[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

Author: Justin Lebar

llvm/lib/Analysis/ValueTracking.cpp

@@ -2602,6 +2602,11 @@ static bool cannotBeOrderedLessThanZeroImpl(const Value *V,
                                             const TargetLibraryInfo *TLI,
                                             bool SignBitOnly,
                                             unsigned Depth) {
+  // TODO: This function does not do the right thing when SignBitOnly is true
+  // and we're lowering to a hypothetical IEEE 754-compliant-but-evil platform
+  // which flips the sign bits of NaNs.  See
+  // https://llvm.org/bugs/show_bug.cgi?id=31702.
+
   if (const ConstantFP *CFP = dyn_cast<ConstantFP>(V)) {
     return !CFP->getValueAPF().isNegative() ||
            (!SignBitOnly && CFP->getValueAPF().isZero());
@@ -2678,8 +2683,22 @@ static bool cannotBeOrderedLessThanZeroImpl(const Value *V,
         if (Exponent->getBitWidth() <= 64 && Exponent->getSExtValue() % 2u == 0)
           return true;
       }
+      // TODO: This is not correct.  Given that exp is an integer, here are the
+      // ways that pow can return a negative value:
+      //
+      //   pow(x, exp)    --> negative if exp is odd and x is negative.
+      //   pow(-0, exp)   --> -inf if exp is negative odd.
+      //   pow(-0, exp)   --> -0 if exp is positive odd.
+      //   pow(-inf, exp) --> -0 if exp is negative odd.
+      //   pow(-inf, exp) --> -inf if exp is positive odd.
+      //
+      // Therefore, if !SignBitOnly, we can return true if x >= +0 or x is NaN,
+      // but we must return false if x == -0.  Unfortunately we do not currently
+      // have a way of expressing this constraint.  See details in
+      // https://llvm.org/bugs/show_bug.cgi?id=31702.
       return cannotBeOrderedLessThanZeroImpl(I->getOperand(0), TLI, SignBitOnly,
                                              Depth + 1);
+
     case Intrinsic::fma:
     case Intrinsic::fmuladd:
       // x*x+y is non-negative if y is non-negative.