Index: llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -469,6 +469,12 @@
 
   KnownBits Known = IC.computeKnownBits(Op0, 0, &II);
 
+  if (!IsTZ && Known.isNegative()) {
+    // ctlz(x) -> 0 if x is known negative.
+    auto *C = Constant::getNullValue(Op0->getType());
+    return IC.replaceInstUsesWith(II, C);
+  }
+
   // Create a mask for bits above (ctlz) or below (cttz) the first known one.
   unsigned PossibleZeros = IsTZ ? Known.countMaxTrailingZeros()
                                 : Known.countMaxLeadingZeros();
Index: llvm/test/Transforms/InstCombine/ctlz.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/InstCombine/ctlz.ll
@@ -0,0 +1,71 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -instcombine -S < %s | FileCheck %s
+declare i32 @llvm.ctlz.i32(i32, i1)
+declare void @llvm.assume(i1)
+declare void @use(i32)
+
+define i32 @ctlz_sext_negative_zero_undef(i16 %x) {
+; CHECK-LABEL: @ctlz_sext_negative_zero_undef(
+; CHECK-NEXT:    [[ASSUME:%.*]] = icmp slt i16 [[X:%.*]], 0
+; CHECK-NEXT:    call void @llvm.assume(i1 [[ASSUME]])
+; CHECK-NEXT:    ret i32 0
+;
+  %assume = icmp slt i16 %x, 0
+  call void @llvm.assume(i1 %assume)
+  %s = sext i16 %x to i32
+  %c = call i32 @llvm.ctlz.i32(i32 %s, i1 true)
+  ret i32 %c
+}
+
+define i32 @ctlz_negative_zero_undef(i32 %x) {
+; CHECK-LABEL: @ctlz_negative_zero_undef(
+; CHECK-NEXT:    [[ASSUME:%.*]] = icmp slt i32 [[X:%.*]], 0
+; CHECK-NEXT:    call void @llvm.assume(i1 [[ASSUME]])
+; CHECK-NEXT:    ret i32 0
+;
+  %assume = icmp slt i32 %x, 0
+  call void @llvm.assume(i1 %assume)
+  %c = call i32 @llvm.ctlz.i32(i32 %x, i1 true)
+  ret i32 %c
+}
+
+define i32 @ctlz_negative_zero_def(i32 %x) {
+; CHECK-LABEL: @ctlz_negative_zero_def(
+; CHECK-NEXT:    [[ASSUME:%.*]] = icmp slt i32 [[X:%.*]], 0
+; CHECK-NEXT:    call void @llvm.assume(i1 [[ASSUME]])
+; CHECK-NEXT:    ret i32 0
+;
+  %assume = icmp slt i32 %x, 0
+  call void @llvm.assume(i1 %assume)
+  %c = call i32 @llvm.ctlz.i32(i32 %x, i1 false)
+  ret i32 %c
+}
+
+define i32 @ctlz_sext_negative_zero_def(i16 %x) {
+; CHECK-LABEL: @ctlz_sext_negative_zero_def(
+; CHECK-NEXT:    [[ASSUME:%.*]] = icmp slt i16 [[X:%.*]], 0
+; CHECK-NEXT:    call void @llvm.assume(i1 [[ASSUME]])
+; CHECK-NEXT:    ret i32 0
+;
+  %assume = icmp slt i16 %x, 0
+  call void @llvm.assume(i1 %assume)
+  %s = sext i16 %x to i32
+  %c = call i32 @llvm.ctlz.i32(i32 %s, i1 false)
+  ret i32 %c
+}
+
+define i32 @ctlz_sext_negative_zero_undef_extra_use(i16 %x) {
+; CHECK-LABEL: @ctlz_sext_negative_zero_undef_extra_use(
+; CHECK-NEXT:    [[ASSUME:%.*]] = icmp slt i16 [[X:%.*]], 0
+; CHECK-NEXT:    call void @llvm.assume(i1 [[ASSUME]])
+; CHECK-NEXT:    [[S:%.*]] = sext i16 [[X]] to i32
+; CHECK-NEXT:    call void @use(i32 [[S]])
+; CHECK-NEXT:    ret i32 0
+;
+  %assume = icmp slt i16 %x, 0
+  call void @llvm.assume(i1 %assume)
+  %s = sext i16 %x to i32
+  call void @use(i32 %s)
+  %c = call i32 @llvm.ctlz.i32(i32 %s, i1 true)
+  ret i32 %c
+}
Index: llvm/test/Transforms/InstSimplify/call.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/call.ll
+++ llvm/test/Transforms/InstSimplify/call.ll
@@ -1465,78 +1465,4 @@
   ret <3 x i33> %r
 }
 
