Index: llvm/lib/Target/ARM/ARMTargetTransformInfo.h
===================================================================
--- llvm/lib/Target/ARM/ARMTargetTransformInfo.h
+++ llvm/lib/Target/ARM/ARMTargetTransformInfo.h
@@ -120,6 +120,11 @@
 
   Optional<Instruction *> instCombineIntrinsic(InstCombiner &IC,
                                                IntrinsicInst &II) const;
+  Optional<Value *> simplifyDemandedVectorEltsIntrinsic(
+      InstCombiner &IC, IntrinsicInst &II, APInt DemandedElts, APInt &UndefElts,
+      APInt &UndefElts2, APInt &UndefElts3,
+      std::function<void(Instruction *, unsigned, APInt, APInt &)>
+          SimplifyAndSetOp) const;
 
   /// \name Scalar TTI Implementations
   /// @{
Index: llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
===================================================================
--- llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
+++ llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
@@ -248,6 +248,45 @@
   return None;
 }
 
+Optional<Value *> ARMTTIImpl::simplifyDemandedVectorEltsIntrinsic(
+    InstCombiner &IC, IntrinsicInst &II, APInt OrigDemandedElts,
+    APInt &UndefElts, APInt &UndefElts2, APInt &UndefElts3,
+    std::function<void(Instruction *, unsigned, APInt, APInt &)>
+        SimplifyAndSetOp) const {
+
+  auto SimplifyNarrowInstrTopBottom =[&](unsigned TopOpc) {
+    unsigned NumElts = cast<FixedVectorType>(II.getType())->getNumElements();
+    unsigned IsTop = cast<ConstantInt>(II.getOperand(TopOpc))->getZExtValue();
+
+    // The only odd/even lanes of operand 0 will only be demanded depending
+    // on whether this is a top/bottom instruction.
+    APInt DemandedElts =
+        APInt::getSplat(NumElts, IsTop ? APInt::getLowBitsSet(2, 1)
+                                       : APInt::getHighBitsSet(2, 1));
+    SimplifyAndSetOp(&II, 0, OrigDemandedElts & DemandedElts, UndefElts);
+    // The other lanes will be defined from the inserted elements.
+    UndefElts &= APInt::getSplat(NumElts, !IsTop ? APInt::getLowBitsSet(2, 1)
+                                                 : APInt::getHighBitsSet(2, 1));
+    return None;
+  };
+
+  switch (II.getIntrinsicID()) {
+  default:
+    break;
+  case Intrinsic::arm_mve_vcvt_narrow:
+    SimplifyNarrowInstrTopBottom(2);
+    break;
+  case Intrinsic::arm_mve_vqmovn:
+    SimplifyNarrowInstrTopBottom(4);
+    break;
+  case Intrinsic::arm_mve_vshrn:
+    SimplifyNarrowInstrTopBottom(7);
+    break;
+  }
+
+  return None;
+}
+
 InstructionCost ARMTTIImpl::getIntImmCost(const APInt &Imm, Type *Ty,
                                           TTI::TargetCostKind CostKind) {
   assert(Ty->isIntegerTy());
Index: llvm/test/Transforms/InstCombine/ARM/mve-narrow.ll
===================================================================
--- llvm/test/Transforms/InstCombine/ARM/mve-narrow.ll
+++ llvm/test/Transforms/InstCombine/ARM/mve-narrow.ll
@@ -7,7 +7,7 @@
 
