#10521: Wrong results in strict Word8 storage on x64 -------------------------------------+------------------------------------- Reporter: VincentBerthoux2 | Owner: Type: bug | Status: new Priority: high | Milestone: 7.10.2 Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: x86_64 Type of failure: Incorrect result | (amd64) at runtime | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Changes (by VincentBerthoux2): * priority: normal => high * milestone: => 7.10.2 Old description:
The following snippet produce two different results in function of the compiler platform used:
{{{#!hs import Data.Word( Word8 )
-- removing the bang patterns on V definition makes -- the problem go away. data V = V !Word8 !Word8 deriving Show
toV :: Float -> V toV d = V (truncate $ d * coeff) (fromIntegral $ exponent d + 128) where coeff = significand d * 255.9999 / d
main :: IO () main = print $ map toV [ 3.56158e-2, 0.7415215, 0.5383201, 0.1289829, 0.45520145 ] }}}
On GHC 7.10.1 x86 (under windows and Linux) the output is: {{{ [V 145 124,V 189 128,V 137 128,V 132 126,V 233 127] }}}
On GHC 7.10.1 x64 (under windows and Linux), the (invalid) output is: {{{ [V 0 124,V 0 128,V 0 128,V 0 126,V 0 127] }}}
The bug appear at the following optimisation levels:
- {{{-O1}}} - {{{-O2}}} - {{{-O3}}}
the results are the same at {{{-O0}}}
This bug was discovered in a bug report in the library JuicyPixels [https://github.com/Twinside/Juicy.Pixels/issues/98].
New description: The following snippet produce two different results in function of the compiler platform used: {{{#!hs import Data.Word( Word8 ) -- removing the bang patterns on V definition makes -- the problem go away. data V = V !Word8 !Word8 deriving Show toV :: Float -> V toV d = V (truncate $ d * coeff) (fromIntegral $ exponent d + 128) where coeff = significand d * 255.9999 / d main :: IO () main = print $ map toV [ 3.56158e-2, 0.7415215, 0.5383201, 0.1289829, 0.45520145 ] }}} On GHC 7.10.1 x86 (under windows and Linux) the output is: {{{ [V 145 124,V 189 128,V 137 128,V 132 126,V 233 127] }}} On GHC 7.10.1 x64 (under windows and Linux), the (invalid) output is: {{{ [V 0 124,V 0 128,V 0 128,V 0 126,V 0 127] }}} The bug appear at the following optimisation levels: - {{{-O1}}} - {{{-O2}}} - {{{-O3}}} the results are the same at {{{-O0}}} This bug was discovered in a bug report in the library JuicyPixels [https://github.com/Twinside/Juicy.Pixels/issues/98]. The same problem has been seen with GHC 7.10.2 RC1 -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10521#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler