Re: [Haskell-cafe] GHC vs GCC

On Sat, Mar 27, 2010 at 12:43 AM, Rafael Almeida wrote:

On Fri, Mar 26, 2010 at 6:49 PM, Jason Dagit wrote:

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On Fri, Mar 26, 2010 at 2:33 PM, Bryan O'Sullivan wrote:

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On Fri, Mar 26, 2010 at 10:46 AM, Rafael Cunha de Almeida wrote:

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During a talk with a friend I came up with two programs, one written in C and another in haskell.

Your Haskell code builds a huge thunked accumulator value, so of course it's slow (put bang patterns on all arguments). Also, you should use rem instead of mod. Make those tiny changes and you'll get a 5x speedup, to

half

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the performance of the C code.

Interesting. I had to add -fvia-C to get within half the performance of C. Just bang patterns and rem and I'm 1/5th of C. I'm on a x86_64 machine. I wonder if that plays in.

Jason

Using bang patterns didn't help almost anything here. Using rem instead of mod made the time go from 45s to 40s. Now, using -fvia-C really helped (when I used rem but not using mod). It went down to 10s.

It's worth pointing out that there's a bit of bang-pattern mysticism going on in this conversation (which has not been uncommon of late!). A non-buggy strictness analyzer should expose the strictness of these functions without difficulty. If bang patterns make any difference at all with a -O flag, either there's a strictness analysis bug, or some very interesting effects from shifting the order of forcing of strict variables. Putting in bang patterns is a good idea to plug the obvious space leak when run without optimization, but isn't going to make a difference for optimizing compilation of obviously-strict functions. -Jan-Willem Maessen