 define <8 x i16> @test_shrn_v8i16_t1(<8 x i16> %a, <8 x i16> %b, <4 x i32> %c, <4 x i32> %d) {
 ; CHECK-LABEL: @test_shrn_v8i16_t1(
-; CHECK-NEXT:    [[X:%.*]] = add <8 x i16> [[A:%.*]], <i16 1, i16 -1, i16 1, i16 -1, i16 1, i16 -1, i16 1, i16 -1>
+; CHECK-NEXT:    [[X:%.*]] = add <8 x i16> [[A:%.*]], <i16 1, i16 poison, i16 1, i16 poison, i16 1, i16 poison, i16 1, i16 poison>
 ; CHECK-NEXT:    [[Z:%.*]] = call <8 x i16> @llvm.arm.mve.vshrn.v8i16.v4i32(<8 x i16> [[X]], <4 x i32> [[D:%.*]], i32 16, i32 0, i32 0, i32 0, i32 0, i32 1)
 ; CHECK-NEXT:    ret <8 x i16> [[Z]]
 ;
@@ -18,7 +18,7 @@
 
 define <8 x i16> @test_shrn_v8i16_t2(<8 x i16> %a, <8 x i16> %b, <4 x i32> %c, <4 x i32> %d) {
 ; CHECK-LABEL: @test_shrn_v8i16_t2(
-; CHECK-NEXT:    [[X:%.*]] = add <8 x i16> [[A:%.*]], <i16 -1, i16 1, i16 -1, i16 1, i16 -1, i16 1, i16 -1, i16 1>
+; CHECK-NEXT:    [[X:%.*]] = add <8 x i16> [[A:%.*]], <i16 -1, i16 poison, i16 -1, i16 poison, i16 -1, i16 poison, i16 -1, i16 poison>
 ; CHECK-NEXT:    [[Z:%.*]] = call <8 x i16> @llvm.arm.mve.vshrn.v8i16.v4i32(<8 x i16> [[X]], <4 x i32> [[D:%.*]], i32 16, i32 0, i32 0, i32 0, i32 0, i32 1)
 ; CHECK-NEXT:    ret <8 x i16> [[Z]]
 ;
@@ -29,7 +29,7 @@
 
 define <8 x i16> @test_shrn_v8i16_b1(<8 x i16> %a, <8 x i16> %b, <4 x i32> %c, <4 x i32> %d) {
 ; CHECK-LABEL: @test_shrn_v8i16_b1(
-; CHECK-NEXT:    [[X:%.*]] = add <8 x i16> [[A:%.*]], <i16 1, i16 -1, i16 1, i16 -1, i16 1, i16 -1, i16 1, i16 -1>
+; CHECK-NEXT:    [[X:%.*]] = add <8 x i16> [[A:%.*]], <i16 poison, i16 -1, i16 poison, i16 -1, i16 poison, i16 -1, i16 poison, i16 -1>
 ; CHECK-NEXT:    [[Z:%.*]] = call <8 x i16> @llvm.arm.mve.vshrn.v8i16.v4i32(<8 x i16> [[X]], <4 x i32> [[D:%.*]], i32 16, i32 0, i32 0, i32 0, i32 0, i32 0)
 ; CHECK-NEXT:    ret <8 x i16> [[Z]]
 ;
@@ -40,7 +40,7 @@
 
 define <8 x i16> @test_shrn_v8i16_b2(<8 x i16> %a, <8 x i16> %b, <4 x i32> %c, <4 x i32> %d) {
 ; CHECK-LABEL: @test_shrn_v8i16_b2(
-; CHECK-NEXT:    [[X:%.*]] = add <8 x i16> [[A:%.*]], <i16 -1, i16 1, i16 -1, i16 1, i16 -1, i16 1, i16 -1, i16 1>
+; CHECK-NEXT:    [[X:%.*]] = add <8 x i16> [[A:%.*]], <i16 poison, i16 1, i16 poison, i16 1, i16 poison, i16 1, i16 poison, i16 1>
 ; CHECK-NEXT:    [[Z:%.*]] = call <8 x i16> @llvm.arm.mve.vshrn.v8i16.v4i32(<8 x i16> [[X]], <4 x i32> [[D:%.*]], i32 16, i32 0, i32 0, i32 0, i32 0, i32 0)
 ; CHECK-NEXT:    ret <8 x i16> [[Z]]
 ;
@@ -51,8 +51,7 @@
 
