Fast sign of integer in C

Question

There is a sign function in C:

int sign(int x)
{
    if(x > 0) return 1;
    if(x < 0) return -1;
    return 0;
}

Unfortunately, comp

Answer 1

First of all, integer comparison is very cheap. It's branching that can be expensive (due to the risk of branch mispredictions).

I have benchmarked your function on a Sandy Bridge box using gcc 4.7.2, and it takes about 1.2ns per call.

The following is about 25% faster, at about 0.9ns per call:

int sign(int x) {
    return (x > 0) - (x < 0);
}

The machine code for the above is completely branchless:

_sign:
    xorl    %eax, %eax
    testl   %edi, %edi
    setg    %al
    shrl    $31, %edi
    subl    %edi, %eax
    ret

Two things are worth pointing out:

1. The base level of performance is very high.
2. Eliminating branching does improve performance here, but not dramatically.