[VPlan] Add ReductionStartVector VPInstruction.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7235,8 +7235,14 @@ static void fixReductionScalarResumeWhenVectorizingEpilog(
       cast<VPReductionPHIRecipe>(EpiRedResult->getOperand(0));
   const RecurrenceDescriptor &RdxDesc =
       EpiRedHeaderPhi->getRecurrenceDescriptor();
-  Value *MainResumeValue =
-      EpiRedHeaderPhi->getStartValue()->getUnderlyingValue();
+  Value *MainResumeValue;
+  if (auto *VPI = dyn_cast<VPInstruction>(EpiRedHeaderPhi->getStartValue())) {
+    assert((VPI->getOpcode() == VPInstruction::Broadcast ||
+            VPI->getOpcode() == VPInstruction::ReductionStartVector) &&
+           "unexpected start recipe");
+    MainResumeValue = VPI->getOperand(0)->getUnderlyingValue();
+  } else
+    MainResumeValue = EpiRedHeaderPhi->getStartValue()->getUnderlyingValue();
   if (RecurrenceDescriptor::isAnyOfRecurrenceKind(
           RdxDesc.getRecurrenceKind())) {
     Value *StartV = EpiRedResult->getOperand(1)->getLiveInIRValue();
@@ -9173,7 +9179,7 @@ void LoopVectorizationPlanner::adjustRecipesForReductions(
       continue;
 
     const RecurrenceDescriptor &RdxDesc = PhiR->getRecurrenceDescriptor();
-    Type *PhiTy = PhiR->getOperand(0)->getLiveInIRValue()->getType();
+    Type *PhiTy = PhiR->getUnderlyingValue()->getType();
     // If tail is folded by masking, introduce selects between the phi
     // and the users outside the vector region of each reduction, at the
     // beginning of the dedicated latch block.
@@ -9311,6 +9317,27 @@ void LoopVectorizationPlanner::adjustRecipesForReductions(
       // start value.
       PhiR->setOperand(0, Plan->getOrAddLiveIn(RdxDesc.getSentinelValue()));
     }
+    RecurKind RK = RdxDesc.getRecurrenceKind();
+    if ((!RecurrenceDescriptor::isAnyOfRecurrenceKind(RK) &&
+         !RecurrenceDescriptor::isFindLastIVRecurrenceKind(RK) &&
+         !RecurrenceDescriptor::isMinMaxRecurrenceKind(RK))) {
+      VPBuilder PHBuilder(Plan->getVectorPreheader());
+      VPValue *Iden = Plan->getOrAddLiveIn(
+          getRecurrenceIdentity(RK, PhiTy, RdxDesc.getFastMathFlags()));
+      // If the PHI is used by a partial reduction, set the scale factor.
+      unsigned ScaleFactor =
+          RecipeBuilder.getScalingForReduction(RdxDesc.getLoopExitInstr())
+              .value_or(1);
+      Type *I32Ty = IntegerType::getInt32Ty(PhiTy->getContext());
+      auto *ScaleFactorVPV =
+          Plan->getOrAddLiveIn(ConstantInt::get(I32Ty, ScaleFactor));
+      VPValue *StartV = PHBuilder.createNaryOp(
+          VPInstruction::ReductionStartVector,
+          {PhiR->getStartValue(), Iden, ScaleFactorVPV},
+          PhiTy->isFloatingPointTy() ? RdxDesc.getFastMathFlags()
+                                     : FastMathFlags());
+      PhiR->setOperand(0, StartV);
+    }
   }
   for (VPRecipeBase *R : ToDelete)
     R->eraseFromParent();
@@ -9816,6 +9843,15 @@ preparePlanForEpilogueVectorLoop(VPlan &Plan, Loop *L,
         Value *Cmp = Builder.CreateICmpEQ(ResumeV, ToFrozen[StartV]);
         ResumeV =
             Builder.CreateSelect(Cmp, RdxDesc.getSentinelValue(), ResumeV);
+      } else {
+        VPValue *StartVal = Plan.getOrAddLiveIn(ResumeV);
+        auto *PhiR = dyn_cast<VPReductionPHIRecipe>(&R);
+        if (auto *VPI = dyn_cast<VPInstruction>(PhiR->getStartValue())) {
+          assert(VPI->getOpcode() == VPInstruction::ReductionStartVector &&
+                 "unexpected start value");
+          VPI->setOperand(0, StartVal);
+          continue;
+        }
       }
     } else {
       // Retrieve the induction resume values for wide inductions from

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -934,6 +934,10 @@ class VPInstruction : public VPRecipeWithIRFlags,
     /// Scale the first operand (vector step) by the second operand
     /// (scalar-step).  Casts both operands to the result type if needed.
     WideIVStep,
+    /// Start vector for reductions with 3 operands: the original start value,
+    /// the identity value for the reduction and an integer indicating the
+    /// scaling factor.
+    ReductionStartVector,
     // Creates a step vector starting from 0 to VF with a step of 1.
     StepVector,
 
@@ -2231,7 +2235,7 @@ class VPReductionPHIRecipe : public VPHeaderPHIRecipe,
   bool onlyFirstLaneUsed(const VPValue *Op) const override {
     assert(is_contained(operands(), Op) &&
            "Op must be an operand of the recipe");
-    return Op == getStartValue();
+    return isOrdered() || isInLoop();
   }
 };
 

llvm/lib/Transforms/Vectorize/VPlanAnalysis.cpp

@@ -74,6 +74,7 @@ Type *VPTypeAnalysis::inferScalarTypeForRecipe(const VPInstruction *R) {
   switch (Opcode) {
   case Instruction::ExtractElement:
   case Instruction::Freeze:
+  case VPInstruction::ReductionStartVector:
     return inferScalarType(R->getOperand(0));
   case Instruction::Select: {
     Type *ResTy = inferScalarType(R->getOperand(1));
@@ -395,6 +396,10 @@ static unsigned getVFScaleFactor(VPRecipeBase *R) {
     return RR->getVFScaleFactor();
   if (auto *RR = dyn_cast<VPPartialReductionRecipe>(R))
     return RR->getVFScaleFactor();
+  assert(
+      (!isa<VPInstruction>(R) || cast<VPInstruction>(R)->getOpcode() !=
+                                     VPInstruction::ReductionStartVector) &&
+      "getting scaling factor of reduction-start-vector not implemented yet");
   return 1;
 }
 

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -604,6 +604,20 @@ Value *VPInstruction::generate(VPTransformState &State) {
     return Builder.CreateVectorSplat(
         State.VF, State.get(getOperand(0), /*IsScalar*/ true), "broadcast");
   }
+  case VPInstruction::ReductionStartVector: {
+    if (State.VF.isScalar())
+      return State.get(getOperand(0), true);
+    IRBuilderBase::FastMathFlagGuard FMFG(Builder);
+    Builder.setFastMathFlags(getFastMathFlags());
+    // If this start vector is scaled then it should produce a vector with fewer
+    // elements than the VF.
+    ElementCount VF = State.VF.divideCoefficientBy(
+        cast<ConstantInt>(getOperand(2)->getLiveInIRValue())->getZExtValue());
+    auto *Iden = Builder.CreateVectorSplat(VF, State.get(getOperand(1), true));
+    Constant *Zero = Builder.getInt32(0);
+    return Builder.CreateInsertElement(Iden, State.get(getOperand(0), true),
+                                       Zero);
+  }
   case VPInstruction::ComputeAnyOfResult: {
     // FIXME: The cross-recipe dependency on VPReductionPHIRecipe is temporary
     // and will be removed by breaking up the recipe further.
@@ -899,6 +913,7 @@ bool VPInstruction::opcodeMayReadOrWriteFromMemory() const {
   case VPInstruction::PtrAdd:
   case VPInstruction::WideIVStep:
   case VPInstruction::StepVector:
+  case VPInstruction::ReductionStartVector:
     return false;
   default:
     return true;
@@ -929,6 +944,7 @@ bool VPInstruction::onlyFirstLaneUsed(const VPValue *Op) const {
   case VPInstruction::CanonicalIVIncrementForPart:
   case VPInstruction::BranchOnCount:
   case VPInstruction::BranchOnCond:
+  case VPInstruction::ReductionStartVector:
     return true;
   case VPInstruction::PtrAdd:
     return Op == getOperand(0) || vputils::onlyFirstLaneUsed(this);
@@ -1034,6 +1050,9 @@ void VPInstruction::print(raw_ostream &O, const Twine &Indent,
   case VPInstruction::FirstActiveLane:
     O << "first-active-lane";
     break;
+  case VPInstruction::ReductionStartVector:
+    O << "reduction-start-vector";
+    break;
   default:
     O << Instruction::getOpcodeName(getOpcode());
   }
@@ -1617,6 +1636,7 @@ bool VPIRFlags::flagsValidForOpcode(unsigned Opcode) const {
            Opcode == Instruction::FDiv || Opcode == Instruction::FRem ||
            Opcode == Instruction::FCmp || Opcode == Instruction::Select ||
            Opcode == VPInstruction::WideIVStep ||
+           Opcode == VPInstruction::ReductionStartVector ||
            Opcode == VPInstruction::ComputeReductionResult;
   case OperationType::NonNegOp:
     return Opcode == Instruction::ZExt;
@@ -3847,17 +3867,19 @@ void VPFirstOrderRecurrencePHIRecipe::print(raw_ostream &O, const Twine &Indent,
 #endif
 
