Proposal: Add Data.Bits.popCount

[Edward Kmett]

+1. I use this operation very heavily in geometric algebra.

On Wed, Aug 17, 2011 at 12:37 PM, Johan Tibell <johan.tibell at gmail.com>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
>
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