[GHC] #9238: Negative zero broken

[GHC]

#9238: Negative zero broken
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       Reporter:          |             Owner:
  augustss                |            Status:  new
           Type:  bug     |         Milestone:
       Priority:  normal  |           Version:  7.8.2
      Component:          |  Operating System:  Windows
  Compiler                |   Type of failure:  Incorrect result at runtime
       Keywords:          |         Test Case:
   Architecture:          |          Blocking:
  Unknown/Multiple        |
     Difficulty:          |
  Unknown                 |
     Blocked By:          |
Related Tickets:          |
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 Try the following program
 {{{
 compareDouble :: Double -> Double -> Ordering
 compareDouble x y =
        case (isNaN x, isNaN y) of
        (True, True)   -> EQ
        (True, False)  -> LT
        (False, True)  -> GT
        (False, False) ->
           -- Make -0 less than 0
           case (x == 0, y == 0, isNegativeZero x, isNegativeZero y) of
           (True, True, True, False) -> LT
           (True, True, False, True) -> GT
           _                         -> x `compare` y

 main = do
     let l = [-0, 0]
     print [ (x, y, compareDouble x y) | x <- l, y <- l ]
 }}}

 Compile and run with -O0
 {{{
 $ ghc -O0 D.hs
 [1 of 1] Compiling Main             ( D.hs, D.o )
 Linking D.exe ...
 $ ./D
 [(-0.0,-0.0,EQ),(-0.0,0.0,LT),(0.0,-0.0,GT),(0.0,0.0,EQ)]
 }}}
 This is the correct output.

 Compile and run with -O1
 {{{
 $ ghc -O1 D.hs
 [1 of 1] Compiling Main             ( D.hs, D.o )
 Linking D.exe ...
 $ ./D
 [(-0.0,-0.0,LT),(-0.0,0.0,LT),(0.0,-0.0,EQ),(0.0,0.0,EQ)]
 }}}
 This is wrong.

 Put a NOINLINE pragma on compareDouble:
 {{{
 $ ghc -O1 D.hs
 [1 of 1] Compiling Main             ( D.hs, D.o )
 Linking D.exe ...
 $ ./D
 [(-0.0,-0.0,EQ),(-0.0,0.0,EQ),(0.0,-0.0,EQ),(0.0,0.0,EQ)]
 }}}
 This is wrong in a different way.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9238>
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