On Thursday, 21 May 2015 at 13:24:57 UTC, Steven Schveighoffer
wrote:
Gah, I messed up, used output from old code that wasn't doing
what I thought it was. You are right about that.
But my whole point there was that x >> bsr(x) is ALWAYS 1.
2 >> bsr(2) == 1
3 >> bsr(3) == 1
4 >> bsr(4) == 1
17 >> bsr(17) == 1
So really, your test checks to see if a value is zero or not,
not whether it's a power of 2.
BUT, the opposite mechanism would work:
1 << bsr(x) == x;
Ha yes. You'd want to use TZCNT.
Alternatively, with bsf on could do:
x << bsf(x) == 1 << [32|64]
0 >> anything is 0.
Hah, good point :) Even if bsr(0) is undefined it doesn't
matter. Didn't think of that.
But that means the opposite solution I mentioned above, doesn't
work, still back to square one.
-Steve
Well no, there all needed to make it work :) Still no idea if
this is actually faster, but there is one less operation to
perform.
