0 is not a power of 2

[Almighty Bob via Digitalmars-d]

On Tuesday, 19 May 2015 at 05:16:48 UTC, Andrei Alexandrescu 
wrote:
> So there's this classic trick:
>
> bool isPowerOf2(uint x)
> {
>     return (x & (x - 1)) == 0;
> }
>
> which has no branches at least with dmd, see 
> http://goo.gl/TVkCwc.
>
> Any ideas for faster code?

If you dont mind asm then after you do...

tmp = x-1;

you could add the borrow/carry flag back onto the tmp, so it'd 
add back up to zero any time there's an underflow in the (x-1) op.

So two extra instructions, (you need a zero for the ADC) no 
branch.