dons:
The following C program was described on #haskell
#include
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!
GCC 4.x seems to do a much better job, turning the inner loop into: .L2: mulsd %xmm3, %xmm0 mulsd %xmm1, %xmm0 movapd %xmm0, %xmm1 mulsd %xmm2, %xmm1 addl $1, %eax cmpl $100000001, %eax jne .L2 -- Don