Optimizer has bugs regarding handling of -0.0

This is most likely related to #9238 (closed)

Perhaps it can be merged into that if it is indeed the case, though it'd be good for an expert to take a look and make sure first that the culprit is indeed the same. In any case, the program in this ticket can at least serve as a test-case.

I observed this on 7.8.3; though I suspect the same holds in the just released 7.10.1 as well. For the following program:

testF :: Float -> Bool
testF x = x == 0 && not (isNegativeZero x)

testD :: Double -> Bool
testD x = x == 0 && not (isNegativeZero x)

main :: IO ()
main = do print $ testF (-0.0)
          print $ testD (-0.0)

If I compile with no optimizations, then I get the correct answers:

$ /bin/rm -f a.hi a.o a; ghc -O0 a; ./a
[1 of 1] Compiling Main             ( a.hs, a.o )
Linking a ...
False
False

But if I turn optimizations on, then I get:

$ /bin/rm -f a.hi a.o a; ghc -O2 a; ./a
[1 of 1] Compiling Main             ( a.hs, a.o )
Linking a ...
True
True

which is just plain wrong.

Trac metadata

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