将星星放在寄存器前是什么意思?

问题描述

我正在研究c ++如何通过汇编语言调用正确的成员函数.我附带的简单程序如下:

I am studying how c++ invoke the right member functions through assembly language.The simple program that I come with is as the following:

class A                        
{                              
    public:                    
        virtual void show() {} 
};                             

class B : public A             
{                              
    public:                    
        void show() {}         
};                             


int main()                     
{                              
    A* pA = new B;             
    pA->show();                

    return 0;                  
}                              

其汇编如下:

main:
.LFB2:
    .cfi_startproc
    .cfi_personality 0x3,__gxx_personality_v0
    pushq   %rbp
    .cfi_def_cfa_offset 16
    movq    %rsp, %rbp
    .cfi_offset 6, -16
    .cfi_def_cfa_register 6
    pushq   %rbx
    subq    $24, %rsp
    movl    $8, %edi
    .cfi_offset 3, -24
    call    _Znwm            <<<================ 1
    movq    %rax, %rbx
    movq    %rbx, %rax
    movq    %rax, %rdi
    call    _ZN1BC1Ev
.L11:
    movq    %rbx, %rax
    movq    %rax, -24(%rbp)
    movq    -24(%rbp), %rax
    movq    (%rax), %rax
    movq    (%rax), %rdx
    movq    -24(%rbp), %rax
    movq    %rax, %rdi
    call    *%rdx             <<<=== 2
    movl    $0, %eax
    addq    $24, %rsp
    popq    %rbx
    leave
    ret
    .cfi_endproc

我的问题是:

1. 我在google中搜索了nwm，它只告诉我c ++使用它来分配内存，有人可以告诉我更多吗?在一个库中吗?如果可能，我可以获取其源代码吗?怎么样?
2. 我对这种语法不太熟悉，它想做什么?

1. I searched google for nwm, it only tells me that c++ uses it to allocate memory, could some one tell me more about it ? is it in one lib? if possible, can i get its source code? how?
2. i am not quite familiar with this syntax, what does it want to do ?

推荐答案

*在AT& T汇编语法中的绝对地址之前用于调用或跳转指令.这意味着它将跳转到寄存器中包含的地址.另一种选择是相对跳转，它相对于当前指令.

The * is used before absolute addresses in AT&T assembly syntax for call or jump instructions. This means it will jump to the address contained in the register. The alternative is a relative jump, which is relative to the current instruction.

从 GNU as手册:

在您的代码中，有必要调用寄存器中的地址.调用pA->show()需要进行查找，以查看要调用的正确函数是什么.这是因为它是类A上的虚函数.

In your code it makes sense for there to be a call to an address in a register. Calling pA->show() requires a look up to see what the right function to be called is. This is because it is a virtual function on class A.

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