C++代码的汇编代码中，static_initialization_and_destruction和_GLOBAL__sub_I_main函数的目的是什么？

Question

以下是C++源代码。该代码具有一个名为HumanBeing的类，其中包含Display和verify函数。每个函数都会打印语句。

#include <iostream>

using namespace std;

class HumanBeing {
public:
    void display() {
        cout << "hello aam a human being" << endl;
    }
    void print() {
        cout << "verify print" << endl;
    }
};

int main() {
    HumanBeing vamshi;

    vamshi.display();
    vamshi.print();
    return 0;
}

这是上述C++代码的相应汇编代码。

        .file   "verify.cpp"
        .local  _ZStL8__ioinit
        .comm   _ZStL8__ioinit,1,1
        .section        .rodata
.LC0:
        .string "hello aam a human being"
        .section        .text._ZN10HumanBeing7displayEv,"axG",@progbits,_ZN10HumanBeing7displayEv,comdat
        .align 2
        .weak   _ZN10HumanBeing7displayEv
        .type   _ZN10HumanBeing7displayEv, @function
_ZN10HumanBeing7displayEv:
.LFB971:
        .cfi_startproc
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        subq    $16, %rsp
        movq    %rdi, -8(%rbp)
        movl    $.LC0, %esi
        movl    $_ZSt4cout, %edi
        call    _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
        movl    $_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_, %esi
        movq    %rax, %rdi
        call    _ZNSolsEPFRSoS_E
        leave
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc
.LFE971:
        .size   _ZN10HumanBeing7displayEv, .-_ZN10HumanBeing7displayEv
        .section        .rodata
.LC1:
        .string "verify print"
        .section        .text._ZN10HumanBeing5printEv,"axG",@progbits,_ZN10HumanBeing5printEv,comdat
        .align 2
        .weak   _ZN10HumanBeing5printEv
        .type   _ZN10HumanBeing5printEv, @function
_ZN10HumanBeing5printEv:
.LFB972:
        .cfi_startproc
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        subq    $16, %rsp
        movq    %rdi, -8(%rbp)
        movl    $.LC1, %esi
        movl    $_ZSt4cout, %edi
        call    _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
        movl    $_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_, %esi
        movq    %rax, %rdi
        call    _ZNSolsEPFRSoS_E
        leave
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc
.LFE972:
        .size   _ZN10HumanBeing5printEv, .-_ZN10HumanBeing5printEv
        .text
        .globl  main
        .type   main, @function
main:
.LFB973:
        .cfi_startproc
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        subq    $16, %rsp
        leaq    -1(%rbp), %rax
        movq    %rax, %rdi
        call    _ZN10HumanBeing7displayEv
        leaq    -1(%rbp), %rax
        movq    %rax, %rdi
        call    _ZN10HumanBeing5printEv
        movl    $0, %eax
        leave
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc
.LFE973:
        .size   main, .-main
        .type   _Z41__static_initialization_and_destruction_0ii, @function
_Z41__static_initialization_and_destruction_0ii:
.LFB982:
        .cfi_startproc
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        subq    $16, %rsp
        movl    %edi, -4(%rbp)
        movl    %esi, -8(%rbp)
        cmpl    $1, -4(%rbp)
        jne     .L5
        cmpl    $65535, -8(%rbp)
        jne     .L5
        movl    $_ZStL8__ioinit, %edi
        call    _ZNSt8ios_base4InitC1Ev
        movl    $__dso_handle, %edx
        movl    $_ZStL8__ioinit, %esi
        movl    $_ZNSt8ios_base4InitD1Ev, %edi
        call    __cxa_atexit
.L5:
        leave
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc
.LFE982:
        .size   _Z41__static_initialization_and_destruction_0ii, .-_Z41__static_initialization_and_destruction_0ii
        .type   _GLOBAL__sub_I_main, @function
_GLOBAL__sub_I_main:
.LFB983:
        .cfi_startproc

在这段代码中，我有以下疑问：

1. static_initialization_and_destruction -- 看起来像是函数，但它的确切工作方式并不清楚。

2. GLOBAL__sub_I_main 函数的目的和工作原理也不太清楚。

请问有人能够清晰地解释一下这段代码中构造函数和析构函数的工作和组织方式，以及 GLOBAL__sub_I_main 函数的目的吗？

- vamshi

Answer 1

两个函数都被gcc在C++中用于任何具有静态存储期并需要在main函数之前构造的实例化类，例如：

class A {
    A();
    ~A();
    ... 
};

A a;

// "a" will need to be constructed before main
int main()
{
    return a.Foo();
}
// "a" will need to be destructed after main

生成的_Z41__static_initialization_and_destruction_0ii函数（解码为__static_initialization_and_destruction_0(int, int)）有两个目的：

• 在预处理/编译的C++源文件中调用具有静态存储期的类的任何构造函数（在必要时）。
• 通过atexit将具有静态存储期的类的析构函数注册为退出函数。

_GLOBAL__sub_I_main是它周围的一个简单包装函数，其函数指针被添加到.init_array节中。在编译多个源文件时，每个单独的对象文件都将添加初始化函数到.init_array中，所有这些函数都将在调用main之前被调用。

虽然这可能不会立即显而易见，但您的代码包含具有静态存储期的实例。<iostream>头文件声明了std::ios_base::Init，需要尽早构造它以在静态对象的构造函数和析构函数中安全访问标准I/O流。

- Elijan9