 void VPReductionPHIRecipe::execute(VPTransformState &State) {
-  // If this phi is fed by a scaled reduction then it should output a
-  // vector with fewer elements than the VF.
-  ElementCount VF = State.VF.divideCoefficientBy(VFScaleFactor);
+  // Reductions do not have to start at zero. They can start with
+  // any loop invariant values.
+  VPValue *StartVPV = getStartValue();
 
   // In order to support recurrences we need to be able to vectorize Phi nodes.
   // Phi nodes have cycles, so we need to vectorize them in two stages. This is
   // stage #1: We create a new vector PHI node with no incoming edges. We'll use
   // this value when we vectorize all of the instructions that use the PHI.
-  auto *ScalarTy = State.TypeAnalysis.inferScalarType(this);
+  BasicBlock *VectorPH =
+      State.CFG.VPBB2IRBB.at(getParent()->getCFGPredecessor(0));
   bool ScalarPHI = State.VF.isScalar() || IsInLoop;
-  Type *VecTy = ScalarPHI ? ScalarTy : VectorType::get(ScalarTy, VF);
+  Value *StartV = State.get(StartVPV, ScalarPHI);
+  Type *VecTy = StartV->getType();
 
   BasicBlock *HeaderBB = State.CFG.PrevBB;
   assert(State.CurrentParentLoop->getHeader() == HeaderBB &&
@@ -3866,49 +3888,7 @@ void VPReductionPHIRecipe::execute(VPTransformState &State) {
   Phi->insertBefore(HeaderBB->getFirstInsertionPt());
   State.set(this, Phi, IsInLoop);
 
-  BasicBlock *VectorPH =
-      State.CFG.VPBB2IRBB.at(getParent()->getCFGPredecessor(0));
-  // Create start and identity vector values for the reduction in the preheader.
-  // TODO: Introduce recipes in VPlan preheader to create initial values.
-  IRBuilderBase::InsertPointGuard IPBuilder(State.Builder);
-  State.Builder.SetInsertPoint(VectorPH->getTerminator());
-
-  // Reductions do not have to start at zero. They can start with
-  // any loop invariant values.
-  VPValue *StartVPV = getStartValue();
-  RecurKind RK = RdxDesc.getRecurrenceKind();
-  if (RecurrenceDescriptor::isMinMaxRecurrenceKind(RK) ||
-      RecurrenceDescriptor::isAnyOfRecurrenceKind(RK) ||
-      RecurrenceDescriptor::isFindLastIVRecurrenceKind(RK)) {
-    // [I|F]FindLastIV will use a sentinel value to initialize the reduction
-    // phi or the resume value from the main vector loop when vectorizing the
-    // epilogue loop. In the exit block, ComputeReductionResult will generate
-    // checks to verify if the reduction result is the sentinel value. If the
-    // result is the sentinel value, it will be corrected back to the start
-    // value.
-    // TODO: The sentinel value is not always necessary. When the start value is
-    // a constant, and smaller than the start value of the induction variable,
-    // the start value can be directly used to initialize the reduction phi.
-    Phi->addIncoming(State.get(StartVPV, ScalarPHI), VectorPH);
-    return;
-  }
-
-  Value *Iden = getRecurrenceIdentity(RK, VecTy->getScalarType(),
-                                      RdxDesc.getFastMathFlags());
-  unsigned CurrentPart = getUnrollPart(*this);
-  Value *StartV = StartVPV->getLiveInIRValue();
-  if (!ScalarPHI) {
-    if (CurrentPart == 0) {
-      Iden = State.Builder.CreateVectorSplat(VF, Iden);
-      Constant *Zero = State.Builder.getInt32(0);
-      StartV = State.Builder.CreateInsertElement(Iden, StartV, Zero);
-    } else {
-      Iden = State.Builder.CreateVectorSplat(VF, Iden);
-    }
-  }
-
-  Value *StartVal = (CurrentPart == 0) ? StartV : Iden;
-  Phi->addIncoming(StartVal, VectorPH);
+  Phi->addIncoming(StartV, VectorPH);
 }
 
