什么是.cfi和.LFE由GCC从C ++程序产生的装配code？ [英] What is .cfi and .LFE in assembly code produced by GCC from c++ program?

问题描述

我有以下的C ++ code

I have the following c++ code

int factorial(int n){

    if(n==0){
        return 1;
    }
    return n*factorial(n-1);

}

int main(void){
    factorial(5);
    return 0;
}

当我创建一个使用g ++的汇编文件-S factorial.cpp我得到以下几点：

When I create an assembly file using g++ -S factorial.cpp I get the following:

    .file   "tail_call_opt.cpp"
    .text
.globl _Z9factoriali
    .type   _Z9factoriali, @function
_Z9factoriali:
.LFB0:
    .cfi_startproc
    .cfi_personality 0x0,__gxx_personality_v0
    pushl   %ebp
    .cfi_def_cfa_offset 8
    movl    %esp, %ebp
    .cfi_offset 5, -8
    .cfi_def_cfa_register 5
    subl    $24, %esp
    cmpl    $0, 8(%ebp)
    jne .L2
    movl    $1, %eax
    jmp .L3
.L2:
    movl    8(%ebp), %eax
    subl    $1, %eax
    movl    %eax, (%esp)
    call    _Z9factoriali
    imull   8(%ebp), %eax
.L3:
    leave
    ret
    .cfi_endproc
.LFE0:
    .size   _Z9factoriali, .-_Z9factoriali
.globl main
    .type   main, @function
main:
.LFB1:
    .cfi_startproc
    .cfi_personality 0x0,__gxx_personality_v0
    pushl   %ebp
    .cfi_def_cfa_offset 8
    movl    %esp, %ebp
    .cfi_offset 5, -8
    .cfi_def_cfa_register 5
    andl    $-16, %esp
    subl    $16, %esp
    movl    $5, (%esp)
    call    _Z9factoriali
    movl    $0, %eax
    leave
    ret
    .cfi_endproc
.LFE1:
    .size   main, .-main
    .ident  "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"
    .section    .note.GNU-stack,"",@progbits

我可以按照这个最，但什么是.cfi和.LFE部分的目的是什么？
我在哪里可以了解更多有关海湾合作委员会生成的汇编？

I can follow most of this, but what is the purpose of the .cfi and the .LFE parts? Where can I learn more about the assembly produced by gcc?

推荐答案

这些指令告诉气为发出，显然是用于重建时，帧指针丢失在堆栈矮人呼叫帧信息标签。你的情况帧指针是present，所以我想它可能被用来执行异常处理时平仓。这样的机制具有比老sjlj（setjump /跳远踏板）一个开销少。请参见这里，也是连锁矮小规范。

These directives tell gas to emit Dwarf Call Frame Information tags which are apparently used to reconstruct a stack backtrace when a frame pointer is missing. In your case the frame pointer is present, so I guess it could be used to perform unwinding during exception handling. Such mechanism has less overhead than the old sjlj (setjump/longjump) one. See here, and also the linked Dwarf spec.

至于.Lxx标签，.L preFIX表明该标签是本地到该文件，因此不会与其他文件相同的名称标签冲突。 GCC通常使用.L为自动生成的标签。在这种情况下，最有可能的FB是指功能开始和FE是指功能结束。

As for .Lxx labels, .L prefix indicates that the label is local to this file and so will not conflict with the same-name labels in other files. GCC generally uses .L for auto-generated labels. In this case most likely "FB" means "function begin" and "FE" means "function end".

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