Re: 0 is not a power of 2

[Atila Neves via Digitalmars-d]

Aren't predictable branches cheap on current architectures?
Atila

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;
}

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.

Any ideas for faster code?


Thanks,

Andrei