[Haskell-cafe] Re: -O2 bug in GHC 6.8.1?

[Christian Maeder]

good bug! -O or -O2 is irrelevant but it works if compiled with -fvia-C

You (or someone else) should add it to
http://hackage.haskell.org/trac/ghc

Christian

Brad Clow wrote:
> I upgraded from GHC 6.6.1 to 6.8.1 and around that time I noticed that
> the output from an app I am working on changed. I have distilled the
> code down to the following example that produces different output
> depending on whether it is compiled with -O2 or not:
> 
> main = do
>     let (T x) = read "T 3"
>     print $ f x
>     print $ g x
> 
> data T = T Int deriving (Read, Show)
> 
> f x = truncate $ 2000 / fromIntegral ((x * 25) + 10)
> 
> g :: Int -> Int
> g x = f x
> 
> Here is the transcript from a shell (example code is saved in test.hs):
> 
> $ ghc --make test.hs
> [1 of 1] Compiling Main             ( test.hs, test.o )
> Linking test ...
> $ ./test
> 23
> 23
> $ rm test.hi test.o test
> $ ghc -O2 --make test.hs
> [1 of 1] Compiling Main             ( test.hs, test.o )
> Linking test ...
> $ ./test
> 23
> 24
> 
> It is a little disconcerting that -O2 changes the output. :-)
> 
> Regards
> brad
>