[Haskell-cafe] Haskell performance question

[Don Stewart]

nominolo:
> On Thu, 2007-11-08 at 10:33 -0800, Dan Piponi wrote:
> > 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
> > 
> > Even though 'n' is 10 times bigger in the C program it runs much
> > faster than the Haskell program on my MacBook Pro with Haskell 6.6.1.
> > I've tried lots of different combinations of flags that I've found in
> > various postings to haskell-cafe but to no avail.
> > 
> > What flags do I need to get at least within a factor of 2 or 3 of C?
> > Am I using the wrong kind of array? Or is Haskell always going to be
> > 15-20 times slower for this kind of numerical work?
> 
> Wow.  You should *really* try using GHC 6.8.1:
> 
> GHC 6.6.1  (-O2)
> 
> real    0m7.062s
> user    0m5.464s
> sys     0m0.288s
> 
> GHC 6.8.0.20071019 
> 
> real    0m0.723s
> user    0m0.616s
> sys     0m0.100s
> 
> result is "2.0e7" for both

So that looks like a bug in the simplifier in 6.6.1 that has been fixed.
This should go in the testsuite, perhaps, Simon?

-- Don