C bit twiddling so that Zero => 1 while Non-Zero => -1? (ideally x86 intrinsic)-CodePudding

I'm looking for a way to achieve what I wrote in the title. I'm doing it now with an "if" and I want to get rid of branching. I've had a look at a couple of pages such as this one, can't find the exact thing I'm looking for.

CodePudding user response：

Converting x to a boolean does not generate any branches on current x86 processors. You can use simple arithmetics to generate your result:

int test_zero(int x) {
    return 1 - 2 * !!x;
}

gcc 11.2 generates this:

test_zero:
    cmp     edi, 1
    sbb     eax, eax
    and     eax, 2
    sub     eax, 1
    ret

clang 13.0.0 generates this:

test_zero:                              # @test_zero
    xor     eax, eax
    test    edi, edi
    sete    al
    add     eax, eax
    add     eax, -1
    ret

As commented by dratenik, even simpler and more readable source compiles to exactly the same branchless executable code:

int test_zero2(int x) {
    return x ? -1 : 1;
}

You check the code generation on Godbolt's compiler explorer.

CodePudding user response：

Without bit-wise operation, you can do something like:

int func(int x)
{
    return int(uint(x-1) / uint(-1)) * 2 - 1;
}

Or:

int func(int x)
{
    return int(uint(~x) / uint(~0)) * 2 - 1;
}