VPlanUnroll.cpp

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//===-- VPlanUnroll.cpp - VPlan unroller ----------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
///
/// \file
/// This file implements explicit unrolling for VPlans.
///
//===----------------------------------------------------------------------===//
 
#include "VPRecipeBuilder.h"
#include "VPlan.h"
#include "VPlanAnalysis.h"
#include "VPlanCFG.h"
#include "VPlanHelpers.h"
#include "VPlanPatternMatch.h"
#include "VPlanTransforms.h"
#include "VPlanUtils.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/Analysis/IVDescriptors.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Intrinsics.h"
 
using namespace llvm;
using namespace llvm::VPlanPatternMatch;
 
namespace {
 
/// Helper to hold state needed for unrolling. It holds the Plan to unroll by
/// UF. It also holds copies of VPValues across UF-1 unroll parts to facilitate
/// the unrolling transformation, where the original VPValues are retained for
/// part zero.
class UnrollState {
  /// Plan to unroll.
  VPlan &Plan;
  /// Unroll factor to unroll by.
  const unsigned UF;
  /// Analysis for types.
  VPTypeAnalysis TypeInfo;
 
  /// Unrolling may create recipes that should not be unrolled themselves.
  /// Those are tracked in ToSkip.
  SmallPtrSet<VPRecipeBase *, 8> ToSkip;
 
  // Associate with each VPValue of part 0 its unrolled instances of parts 1,
  // ..., UF-1.
  DenseMap<VPValue *, SmallVector<VPValue *>> VPV2Parts;
 
  /// Unroll replicate region \p VPR by cloning the region UF - 1 times.
  void unrollReplicateRegionByUF(VPRegionBlock *VPR);
 
  /// Unroll recipe \p R by cloning it UF - 1 times, unless it is uniform across
  /// all parts.
  void unrollRecipeByUF(VPRecipeBase &R);
 
  /// Unroll header phi recipe \p R. How exactly the recipe gets unrolled
  /// depends on the concrete header phi. Inserts newly created recipes at \p
  /// InsertPtForPhi.
  void unrollHeaderPHIByUF(VPHeaderPHIRecipe *R,
                           VPBasicBlock::iterator InsertPtForPhi);
 
  /// Unroll a widen induction recipe \p IV. This introduces recipes to compute
  /// the induction steps for each part.
  void unrollWidenInductionByUF(VPWidenInductionRecipe *IV,
                                VPBasicBlock::iterator InsertPtForPhi);
 
  VPValue *getConstantInt(unsigned Part) {
    Type *CanIVIntTy = Plan.getVectorLoopRegion()->getCanonicalIVType();
    return Plan.getConstantInt(CanIVIntTy, Part);
  }
 
public:
  UnrollState(VPlan &Plan, unsigned UF) : Plan(Plan), UF(UF), TypeInfo(Plan) {}
 
  void unrollBlock(VPBlockBase *VPB);
 
  VPValue *getValueForPart(VPValue *V, unsigned Part) {
    if (Part == 0 || isa<VPIRValue, VPSymbolicValue>(V))
      return V;
    assert((VPV2Parts.contains(V) && VPV2Parts[V].size() >= Part) &&
           "accessed value does not exist");
    return VPV2Parts[V][Part - 1];
  }
 
  /// Given a single original recipe \p OrigR (of part zero), and its copy \p
  /// CopyR for part \p Part, map every VPValue defined by \p OrigR to its
  /// corresponding VPValue defined by \p CopyR.
  void addRecipeForPart(VPRecipeBase *OrigR, VPRecipeBase *CopyR,
                        unsigned Part) {
    for (const auto &[Idx, VPV] : enumerate(OrigR->definedValues())) {
      const auto &[V, _] = VPV2Parts.try_emplace(VPV);
      assert(V->second.size() == Part - 1 && "earlier parts not set");
      V->second.push_back(CopyR->getVPValue(Idx));
    }
  }
 
  /// Given a uniform recipe \p R, add it for all parts.
  void addUniformForAllParts(VPSingleDefRecipe *R) {
    const auto &[V, Inserted] = VPV2Parts.try_emplace(R);
    assert(Inserted && "uniform value already added");
    for (unsigned Part = 0; Part != UF; ++Part)
      V->second.push_back(R);
  }
 
  bool contains(VPValue *VPV) const { return VPV2Parts.contains(VPV); }
 
  /// Update \p R's operand at \p OpIdx with its corresponding VPValue for part
  /// \p P.
  void remapOperand(VPRecipeBase *R, unsigned OpIdx, unsigned Part) {
    auto *Op = R->getOperand(OpIdx);
    R->setOperand(OpIdx, getValueForPart(Op, Part));
  }
 
