Index: lib/Analysis/InstructionSimplify.cpp
===================================================================
--- lib/Analysis/InstructionSimplify.cpp
+++ lib/Analysis/InstructionSimplify.cpp
@@ -4144,6 +4144,9 @@
   auto *Op0Const = dyn_cast<Constant>(Op0);
   auto *Op1Const = dyn_cast<Constant>(Op1);
 
+  if (isa<UndefValue>(Mask))
+    return UndefValue::get(RetTy);
+
   // If all operands are constant, constant fold the shuffle.
   if (Op0Const && Op1Const)
     return ConstantFoldShuffleVectorInstruction(Op0Const, Op1Const, Mask);
Index: test/Transforms/InstSimplify/shufflevector.ll
===================================================================
--- test/Transforms/InstSimplify/shufflevector.ll
+++ 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:%.*]]