 define <8 x i16> @test_shrn_v8i16_bt(<8 x i16> %a, <8 x i16> %b, <4 x i32> %c, <4 x i32> %d) {
 ; CHECK-LABEL: @test_shrn_v8i16_bt(
-; CHECK-NEXT:    [[X:%.*]] = add <8 x i16> [[A:%.*]], [[B:%.*]]
-; CHECK-NEXT:    [[Y:%.*]] = call <8 x i16> @llvm.arm.mve.vshrn.v8i16.v4i32(<8 x i16> [[X]], <4 x i32> [[C:%.*]], i32 16, i32 0, i32 0, i32 0, i32 0, i32 0)
+; CHECK-NEXT:    [[Y:%.*]] = call <8 x i16> @llvm.arm.mve.vshrn.v8i16.v4i32(<8 x i16> poison, <4 x i32> [[C:%.*]], i32 16, i32 0, i32 0, i32 0, i32 0, i32 0)
 ; CHECK-NEXT:    [[Z:%.*]] = call <8 x i16> @llvm.arm.mve.vshrn.v8i16.v4i32(<8 x i16> [[Y]], <4 x i32> [[D:%.*]], i32 16, i32 0, i32 0, i32 0, i32 0, i32 1)
 ; CHECK-NEXT:    ret <8 x i16> [[Z]]
 ;
@@ -64,8 +63,7 @@
 
 define <8 x i16> @test_shrn_v8i16_tb(<8 x i16> %a, <8 x i16> %b, <4 x i32> %c, <4 x i32> %d) {
 ; CHECK-LABEL: @test_shrn_v8i16_tb(
-; CHECK-NEXT:    [[X:%.*]] = add <8 x i16> [[A:%.*]], [[B:%.*]]
-; CHECK-NEXT:    [[Y:%.*]] = call <8 x i16> @llvm.arm.mve.vshrn.v8i16.v4i32(<8 x i16> [[X]], <4 x i32> [[C:%.*]], i32 16, i32 0, i32 0, i32 0, i32 0, i32 1)
+; CHECK-NEXT:    [[Y:%.*]] = call <8 x i16> @llvm.arm.mve.vshrn.v8i16.v4i32(<8 x i16> poison, <4 x i32> [[C:%.*]], i32 16, i32 0, i32 0, i32 0, i32 0, i32 1)
 ; CHECK-NEXT:    [[Z:%.*]] = call <8 x i16> @llvm.arm.mve.vshrn.v8i16.v4i32(<8 x i16> [[Y]], <4 x i32> [[D:%.*]], i32 16, i32 0, i32 0, i32 0, i32 0, i32 0)
 ; CHECK-NEXT:    ret <8 x i16> [[Z]]
 ;
@@ -105,8 +103,7 @@
 
 define <16 x i8> @test_shrn_v16i8_bt(<16 x i8> %a, <16 x i8> %b, <8 x i16> %c, <8 x i16> %d) {
 ; CHECK-LABEL: @test_shrn_v16i8_bt(
-; CHECK-NEXT:    [[X:%.*]] = add <16 x i8> [[A:%.*]], [[B:%.*]]
-; CHECK-NEXT:    [[Y:%.*]] = call <16 x i8> @llvm.arm.mve.vshrn.v16i8.v8i16(<16 x i8> [[X]], <8 x i16> [[C:%.*]], i32 16, i32 0, i32 0, i32 0, i32 0, i32 0)
+; CHECK-NEXT:    [[Y:%.*]] = call <16 x i8> @llvm.arm.mve.vshrn.v16i8.v8i16(<16 x i8> poison, <8 x i16> [[C:%.*]], i32 16, i32 0, i32 0, i32 0, i32 0, i32 0)
 ; CHECK-NEXT:    [[Z:%.*]] = call <16 x i8> @llvm.arm.mve.vshrn.v16i8.v8i16(<16 x i8> [[Y]], <8 x i16> [[D:%.*]], i32 16, i32 0, i32 0, i32 0, i32 0, i32 1)
 ; CHECK-NEXT:    ret <16 x i8> [[Z]]
 ;
@@ -171,8 +168,7 @@
 
