diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -5411,6 +5411,9 @@
       return X;
     if (match(Op0, m_AShr(m_Negative(), m_Value())))
       return Constant::getNullValue(ReturnType);
+    if (match(Op0, m_SExt(m_Value(X))) &&
+        isKnownNegative(X, Q.DL, /*Depth=*/0, Q.AC, Q.CxtI, Q.DT))
+      return Constant::getNullValue(ReturnType);
     break;
   }
   case Intrinsic::smax:
diff --git a/llvm/test/Transforms/InstSimplify/call.ll b/llvm/test/Transforms/InstSimplify/call.ll
--- a/llvm/test/Transforms/InstSimplify/call.ll
+++ b/llvm/test/Transforms/InstSimplify/call.ll
@@ -1469,9 +1469,7 @@
 ; 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]]
+; CHECK-NEXT:    ret i32 0
 ;
   %assume = icmp slt i16 %x, 0
   call void @llvm.assume(i1 %assume)
@@ -1484,9 +1482,7 @@
 ; 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]]
+; CHECK-NEXT:    ret i32 0
 ;
   %assume = icmp slt i16 %x, 0
   call void @llvm.assume(i1 %assume)
@@ -1502,8 +1498,7 @@
 ; 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]]
+; CHECK-NEXT:    ret i32 0
 ;
   %assume = icmp slt i16 %x, 0
   call void @llvm.assume(i1 %assume)