Does using xor reg, reg give advantage over mov reg, 0?

Question

There're two well-known ways to set an integer register to zero value on x86.

Either

mov reg, 0

or

xor reg, reg

There's an opinion that the second variant is better since the value 0 is not stored in the code and that saves several bytes of produced machine code. This is definitely good - less instruction cache is used and this can sometimes allow for faster code execution. Many compilers produce such code.

However there's formally an inter-instruction dependency between the xor instruction and whatever earlier instruction that changes the same register. Since there's a depedency the latter instruction needs to wait until the former completes and this could reduce the processor units load and hurt performance.

add reg, 17
;do something else with reg here
xor reg, reg

It's obvious that the result of xor will be exactly the same regardless of the initial register value. But it the processor able to recognize this?

I tried the following test in VC++7:

const int Count = 10 * 1000 * 1000 * 1000;
int _tmain(int argc, _TCHAR* argv[])
{
    int i;
    DWORD start = GetTickCount();
    for( i = 0; i < Count ; i++ ) {
        __asm {
            mov eax, 10
            xor eax, eax
        };
    }
    DWORD diff = GetTickCount() - start;
    start = GetTickCount();
    for( i = 0; i < Count ; i++ ) {
        __asm {
            mov eax, 10
            mov eax, 0
        };
    }
    diff = GetTickCount() - start;
    return 0;
}

With optimizations off both loops take exactly the same time. Does this reasonably prove that the processor recognizes that there's no dependency of xor reg, reg instruction on the earlier mov eax, 0 instruction? What could be a better test to check this?

Answer 1

an actual answer for you:

Intel 64 and IA-32 Architectures Optimization Reference Manual

Section 3.5.1.8 is where you want to look.

In short there are situations where an xor or a mov may be preferred. The issues center around dependency chains and preservation of condition codes.