Index: llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -3978,23 +3978,24 @@
   if (TypeIdx != 1)
     return UnableToLegalize;
 
-  LLT SrcTy = MRI.getType(MI.getOperand(1).getReg());
+  Register DstReg = MI.getOperand(0).getReg();
+  Register SrcReg = MI.getOperand(1).getReg();
+  LLT DstTy = MRI.getType(DstReg);
+  LLT SrcTy = MRI.getType(SrcReg);
   unsigned NarrowSize = NarrowTy.getSizeInBits();
 
   if (SrcTy.isScalar() && SrcTy.getSizeInBits() == 2 * NarrowSize) {
     MachineIRBuilder &B = MIRBuilder;
-    auto UnmergeSrc = B.buildUnmerge(NarrowTy, MI.getOperand(1));
+    auto UnmergeSrc = B.buildUnmerge(NarrowTy, SrcReg);
     // ctlz(Hi:Lo) -> Hi == 0 ? (NarrowSize + ctlz(Lo)) : ctlz(Hi)
     auto C_0 = B.buildConstant(NarrowTy, 0);
     auto HiIsZero = B.buildICmp(CmpInst::ICMP_EQ, LLT::scalar(1),
                                 UnmergeSrc.getReg(1), C_0);
-    auto LoCTLZ = B.buildCTLZ(NarrowTy, UnmergeSrc.getReg(0));
+    auto LoCTLZ = B.buildCTLZ(DstTy, UnmergeSrc.getReg(0));
     auto C_NarrowSize = B.buildConstant(NarrowTy, NarrowSize);
-    auto HiIsZeroCTLZ = B.buildAdd(NarrowTy, LoCTLZ, C_NarrowSize);
-    auto HiCTLZ = B.buildCTLZ_ZERO_UNDEF(NarrowTy, UnmergeSrc.getReg(1));
-    auto LoOut = B.buildSelect(NarrowTy, HiIsZero, HiIsZeroCTLZ, HiCTLZ);
-
-    B.buildMerge(MI.getOperand(0), {LoOut.getReg(0), C_0.getReg(0)});
+    auto HiIsZeroCTLZ = B.buildAdd(DstTy, LoCTLZ, C_NarrowSize);
+    auto HiCTLZ = B.buildCTLZ_ZERO_UNDEF(DstTy, UnmergeSrc.getReg(1));
+    B.buildSelect(DstReg, HiIsZero, HiIsZeroCTLZ, HiCTLZ);
 
     MI.eraseFromParent();
     return Legalized;
@@ -4009,24 +4010,24 @@
   if (TypeIdx != 1)
     return UnableToLegalize;
 
-  LLT SrcTy = MRI.getType(MI.getOperand(1).getReg());
+  Register DstReg = MI.getOperand(0).getReg();
+  Register SrcReg = MI.getOperand(1).getReg();
+  LLT DstTy = MRI.getType(DstReg);
+  LLT SrcTy = MRI.getType(SrcReg);
   unsigned NarrowSize = NarrowTy.getSizeInBits();
 
   if (SrcTy.isScalar() && SrcTy.getSizeInBits() == 2 * NarrowSize) {
     MachineIRBuilder &B = MIRBuilder;
-    auto UnmergeSrc = B.buildUnmerge(NarrowTy, MI.getOperand(1));
+    auto UnmergeSrc = B.buildUnmerge(NarrowTy, SrcReg);
     // cttz(Hi:Lo) -> Lo == 0 ? (cttz(Hi) + NarrowSize) : cttz(Lo)
     auto C_0 = B.buildConstant(NarrowTy, 0);
     auto LoIsZero = B.buildICmp(CmpInst::ICMP_EQ, LLT::scalar(1),
                                 UnmergeSrc.getReg(0), C_0);
-    auto HiCTTZ = B.buildCTTZ(NarrowTy, UnmergeSrc.getReg(1));
-    auto C_NarrowSize = B.buildConstant(NarrowTy, NarrowSize);
-    auto LoIsZeroCTTZ = B.buildAdd(NarrowTy, HiCTTZ, C_NarrowSize);
-    auto LoCTTZ = B.buildCTTZ_ZERO_UNDEF(NarrowTy, UnmergeSrc.getReg(0));
-    auto LoOut = B.buildSelect(NarrowTy, LoIsZero, LoIsZeroCTTZ, LoCTTZ);
-
-    B.buildMerge(MI.getOperand(0), {LoOut.getReg(0), C_0.getReg(0)});
-
+    auto HiCTTZ = B.buildCTTZ(DstTy, UnmergeSrc.getReg(1));
+    auto C_NarrowSize = B.buildConstant(DstTy, NarrowSize);
+    auto LoIsZeroCTTZ = B.buildAdd(DstTy, HiCTTZ, C_NarrowSize);
+    auto LoCTTZ = B.buildCTTZ_ZERO_UNDEF(DstTy, UnmergeSrc.getReg(0));
+    B.buildSelect(DstReg, LoIsZero, LoIsZeroCTTZ, LoCTTZ);
     MI.eraseFromParent();
     return Legalized;
   }
Index: llvm/unittests/CodeGen/GlobalISel/LegalizerHelperTest.cpp
===================================================================
--- llvm/unittests/CodeGen/GlobalISel/LegalizerHelperTest.cpp
+++ llvm/unittests/CodeGen/GlobalISel/LegalizerHelperTest.cpp
@@ -90,6 +90,76 @@
   EXPECT_TRUE(CheckMachineFunction(*MF, CheckStr)) << *MF;
 }
 
