On Tuesday, 19 May 2015 at 09:34:04 UTC, Almighty Bob wrote:
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.
Oops, should be either add the carry onto x, or subtract the
carry from tmp. Either way the result of the & is non zero.