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -1153,6 +1153,16 @@ static void simplifyRecipe(VPRecipeBase &R, VPTypeAnalysis &TypeInfo) {
       return;
     }
   }
+  // Simplify redundant ReductionStartVector recipes after unrolling.
+  VPValue *StartV;
+  if (match(Def, m_VPInstruction<VPInstruction::ReductionStartVector>(
+                     m_VPValue(StartV), m_VPValue(), m_VPValue()))) {
+    Def->replaceUsesWithIf(StartV, [](const VPUser &U, unsigned Idx) {
+      auto *PhiR = dyn_cast<VPReductionPHIRecipe>(&U);
+      return PhiR && PhiR->isInLoop();
+    });
+    return;
+  }
 }
 
 void VPlanTransforms::simplifyRecipes(VPlan &Plan, Type &CanonicalIVTy) {

llvm/lib/Transforms/Vectorize/VPlanUnroll.cpp

@@ -25,6 +25,7 @@
 #include "llvm/IR/Intrinsics.h"
 
 using namespace llvm;
+using namespace llvm::VPlanPatternMatch;
 
 namespace {
 
@@ -223,6 +224,22 @@ void UnrollState::unrollHeaderPHIByUF(VPHeaderPHIRecipe *R,
       Copy->addOperand(R);
       Copy->addOperand(getConstantVPV(Part));
     } else if (RdxPhi) {
+      // If the start value is a ReductionStartVector, use the identity value
+      // (second operand) for unrolled parts. If the scaling factor is > 1,
+      // create a new ReductionStartVector with the scale factor and both
+      // operands set to the identity value.
+      if (auto *VPI = dyn_cast<VPInstruction>(RdxPhi->getStartValue())) {
+        assert(VPI->getOpcode() == VPInstruction::ReductionStartVector &&
+               "unexpected start VPInstruction");
+        if (match(VPI->getOperand(2), m_SpecificInt(1))) {
+          Copy->setOperand(0, VPI->getOperand(1));
+        } else if (Part == 1) {
+          auto *C = VPI->clone();
+          C->setOperand(0, C->getOperand(1));
+          C->insertAfter(VPI);
+          addUniformForAllParts(C);
+        }
+      }
       Copy->addOperand(getConstantVPV(Part));
     } else {
       assert(isa<VPActiveLaneMaskPHIRecipe>(R) &&
@@ -233,7 +250,6 @@ void UnrollState::unrollHeaderPHIByUF(VPHeaderPHIRecipe *R,
 
 /// Handle non-header-phi recipes.
 void UnrollState::unrollRecipeByUF(VPRecipeBase &R) {
-  using namespace llvm::VPlanPatternMatch;
   if (match(&R, m_BranchOnCond(m_VPValue())) ||
       match(&R, m_BranchOnCount(m_VPValue(), m_VPValue())))
     return;
@@ -301,7 +317,6 @@ void UnrollState::unrollRecipeByUF(VPRecipeBase &R) {
   }
 }
 
