On Mon, Aug 20, 2007 at 09:57:38PM +0100, Simon Peyton-Jones wrote:
GHC does some constant folding, but little by way of strength
reduction, or using shifts instead of multiplication. It's pretty
easy to add more: it's all done in a single module. Look at
primOpRules in the module PrelRules.
Patches welcome! But please also supply test-suite tests that check
the correctness of the rules.
Sucking another example out of comp.lang.functional:
This:
import System
f :: Int -> Int -> Int
f s n = if n > 0 then f (s+n) (n-1) else s
main = do
[n] <- getArgs
putStrLn $ show $ f 0 (read n)
is 3-4x slower than this:
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
int f(int s, int n) {
return n > 0 ? f(s+n, n-1) : s;
}
int main(int argc, char *argv[]) {
assert(argc == 2);
printf("%d\n", f(0, strtol(argv[1],0,0)));
}
The generated assembler suggests (if I've read it correctly) that gcc
is spotting that it can replace the tail call with a jump in the C
version, but for some reason it can't spot it for the Haskell version
when compiling with -fvia-C (and neither does ghc itself using
-fasm). So the haskell version ends up pushing and popping values on
and off the stack for every call to f, which is a bit sad.
Phil
--
http://www.kantaka.co.uk/ .oOo. public key: http://www.kantaka.co.uk/gpg.txt
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