[GHC] #2156: compilation math/truncate bug with optimization enabled

[GHC]

#2156: compilation math/truncate bug with optimization enabled
------------------------+---------------------------------------------------
    Reporter:  trevorm  |       Owner:          
        Type:  bug      |      Status:  new     
    Priority:  normal   |   Component:  Compiler
     Version:  6.8.2    |    Severity:  critical
    Keywords:           |    Testcase:          
Architecture:  x86      |          Os:  Linux   
------------------------+---------------------------------------------------
 The following code produces different results depending on whether it is
 compiled with optimization (-O1/-O2) or not. The unoptimized program when
 run outputs the correct value (3), the optimized one outputs an incorrect
 value (2)

 {{{
 module Main where

 import IO

 lg8base2 :: Int
 lg8base2 = truncate (log 8 / log 2)

 main :: IO()
 main = do
         hPutStrLn stdout $ show lg8base2



 }}}

 Output of gcc -v

 {{{
 Using built-in specs.
 Target: i486-linux-gnu
 Configured with: ../src/configure -v --enable-
 languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr --enable-
 shared --with-system-zlib --libexecdir=/usr/lib --without-included-gettext
 --enable-threads=posix --enable-nls --with-gxx-include-
 dir=/usr/include/c++/4.2 --program-suffix=-4.2 --enable-clocale=gnu
 --enable-libstdcxx-debug --enable-mpfr --enable-targets=all --enable-
 checking=release --build=i486-linux-gnu --host=i486-linux-gnu
 --target=i486-linux-gnu
 Thread model: posix
 gcc version 4.2.3 20071123 (prerelease) (Debian 4.2.2-4)
 }}}

 Sequence of compiles / runs
 {{{
 trevor at tmlinux:~/haskell$ ghc --make test
 [1 of 1] Compiling Main             ( test.hs, test.o )
 Linking test ...
 trevor at tmlinux:~/haskell$ ./test
 3
 trevor at tmlinux:~/haskell$ rm test.o
 trevor at tmlinux:~/haskell$ ghc -O2 --make test
 [1 of 1] Compiling Main             ( test.hs, test.o )
 Linking test ...
 trevor at tmlinux:~/haskell$ ./test
 2
 }}}
 Output of compilation phase with -v is attached (test.unoptimized.output &
 test.optimized.output)

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2156>
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