diff --git a/llvm/include/llvm/IR/PatternMatch.h b/llvm/include/llvm/IR/PatternMatch.h
--- a/llvm/include/llvm/IR/PatternMatch.h
+++ b/llvm/include/llvm/IR/PatternMatch.h
@@ -2285,6 +2285,31 @@
   return m_c_Xor(V, m_AllOnes());
 }
 
+template <typename ValTy> struct NotForbidUndef_match {
+  ValTy Val;
+  NotForbidUndef_match(const ValTy &V) : Val(V) {}
+
+  template <typename OpTy> bool match(OpTy *V) {
+    // We do not use m_c_Xor because that could match an arbitrary APInt that is
+    // not -1 as C and then fail to match the other operand if it is -1.
+    // This code should still work even when both operands are constants.
+    Value *X;
+    const APInt *C;
+    if (m_Xor(m_Value(X), m_APIntForbidUndef(C)).match(V) && C->isAllOnes())
+      return Val.match(X);
+    if (m_Xor(m_APIntForbidUndef(C), m_Value(X)).match(V) && C->isAllOnes())
+      return Val.match(X);
+    return false;
+  }
+};
+
+/// Matches a bitwise 'not' as 'xor V, -1' or 'xor -1, V'. For vectors, the
+/// constant value must be composed of only -1 scalar elements.
+template <typename ValTy>
+inline NotForbidUndef_match<ValTy> m_NotForbidUndef(const ValTy &V) {
+  return NotForbidUndef_match<ValTy>(V);
+}
+
 /// Matches an SMin with LHS and RHS in either order.
 template <typename LHS, typename RHS>
 inline MaxMin_match<ICmpInst, LHS, RHS, smin_pred_ty, true>
diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -2408,11 +2408,10 @@
     // The 'not' op must contain a complete -1 operand (no undef elements for
     // vector) for the transform to be safe.
     Value *NotA;
-    const APInt *C;
-    if (match(X, m_c_Or(m_CombineAnd(m_Xor(m_Value(A), m_APIntForbidUndef(C)),
-                                     m_Value(NotA)),
-                        m_Value(B))) &&
-        match(Y, m_c_And(m_Specific(A), m_Specific(B))) && C->isAllOnes())
+    if (match(X,
+              m_c_Or(m_CombineAnd(m_NotForbidUndef(m_Value(A)), m_Value(NotA)),
+                     m_Value(B))) &&
+        match(Y, m_c_And(m_Specific(A), m_Specific(B))))
       return NotA;
 
     return nullptr;
diff --git a/llvm/unittests/IR/PatternMatch.cpp b/llvm/unittests/IR/PatternMatch.cpp
--- a/llvm/unittests/IR/PatternMatch.cpp
+++ b/llvm/unittests/IR/PatternMatch.cpp
@@ -1727,6 +1727,41 @@
   EXPECT_FALSE(match(PtrToInt2, m_VScale(DL)));
 }
 
+TEST_F(PatternMatchTest, NotForbidUndef) {
+  Type *ScalarTy = IRB.getInt8Ty();
+  Type *VectorTy = FixedVectorType::get(ScalarTy, 3);
+  Constant *ScalarUndef = UndefValue::get(ScalarTy);
+  Constant *ScalarOnes = Constant::getAllOnesValue(ScalarTy);
+  Constant *VectorZero = Constant::getNullValue(VectorTy);
+  Constant *VectorOnes = Constant::getAllOnesValue(VectorTy);
+
+  SmallVector<Constant *, 3> MixedElems;
+  MixedElems.push_back(ScalarOnes);
+  MixedElems.push_back(ScalarOnes);
+  MixedElems.push_back(ScalarUndef);
+  Constant *VectorMixed = ConstantVector::get(MixedElems);
+
+  Value *Not = IRB.CreateXor(VectorZero, VectorOnes);
+  Value *X;
+  EXPECT_TRUE(match(Not, m_Not(m_Value())));
+  EXPECT_TRUE(match(Not, m_NotForbidUndef(m_Value(X))));
+  EXPECT_TRUE(match(X, m_Zero()));
+
+  Value *NotCommute = IRB.CreateXor(VectorOnes, VectorZero);
+  Value *Y;
+  EXPECT_TRUE(match(NotCommute, m_Not(m_Value())));
+  EXPECT_TRUE(match(NotCommute, m_NotForbidUndef(m_Value(Y))));
+  EXPECT_TRUE(match(Y, m_Zero()));
+
+  Value *NotWithUndefs = IRB.CreateXor(VectorZero, VectorMixed);
+  EXPECT_TRUE(match(NotWithUndefs, m_Not(m_Value())));
+  EXPECT_FALSE(match(NotWithUndefs, m_NotForbidUndef(m_Value())));
+
+  Value *NotWithUndefsCommute = IRB.CreateXor(VectorMixed, VectorZero);
+  EXPECT_TRUE(match(NotWithUndefsCommute, m_Not(m_Value())));
+  EXPECT_FALSE(match(NotWithUndefsCommute, m_NotForbidUndef(m_Value(X))));
+}
+
 template <typename T> struct MutableConstTest : PatternMatchTest { };
 
 typedef ::testing::Types<std::tuple<Value*, Instruction*>,