 输出为：

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

现在我可以看到它已被删除局部变量，但在那里仍然有相当多的浪费。在我看来，整个：

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

更简单：

section could be replaced with the simpler:

    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 本身会节省很少，但是，如果这也是用普通函数完成的话，那么在大量循环中不必要地调整堆栈指针的效果将是相当大的。

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.

用于生成程序集的命令为：

The command used to generate the assembly was:

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

推荐答案

您可以使用 -fomit-frame-pointer 编译器标志启用特定的优化。这样做使得在一些机器上调试变得不可能，而在其他所有机器上调试都变得困难，这就是为什么它通常被禁用的原因。

You can enable that particular optimization with the -fomit-frame-pointer compiler flag. Doing so makes debugging impossible on some machines and substantially more difficult on everything else, which is why it's usually disabled.

虽然你的GCC文档可能会说 -fomit-frame-pointer 在各种优化级别启用，您可能会发现情况并非如此 - 您几乎肯定必须自己明确启用它。

Although your GCC documentation may say that -fomit-frame-pointer is enabled at various optimization levels, you'll likely find that that's not the case—you'll almost certainly have to explicitly enable it yourself.

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