Index: lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -1400,7 +1400,8 @@
   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());
+    auto 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();
@@ -1412,13 +1413,15 @@
     if ((Mask & KnownZero) == Mask)
       return replaceInstUsesWith(CI, ConstantInt::get(IT,
                                  APInt(BitWidth, TrailingZeros)));
-
+    if (KnownOne != 0 || isKnownNonZero(Op0, DL))
+      goto MakeCttzCltzUndef;
     }
     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());
+    auto 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();
@@ -1430,10 +1433,21 @@
     if ((Mask & KnownZero) == Mask)
       return replaceInstUsesWith(CI, ConstantInt::get(IT,
                                  APInt(BitWidth, LeadingZeros)));
-
+    if (KnownOne != 0 || isKnownNonZero(Op0, DL))
+      goto MakeCttzCltzUndef;
     }
     break;
-
+  MakeCttzCltzUndef: {
+    bool IsZeroUndef = false;
+    auto Op1 = II->getArgOperand(1);
+    if (auto Op1C = dyn_cast<ConstantInt>(Op1))
+      IsZeroUndef = Op1C->getZExtValue() != 0;
+    if (!IsZeroUndef) {
+      Worklist.Add(II);
+      II->setOperand(1, ConstantInt::getAllOnesValue(Op1->getType()));
+    }
+    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
@@ -379,6 +379,18 @@
 ; CHECK-NEXT: ret i32 undef
 }
 
+define i32 @ctlz_make_undef(i32 %a) {
+entry:
+  %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: entry:
+; 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
@@ -387,6 +399,18 @@
 ; 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)