GCC 内联汇编错误：不能取'this'的地址，这是一个右值表达式

Question

I'm still fighting with GCC - compiling the following inline assembly code (with -fasm-blocks , which enables Intel style assembly syntax) nets me a strange error Cannot take the address of 'this', which is an rvalue expression ...

MyClass::MyFunction()
{
    _asm
    {
        //...
        mov ebx, this // error: Cannot take the address of 'this', which is an rvalue expression
        //...
        mov eax, this // error: Cannot take the address of 'this', which is an rvalue expression
        //...
    };
}

Why can I store pointers to different objects in registers, but can't use pointer to MyClass instance?

Answer 1

That's because the compiler might decide on its own to store this in a register (generally ECX ) instead of a memory cell, for optimization purposes, or because the calling convention explicitly specifies it should do that.

In that case, you cannot take its address, because registers are not addressable memory.

Answer 2

You can use something like this:

#include <stdio.h>

class A{
public:
    void* work(){
        void* result;
        asm( "mov %%eax, %%eax"
            : "=a" (result) /* put contents of EAX to result*/
            : "a"(this)     /* put this to EAX */
            );
        return result;
    }
};


int main(){
    A a;
    printf("%x - %x\n", &a, a.work());
}

See more details on operands passing to inline asm here

Answer 3

Practically speaking, each implementation defines its own rules with regards to asm. In the case of g++, it looks like when you write mov ebx, something , g++ needs the address of something in order to generate the instruction. (Not to surprising, really, given the way assemblers work.) this doesn't have an address. (That's what being an rvalue means.) The implementation could treat this as a special case in inline assembler, and replace it with whatever is appropriate at that spot in the code. g++ doesn't do this, probably because it has another, more general mechanism (elder_george's solution) which handles the problem.