Re: [GHC] #7858: Fix definitions of abs/signum for Floats/Doubles

21 Apr 2014

      #7858: Fix definitions of abs/signum for Floats/Doubles
-------------------------------------+------------------------------------
        Reporter:  lerkok            |            Owner:
            Type:  bug               |           Status:  patch
        Priority:  normal            |        Milestone:
       Component:  libraries/base    |          Version:  7.6.3
      Resolution:                    |         Keywords:  floating point
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
-------------------------------------+------------------------------------
Changes (by bernalex):

 * cc: ekmett (added)
 * status:  new => patch

Comment:

 I added my patches for this here per SPJ's suggestion.

 The signum patch does not work. I do not know why. It must be due to GHC
 internals, because the function itself is fine as may be verified by
 copying

 {{{
 signumFix x
     | x == 0.0  = x
     | isNaN x   = x
     | x > 0.0   = 1
     | otherwise = negate 1
 }}}

 into a file signumfix.hs and :l signumfix.hs into GHCi. signumFix (-0.0 ::
 Float) will now return -0.0, where Prelude.signum returns 0.0 with the
 same argument.

 Here is a core dump of me invoking my Prelude.signum after building
 GHC/GHCi with my patches:

 {{{
 Prelude> abs (-0.0 :: Float)

 ==================== Simplified expression ====================
 let {
   it_aqt :: GHC.Types.Float
   [LclId,
    Str=DmdType,
    Unf=Unf{Src=<vanilla>, TopLvl=False, Arity=0, Value=False,
            ConLike=False, WorkFree=False, Expandable=False,
            Guidance=IF_ARGS [] 70 0}]
   it_aqt =
     GHC.Num.abs
       @ GHC.Types.Float
       GHC.Float.$fNumFloat
       (GHC.Num.negate
          @ GHC.Types.Float
          GHC.Float.$fNumFloat
          (GHC.Types.F# (__float 0.0))) } in
 GHC.Base.thenIO
   @ ()
   @ [()]
   (System.IO.print @ GHC.Types.Float GHC.Float.$fShowFloat it_aqt)
   (GHC.Base.returnIO
      @ [()]
      (GHC.Types.:
         @ ()
         (it_aqt
          `cast` (UnivCo representational GHC.Types.Float ()
                  :: GHC.Types.Float ~# ()))
         (GHC.Types.[] @ ())))

 0.0

 Prelude> signum (-0.0 :: Float)

 ==================== Simplified expression ====================
 let {
   it_aKQ :: GHC.Types.Float
   [LclId,
    Str=DmdType,
    Unf=Unf{Src=<vanilla>, TopLvl=False, Arity=0, Value=False,
            ConLike=False, WorkFree=False, Expandable=False,
            Guidance=IF_ARGS [] 70 0}]
   it_aKQ =
     GHC.Num.signum
       @ GHC.Types.Float
       GHC.Float.$fNumFloat
       (GHC.Num.negate
          @ GHC.Types.Float
          GHC.Float.$fNumFloat
          (GHC.Types.F# (__float 0.0))) } in
 GHC.Base.thenIO
   @ ()
   @ [()]
   (System.IO.print @ GHC.Types.Float GHC.Float.$fShowFloat it_aKQ)
   (GHC.Base.returnIO
      @ [()]
      (GHC.Types.:
         @ ()
         (it_aKQ
          `cast` (UnivCo representational GHC.Types.Float ()
                  :: GHC.Types.Float ~# ()))
         (GHC.Types.[] @ ())))

 0.0
 }}}

 Please note that the abs patch works well and is likely ready to go.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7858#comment:3
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