0 is not a power of 2

[John Colvin via Digitalmars-d]

On Tuesday, 19 May 2015 at 15:39:16 UTC, safety0ff wrote:
> On Tuesday, 19 May 2015 at 08:28:11 UTC, John Colvin wrote:
>>
>> I tested with a few different (modern) backends to see what 
>> was generated, they all essentially give you this (gcc 5.1.0 
>> -O3 -march=broadwell):
>>
>> isPowerOf2:
>> 	xorl	%eax, %eax
>> 	testl	%edi, %edi
>> 	je	.L5
>> 	blsr	%edi, %edi
>> 	testl	%edi, %edi
>> 	sete	%al
>> .L5:
>> 	ret
>
> I think you used:
>     return x && (x & (x - 1)) == 0;
> instead of
>     return (x & (x - 1)) == 0 && x;
>
> Which influences code generation (more weight on the x == 0 
> test,) hence the branch.

I used what andrei posted, but yes i do see the difference. How 
infuriating. A compiler that will entirely restructure your code 
leaving you with something unrecognisable in many cases, but in 
others is sensitive to the order of operands around an &&.