Index: llvm/lib/IR/Instructions.cpp
===================================================================
--- llvm/lib/IR/Instructions.cpp
+++ llvm/lib/IR/Instructions.cpp
@@ -2053,8 +2053,8 @@
     if (UsesLHS && UsesRHS)
       return false;
   }
-  assert((UsesLHS ^ UsesRHS) && "Should have selected from exactly 1 source");
-  return true;
+  // Allow for degenerate case: completely undef mask means neither source is used.
+  return UsesLHS || UsesRHS;
 }
 
 bool ShuffleVectorInst::isSingleSourceMask(ArrayRef<int> Mask) {
@@ -2182,6 +2182,8 @@
 }
 
 bool ShuffleVectorInst::isIdentityWithPadding() const {
+  if (isa<UndefValue>(Op<2>()))
+    return false;
   int NumOpElts = cast<VectorType>(Op<0>()->getType())->getNumElements();
   int NumMaskElts = cast<VectorType>(getType())->getNumElements();
   if (NumMaskElts <= NumOpElts)
@@ -2201,6 +2203,8 @@
 }
 
 bool ShuffleVectorInst::isIdentityWithExtract() const {
+  if (isa<UndefValue>(Op<2>()))
+    return false;
   int NumOpElts = cast<VectorType>(Op<0>()->getType())->getNumElements();
   int NumMaskElts = getType()->getNumElements();
   if (NumMaskElts >= NumOpElts)
@@ -2211,7 +2215,8 @@
 
 bool ShuffleVectorInst::isConcat() const {
   // Vector concatenation is differentiated from identity with padding.
-  if (isa<UndefValue>(Op<0>()) || isa<UndefValue>(Op<1>()))
+  if (isa<UndefValue>(Op<0>()) || isa<UndefValue>(Op<1>()) ||
+      isa<UndefValue>(Op<2>()))
     return false;
 
   int NumOpElts = cast<VectorType>(Op<0>()->getType())->getNumElements();
Index: llvm/test/Transforms/CodeGenPrepare/X86/cgp_shuffle_crash.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/CodeGenPrepare/X86/cgp_shuffle_crash.ll
@@ -0,0 +1,23 @@
+; RUN: opt -codegenprepare -S %s | FileCheck %s
+
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK-LABEL: shuffle_one_source
+
+define void @shuffle_one_source() {
+BB:
+  %Shuff7 = shufflevector <2 x i8> zeroinitializer, <2 x i8> zeroinitializer, <2 x i32> undef
+  br label %CF
+
+CF:                                               ; preds = %CF, %BB
+  %Cmp82 = icmp slt i32 480483, undef
+  br i1 undef, label %CF, label %CF242
+
+CF242:                                            ; preds = %CF
+  %B109 = mul <2 x i8> undef, %Shuff7
+  br label %CF245
+
+CF245:                                            ; preds = %CF245, %CF242
+  %Sl187 = select i1 %Cmp82, <2 x i8> %B109, <2 x i8> undef
+  br label %CF245
+}