-using namespace llvm::VPlanPatternMatch;
 void UnrollState::unrollBlock(VPBlockBase *VPB) {
   auto *VPR = dyn_cast<VPRegionBlock>(VPB);
   if (VPR) {

llvm/test/Transforms/LoopVectorize/AArch64/epilog-iv-select-cmp.ll

@@ -60,16 +60,16 @@ define i8 @select_icmp_var_start(ptr %a, i8 %n, i8 %start) {
 ; CHECK-NEXT:    [[N_MOD_VF4:%.*]] = urem i32 [[TMP2]], 8
 ; CHECK-NEXT:    [[N_VEC5:%.*]] = sub i32 [[TMP2]], [[N_MOD_VF4]]
 ; CHECK-NEXT:    [[TMP16:%.*]] = trunc i32 [[N_VEC5]] to i8
-; CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <8 x i8> poison, i8 [[BC_RESUME_VAL]], i64 0
+; CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <8 x i8> poison, i8 [[TMP15]], i64 0
 ; CHECK-NEXT:    [[DOTSPLAT:%.*]] = shufflevector <8 x i8> [[DOTSPLATINSERT]], <8 x i8> poison, <8 x i32> zeroinitializer
-; CHECK-NEXT:    [[INDUCTION:%.*]] = add <8 x i8> [[DOTSPLAT]], <i8 0, i8 1, i8 2, i8 3, i8 4, i8 5, i8 6, i8 7>
-; CHECK-NEXT:    [[DOTSPLATINSERT10:%.*]] = insertelement <8 x i8> poison, i8 [[TMP15]], i64 0
+; CHECK-NEXT:    [[DOTSPLATINSERT10:%.*]] = insertelement <8 x i8> poison, i8 [[BC_RESUME_VAL]], i64 0
 ; CHECK-NEXT:    [[DOTSPLAT11:%.*]] = shufflevector <8 x i8> [[DOTSPLATINSERT10]], <8 x i8> poison, <8 x i32> zeroinitializer
+; CHECK-NEXT:    [[INDUCTION:%.*]] = add <8 x i8> [[DOTSPLAT11]], <i8 0, i8 1, i8 2, i8 3, i8 4, i8 5, i8 6, i8 7>
 ; CHECK-NEXT:    br label %[[VEC_EPILOG_VECTOR_BODY:.*]]
 ; CHECK:       [[VEC_EPILOG_VECTOR_BODY]]:
 ; CHECK-NEXT:    [[INDEX6:%.*]] = phi i32 [ [[VEC_EPILOG_RESUME_VAL]], %[[VEC_EPILOG_PH]] ], [ [[INDEX_NEXT13:%.*]], %[[VEC_EPILOG_VECTOR_BODY]] ]
 ; CHECK-NEXT:    [[VEC_IND7:%.*]] = phi <8 x i8> [ [[INDUCTION]], %[[VEC_EPILOG_PH]] ], [ [[VEC_IND_NEXT8:%.*]], %[[VEC_EPILOG_VECTOR_BODY]] ]
-; CHECK-NEXT:    [[VEC_PHI9:%.*]] = phi <8 x i8> [ [[DOTSPLAT11]], %[[VEC_EPILOG_PH]] ], [ [[TMP20:%.*]], %[[VEC_EPILOG_VECTOR_BODY]] ]
+; CHECK-NEXT:    [[VEC_PHI9:%.*]] = phi <8 x i8> [ [[DOTSPLAT]], %[[VEC_EPILOG_PH]] ], [ [[TMP20:%.*]], %[[VEC_EPILOG_VECTOR_BODY]] ]
 ; CHECK-NEXT:    [[IV:%.*]] = trunc i32 [[INDEX6]] to i8
 ; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds i8, ptr [[A]], i8 [[IV]]
 ; CHECK-NEXT:    [[TMP18:%.*]] = getelementptr inbounds i8, ptr [[GEP]], i32 0
@@ -87,12 +87,12 @@ define i8 @select_icmp_var_start(ptr %a, i8 %n, i8 %start) {
 ; CHECK-NEXT:    [[CMP_N16:%.*]] = icmp eq i32 [[TMP2]], [[N_VEC5]]
 ; CHECK-NEXT:    br i1 [[CMP_N16]], label %[[EXIT]], label %[[VEC_EPILOG_SCALAR_PH]]
 ; CHECK:       [[VEC_EPILOG_SCALAR_PH]]:
-; CHECK-NEXT:    [[BC_RESUME_VAL17:%.*]] = phi i8 [ [[TMP16]], %[[VEC_EPILOG_MIDDLE_BLOCK]] ], [ [[IND_END]], %[[VEC_EPILOG_ITER_CHECK]] ], [ 0, %[[ITER_CHECK]] ]
-; CHECK-NEXT:    [[BC_MERGE_RDX18:%.*]] = phi i8 [ [[RDX_SELECT15]], %[[VEC_EPILOG_MIDDLE_BLOCK]] ], [ [[RDX_SELECT]], %[[VEC_EPILOG_ITER_CHECK]] ], [ [[START]], %[[ITER_CHECK]] ]
+; CHECK-NEXT:    [[BC_RESUME_VAL15:%.*]] = phi i8 [ [[TMP16]], %[[VEC_EPILOG_MIDDLE_BLOCK]] ], [ [[IND_END]], %[[VEC_EPILOG_ITER_CHECK]] ], [ 0, %[[ITER_CHECK]] ]
+; CHECK-NEXT:    [[BC_MERGE_RDX16:%.*]] = phi i8 [ [[RDX_SELECT15]], %[[VEC_EPILOG_MIDDLE_BLOCK]] ], [ [[RDX_SELECT]], %[[VEC_EPILOG_ITER_CHECK]] ], [ [[START]], %[[ITER_CHECK]] ]
 ; CHECK-NEXT:    br label %[[LOOP:.*]]
 ; CHECK:       [[LOOP]]:
-; CHECK-NEXT:    [[IV1:%.*]] = phi i8 [ [[BC_RESUME_VAL17]], %[[VEC_EPILOG_SCALAR_PH]] ], [ [[IV_NEXT:%.*]], %[[LOOP]] ]
-; CHECK-NEXT:    [[RDX:%.*]] = phi i8 [ [[BC_MERGE_RDX18]], %[[VEC_EPILOG_SCALAR_PH]] ], [ [[SEL:%.*]], %[[LOOP]] ]
+; CHECK-NEXT:    [[IV1:%.*]] = phi i8 [ [[BC_RESUME_VAL15]], %[[VEC_EPILOG_SCALAR_PH]] ], [ [[IV_NEXT:%.*]], %[[LOOP]] ]
+; CHECK-NEXT:    [[RDX:%.*]] = phi i8 [ [[BC_MERGE_RDX16]], %[[VEC_EPILOG_SCALAR_PH]] ], [ [[SEL:%.*]], %[[LOOP]] ]
 ; CHECK-NEXT:    [[GEP1:%.*]] = getelementptr inbounds i8, ptr [[A]], i8 [[IV1]]
 ; CHECK-NEXT:    [[L:%.*]] = load i8, ptr [[GEP1]], align 8
 ; CHECK-NEXT:    [[C:%.*]] = icmp eq i8 [[L]], 3

