[Haskell-cafe] Very fast loops. Now!

[Donald Bruce Stewart]

The following C program was described on #haskell

    #include <stdio.h>

    int main()
    {
        double x = 1.0/3.0;
        double y = 3.0;
        int i    = 1;
        for (; i<=1000000000; i++) {
            x = x*y/3.0;
            y = x*9.0;
        }
        printf("%f\n", x+y);
    }


Which was translated to the following Haskell:

    {-# OPTIONS -fexcess-precision #-}

    import Text.Printf

    main = go (1/3) 3 1

    go :: Double -> Double -> Int -> IO ()
    go !x !y !i
        | i == 1000000000 = printf "%f\n" (x+y)
        | otherwise       = go (x*y/3) (x*9) (i+1)


To everyone's surprise, GHC 6.6 beats GCC (3.3.5) here, at least the two test machines:


    $ ghc -O -fexcess-precision -fbang-patterns -optc-O3 -optc-ffast-math -optc-mfpmath=sse -optc-msse2 A.hs -o a

    $ time ./a
    3.333333
    ./a  0.96s user 0.01s system 99% cpu 0.969 total
                                         ^^^^^

Versus gcc 3.3.5:

    $ gcc -O3 -ffast-math -mfpmath=sse -msse2 -std=c99 t.c -o c_loop
    $ time ./c_loop 
    3.333333
    ./c_loop  1.01s user 0.01s system 97% cpu 1.046 total
                                              ^^^^^

Note that newer gcc's will statically compute that loop. Note also that
-fexcess-precision must currently be provided as a pragma only.

I declare GHC Haskell numerics (with -fexcess-precision) not so shabby!

-- Don