[Haskell-cafe] Ghc: -O harmful?

[Don Stewart]

-fno-state-hack?

xofon:
> 
>      Hello all,
> 
>      I have a haskell program that runs an order of magnitude slower
> when compiled with optimisations turned on.  This happens on 6.8.2
> as well as 6.10.1:
> 
> 
> petr at r4at184:/tmp[1]% ghc --make -fforce-recomp -o out buga.hs   
> [1 of 1] Compiling Main             ( buga.hs, buga.o )
> Linking out ...
> petr at r4at184:/tmp% time ./out < 3-med.in >|/dev/null
> ./out < 3-med.in >| /dev/null  0,03s user 0,00s system 99% cpu 0,038 total
> 
> petr at r4at184:/tmp% ghc --make -fforce-recomp -o out buga.hs -O
> [1 of 1] Compiling Main             ( buga.hs, buga.o )
> Linking out ...
> petr at r4at184:/tmp% time ./out < 3-med.in >|/dev/null
> ./out < 3-med.in >| /dev/null  0,99s user 0,01s system 99% cpu 1,001 total
> 
> petr at r4at184:/tmp[1]% ghc --make -fforce-recomp -o out buga.hs -O2
> [1 of 1] Compiling Main             ( buga.hs, buga.o )
> Linking out ...
> petr at r4at184:/tmp% time ./out < 3-med.in >|/dev/null
> ./out < 3-med.in >| /dev/null  0,99s user 0,01s system 99% cpu 1,004 total
> 
> petr at r4at184:/tmp% ghc --version
> The Glorious Glasgow Haskell Compilation System, version 6.8.2
> 
> 
> psxxxxxx:/tmp$ ghc --make -fforce-recomp -o out buga.hs   
> [1 of 1] Compiling Main             ( buga.hs, buga.o )
> Linking out ...
> psxxxxxx:/tmp$ time ./out < 3-med.in >|/dev/null
> real	0m0.028s
> user	0m0.011s
> sys	0m0.007s
> 
> psxxxxxx:/tmp$ ghc --make -fforce-recomp -o out buga.hs -O
> [1 of 1] Compiling Main             ( buga.hs, buga.o )
> Linking out ...
> psxxxxxx:/tmp$ time ./out < 3-med.in >|/dev/null
> real	0m0.252s
> user	0m0.225s
> sys	0m0.011s
> 
> psxxxxxx:/tmp$ ghc --make -fforce-recomp -o out buga.hs -O2
> [1 of 1] Compiling Main             ( buga.hs, buga.o )
> Linking out ...
> Vazeny pane, cekam na Vase rozkazy.
> psxxxxxx:/tmp$ time ./out < 3-med.in >|/dev/null
> real	0m0.239s
> user	0m0.225s
> sys	0m0.010s
> 
> psxxxxxx:/tmp$ ghc --version
> The Glorious Glasgow Haskell Compilation System, version 6.10.1
> 
> 
> The GHC documentation states that
> 
> > -O2:
> >     Means: "Apply every non-dangerous optimisation, even if it means
> > significantly longer compile times."  The avoided "dangerous"
> > optimisations are those that can make runtime or space worse if
> > you're unlucky. They are normally turned on or off individually.
> 
> If I understand this correctly, the dangerous optimisations should not
> be performed with -O.  Am I doing anything wrong?  Am I hitting a bug
> in GHC optimiser? A known one?
> 
> 
>      The program source:
> 
> --------------
> petr at r4at184:/tmp$ cat buga.hs 
> {-# LANGUAGE FlexibleContexts #-}
> 
> import Control.Monad
> import Control.Monad.State
> 
> data Predpoc a = Nic | Kus { delka :: Int,
>                              levy :: Predpoc a,
>                              pravy :: Predpoc a }
>   deriving Show
> 
> gList i = Kus 1 Nic Nic
> 
> gPredpoc kus1 kus2 =
>   Kus (delka kus1 + delka kus2)
>       kus1 kus2
> 
> 
> pop :: MonadState [a] m => m a
> pop = get >>= \(x:xs) -> put xs >> return x
> 
> mbuild :: MonadState [Int] m => Int -> m (Predpoc Int)
> mbuild 1 = gList `liftM` pop
> mbuild n = liftM2 gPredpoc (mbuild n1) (mbuild n2)
>   where n1 = n`div`2
>         n2 = n - n1
> 
> build n li = evalState (mbuild n) li
> 
> best :: Predpoc Int -> Int -> Int -> Int
> best kus i j
>   | i == 1 && j == delka kus
>                     = delka kus
>   | j <= del1       = best (levy kus) i j
>   | i > del1        = best (pravy kus) (i-del1) (j-del1)
>   | otherwise       = best (levy kus) i del1 +
>                       best (pravy kus) 1 (j-del1)
>   where
>     del1 = delka (levy kus)
> 
> main = do
>   n <- read `liftM` getLine
>   pole <- liftM (build n . map read . words) getLine
>   replicateM_ 100 $ do
>     getLine
>     print $ best pole 42 420
> --------------
> 
>      The sample input file (30k) can be downloaded from
> http://pikomat.mff.cuni.cz/petr/3-med.in
> 
>      I suspect that the main's variable "pole" (which is a large
> binary tree of type Predpoc Int) may be built from scratch in each
> iteration of the replicateM_ loop.
> 
>      Petr
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