  /// Update \p R's operands with their corresponding VPValues for part \p P.
  void remapOperands(VPRecipeBase *R, unsigned Part) {
    for (const auto &[OpIdx, Op] : enumerate(R->operands()))
      R->setOperand(OpIdx, getValueForPart(Op, Part));
  }
};
} // namespace
 
static void addStartIndexForScalarSteps(VPScalarIVStepsRecipe *Steps,
                                        unsigned Part, VPlan &Plan,
                                        VPTypeAnalysis &TypeInfo) {
  if (Part == 0)
    return;
 
  VPBuilder Builder(Steps);
  Type *BaseIVTy = TypeInfo.inferScalarType(Steps->getOperand(0));
  Type *IntStepTy =
      IntegerType::get(BaseIVTy->getContext(), BaseIVTy->getScalarSizeInBits());
  VPValue *StartIndex = Steps->getVFValue();
  if (Part > 1) {
    StartIndex = Builder.createOverflowingOp(
        Instruction::Mul,
        {StartIndex,
         Plan.getConstantInt(TypeInfo.inferScalarType(StartIndex), Part)});
  }
  StartIndex = Builder.createScalarSExtOrTrunc(
      StartIndex, IntStepTy, TypeInfo.inferScalarType(StartIndex),
      Steps->getDebugLoc());
 
  if (BaseIVTy->isFloatingPointTy())
    StartIndex = Builder.createScalarCast(Instruction::SIToFP, StartIndex,
                                          BaseIVTy, Steps->getDebugLoc());
 
  Steps->setStartIndex(StartIndex);
}
 
void UnrollState::unrollReplicateRegionByUF(VPRegionBlock *VPR) {
  VPBlockBase *InsertPt = VPR->getSingleSuccessor();
  for (unsigned Part = 1; Part != UF; ++Part) {
    auto *Copy = VPR->clone();
    VPBlockUtils::insertBlockBefore(Copy, InsertPt);
 
    auto PartI = vp_depth_first_shallow(Copy->getEntry());
    auto Part0 = vp_depth_first_shallow(VPR->getEntry());
    for (const auto &[PartIVPBB, Part0VPBB] :
         zip(VPBlockUtils::blocksOnly<VPBasicBlock>(PartI),
             VPBlockUtils::blocksOnly<VPBasicBlock>(Part0))) {
      for (const auto &[PartIR, Part0R] : zip(*PartIVPBB, *Part0VPBB)) {
        remapOperands(&PartIR, Part);
        if (auto *Steps = dyn_cast<VPScalarIVStepsRecipe>(&PartIR))
          addStartIndexForScalarSteps(Steps, Part, Plan, TypeInfo);
 
        addRecipeForPart(&Part0R, &PartIR, Part);
      }
    }
  }
}
 
void UnrollState::unrollWidenInductionByUF(
    VPWidenInductionRecipe *IV, VPBasicBlock::iterator InsertPtForPhi) {
  VPBasicBlock *PH = cast<VPBasicBlock>(
      IV->getParent()->getEnclosingLoopRegion()->getSinglePredecessor());
  Type *IVTy = TypeInfo.inferScalarType(IV);
  auto &ID = IV->getInductionDescriptor();
  FastMathFlags FMF;
  VPIRFlags::WrapFlagsTy WrapFlags(false, false);
  if (auto *IntOrFPInd = dyn_cast<VPWidenIntOrFpInductionRecipe>(IV)) {
    if (IntOrFPInd->hasFastMathFlags())
      FMF = IntOrFPInd->getFastMathFlags();
    if (IntOrFPInd->hasNoWrapFlags())
      WrapFlags = IntOrFPInd->getNoWrapFlags();
  }
 
  VPValue *ScalarStep = IV->getStepValue();
  VPBuilder Builder(PH);
  Type *VectorStepTy =
      IVTy->isPointerTy() ? TypeInfo.inferScalarType(ScalarStep) : IVTy;
  VPInstruction *VectorStep = Builder.createNaryOp(
      VPInstruction::WideIVStep, {&Plan.getVF(), ScalarStep}, VectorStepTy, FMF,
      IV->getDebugLoc());
 
  ToSkip.insert(VectorStep);
 
