Index: lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -1237,6 +1237,20 @@
   return nullptr;
 }
 
+static void makeCttzCtlzZeroUndef(InstCombiner &IC, IntrinsicInst *II) {
+  assert((II->getIntrinsicID() == Intrinsic::cttz ||
+          II->getIntrinsicID() == Intrinsic::ctlz) &&
+         "Expected cttz or ctlz intrinsic");
+  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()));
+  }
+}
+
 /// CallInst simplification. This mostly only handles folding of intrinsic
 /// instructions. For normal calls, it allows visitCallSite to do the heavy
 /// lifting.
@@ -1400,40 +1414,43 @@
   case Intrinsic::cttz: {
     // If all bits below the first known one are known zero,
     // this value is constant.
-    IntegerType *IT = dyn_cast<IntegerType>(II->getArgOperand(0)->getType());
+    Value *Op0 = II->getArgOperand(0);
+    IntegerType *IT = dyn_cast<IntegerType>(Op0->getType());
     // FIXME: Try to simplify vectors of integers.
     if (!IT) break;
     uint32_t BitWidth = IT->getBitWidth();
     APInt KnownZero(BitWidth, 0);
     APInt KnownOne(BitWidth, 0);
-    computeKnownBits(II->getArgOperand(0), KnownZero, KnownOne, 0, II);
+    computeKnownBits(Op0, KnownZero, KnownOne, 0, II);
     unsigned TrailingZeros = KnownOne.countTrailingZeros();
     APInt Mask(APInt::getLowBitsSet(BitWidth, TrailingZeros));
     if ((Mask & KnownZero) == Mask)
       return replaceInstUsesWith(CI, ConstantInt::get(IT,
                                  APInt(BitWidth, TrailingZeros)));
-
+    if (KnownOne != 0 || isKnownNonZero(Op0, DL))
+      makeCttzCtlzZeroUndef(*this, II);
     }
     break;
   case Intrinsic::ctlz: {
     // If all bits above the first known one are known zero,
     // this value is constant.
-    IntegerType *IT = dyn_cast<IntegerType>(II->getArgOperand(0)->getType());
+    Value *Op0 = II->getArgOperand(0);
+    IntegerType *IT = dyn_cast<IntegerType>(Op0->getType());
     // FIXME: Try to simplify vectors of integers.
     if (!IT) break;
     uint32_t BitWidth = IT->getBitWidth();
     APInt KnownZero(BitWidth, 0);
     APInt KnownOne(BitWidth, 0);
-    computeKnownBits(II->getArgOperand(0), KnownZero, KnownOne, 0, II);
+    computeKnownBits(Op0, KnownZero, KnownOne, 0, II);
     unsigned LeadingZeros = KnownOne.countLeadingZeros();
     APInt Mask(APInt::getHighBitsSet(BitWidth, LeadingZeros));
     if ((Mask & KnownZero) == Mask)
       return replaceInstUsesWith(CI, ConstantInt::get(IT,
                                  APInt(BitWidth, LeadingZeros)));
-
+    if (KnownOne != 0 || isKnownNonZero(Op0, DL))
+      makeCttzCtlzZeroUndef(*this, II);
     }
     break;
-
   case Intrinsic::uadd_with_overflow:
   case Intrinsic::sadd_with_overflow:
   case Intrinsic::umul_with_overflow:
Index: test/Transforms/InstCombine/intrinsics.ll
===================================================================
--- test/Transforms/InstCombine/intrinsics.ll
+++ test/Transforms/InstCombine/intrinsics.ll
@@ -384,6 +384,16 @@
 ; CHECK-NEXT: ret i32 undef
 }
 
+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 {
   %cttz = call i32 @llvm.cttz.i32(i32 0, i1 true)
   ret i32 %cttz
@@ -392,6 +402,18 @@
 ; CHECK-NEXT: ret i32 undef
 }
 
+define i32 @cttz_make_undef(i32 %a) {
+entry:
+  %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: entry:
+; 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 {
   %tobool = icmp ne i32 %Value, 0
   %ctlz = call i32 @llvm.ctlz.i32(i32 %Value, i1 true)