+// CTLZ scalar narrowing
+TEST_F(GISelMITest, NarrowScalarCTLZ) {
+  setUp();
+  if (!TM)
+    return;
+
+  // Declare your legalization info
+  DefineLegalizerInfo(A, {
+    getActionDefinitionsBuilder(G_CTLZ).legalFor({{s32, s64}});
+  });
+  // Build Instr
+  auto CTLZ =
+      B.buildInstr(TargetOpcode::G_CTLZ, {LLT::scalar(32)}, {Copies[0]});
+  AInfo Info(MF->getSubtarget());
+  DummyGISelObserver Observer;
+  LegalizerHelper Helper(*MF, Info, Observer, B);
+  // Perform Legalization
+  EXPECT_EQ(LegalizerHelper::LegalizeResult::Legalized,
+            Helper.narrowScalar(*CTLZ, 1, LLT::scalar(32)));
+
+  auto CheckStr = R"(
+  CHECK: [[UNMERGE_LO:%[0-9]+]]:_(s32), [[UNMERGE_HI:%[0-9]+]]:_(s32) = G_UNMERGE_VALUES %0:_(s64)
+  CHECK: [[ZERO:%[0-9]+]]:_(s32) = G_CONSTANT i32 0
+  CHECK: [[CMP:%[0-9]+]]:_(s1) = G_ICMP intpred(eq), [[UNMERGE_HI]]:_(s32), [[ZERO]]:_
+  CHECK: [[CTLZ_LO:%[0-9]+]]:_(s32) = G_CTLZ [[UNMERGE_LO]]:_(s32)
+  CHECK: [[THIRTYTWO:%[0-9]+]]:_(s32) = G_CONSTANT i32 32
+  CHECK: [[ADD:%[0-9]+]]:_(s32) = G_ADD [[CTLZ_LO]]:_, [[THIRTYTWO]]:_
+  CHECK: [[CTLZ_HI:%[0-9]+]]:_(s32) = G_CTLZ_ZERO_UNDEF [[UNMERGE_HI]]:_(s32)
+  CHECK: %{{[0-9]+}}:_(s32) = G_SELECT [[CMP]]:_(s1), [[ADD]]:_, [[CTLZ_HI]]:_
+  )";
+
+  // Check
+  EXPECT_TRUE(CheckMachineFunction(*MF, CheckStr)) << *MF;
+}
+
+// CTTZ scalar narrowing
+TEST_F(GISelMITest, NarrowScalarCTTZ) {
+  setUp();
+  if (!TM)
+    return;
+
+  // Declare your legalization info
+  DefineLegalizerInfo(A, {
+    getActionDefinitionsBuilder(G_CTTZ).legalFor({{s32, s64}});
+  });
+  // Build Instr
+  auto CTTZ =
+      B.buildInstr(TargetOpcode::G_CTTZ, {LLT::scalar(32)}, {Copies[0]});
+  AInfo Info(MF->getSubtarget());
+  DummyGISelObserver Observer;
+  LegalizerHelper Helper(*MF, Info, Observer, B);
+  // Perform Legalization
+  EXPECT_EQ(LegalizerHelper::LegalizeResult::Legalized,
+            Helper.narrowScalar(*CTTZ, 1, LLT::scalar(32)));
+
+  auto CheckStr = R"(
+  CHECK: [[UNMERGE_LO:%[0-9]+]]:_(s32), [[UNMERGE_HI:%[0-9]+]]:_(s32) = G_UNMERGE_VALUES %0:_(s64)
+  CHECK: [[ZERO:%[0-9]+]]:_(s32) = G_CONSTANT i32 0
+  CHECK: [[CMP:%[0-9]+]]:_(s1) = G_ICMP intpred(eq), [[UNMERGE_LO]]:_(s32), [[ZERO]]:_
+  CHECK: [[CTTZ_HI:%[0-9]+]]:_(s32) = G_CTTZ [[UNMERGE_HI]]:_(s32)
+  CHECK: [[THIRTYTWO:%[0-9]+]]:_(s32) = G_CONSTANT i32 32
+  CHECK: [[ADD:%[0-9]+]]:_(s32) = G_ADD [[CTTZ_HI]]:_, [[THIRTYTWO]]:_
+  CHECK: [[CTTZ_LO:%[0-9]+]]:_(s32) = G_CTTZ_ZERO_UNDEF [[UNMERGE_LO]]:_(s32)
+  CHECK: %{{[0-9]+}}:_(s32) = G_SELECT [[CMP]]:_(s1), [[ADD]]:_, [[CTTZ_LO]]:_
+  )";
+
+  // Check
+  EXPECT_TRUE(CheckMachineFunction(*MF, CheckStr)) << *MF;
+}
+
 // CTTZ expansion in terms of CTPOP
 TEST_F(GISelMITest, LowerBitCountingCTTZ2) {
   setUp();