  // Now create recipes to compute the induction steps for part 1 .. UF. Part 0
  // remains the header phi. Parts > 0 are computed by adding Step to the
  // previous part. The header phi recipe will get 2 new operands: the step
  // value for a single part and the last part, used to compute the backedge
  // value during VPWidenInductionRecipe::execute.
  // %Part.0 = VPWidenInductionRecipe %Start, %ScalarStep, %VectorStep, %Part.3
  // %Part.1 = %Part.0 + %VectorStep
  // %Part.2 = %Part.1 + %VectorStep
  // %Part.3 = %Part.2 + %VectorStep
  //
  // The newly added recipes are added to ToSkip to avoid interleaving them
  // again.
  VPValue *Prev = IV;
  Builder.setInsertPoint(IV->getParent(), InsertPtForPhi);
  unsigned AddOpc;
  VPIRFlags AddFlags;
  if (IVTy->isPointerTy()) {
    AddOpc = VPInstruction::WidePtrAdd;
    AddFlags = GEPNoWrapFlags::none();
  } else if (IVTy->isFloatingPointTy()) {
    AddOpc = ID.getInductionOpcode();
    AddFlags = FMF;
  } else {
    AddOpc = Instruction::Add;
    AddFlags = WrapFlags;
    if (cast<VPWidenIntOrFpInductionRecipe>(IV)->isCanonical())
      AddFlags = VPIRFlags::WrapFlagsTy(/*NUW=*/true, /*NSW=*/false);
  }
  for (unsigned Part = 1; Part != UF; ++Part) {
    std::string Name =
        Part > 1 ? "step.add." + std::to_string(Part) : "step.add";
 
    VPInstruction *Add =
        Builder.createNaryOp(AddOpc,
                             {
                                 Prev,
                                 VectorStep,
                             },
                             AddFlags, IV->getDebugLoc(), Name);
    ToSkip.insert(Add);
    addRecipeForPart(IV, Add, Part);
    Prev = Add;
  }
  IV->addOperand(VectorStep);
  IV->addOperand(Prev);
}
 
void UnrollState::unrollHeaderPHIByUF(VPHeaderPHIRecipe *R,
                                      VPBasicBlock::iterator InsertPtForPhi) {
  // First-order recurrences pass a single vector or scalar through their header
  // phis, irrespective of interleaving.
  if (isa<VPFirstOrderRecurrencePHIRecipe>(R))
    return;
 
  // Generate step vectors for each unrolled part.
  if (auto *IV = dyn_cast<VPWidenInductionRecipe>(R)) {
    unrollWidenInductionByUF(IV, InsertPtForPhi);
    return;
  }
 
  auto *RdxPhi = dyn_cast<VPReductionPHIRecipe>(R);
  if (RdxPhi && RdxPhi->isOrdered())
    return;
 
  auto InsertPt = std::next(R->getIterator());
  for (unsigned Part = 1; Part != UF; ++Part) {
    VPRecipeBase *Copy = R->clone();
    Copy->insertBefore(*R->getParent(), InsertPt);
    addRecipeForPart(R, Copy, Part);
    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 (Part != 1)
          continue;
        VPValue *StartV;
        if (match(VPI->getOperand(2), m_One())) {
          StartV = VPI->getOperand(1);
        } else {
          auto *C = VPI->clone();
          C->setOperand(0, C->getOperand(1));
          C->insertAfter(VPI);
          StartV = C;
        }
        for (unsigned Part = 1; Part != UF; ++Part)
          VPV2Parts[VPI][Part - 1] = StartV;
      }
    } else {
      assert(isa<VPActiveLaneMaskPHIRecipe>(R) &&
             "unexpected header phi recipe not needing unrolled part");
    }
  }
}
 
/// Handle non-header-phi recipes.
void UnrollState::unrollRecipeByUF(VPRecipeBase &R) {
  if (match(&R, m_CombineOr(m_BranchOnCond(), m_BranchOnCount())))
    return;
 
  if (auto *VPI = dyn_cast<VPInstruction>(&R)) {
    if (vputils::onlyFirstPartUsed(VPI)) {
      addUniformForAllParts(VPI);
      return;
    }
  }
  if (auto *RepR = dyn_cast<VPReplicateRecipe>(&R)) {
    if (isa<StoreInst>(RepR->getUnderlyingValue()) &&
        RepR->getOperand(1)->isDefinedOutsideLoopRegions()) {
      // Stores to an invariant address only need to store the last part.
      remapOperands(&R, UF - 1);
      return;
    }
    if (match(RepR,
              m_Intrinsic<Intrinsic::experimental_noalias_scope_decl>())) {
      addUniformForAllParts(RepR);
      return;
    }
  }
 
