Index: lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -1105,6 +1105,9 @@
 }
 
 static Value *foldCttzCtlz(IntrinsicInst &II, InstCombiner &IC) {
+  assert((II.getIntrinsicID() == Intrinsic::cttz ||
+          II.getIntrinsicID() == Intrinsic::ctlz) &&
+         "Expected cttz or ctlz intrinsic");
   Value *Op0 = II.getArgOperand(0);
   // FIXME: Try to simplify vectors of integers.
   auto *IT = dyn_cast<IntegerType>(Op0->getType());
@@ -1130,6 +1133,17 @@
   if ((Mask & KnownZero) == Mask)
     return ConstantInt::get(IT, APInt(BitWidth, NumMaskBits));
 
+  if (KnownOne != 0 || isKnownNonZero(Op0, IC.getDataLayout())) {
+    bool IsZeroUndef = false;
+    Value *Op1 = II.getArgOperand(1);
+    if (auto *Op1C = dyn_cast<ConstantInt>(Op1))
+      IsZeroUndef = Op1C->getZExtValue() != 0;
+    if (!IsZeroUndef) {
+      IC.Worklist.Add(&II);
+      II.setOperand(1, ConstantInt::getAllOnesValue(Op1->getType()));
+    }
+  }
+
   return nullptr;
 }
 
Index: test/Transforms/InstCombine/intrinsics.ll
===================================================================
--- test/Transforms/InstCombine/intrinsics.ll
+++ test/Transforms/InstCombine/intrinsics.ll
@@ -380,6 +380,16 @@
 
 }
 
+define i32 @ctlz_make_undef(i32 %a) {
+  %or = or i32 %a, 8
+  %ctlz = tail call i32 @llvm.ctlz.i32(i32 %or, i1 false)
+  ret i32 %ctlz
+; CHECK-LABEL: @ctlz_make_undef(
+; CHECK-NEXT: %or = or i32 %a, 8
+; CHECK-NEXT: %ctlz = tail call i32 @llvm.ctlz.i32(i32 %or, i1 true)
+; CHECK-NEXT: ret i32 %ctlz
+}
+
 define i32 @cttz_undef(i32 %Value) nounwind {
 ; CHECK-LABEL: @cttz_undef(
 ; CHECK-NEXT:    ret i32 undef
@@ -389,6 +399,16 @@
 
 }
 
+define i32 @cttz_make_undef(i32 %a) {
+  %or = or i32 %a, 8
+  %cttz = tail call i32 @llvm.cttz.i32(i32 %or, i1 false)
+  ret i32 %cttz
+; CHECK-LABEL: @cttz_make_undef(
+; CHECK-NEXT: %or = or i32 %a, 8
+; CHECK-NEXT: %cttz = tail call i32 @llvm.cttz.i32(i32 %or, i1 true)
+; CHECK-NEXT: ret i32 %cttz
+}
+
 define i32 @ctlz_select(i32 %Value) nounwind {
 ; CHECK-LABEL: @ctlz_select(
 ; CHECK-NEXT:    [[TMP1:%.*]] = call i32 @llvm.ctlz.i32(i32 %Value, i1 false)