[InstCombine] Avoid incorrect folding of select into phi nodes when incoming element is a vector type

Summary:
We are incorrectly folding selects into phi nodes when the incoming value of a phi
node is a constant vector. This optimization is done in `FoldOpIntoPhi` when the
select condition is a phi node with constant incoming values.
Without the fix, we are miscompiling (i.e. incorrectly folding the
select into the phi node) when the vector contains non-zero
elements.
This patch fixes the miscompile and we will correctly fold based on the
select vector operand (see added test cases).

Reviewers: majnemer, sanjoy, spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31189

llvm-svn: 298845

1 files changed, 5 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 9cd22f65c99..020e8c9f046 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -908,7 +908,11 @@ Instruction *InstCombiner::FoldOpIntoPhi(Instruction &I) {
       // Beware of ConstantExpr:  it may eventually evaluate to getNullValue,
       // even if currently isNullValue gives false.
       Constant *InC = dyn_cast<Constant>(PN->getIncomingValue(i));
-      if (InC && !isa<ConstantExpr>(InC))
+      // For vector constants, we cannot use isNullValue to fold into
+      // FalseVInPred versus TrueVInPred. When we have individual nonzero
+      // elements in the vector, we will incorrectly fold InC to
+      // `TrueVInPred`.
+      if (InC && !isa<ConstantExpr>(InC) && !isa<VectorType>(InC->getType()))
         InV = InC->isNullValue() ? FalseVInPred : TrueVInPred;
       else
         InV = Builder->CreateSelect(PN->getIncomingValue(i),