  // Unroll non-uniform recipes.
  auto InsertPt = std::next(R.getIterator());
  VPBasicBlock &VPBB = *R.getParent();
  for (unsigned Part = 1; Part != UF; ++Part) {
    VPRecipeBase *Copy = R.clone();
    Copy->insertBefore(VPBB, InsertPt);
    addRecipeForPart(&R, Copy, Part);
 
    // Phi operands are updated once all other recipes have been unrolled.
    if (isa<VPWidenPHIRecipe>(Copy))
      continue;
 
    VPValue *Op;
    if (match(&R, m_VPInstruction<VPInstruction::FirstOrderRecurrenceSplice>(
                      m_VPValue(), m_VPValue(Op)))) {
      Copy->setOperand(0, getValueForPart(Op, Part - 1));
      Copy->setOperand(1, getValueForPart(Op, Part));
      continue;
    }
    if (auto *VPR = dyn_cast<VPVectorPointerRecipe>(&R)) {
      VPBuilder Builder(VPR);
      const DataLayout &DL = Plan.getDataLayout();
      Type *IndexTy = DL.getIndexType(TypeInfo.inferScalarType(VPR));
      Type *VFTy = TypeInfo.inferScalarType(&Plan.getVF());
      VPValue *VF = Builder.createScalarZExtOrTrunc(
          &Plan.getVF(), IndexTy, VFTy, DebugLoc::getUnknown());
      // VFxUF does not wrap, so VF * Part also cannot wrap.
      VPValue *VFxPart = Builder.createOverflowingOp(
          Instruction::Mul, {VF, Plan.getConstantInt(IndexTy, Part)},
          {true, true});
      Copy->setOperand(0, VPR->getOperand(0));
      Copy->addOperand(VFxPart);
      continue;
    }
    if (auto *Red = dyn_cast<VPReductionRecipe>(&R)) {
      auto *Phi = dyn_cast<VPReductionPHIRecipe>(R.getOperand(0));
      if (Phi && Phi->isOrdered()) {
        auto &Parts = VPV2Parts[Phi];
        if (Part == 1) {
          Parts.clear();
          Parts.push_back(Red);
        }
        Parts.push_back(Copy->getVPSingleValue());
        Phi->setOperand(1, Copy->getVPSingleValue());
      }
    }
    if (auto *VEPR = dyn_cast<VPVectorEndPointerRecipe>(Copy)) {
      // Materialize PartN offset for VectorEndPointer.
      VEPR->setOperand(0, R.getOperand(0));
      VEPR->setOperand(1, R.getOperand(1));
      VEPR->materializeOffset(Part);
      continue;
    }
 
    remapOperands(Copy, Part);
 
    if (auto *ScalarIVSteps = dyn_cast<VPScalarIVStepsRecipe>(Copy))
      addStartIndexForScalarSteps(ScalarIVSteps, Part, Plan, TypeInfo);
 
    // Add operand indicating the part to generate code for, to recipes still
    // requiring it.
    if (isa<VPWidenCanonicalIVRecipe>(Copy))
      Copy->addOperand(getConstantInt(Part));
 
    if (match(Copy,
              m_VPInstruction<VPInstruction::CanonicalIVIncrementForPart>())) {
      VPBuilder Builder(Copy);
      VPValue *ScaledByPart = Builder.createOverflowingOp(
          Instruction::Mul, {Copy->getOperand(1), getConstantInt(Part)});
      Copy->setOperand(1, ScaledByPart);
    }
  }
  if (auto *VEPR = dyn_cast<VPVectorEndPointerRecipe>(&R)) {
    // Materialize Part0 offset for VectorEndPointer.
    VEPR->materializeOffset();
  }
}
 
void UnrollState::unrollBlock(VPBlockBase *VPB) {
  auto *VPR = dyn_cast<VPRegionBlock>(VPB);
  if (VPR) {
    if (VPR->isReplicator())
      return unrollReplicateRegionByUF(VPR);
 
    // Traverse blocks in region in RPO to ensure defs are visited before uses
    // across blocks.
    ReversePostOrderTraversal<VPBlockShallowTraversalWrapper<VPBlockBase *>>
        RPOT(VPR->getEntry());
    for (VPBlockBase *VPB : RPOT)
      unrollBlock(VPB);
    return;
  }
 
  // VPB is a VPBasicBlock; unroll it, i.e., unroll its recipes.
  auto *VPBB = cast<VPBasicBlock>(VPB);
  auto InsertPtForPhi = VPBB->getFirstNonPhi();
  for (VPRecipeBase &R : make_early_inc_range(*VPBB)) {
    if (ToSkip.contains(&R) || isa<VPIRInstruction>(&R))
      continue;
 
