Index: llvm/trunk/lib/Analysis/InstructionSimplify.cpp
===================================================================
--- llvm/trunk/lib/Analysis/InstructionSimplify.cpp
+++ llvm/trunk/lib/Analysis/InstructionSimplify.cpp
@@ -4049,6 +4049,9 @@
 static Value *SimplifyShuffleVectorInst(Value *Op0, Value *Op1, Constant *Mask,
                                         Type *RetTy, const SimplifyQuery &Q,
                                         unsigned MaxRecurse) {
+  if (isa<UndefValue>(Mask))
+    return UndefValue::get(RetTy);
+
   Type *InVecTy = Op0->getType();
   unsigned MaskNumElts = Mask->getType()->getVectorNumElements();
   unsigned InVecNumElts = InVecTy->getVectorNumElements();
Index: llvm/trunk/test/Transforms/InstSimplify/shufflevector.ll
===================================================================
--- llvm/trunk/test/Transforms/InstSimplify/shufflevector.ll
+++ llvm/trunk/test/Transforms/InstSimplify/shufflevector.ll
@@ -118,6 +118,14 @@
   ret <4 x i32> %shuf
 }
 
+define <4 x i32> @undef_mask_1(<4 x i32> %x, <4 x i32> %y) {
+; CHECK-LABEL: @undef_mask_1(
+; CHECK-NEXT:    ret <4 x i32> undef
+;
+  %shuf = shufflevector <4 x i32> %x, <4 x i32> %y, <4 x i32> undef
+  ret <4 x i32> %shuf
+}
+
 define <4 x i32> @identity_mask_0(<4 x i32> %x) {
 ; CHECK-LABEL: @identity_mask_0(
 ; CHECK-NEXT:    ret <4 x i32> [[X:%.*]]