Checking if an Integer is an Exact Binary Power

[Andrei Alexandrescu via Digitalmars-d]

On 4/24/16 9:17 PM, Xinok wrote:
> I modified David's solution a bit to (hopefully) eliminate the branch:
> bool powerOf2(uint x){ return !x ^ !(x & (x - 1)); }
>
>      0000000000000000 <_D4test8powerOf2FkZb>:
>         0:   50                      push   %rax
>         1:   53                      push   %rbx
>         2:   48 89 fa                mov    %rdi,%rdx
>         5:   85 d2                   test   %edx,%edx
>         7:   0f 94 c0                sete   %al
>         a:   8d 8a ff ff ff ff       lea    -0x1(%rdx),%ecx
>        10:   85 ca                   test   %ecx,%edx
>        12:   0f 94 c3                sete   %bl
>        15:   32 c3                   xor    %bl,%al
>        17:   5b                      pop    %rbx
>        18:   59                      pop    %rcx
>        19:   c3                      retq
>        1a:   66 0f 1f 44 00 00       nopw   0x0(%rax,%rax,1)
>
> The branches are gone but I'm really not sure if this is better or worse.

With gdc https://godbolt.org/g/jcU4np isPow2B is the winner (shortest 
code, simplest instructions). -- Andrei