    // Add all VPValues for all parts to AnyOf, FirstActiveLaneMask and
    // Compute*Result which combine all parts to compute the final value.
    VPValue *Op1;
    if (match(&R, m_VPInstruction<VPInstruction::AnyOf>(m_VPValue(Op1))) ||
        match(&R, m_FirstActiveLane(m_VPValue(Op1))) ||
        match(&R, m_LastActiveLane(m_VPValue(Op1))) ||
        match(&R,
              m_ComputeAnyOfResult(m_VPValue(), m_VPValue(), m_VPValue(Op1))) ||
        match(&R, m_ComputeReductionResult(m_VPValue(Op1)))) {
      addUniformForAllParts(cast<VPInstruction>(&R));
      for (unsigned Part = 1; Part != UF; ++Part)
        R.addOperand(getValueForPart(Op1, Part));
      continue;
    }
    VPValue *Op0;
    if (match(&R, m_ExtractLane(m_VPValue(Op0), m_VPValue(Op1)))) {
      addUniformForAllParts(cast<VPInstruction>(&R));
      for (unsigned Part = 1; Part != UF; ++Part)
        R.addOperand(getValueForPart(Op1, Part));
      continue;
    }
 
    VPValue *Op2;
    if (match(&R, m_ExtractLastActive(m_VPValue(), m_VPValue(Op1),
                                      m_VPValue(Op2)))) {
      addUniformForAllParts(cast<VPInstruction>(&R));
      for (unsigned Part = 1; Part != UF; ++Part) {
        R.addOperand(getValueForPart(Op1, Part));
        R.addOperand(getValueForPart(Op2, Part));
      }
      continue;
    }
 
    if (Plan.hasScalarVFOnly()) {
      if (match(&R, m_ExtractLastPart(m_VPValue(Op0))) ||
          match(&R, m_ExtractPenultimateElement(m_VPValue(Op0)))) {
        auto *I = cast<VPInstruction>(&R);
        bool IsPenultimatePart =
            I->getOpcode() == VPInstruction::ExtractPenultimateElement;
        unsigned PartIdx = IsPenultimatePart ? UF - 2 : UF - 1;
        // For scalar VF, directly use the scalar part value.
        I->replaceAllUsesWith(getValueForPart(Op0, PartIdx));
        continue;
      }
    }
    // For vector VF, the penultimate element is always extracted from the last part.
    if (match(&R, m_ExtractLastLaneOfLastPart(m_VPValue(Op0))) ||
        match(&R, m_ExtractPenultimateElement(m_VPValue(Op0)))) {
      addUniformForAllParts(cast<VPSingleDefRecipe>(&R));
      R.setOperand(0, getValueForPart(Op0, UF - 1));
      continue;
    }
 
    auto *SingleDef = dyn_cast<VPSingleDefRecipe>(&R);
    if (SingleDef && vputils::isUniformAcrossVFsAndUFs(SingleDef)) {
      addUniformForAllParts(SingleDef);
      continue;
    }
 
    if (auto *H = dyn_cast<VPHeaderPHIRecipe>(&R)) {
      unrollHeaderPHIByUF(H, InsertPtForPhi);
      continue;
    }
 
    unrollRecipeByUF(R);
  }
}
 
void VPlanTransforms::unrollByUF(VPlan &Plan, unsigned UF) {
  assert(UF > 0 && "Unroll factor must be positive");
  Plan.setUF(UF);
  llvm::scope_exit Cleanup([&Plan, UF]() {
    auto Iter = vp_depth_first_deep(Plan.getEntry());
    // Remove recipes that are redundant after unrolling.
    for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(Iter)) {
      for (VPRecipeBase &R : make_early_inc_range(*VPBB)) {
        auto *VPI = dyn_cast<VPInstruction>(&R);
        if (VPI &&
            VPI->getOpcode() == VPInstruction::CanonicalIVIncrementForPart &&
            VPI->getOperand(1) == &Plan.getVF()) {
          VPI->replaceAllUsesWith(VPI->getOperand(0));
          VPI->eraseFromParent();
        }
      }
    }
 
    Type *TCTy = VPTypeAnalysis(Plan).inferScalarType(Plan.getTripCount());
    Plan.getUF().replaceAllUsesWith(Plan.getConstantInt(TCTy, UF));
  });
  if (UF == 1) {
    return;
  }
 
  UnrollState Unroller(Plan, UF);
 