llvm/test/Transforms/LoopVectorize/AArch64/partial-reduce-dot-product-epilogue.ll

@@ -161,8 +161,8 @@ define void @dotp_small_epilogue_vf(i64 %idx.neg, i8 %a) #1 {
 ; CHECK-NEXT:    [[BROADCAST_SPLATINSERT6:%.*]] = insertelement <4 x i8> poison, i8 [[A]], i64 0
 ; CHECK-NEXT:    [[BROADCAST_SPLAT7:%.*]] = shufflevector <4 x i8> [[BROADCAST_SPLATINSERT6]], <4 x i8> poison, <4 x i32> zeroinitializer
 ; CHECK-NEXT:    [[IND_END:%.*]] = add i64 [[IDX_NEG]], [[N_VEC5]]
-; CHECK-NEXT:    [[TMP8:%.*]] = sext <4 x i8> [[BROADCAST_SPLAT7]] to <4 x i32>
 ; CHECK-NEXT:    [[TMP10:%.*]] = insertelement <4 x i32> zeroinitializer, i32 [[BC_MERGE_RDX]], i32 0
+; CHECK-NEXT:    [[TMP8:%.*]] = sext <4 x i8> [[BROADCAST_SPLAT7]] to <4 x i32>
 ; CHECK-NEXT:    br label [[VEC_EPILOG_VECTOR_BODY:%.*]]
 ; CHECK:       vec.epilog.vector.body:
 ; CHECK-NEXT:    [[INDEX9:%.*]] = phi i64 [ [[IV]], [[VEC_EPILOG_PH]] ], [ [[INDEX_NEXT14:%.*]], [[VEC_EPILOG_VECTOR_BODY]] ]

