了解gcc -S输出

问题描述

我在下面的x86_64上非常复杂的程序上做了gcc -S:

I did gcc -S on the very complex program below on x86_64:

int main() {
    int x = 3;
    x = 5;
    return 0;
}

我得到的是:

       .file   "main.c"
        .text
.globl main
        .type   main, @function
main:
.LFB0:
        .cfi_startproc
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        movl    $3, -4(%rbp)
        movl    $5, -4(%rbp)
        movl    $0, %eax
        leave
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc
.LFE0:
        .size   main, .-main
        .ident  "GCC: (GNU) 4.4.7 20120313 (Red Hat 4.4.7-3)"
        .section        .note.GNU-stack,"",@progbits

我在想，是否有人可以帮助我理解输出或使我参考一些链接说明.具体来说，cfi ,LFB0,LFE0 , leave是什么意思?我所能找到的关于这些的是这篇文章但无法完全理解它的用途.另外，在这种情况下ret会做什么?我猜它正在返回到__libc_start_main()，这反过来又叫do_exit()，对吗?

I was wondering if someone could help me understand the output or refer me to some link explaining. Specifically, What does cfi ,LFB0,LFE0 , leave mean? All I could find regarding these is this post but couldn't fully understand what it was for. Also, what does ret do in this case? I'm guessing it's returning to __libc_start_main() which in turn would call do_exit() , is that correct?

推荐答案

那些.cfisomething指令导致编译器生成其他数据.当指令导致异常时，此数据有助于遍历调用堆栈，因此可以找到并正确执行异常处理程序(如果有).调用堆栈信息对于调试很有用.此数据最有可能进入可执行文件的单独部分.不会在您的代码说明之间插入它.

Those .cfisomething directives result in generation of additional data by the compiler. This data helps traverse the call stack when an instruction causes an exception, so the exception handler (if any) can be found and correctly executed. The call stack information is useful for debugging. This data most probably goes into a separate section of the executable. It's not inserted between the instructions of your code.

.LFsomething:只是常规标签，可能被这些额外的异常相关数据引用.

.LFsomething: are just regular labels that are probably referenced by that extra exception-related data.

leave和ret是CPU指令.

leave等效于:

movq    %rbp, %rsp
popq    %rbp

并且取消了这两个指令的作用

and it undoes the effect of these two instructions

pushq   %rbp
movq    %rsp, %rbp

以及通过从rsp减去一些东西来分配堆栈空间的指令.

and instructions that allocate space on the stack by subtracting something from rsp.

ret从函数返回.它从堆栈中弹出返回地址并跳转到该地址.如果是__libc_start_main()调用了main()，那么它将返回那里.

ret returns from the function. It pops the return address from the stack and jumps to that address. If it was __libc_start_main() that called main(), then it returns there.

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