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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@tmlinux:~/haskell$ ghc --make test
[1 of 1] Compiling Main             ( test.hs, test.o )
Linking test ...
trevor@tmlinux:~/haskell$ ./test
3
trevor@tmlinux:~/haskell$ rm test.o
trevor@tmlinux:~/haskell$ ghc -O2 --make test
[1 of 1] Compiling Main             ( test.hs, test.o )
Linking test ...
trevor@tmlinux:~/haskell$ ./test
2

Output of compilation phase with -v is attached (test.unoptimized.output & test.optimized.output)

Trac metadata

Trac field	Value
Version	6.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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