[GHC] #10521: Wrong results in strict Word8 storage on x64

[GHC]

#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>
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