Re: [Haskell-cafe] Haskell performance question

dpiponi:

I see lots of shootout examples where Haskell programs seem to perform comparably with C programs, but I find it hard to reproduce anything like those figures when testing with my own code. So here's a simple case:

I have this C program:

#include

#define n 100000000

double a[n];

int main() { int i; for (i = 0; i

And this Haskell program:

import Data.Array.IO import Control.Monad

n = 10000000

main = do a <- newArray (0,n-1) 1.0 :: IO (IOUArray Int Double) forM_ [0..n-2] $ \i -> do { x <- readArray a i; y <- readArray a (i+1); writeArray a (i+1) (x+y) } x <- readArray a (n-1) print x

Be really careful with Doubles (i.e. check what Int arrays also do), since you need special flags to get good performance from unboxed Double arrays. The best effort I know of is: http://shootout.alioth.debian.org/gp4/benchmark.php?test=spectralnorm&lang=all which took some time to work out the right path through the compiler. In essence: * some gcc flags: -optc-O3 -optc-march=pentium4 -optc-mfpmath=sse -optc-msse2 -optc-ffast-math * Foreign.Marshal.Array * type Reals = Ptr Double * inlinePerformIO in the end I'm stunned at how well we did there, since Double math has been a long term issue. Perhaps there are other lessons we can extract from that code? -- Don