  // Iterate over all blocks in the plan starting from Entry, and unroll
  // recipes inside them. This includes the vector preheader and middle blocks,
  // which may set up or post-process per-part values.
  ReversePostOrderTraversal<VPBlockShallowTraversalWrapper<VPBlockBase *>> RPOT(
      Plan.getEntry());
  for (VPBlockBase *VPB : RPOT)
    Unroller.unrollBlock(VPB);
 
  unsigned Part = 1;
  // Remap operands of cloned header phis to update backedge values. The header
  // phis cloned during unrolling are just after the header phi for part 0.
  // Reset Part to 1 when reaching the first (part 0) recipe of a block.
  for (VPRecipeBase &H :
       Plan.getVectorLoopRegion()->getEntryBasicBlock()->phis()) {
    // The second operand of Fixed Order Recurrence phi's, feeding the spliced
    // value across the backedge, needs to remap to the last part of the spliced
    // value.
    if (isa<VPFirstOrderRecurrencePHIRecipe>(&H)) {
      Unroller.remapOperand(&H, 1, UF - 1);
      continue;
    }
    if (Unroller.contains(H.getVPSingleValue())) {
      Part = 1;
      continue;
    }
    Unroller.remapOperands(&H, Part);
    Part++;
  }
 
  VPlanTransforms::removeDeadRecipes(Plan);
}
 
/// Add a lane offset to the start index of \p Steps.
static void addLaneToStartIndex(VPScalarIVStepsRecipe *Steps, unsigned Lane,
                                VPlan &Plan, VPRecipeBase *InsertPt) {
  assert(Lane > 0 && "Zero lane adds no offset to start index");
  VPTypeAnalysis TypeInfo(Plan);
  Type *BaseIVTy = TypeInfo.inferScalarType(Steps->getOperand(0));
 
  VPValue *OldStartIndex = Steps->getStartIndex();
  VPValue *LaneOffset;
  unsigned AddOpcode;
  // TODO: Retrieve the flags from Steps unconditionally.
  VPIRFlags Flags;
  if (BaseIVTy->isFloatingPointTy()) {
    int SignedLane = static_cast<int>(Lane);
    if (!OldStartIndex && Steps->getInductionOpcode() == Instruction::FSub)
      SignedLane = -SignedLane;
    LaneOffset = Plan.getOrAddLiveIn(ConstantFP::get(BaseIVTy, SignedLane));
    AddOpcode = Steps->getInductionOpcode();
    Flags = VPIRFlags(FastMathFlags());
  } else {
    unsigned BaseIVBits = BaseIVTy->getScalarSizeInBits();
    LaneOffset = Plan.getConstantInt(
        APInt(BaseIVBits, Lane, /*isSigned*/ false, /*implicitTrunc*/ true));
    AddOpcode = Instruction::Add;
    Flags = VPIRFlags(VPIRFlags::WrapFlagsTy(false, false));
  }
 
  VPValue *NewStartIndex = LaneOffset;
  if (OldStartIndex) {
    VPBuilder Builder(InsertPt);
    NewStartIndex =
        Builder.createNaryOp(AddOpcode, {OldStartIndex, LaneOffset}, Flags);
  }
  Steps->setStartIndex(NewStartIndex);
}
 
/// Create a single-scalar clone of \p DefR (must be a VPReplicateRecipe,
/// VPInstruction or VPScalarIVStepsRecipe) for lane \p Lane. Use \p
/// Def2LaneDefs to look up scalar definitions for operands of \DefR.
static VPValue *
cloneForLane(VPlan &Plan, VPBuilder &Builder, Type *IdxTy,
             VPSingleDefRecipe *DefR, VPLane Lane,
             const DenseMap<VPValue *, SmallVector<VPValue *>> &Def2LaneDefs) {
  assert((isa<VPInstruction, VPReplicateRecipe, VPScalarIVStepsRecipe>(DefR)) &&
         "DefR must be a VPReplicateRecipe, VPInstruction or "
         "VPScalarIVStepsRecipe");
  VPValue *Op;
  if (match(DefR, m_VPInstruction<VPInstruction::Unpack>(m_VPValue(Op)))) {
    auto LaneDefs = Def2LaneDefs.find(Op);
    if (LaneDefs != Def2LaneDefs.end())
      return LaneDefs->second[Lane.getKnownLane()];
 
    VPValue *Idx = Plan.getConstantInt(IdxTy, Lane.getKnownLane());
    return Builder.createNaryOp(Instruction::ExtractElement, {Op, Idx});
  }
 
