> > It's cheaper again to use quotInt# and remInt# as I did in my code. >
Just want to point out that this is actually surprisingly less of an impact than I thought. As I did the step-by-step break down (you can see my blog post), this one (changing `quotRem` to `quotInt#` and `remInt#`) yielded almost negligible differences. Every other change had more impact than this, including turning the `Data.Array.Unboxed` into "lookup functions" (like `lenOnes 0 = 0; lenOnes 1 = 3; lenOnes 2 = 3; ...`) Chris On Wed, Aug 10, 2011 at 2:16 AM, Bryan O'Sullivan <b...@serpentine.com>wrote: > On Tue, Aug 9, 2011 at 9:47 AM, Chris Yuen <kizzx2+hask...@gmail.com>wrote: > >> >> - I was using GHC 32-bit. Int is 32-bit there, so I needed Int64. It turns >> out 64-bit operations in 32-bit programs are just darn slow. Maybe it's a >> Windows problem. > > > No, GHC calls out to C for 64-bit integer ops on all 32-bit platforms. > > >> On Linux 64 bit GHC Int is 64 bit so everything just works. Changing Int64 >> to Int liberates me from many `fromIntegral` which saved 20% >> > > Actually, fromIntegral is usually a no-op, so chances are you're seeing the > effect of something else. > > >> - Changing `divMod` to `quotRem` saved another 20% >> > > It's cheaper again to use quotInt# and remInt# as I did in my code. > > >> 1. Why are bangs needed on the length arrays? > > > GHC has to deconstruct the Vector in order to get at the real underlying > array, so that unsafeIndex can perform the actual index into the real > underlying array. > > Without bang patterns, the code performs at least one deconstruction on > every iteration through the loop. Each deconstruction has a cost. With the > bang patterns, they're all hoisted out and performed just once. >
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