LLVM/optimized code for sqrt incorrect for negative values

The LLVM sqrt intrinsic is undefined below values of -0.0

See http://llvm.org/docs/LangRef.html#llvm-sqrt-intrinsic

In some versions of LLVM it returns 0.0 in this case, but in newer versions the return value is undefined.

This causes the result of Haskell's sqrt to vary at negative values based on the codegen and optimization flags chosen.

main = print (sqrt (-7 :: Double))

$ ghc -O -fllvm Sqrt.hs
[1 of 1] Compiling Main             ( Sqrt.hs, Sqrt.o )
Linking Sqrt ...
$ ./Sqrt 
0.0
$ ghc -O  Sqrt.hs
[1 of 1] Compiling Main             ( Sqrt.hs, Sqrt.o )
Linking Sqrt ...
$ ./Sqrt 
NaN

Trac metadata

Trac field	Value
Version	7.8.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (LLVM)
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