[Haskell-cafe] To my boss: The code is cool, but it is about 100 times slower than the old one...

[Fixie Fixie]

Oh, my - what an indentation :-)

New try:

----- Videresendt melding ----
Fra: Fixie Fixie <fixie.fixie at rocketmail.com>
Til: "haskell-cafe at haskell.org" <haskell-cafe at haskell.org> 
Kopi: Clark Gaebel <cgaebel at uwaterloo.ca> 
Sendt: Torsdag, 29. november 2012 20.57
Emne: Vedr: [Haskell-cafe] To my boss: The code is cool, but it is about 100 times slower than the old one...

Sure, the code is from the url I mentioned, both in C and in Haskell.

It allready seems like the haskell-version should run fast to me - it uses unboxed arrays and even unsafe functions to make it run faster. Well, have a look:

C-version:

#include <stdint.h>
#include <stdio.h>


#define VDIM    100
#define VNUM    100000



uint64_t prng (uint64_t w) {
    w ^= w << 13;
    w ^= w >> 7;
    w ^= w << 17;
    return w;
};

void kahanStep (double *s, double *c, double x) {
    double y, t;
    y  = x - *c;
    t  = *s + y;
    *c = (t - *s) - y;
    *s = t;
}

void kahan(double s[], double c[]) {
    for (int i = 1; i <= VNUM; i++) {
        uint64_t w = i;
        for (int j = 0; j
 < VDIM; j++) {
                kahanStep(&s[j], &c[j], w);
                w = prng(w);
        }
    }
};


int main (int argc, char* argv[]) {
    double acc[VDIM], err[VDIM];
    for (int i = 0; i < VDIM; i++) {
        acc[i] = err[i] = 0.0;
    };
    kahan(acc, err);
    printf("[ ");
    for (int i = 0; i < VDIM; i++) {
        printf("%g ", acc[i]);
    };
    printf("]\n");
};

And the haskell version:

{-# LANGUAGE CPP, BangPatterns #-}
module Main (main) where

#define VDIM 100
#define VNUM 100000

import Data.Array.Base
import Data.Array.ST
import Data.Array.Unboxed
import Control.Monad.ST
import GHC.Word
import Control.Monad
import Data.Bits

prng :: Word -> Word
prng w = w'
  where
    !w1 = w `xor` (w `shiftL` 13)
    !w2 = w1 `xor` (w1 `shiftR` 7)
    !w' = w2 `xor` (w2 `shiftL` 17)

type Vec s = STUArray s Int Double

kahan :: Vec s -> Vec s -> ST s ()
kahan s c = do
    let inner w j
            | j < VDIM  = do
                !cj <- unsafeRead c j
                !sj <- unsafeRead s j
                let !y = fromIntegral w - cj
                    !t = sj + y
                    !w' = prng w
                unsafeWrite c j ((t-sj)-y)
                unsafeWrite s j t
                inner w' (j+1)
            | otherwise = return ()
    forM_ [1 .. VNUM] $ \i -> inner (fromIntegral i) 0

calc :: ST s (Vec s)
calc = do
    s <- newArray (0,VDIM-1) 0
    c <- newArray (0,VDIM-1) 0
    kahan s c
    return s

main :: IO ()
main = print . elems $ runSTUArray calc

Cheers,

Felix
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