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    <p>Out of curiosity, I ran this with llvm:</p> <blockquote> <p>ghc Test.hs -O2 -XBangPatterns -fllvm -optlo-O3</p> </blockquote> <p>and it took it down from 60ms to 24ms. Still not ideal.</p> <p>So, one of the first things I'll do when I want to know why a benchmark like this is so slow, is dump the prepared core. That is, the core after optimisations.</p> <blockquote> <p>ghc Test.hs -O2 -ddump-prep -dsuppress-all -XBangPatterns > Test.hscore</p> </blockquote> <p>After looking through the core, I eventually found $wa, where the iterative loop is defined. It turns out it's making surprisingly many index bound checks. See, I usually use Data.Vector.Unboxed, which has "unsafeRead" and "unsafeIndex" functions, to remove bounds checks. These would be useful here. Personally, I think the vector package is superior.</p> <p>If you look at $wa, you'll notice it's unboxing the arguments at the start:</p> <pre><code>case w_s3o9 of _ { STUArray l_s3of u_s3oi ds1_s3ol ds2_s3oH -&gt; case l_s3of of wild2_s3os { I# m_s3oo -&gt; case u_s3oi of wild3_s3ot { I# n1_s3ov -&gt; case ds1_s3ol of wild4_s3oC { I# y1_s3oE -&gt; </code></pre> <p>this looks bad, but it turns out in the recursive call it's using a specialised version, integrate_$s$wa, with unboxed integers etc. This is good.</p> <p>In summary, I think you should get a good improvement by using vector with unsafe indexing.</p> <p>Edit: here is a modified version with Data.Vector. It runs in about 7ms. For <em>good</em> vector code, I think the only slowness compared to C should be due to incomplete alias analysis. <a href="https://gist.github.com/amosr/6026995">https://gist.github.com/amosr/6026995</a></p>
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    1. POWhy does Haskell perform so poorly when executing C-like codes? (in this instance at least)
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    1. COThe `array` package has `unsafeRead` and `unsafeWrite` too, there's no need to switch to `vector` for that.
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      1. USDaniel Fischer
    2. COOh, okay. I had a quick look but couldn't see them. Obviously too quick
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    3. CO@DanielFischer I do not see those methods in the MArray interface. Can they really be implemented when the arrays can be indexed by any element implementing `Ix`?
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