+1 Excerpts from Johan Tibell's message of Wed Aug 17 12:37:25 -0400 2011:
Hi!
Now when we have a fast popCnt# primop* it would be nice to expose it to the world through an official API. I propose we add this method to the Bits class in Data.Bits:
-- | Return the number of set bits in the argument, know as the -- population count or the Hamming weight. popCount :: a -> Int
I will provide a default implementation, which means that this change won't break any existing user defined instances. All the instances for the basic types (Ints, Words, etc) will use the primops.
* These primops compile to a single POPCNT instruction if the user compile using -msse4.2 and a fast lookup table based implementation otherwise.
Discussion period: 2 weeks
Cheers, Johan