On Mar 19, 2007, at 2:21 PM, Reid Spencer wrote:
> On Mon, 2007-03-19 at 14:16 -0700, Chris Lattner wrote:
>>> Implement isOneBitSet in terms of APInt::countPopulation.
>>
>>> @@ -3474,8 +3474,7 @@
>>> // isOneBitSet - Return true if there is exactly one bit set in
>>> the specified
>>> // constant.
>>> static bool isOneBitSet(const ConstantInt *CI) {
>>> - uint64_t V = CI->getZExtValue();
>>> - return V && (V & (V-1)) == 0;
>>> + return CI->getValue().countPopulation() == 1;
>>> }
>>
>> Are you sure this is a good idea? countPopulation is *much* slower
>> than a couple of and's and a subtract.
>
> Its the temporary construction of APInts that makes the performance of
> the existing algorithm poor. This will construct 3 temporaries, each
> potentially with a malloc. Using countPopulation is constant time (I
> agree, not super fast, but consistent) and much easier to read in the
> code.
Optimizing for the "big" apint case isn't interesting, please
optimize for the small case.
-Chris
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