Index: llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -448,6 +448,14 @@
       return IC.replaceInstUsesWith(II, ZextCttz);
     }
 
+    // Sext doesn't change the number of trailing zeros, so narrow:
+    // cttz(sext(x)) -> zext(cttz(x))
+    if (match(Op0, m_OneUse(m_SExt(m_Value(X))))) {
+      auto *Cttz = IC.Builder.CreateBinaryIntrinsic(Intrinsic::cttz, X, Op1);
+      auto *ZextCttz = IC.Builder.CreateZExt(Cttz, II.getType());
+      return IC.replaceInstUsesWith(II, ZextCttz);
+    }
+
     // cttz(abs(x)) -> cttz(x)
     // cttz(nabs(x)) -> cttz(x)
     Value *Y;
Index: llvm/test/Transforms/InstCombine/cttz.ll
===================================================================
--- llvm/test/Transforms/InstCombine/cttz.ll
+++ llvm/test/Transforms/InstCombine/cttz.ll
@@ -64,9 +64,9 @@
 
 define i32 @cttz_sext_zero_undef(i16 %x) {
 ; CHECK-LABEL: @cttz_sext_zero_undef(
-; CHECK-NEXT:    [[S:%.*]] = sext i16 [[X:%.*]] to i32
-; CHECK-NEXT:    [[TZ:%.*]] = call i32 @llvm.cttz.i32(i32 [[S]], i1 true), !range [[RNG1]]
-; CHECK-NEXT:    ret i32 [[TZ]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call i16 @llvm.cttz.i16(i16 [[X:%.*]], i1 true), !range [[RNG0]]
+; CHECK-NEXT:    [[TMP2:%.*]] = zext i16 [[TMP1]] to i32
+; CHECK-NEXT:    ret i32 [[TMP2]]
 ;
   %s = sext i16 %x to i32
   %tz = call i32 @llvm.cttz.i32(i32 %s, i1 true)
@@ -99,9 +99,9 @@
 
 define <2 x i64> @cttz_sext_zero_undef_vec(<2 x i32> %x) {
 ; CHECK-LABEL: @cttz_sext_zero_undef_vec(
-; CHECK-NEXT:    [[S:%.*]] = sext <2 x i32> [[X:%.*]] to <2 x i64>
-; CHECK-NEXT:    [[TZ:%.*]] = tail call <2 x i64> @llvm.cttz.v2i64(<2 x i64> [[S]], i1 true)
-; CHECK-NEXT:    ret <2 x i64> [[TZ]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call <2 x i32> @llvm.cttz.v2i32(<2 x i32> [[X:%.*]], i1 true)
+; CHECK-NEXT:    [[TMP2:%.*]] = zext <2 x i32> [[TMP1]] to <2 x i64>
+; CHECK-NEXT:    ret <2 x i64> [[TMP2]]
 ;
   %s = sext <2 x i32> %x to <2 x i64>
   %tz = tail call <2 x i64> @llvm.cttz.v2i64(<2 x i64> %s, i1 true)
@@ -110,9 +110,9 @@
 
 define <2 x i64> @cttz_sext_zero_def_vec(<2 x i32> %x) {
 ; CHECK-LABEL: @cttz_sext_zero_def_vec(
-; CHECK-NEXT:    [[S:%.*]] = sext <2 x i32> [[X:%.*]] to <2 x i64>
-; CHECK-NEXT:    [[TZ:%.*]] = tail call <2 x i64> @llvm.cttz.v2i64(<2 x i64> [[S]], i1 false)
-; CHECK-NEXT:    ret <2 x i64> [[TZ]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call <2 x i32> @llvm.cttz.v2i32(<2 x i32> [[X:%.*]], i1 false)
+; CHECK-NEXT:    [[TMP2:%.*]] = zext <2 x i32> [[TMP1]] to <2 x i64>
+; CHECK-NEXT:    ret <2 x i64> [[TMP2]]
 ;
   %s = sext <2 x i32> %x to <2 x i64>
   %tz = tail call <2 x i64> @llvm.cttz.v2i64(<2 x i64> %s, i1 false)