Checking if an Integer is an Exact Binary Power

[Xinok via Digitalmars-d]

On Sunday, 24 April 2016 at 23:17:53 UTC, David Nadlinger wrote:
> On Sunday, 24 April 2016 at 23:00:56 UTC, Temtaime wrote:
>> Please no cmp.
>> Just
>> bool isPowerOf2(uint x) { return x && !(x & (x - 1)); }
>
> You do realise that this will (typically) emit a branch?
>
>  — David

I compiled using dmd -O and looked at the disassembly using 
objdump -D and there are indeed a couple branches in the 
disassembly:

     0000000000000000 <_D4test8powerOf2FkZk>:
        0:   50                      push   %rax
        1:   48 89 f9                mov    %rdi,%rcx
        4:   85 c9                   test   %ecx,%ecx
        6:   74 0a                   je     12 
<_D4test8powerOf2FkZk+0x12>
        8:   8d 81 ff ff ff ff       lea    -0x1(%rcx),%eax
        e:   85 c1                   test   %eax,%ecx
       10:   74 04                   je     16 
<_D4test8powerOf2FkZk+0x16>
       12:   31 c0                   xor    %eax,%eax
       14:   eb 05                   jmp    1b 
<_D4test8powerOf2FkZk+0x1b>
       16:   b8 01 00 00 00          mov    $0x1,%eax
       1b:   59                      pop    %rcx
       1c:   c3                      retq
       1d:   0f 1f 00                nopl   (%rax)


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.