 define <8 x i16> @test_qmovn_v8i16_bt(<8 x i16> %a, <8 x i16> %b, <4 x i32> %c, <4 x i32> %d) {
 ; CHECK-LABEL: @test_qmovn_v8i16_bt(
-; CHECK-NEXT:    [[X:%.*]] = add <8 x i16> [[A:%.*]], [[B:%.*]]
-; CHECK-NEXT:    [[Y:%.*]] = call <8 x i16> @llvm.arm.mve.vqmovn.v8i16.v4i32(<8 x i16> [[X]], <4 x i32> [[C:%.*]], i32 0, i32 0, i32 0)
+; CHECK-NEXT:    [[Y:%.*]] = call <8 x i16> @llvm.arm.mve.vqmovn.v8i16.v4i32(<8 x i16> poison, <4 x i32> [[C:%.*]], i32 0, i32 0, i32 0)
 ; CHECK-NEXT:    [[Z:%.*]] = call <8 x i16> @llvm.arm.mve.vqmovn.v8i16.v4i32(<8 x i16> [[Y]], <4 x i32> [[D:%.*]], i32 0, i32 0, i32 1)
 ; CHECK-NEXT:    ret <8 x i16> [[Z]]
 ;
@@ -184,8 +180,7 @@
 
 define <16 x i8> @test_qmovn_v16i8_bt(<16 x i8> %a, <16 x i8> %b, <8 x i16> %c, <8 x i16> %d) {
 ; CHECK-LABEL: @test_qmovn_v16i8_bt(
-; CHECK-NEXT:    [[X:%.*]] = add <16 x i8> [[A:%.*]], [[B:%.*]]
-; CHECK-NEXT:    [[Y:%.*]] = call <16 x i8> @llvm.arm.mve.vqmovn.v16i8.v8i16(<16 x i8> [[X]], <8 x i16> [[C:%.*]], i32 0, i32 0, i32 0)
+; CHECK-NEXT:    [[Y:%.*]] = call <16 x i8> @llvm.arm.mve.vqmovn.v16i8.v8i16(<16 x i8> poison, <8 x i16> [[C:%.*]], i32 0, i32 0, i32 0)
 ; CHECK-NEXT:    [[Z:%.*]] = call <16 x i8> @llvm.arm.mve.vqmovn.v16i8.v8i16(<16 x i8> [[Y]], <8 x i16> [[D:%.*]], i32 0, i32 0, i32 1)
 ; CHECK-NEXT:    ret <16 x i8> [[Z]]
 ;
@@ -223,8 +218,7 @@
 
 define <8 x half> @test_cvtn_v8i16_bt(<8 x half> %a, <8 x half> %b, <4 x float> %c, <4 x float> %d) {
 ; CHECK-LABEL: @test_cvtn_v8i16_bt(
-; CHECK-NEXT:    [[X:%.*]] = fadd <8 x half> [[A:%.*]], [[B:%.*]]
-; CHECK-NEXT:    [[Y:%.*]] = call <8 x half> @llvm.arm.mve.vcvt.narrow(<8 x half> [[X]], <4 x float> [[C:%.*]], i32 0)
+; CHECK-NEXT:    [[Y:%.*]] = call <8 x half> @llvm.arm.mve.vcvt.narrow(<8 x half> poison, <4 x float> [[C:%.*]], i32 0)
 ; CHECK-NEXT:    [[Z:%.*]] = call <8 x half> @llvm.arm.mve.vcvt.narrow(<8 x half> [[Y]], <4 x float> [[D:%.*]], i32 1)
 ; CHECK-NEXT:    ret <8 x half> [[Z]]
 ;