Re: [Haskell-cafe] performance issues with popCount

On Fri, 2012-09-07 at 15:55 -0700, Johan Tibell wrote:

On Fri, Sep 7, 2012 at 4:54 AM, Nicolas Trangez wrote:

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On Thu, 2012-09-06 at 12:07 -0700, Johan Tibell wrote:

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Have a look at the popCount implementation for e.g. Int, which are written in C and called through the FFI:

https://github.com/ghc/packages-ghc-prim/blob/master/cbits/popcnt.c

Out of interest: isn't this compiled into the popCnt# primop (and popcnt instruction on SSE4.2)?

It's the other way around the popCnt# primop is compiled into either calls to these C functions or into the popcnt instruction, if -msse4.2 is given.

Yes, indeed. I was referring to Data.Bits.popCount, but looking back this was indeed completely non-obvious. My bad.

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I recently noticed Data.IntSet also contains a fairly basic "bitcount" implementation [1]. Is this kept as-is for a reason, instead of using popCount from Data.Bits?

I don't think so, except that we want to support the last 3 released versions of GHC so we need to have a fallback if Data.Bits.popCount isn't defined.

I might look into creating a (CPP-based) patch then. Nicolas