Index: lib/Transforms/Scalar/NewGVN.cpp
===================================================================
--- lib/Transforms/Scalar/NewGVN.cpp
+++ lib/Transforms/Scalar/NewGVN.cpp
@@ -1151,7 +1151,9 @@
       return SimplifiedE;
   } else if (isa<SelectInst>(I)) {
     if (isa<Constant>(E->getOperand(0)) ||
-        E->getOperand(1) == E->getOperand(2)) {
+        E->getOperand(1) == E->getOperand(2) ||
+        isa<UndefValue>(E->getOperand(1)) ||
+        isa<UndefValue>(E->getOperand(2))) {
       assert(E->getOperand(1)->getType() == I->getOperand(1)->getType() &&
              E->getOperand(2)->getType() == I->getOperand(2)->getType());
       Value *V = SimplifySelectInst(E->getOperand(0), E->getOperand(1),
Index: test/Transforms/NewGVN/pr33165.ll
===================================================================
--- /dev/null
+++ test/Transforms/NewGVN/pr33165.ll
@@ -0,0 +1,24 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -newgvn -S < %s | FileCheck %s
+
+define i2 @t1(i2, i1) {
+; CHECK-LABEL: @t1(
+; CHECK-NEXT:    [[A:%.*]] = xor i2 [[TMP0:%.*]], -1
+; CHECK-NEXT:    ret i2 [[A]]
+;
+  %a = xor i2 %0, -1
+  %b = select i1 %1, i2 %a, i2 undef
+  %c = and i2 %a, %b
+  ret i2 %c
+}
+
+define i2 @t2(i2, i1) {
+; CHECK-LABEL: @t2(
+; CHECK-NEXT:    [[A:%.*]] = xor i2 [[TMP0:%.*]], -1
+; CHECK-NEXT:    ret i2 [[A]]
+;
+  %a = xor i2 %0, -1
+  %b = select i1 %1, i2 undef, i2 %a
+  %c = and i2 %a, %b
+  ret i2 %c
+}