Bulat Ziganshin
Hello Dan,
Thursday, November 8, 2007, 9:33:12 PM, you wrote:
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
1. ghc doesn't implement loop unrolling, even in -O2 mode 2. forM_ may have its own overheads, i'm not sure 3. add strictness annotations:
forM_ [0..n-2] $ \i -> do { return $! i; x <- readArray a i; return $! x; y <- readArray a (i+1); return $! y; writeArray a (i+1) (x+y) }
such cycle should be approx. 3-10 times slower than C one
I didn't know you can do that. Anyway, I tried OP's C and Haskell versions and the one with your strictness annotations. It seems that the `$!' thing doesn't do much, or it's not doing anything. OP's C code: $ time ./test 100000000.000000 real 0m1.128s user 0m0.572s sys 0m0.555s OP's Haskell code with n=1e8: $ time ./test-ghc 1.0e8 real 0m2.065s user 0m1.505s sys 0m0.558s OP's Haskell code with n=1e8 and your strictness annotations: $ time ./test-ghc 1.0e8 real 0m2.064s user 0m1.472s sys 0m0.591s However, bad performance observed by OP is not observed by me. $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.1 $ gcc --version gcc (GCC) 4.1.2 (Gentoo 4.1.2 p1.0.2) Copyright (C) 2006 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. -- c/* __o/* <\ * (__ */\ <