Question

Question says it all. Does anyone know if the following…

size_t div(size_t value) {
    const size_t x = 64;
    return value / x;
}

…is optimized into?

size_t div(size_t value) {
    return value >> 6;
}

Do compilers do this? (My interest lies in GCC). Are there situations where it does and others where it doesn’t?

I would really like to know, because every time I write a division that could be optimized like this I spend some mental energy wondering about whether precious nothings of a second is wasted doing a division where a shift would suffice.

Answer 1

Even with g++ -O0 (yes, -O0!), this happens. Your function compiles down to:

_Z3divm:
.LFB952:
        pushq   %rbp
.LCFI0:
        movq    %rsp, %rbp
.LCFI1:
        movq    %rdi, -24(%rbp)
        movq    $64, -8(%rbp)
        movq    -24(%rbp), %rax
        shrq    $6, %rax
        leave
        ret

Note the shrq $6, which is a right shift by 6 places.

With -O1, the unnecessary junk is removed:

_Z3divm:
.LFB1023:
        movq    %rdi, %rax
        shrq    $6, %rax
        ret

Results on g++ 4.3.3, x64.