[Haskell-cafe] Haskell performance question

[Don Stewart]

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 <stdio.h>
> 
> #define n 100000000
> 
> double a[n];
> 
> int main()
> {
>    int i;
>    for (i = 0; i<n; ++i)
>    {
>        a[i] = 1.0f;
>    }
>    for (i = 0; i<n-1; ++i)
>    {
>        a[i+1] = a[i]+a[i+1];
>    }
>    printf("%f\n",a[n-1]);
> }
> 
> 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