[GHC] #8098: Faulty Word64 arithmetic if optimized

27 Jul 2013

      #8098: Faulty Word64 arithmetic if optimized
----------------------------+---------------------------------
       Reporter:  achp      |             Owner:
           Type:  bug       |            Status:  new
       Priority:  normal    |         Milestone:
      Component:  Compiler  |           Version:  7.4.2
       Keywords:            |  Operating System:  Windows
   Architecture:  x86       |   Type of failure:  None/Unknown
     Difficulty:  Unknown   |         Test Case:
     Blocked By:            |          Blocking:
Related Tickets:            |
----------------------------+---------------------------------
 The following program produces different results depending on
 optimization:

 Checked.hs:
 {{{
 module Checked ((.-.)) where

 (.-.) :: (Bounded a, Integral a) => a -> a -> Maybe a
 x .-. y | x >= 0 && y < 0 = if z > x  then Just z
                                       else Nothing
         | x >= 0          = if z <= x then Just z
                                       else Nothing
         | x < 0 && y > 0  = if z < x  then Just z
                                       else Nothing
         | otherwise       = Just z
     where z = x - y
 }}}

 Main.hs:
 {{{
 module Main (main) where

 import Data.Maybe
 import Data.Word
 import Numeric
 import Checked

 main :: IO ()
 main = print $ showHex (fromJust ((0xFFFFFFFF00000000 :: Word64) .-.
 0x00000000FFFFFFFF)) ""
 }}}

 {{{
 D:\dev\haskell\ch1>ghc --make Checked.hs Main.hs
 [1 of 2] Compiling Checked          ( Checked.hs, Checked.o )
 [2 of 2] Compiling Main             ( Main.hs, Main.o )
 Linking Main.exe ...

 D:\dev\haskell\ch1>Main.exe
 "fffffffe00000001"

 D:\dev\haskell\ch1>del *.hi *.o *.exe

 D:\dev\haskell\ch1>ghc --make -O2 Checked.hs Main.hs
 [1 of 2] Compiling Checked          ( Checked.hs, Checked.o )
 [2 of 2] Compiling Main             ( Main.hs, Main.o )
 Linking Main.exe ...

 D:\dev\haskell\ch1>Main.exe
 "fffe00000001"

 D:\dev\haskell\ch1>
 }}}

 The bug disappears if definition and usage of (.-.) are in the same
 module.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8098
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