[GVN] don't propagate equality comparisons of FP zero (PR22376)

In http://reviews.llvm.org/D6911, we allowed GVN to propagate FP equalities
to allow some simple value range optimizations. But that introduced a bug
when comparing to -0.0 or 0.0: these compare equal even though they are not
bitwise identical.

This patch disallows propagating zero constants in equality comparisons. 
Fixes: http://llvm.org/bugs/show_bug.cgi?id=22376

Differential Revision: http://reviews.llvm.org/D7257

llvm-svn: 227491

1 files changed, 10 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index 29ea432e4a4..7519b598659 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -2182,9 +2182,16 @@ bool GVN::propagateEquality(Value *LHS, Value *RHS,
 
       // Handle the floating point versions of equality comparisons too.
       if ((isKnownTrue && Cmp->getPredicate() == CmpInst::FCMP_OEQ) ||
-          (isKnownFalse && Cmp->getPredicate() == CmpInst::FCMP_UNE))
-        Worklist.push_back(std::make_pair(Op0, Op1));
-
+          (isKnownFalse && Cmp->getPredicate() == CmpInst::FCMP_UNE)) {
+        // Floating point -0.0 and 0.0 compare equal, so we can't
+        // propagate a constant based on that comparison.
+        // FIXME: We should do this optimization if 'no signed zeros' is
+        // applicable via an instruction-level fast-math-flag or some other
+        // indicator that relaxed FP semantics are being used.
+        if (!isa<ConstantFP>(Op1) || !cast<ConstantFP>(Op1)->isZero())
+          Worklist.push_back(std::make_pair(Op0, Op1));
+      }
+ 
       // If "A >= B" is known true, replace "A < B" with false everywhere.
       CmpInst::Predicate NotPred = Cmp->getInversePredicate();
       Constant *NotVal = ConstantInt::get(Cmp->getType(), isKnownFalse);