Proposal: Add Data.Bits.popCount

[Edward Z. Yang]

+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
>