1 files changed, 16 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 13903d16529..7c9dbcb66cc 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1455,6 +1455,22 @@ Instruction *InstCombiner::foldVectorBinop(BinaryOperator &Inst) {
         }
         NewVecC[ShMask[I]] = CElt;
       }
+      // If this is a widening shuffle, we must be able to extend with undef
+      // elements. If the original binop does not produce an undef in the high
+      // lanes, then this transform is not safe.
+      // TODO: We could shuffle those non-undef constant values into the
+      //       result by using a constant vector (rather than an undef vector)
+      //       as operand 1 of the new binop, but that might be too aggressive
+      //       for target-independent shuffle creation.
+      if (I >= SrcVecNumElts) {
+        Constant *MaybeUndef =
+            ConstOp1 ? ConstantExpr::get(Opcode, UndefScalar, CElt)
+                     : ConstantExpr::get(Opcode, CElt, UndefScalar);
+        if (!isa<UndefValue>(MaybeUndef)) {
+          MayChange = false;
+          break;
+        }
+      }
     }
     if (MayChange) {
       Constant *NewC = ConstantVector::get(NewVecC);