Proposal: Add Data.Bits.popCount

Maciej Marcin Piechotka uzytkownik2 at
Wed Aug 17 19:44:17 CEST 2011

On Wed, 2011-08-17 at 18:37 +0200, Johan Tibell wrote:
> 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

I posted bug for various similar instructions some time ago:

Probably the update should be done in batch (i.e. popCount,
trailing/leading zeros etc.)


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