  // Collect the operands at Lane, creating extracts as needed.
  SmallVector<VPValue *> NewOps;
  for (VPValue *Op : DefR->operands()) {
    // If Op is a definition that has been unrolled, directly use the clone for
    // the corresponding lane.
    auto LaneDefs = Def2LaneDefs.find(Op);
    if (LaneDefs != Def2LaneDefs.end()) {
      NewOps.push_back(LaneDefs->second[Lane.getKnownLane()]);
      continue;
    }
    if (Lane.getKind() == VPLane::Kind::ScalableLast) {
      // Look through mandatory Unpack.
      [[maybe_unused]] bool Matched =
          match(Op, m_VPInstruction<VPInstruction::Unpack>(m_VPValue(Op)));
      assert(Matched && "original op must have been Unpack");
      auto *ExtractPart =
          Builder.createNaryOp(VPInstruction::ExtractLastPart, {Op});
      NewOps.push_back(
          Builder.createNaryOp(VPInstruction::ExtractLastLane, {ExtractPart}));
      continue;
    }
    if (vputils::isSingleScalar(Op)) {
      NewOps.push_back(Op);
      continue;
    }
 
    // Look through buildvector to avoid unnecessary extracts.
    if (match(Op, m_BuildVector())) {
      NewOps.push_back(
          cast<VPInstruction>(Op)->getOperand(Lane.getKnownLane()));
      continue;
    }
    VPValue *Idx = Plan.getConstantInt(IdxTy, Lane.getKnownLane());
    VPValue *Ext = Builder.createNaryOp(Instruction::ExtractElement, {Op, Idx});
    NewOps.push_back(Ext);
  }
 
  VPSingleDefRecipe *New;
  if (auto *RepR = dyn_cast<VPReplicateRecipe>(DefR)) {
    // TODO: have cloning of replicate recipes also provide the desired result
    // coupled with setting its operands to NewOps (deriving IsSingleScalar and
    // Mask from the operands?)
    New = new VPReplicateRecipe(RepR->getUnderlyingInstr(), NewOps,
                                /*IsSingleScalar=*/true, /*Mask=*/nullptr,
                                *RepR, *RepR, RepR->getDebugLoc());
  } else {
    New = DefR->clone();
    for (const auto &[Idx, Op] : enumerate(NewOps)) {
      New->setOperand(Idx, Op);
    }
    if (auto *Steps = dyn_cast<VPScalarIVStepsRecipe>(New)) {
      // Skip lane 0: an absent start index is implicitly zero.
      unsigned KnownLane = Lane.getKnownLane();
      if (KnownLane != 0)
        addLaneToStartIndex(Steps, KnownLane, Plan, DefR);
    }
  }
  New->insertBefore(DefR);
  return New;
}
 
/// Convert recipes in region blocks to operate on a single lane 0.
/// VPReplicateRecipes are converted to single-scalar ones, branch-on-mask is
/// converted into BranchOnCond and extracts are created as needed.
static void convertRecipesInRegionBlocksToSingleScalar(VPlan &Plan, Type *IdxTy,
                                                       VPBlockBase *Entry,
                                                       ElementCount VF) {
  VPValue *Idx0 = Plan.getZero(IdxTy);
  VPTypeAnalysis TypeInfo(Plan);
  for (VPBlockBase *VPB : vp_depth_first_shallow(Entry)) {
    for (VPRecipeBase &OldR : make_early_inc_range(cast<VPBasicBlock>(*VPB))) {
      VPBuilder Builder(&OldR);
      assert(!match(&OldR, m_ExtractElement(m_VPValue(), m_VPValue())) &&
             "must not contain extracts before conversion");
 
      // For scalar VF, operands are already scalar; no extraction needed.
      if (!VF.isScalar()) {
        for (const auto &[I, Op] : enumerate(OldR.operands())) {
          // Skip operands that don't need extraction: values defined in the
          // same block (already scalar), or values that are already single
          // scalars.
          auto *DefR = Op->getDefiningRecipe();
          if ((isa_and_present<VPScalarIVStepsRecipe>(DefR) &&
               DefR->getParent() == VPB) ||
              vputils::isSingleScalar(Op))
            continue;
 
