0 is not a power of 2

Andrei Alexandrescu via Digitalmars-d digitalmars-d at puremagic.com
Mon May 18 22:16:47 PDT 2015


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


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