How to take value by reference in visual c++ inline assembler?

Question

I'd written the function to swap two integers, I want to take both values and then place it to the variables:

inline void swap_fast(int32& a, int32& b)
{
    ASM // __asm {
        mov eax, [a]
        mov ebx, [b]
        mov [a], ebx
        mov [b], eax
    ENDASM // }
}

But it seems like I'm taking the addresses, not the values.

EAX = 0023FEB0 EBX = 0023FEA4

Where did I mistaken?

Answer 1

From compiled code point of view references are pointers (that's why they behave like pointers). It's compiler that doesn't let you manage them as pointers in your C++ code (C++, not C because C has not references).

For this reason yes, you have pointers then you have to deference them to swap variable values. For an example of (working) assembly code take a look to this post here on SO.

Moreover I'd suggest to use __declspec(naked) too so you can write your own short prolog/epilog (otherwise performance benefit of a custom assembly function is pretty small, if there is a benefit at all).

That said you don't even need to use a register for that and you can do it with stack, just pop in reverse order:

__declspec(naked) void swap_fast(int32& a, int32& b)
{
    __asm
    {
        push [a]
        push [b]
        pop [a]
        pop [b]
    }
}

Same can be done through a register (as temporary variable as you're doing, just do not deference pointers) or with xchg, I suppose out there there are a plethora of different algorithms to perform swap in C, C++ and assembly.

Important note about performance: actually I don't know which method is faster; probably a macro with a temporary (register?) variable has same speed as our smart assembly code because compiler will be free to optimize generated assembly much better...