0 is not a power of 2

[Steven Schveighoffer via Digitalmars-d]

On 5/19/15 1:16 AM, Andrei Alexandrescu wrote:
> So there's this classic trick:
>
> bool isPowerOf2(uint x)
> {
>      return (x & (x - 1)) == 0;
> }
>
> Pretty neat, but it wrongly returns true for x == 0. So the obvious fix is:
>
> bool isPowerOf2(uint x)
> {
>      return x && (x & (x - 1)) == 0;
> }
>
> But that has branches in it. So I came up with:
>
> bool isPowerOf2(uint x)
> {
>      return (x & (x - 1) | !x) == 0;
> }
>
> which has no branches at least with dmd, see http://goo.gl/TVkCwc.

Is it just me, or is it odd that your first version generates xor and a 
bunch of jumps?

I don't see xor anywhere in the "fast" version.

Another possibility is to avoid the zero check altogether. There may be 
cases where you already know before calling isPowerOf2 that it's not 
zero, and checking for zero again is wasteful.

Note that bsr is undefined for 0, so there is precedent for this.

-Steve