llvm/test/Transforms/LoopVectorize/AArch64/vplan-printing.ll

@@ -17,12 +17,13 @@ define i32 @print_partial_reduction(ptr %a, ptr %b) {
 ; CHECK-NEXT: Successor(s): scalar.ph, vector.ph
 ; CHECK-EMPTY:
 ; CHECK-NEXT: vector.ph:
+; CHECK-NEXT:   EMIT vp<[[RDX_START:%.+]]> = reduction-start-vector ir<0>, ir<0>, ir<4>
 ; CHECK-NEXT: Successor(s): vector loop
 ; CHECK-EMPTY:
 ; CHECK-NEXT: <x1> vector loop: {
 ; CHECK-NEXT: vector.body:
 ; CHECK-NEXT:   EMIT vp<[[CAN_IV:%.+]]> = CANONICAL-INDUCTION ir<0>, vp<[[CAN_IV_NEXT:%.+]]>
-; CHECK-NEXT:   WIDEN-REDUCTION-PHI ir<[[ACC:%.+]]> = phi ir<0>, ir<[[REDUCE:%.+]]> (VF scaled by 1/4)
+; CHECK-NEXT:   WIDEN-REDUCTION-PHI ir<[[ACC:%.+]]> = phi vp<[[RDX_START]]>, ir<[[REDUCE:%.+]]> (VF scaled by 1/4)
 ; CHECK-NEXT:   vp<[[STEPS:%.+]]> = SCALAR-STEPS vp<[[CAN_IV]]>, ir<1>, vp<[[VF]]>
 ; CHECK-NEXT:   CLONE ir<%gep.a> = getelementptr ir<%a>, vp<[[STEPS]]>
 ; CHECK-NEXT:   vp<[[PTR_A:%.+]]> = vector-pointer ir<%gep.a>
@@ -83,11 +84,12 @@ define i32 @print_partial_reduction(ptr %a, ptr %b) {
 ; CHECK-NEXT: Successor(s): ir-bb<scalar.ph>, ir-bb<vector.ph>
 ; CHECK-EMPTY:
 ; CHECK-NEXT: ir-bb<vector.ph>:
+; CHECK-NEXT:   EMIT vp<[[RDX_START:%.+]]> = reduction-start-vector ir<0>, ir<0>, ir<4>
 ; CHECK-NEXT: Successor(s): vector.body
 ; CHECK-EMPTY:
 ; CHECK-NEXT: vector.body:
 ; CHECK-NEXT:   EMIT-SCALAR vp<[[EP_IV:%.+]]> = phi [ ir<0>, ir-bb<vector.ph> ], [ vp<%index.next>, vector.body ]
-; CHECK-NEXT:   WIDEN-REDUCTION-PHI ir<%accum> = phi ir<0>, ir<%add> (VF scaled by 1/4)
+; CHECK-NEXT:   WIDEN-REDUCTION-PHI ir<%accum> = phi vp<[[RDX_START]]>, ir<%add> (VF scaled by 1/4)
 ; CHECK-NEXT:   CLONE ir<%gep.a> = getelementptr ir<%a>, vp<[[EP_IV]]>
 ; CHECK-NEXT:   vp<[[PTR_A:%.+]]> = vector-pointer ir<%gep.a>
 ; CHECK-NEXT:   WIDEN ir<%load.a> = load vp<[[PTR_A]]>