-define i32 @ctlz_sext_negative_zero_undef(i16 %x) {
-; CHECK-LABEL: @ctlz_sext_negative_zero_undef(
-; CHECK-NEXT:    [[ASSUME:%.*]] = icmp slt i16 [[X:%.*]], 0
-; CHECK-NEXT:    call void @llvm.assume(i1 [[ASSUME]])
-; CHECK-NEXT:    [[S:%.*]] = sext i16 [[X]] to i32
-; CHECK-NEXT:    [[C:%.*]] = call i32 @llvm.ctlz.i32(i32 [[S]], i1 true)
-; CHECK-NEXT:    ret i32 [[C]]
-;
-  %assume = icmp slt i16 %x, 0
-  call void @llvm.assume(i1 %assume)
-  %s = sext i16 %x to i32
-  %c = call i32 @llvm.ctlz.i32(i32 %s, i1 true)
-  ret i32 %c
-}
-
-define i32 @ctlz_negative_zero_undef(i32 %x) {
-; CHECK-LABEL: @ctlz_negative_zero_undef(
-; CHECK-NEXT:    [[ASSUME:%.*]] = icmp slt i32 [[X:%.*]], 0
-; CHECK-NEXT:    call void @llvm.assume(i1 [[ASSUME]])
-; CHECK-NEXT:    [[C:%.*]] = call i32 @llvm.ctlz.i32(i32 [[X]], i1 true)
-; CHECK-NEXT:    ret i32 [[C]]
-;
-  %assume = icmp slt i32 %x, 0
-  call void @llvm.assume(i1 %assume)
-  %c = call i32 @llvm.ctlz.i32(i32 %x, i1 true)
-  ret i32 %c
-}
-
-define i32 @ctlz_negative_zero_def(i32 %x) {
-; CHECK-LABEL: @ctlz_negative_zero_def(
-; CHECK-NEXT:    [[ASSUME:%.*]] = icmp slt i32 [[X:%.*]], 0
-; CHECK-NEXT:    call void @llvm.assume(i1 [[ASSUME]])
-; CHECK-NEXT:    [[C:%.*]] = call i32 @llvm.ctlz.i32(i32 [[X]], i1 false)
-; CHECK-NEXT:    ret i32 [[C]]
-;
-  %assume = icmp slt i32 %x, 0
-  call void @llvm.assume(i1 %assume)
-  %c = call i32 @llvm.ctlz.i32(i32 %x, i1 false)
-  ret i32 %c
-}
-
-define i32 @ctlz_sext_negative_zero_def(i16 %x) {
-; CHECK-LABEL: @ctlz_sext_negative_zero_def(
-; CHECK-NEXT:    [[ASSUME:%.*]] = icmp slt i16 [[X:%.*]], 0
-; CHECK-NEXT:    call void @llvm.assume(i1 [[ASSUME]])
-; CHECK-NEXT:    [[S:%.*]] = sext i16 [[X]] to i32
-; CHECK-NEXT:    [[C:%.*]] = call i32 @llvm.ctlz.i32(i32 [[S]], i1 false)
-; CHECK-NEXT:    ret i32 [[C]]
-;
-  %assume = icmp slt i16 %x, 0
-  call void @llvm.assume(i1 %assume)
-  %s = sext i16 %x to i32
-  %c = call i32 @llvm.ctlz.i32(i32 %s, i1 false)
-  ret i32 %c
-}
-
-declare void @use(i32)
-define i32 @ctlz_sext_negative_zero_undef_extra_use(i16 %x) {
-; CHECK-LABEL: @ctlz_sext_negative_zero_undef_extra_use(
-; CHECK-NEXT:    [[ASSUME:%.*]] = icmp slt i16 [[X:%.*]], 0
-; CHECK-NEXT:    call void @llvm.assume(i1 [[ASSUME]])
-; CHECK-NEXT:    [[S:%.*]] = sext i16 [[X]] to i32
-; CHECK-NEXT:    call void @use(i32 [[S]])
-; CHECK-NEXT:    [[C:%.*]] = call i32 @llvm.ctlz.i32(i32 [[S]], i1 true)
-; CHECK-NEXT:    ret i32 [[C]]
-;
-  %assume = icmp slt i16 %x, 0
-  call void @llvm.assume(i1 %assume)
-  %s = sext i16 %x to i32
-  call void @use(i32 %s)
-  %c = call i32 @llvm.ctlz.i32(i32 %s, i1 true)
-  ret i32 %c
-}
-
 attributes #0 = { nobuiltin readnone }