Incorrect optimizations involving negative zero

The program

main = print $ if negativeZero == 0.0 then 1 / negativeZero else 0.0
  where
    negativeZero = negate 0.0

outputs -Infinity when compiled with GHC 7.6.3 and Infinity when compiled with GHC 7.8.3 and the optimization level -O2. The relevant pieces of Core:

-- GHC 7.6.3
Main.main2 =
  case GHC.Prim.==## (GHC.Prim.negateDouble# 0.0) 0.0 of _ {
    GHC.Types.False -> Main.main3 (GHC.Types.[] @ GHC.Types.Char);
    GHC.Types.True ->
      case GHC.Prim./## 1.0 (GHC.Prim.negateDouble# 0.0)
      of wild2_aK6 { __DEFAULT ->
      GHC.Float.$w$sshowSignedFloat
        GHC.Float.$fShowDouble_$sshowFloat
        GHC.Show.shows26
        wild2_aK6
        (GHC.Types.[] @ GHC.Types.Char)
      }
  }

-- GHC 7.8.3
Main.main2 =
  case GHC.Prim.negateDouble# 0.0 of _ [Occ=Dead] {
    __DEFAULT -> Main.main3 (GHC.Types.[] @ GHC.Types.Char);
    0.0 ->
      case GHC.Prim./## 1.0 0.0 of wild2_a1NU { __DEFAULT ->
      GHC.Float.$w$sshowSignedFloat
        GHC.Float.$fShowDouble_$sshowFloat
        GHC.Show.shows27
        wild2_a1NU
        (GHC.Types.[] @ GHC.Types.Char)
      }
  }

I don't think this problem is operating system/architecture dependent, but for the sake of completeness I'm on a 32-bit Linux, and this issue was first discovered on 64-bit Windows running in a virtual machine.

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

mentioned in issue #9238 (closed)

changed weight to 5

added Tbug Trac import labels

closed

Thanks for the report. This is a duplicate of #9238 (closed), but interesting to know that it is a regression from 7.6.3.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedDuplicate

added incorrect runtime result label

added Pnormal label