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[ValueTracking] Add comment that CannotBeOrderedLessThanZero does the…
… wrong thing for powi. Summary: CannotBeOrderedLessThanZero(powi(x, exp)) returns true if CannotBeOrderedLessThanZero(x). But powi(-0, exp) is negative if exp is odd, so we actually want to return SignBitMustBeZero(x). Except that also isn't right, because we want to return true if x is NaN, even if x has a negative sign bit. What we really need in order to fix this is a consistent approach in this function to handling the sign bit of NaNs. Without this it's very difficult to say what the correct behavior here is. Reviewers: hfinkel, efriedma, sanjoy Subscribers: llvm-commits Differential Revision: https://reviews.llvm.org/D28927 llvm-svn: 293243
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