          // Extract lane zero from values defined outside the region.
          VPValue *Extract = Builder.createNaryOp(
              Instruction::ExtractElement, {Op, Idx0}, OldR.getDebugLoc());
          OldR.setOperand(I, Extract);
        }
      }
 
      if (auto *RepR = dyn_cast<VPReplicateRecipe>(&OldR)) {
        auto *NewR =
            new VPReplicateRecipe(RepR->getUnderlyingInstr(), RepR->operands(),
                                  /* IsSingleScalar=*/true, /*Mask=*/nullptr,
                                  *RepR, *RepR, RepR->getDebugLoc());
        NewR->insertBefore(RepR);
        RepR->replaceAllUsesWith(NewR);
        RepR->eraseFromParent();
      } else if (auto *BranchOnMask = dyn_cast<VPBranchOnMaskRecipe>(&OldR)) {
        Builder.createNaryOp(VPInstruction::BranchOnCond,
                             {BranchOnMask->getOperand(0)},
                             BranchOnMask->getDebugLoc());
        BranchOnMask->eraseFromParent();
      } else if (auto *PredPhi = dyn_cast<VPPredInstPHIRecipe>(&OldR)) {
        VPValue *PredOp = PredPhi->getOperand(0);
        Type *PredTy = TypeInfo.inferScalarType(PredOp);
        VPValue *PoisonVal = Plan.getOrAddLiveIn(PoisonValue::get(PredTy));
 
        VPPhi *NewPhi = Builder.createScalarPhi({PoisonVal, PredOp},
                                                PredPhi->getDebugLoc());
        PredPhi->replaceAllUsesWith(NewPhi);
        PredPhi->eraseFromParent();
      } else {
        assert((isa<VPScalarIVStepsRecipe>(OldR) ||
                (isa<VPInstruction>(OldR) &&
                 vputils::isSingleScalar(OldR.getVPSingleValue()))) &&
               "unexpected unhandled recipe");
      }
    }
  }
}
 
/// Update recipes in the cloned blocks rooted at \p NewEntry to match \p Lane,
/// using the original blocks rooted at \p OldEntry as reference.
static void processLaneForReplicateRegion(VPlan &Plan, Type *IdxTy,
                                          unsigned Lane, VPBasicBlock *OldEntry,
                                          VPBasicBlock *NewEntry) {
  DenseMap<VPValue *, VPValue *> Old2NewVPValues;
  VPValue *IdxLane = Plan.getConstantInt(IdxTy, Lane);
  for (const auto &[OldBB, NewBB] :
       zip_equal(vp_depth_first_shallow(OldEntry),
                 vp_depth_first_shallow(NewEntry))) {
    for (auto &&[OldR, NewR] :
         zip_equal(*cast<VPBasicBlock>(OldBB), *cast<VPBasicBlock>(NewBB))) {
      for (const auto &[OldV, NewV] :
           zip_equal(OldR.definedValues(), NewR.definedValues()))
        Old2NewVPValues[OldV] = NewV;
 
      // Remap operands to use lane-specific values.
      for (const auto &[I, OldOp] : enumerate(NewR.operands())) {
        // Use cloned value if operand was defined in the region.
        if (auto *NewOp = Old2NewVPValues.lookup(OldOp))
          NewR.setOperand(I, NewOp);
      }
 
      if (auto *Steps = dyn_cast<VPScalarIVStepsRecipe>(&NewR))
        addLaneToStartIndex(Steps, Lane, Plan, Steps);
      else if (match(&NewR, m_ExtractElement(m_VPValue(), m_ZeroInt())))
        NewR.setOperand(1, IdxLane);
    }
  }
}
 
/// Dissolve a single replicate region by replicating its blocks for each lane
/// of \p VF. The region is disconnected, its blocks are reparented, cloned for
/// each lane, and reconnected in sequence.
static void dissolveReplicateRegion(VPRegionBlock *Region, ElementCount VF,
                                    VPlan &Plan, Type *IdxTy) {
  VPBlockBase *FirstLaneEntry = Region->getEntry();
  VPBlockBase *FirstLaneExiting = Region->getExiting();
 
  // Disconnect and dissolve the region.
  VPBlockBase *Predecessor = Region->getSinglePredecessor();
  assert(Predecessor && "Replicate region must have a single predecessor");
  VPBlockBase *Successor = Region->getSingleSuccessor();
  assert(Successor && "Replicate region must have a single successor");
  VPBlockUtils::disconnectBlocks(Predecessor, Region);
  VPBlockUtils::disconnectBlocks(Region, Successor);
 
  VPRegionBlock *ParentRegion = Region->getParent();
  for (VPBlockBase *VPB : vp_depth_first_shallow(FirstLaneEntry))
    VPB->setParent(ParentRegion);
 
  // Process the original blocks for lane 0: converting their recipes to
  // single-scalar.
  convertRecipesInRegionBlocksToSingleScalar(Plan, IdxTy, FirstLaneEntry, VF);
 
  // Clone converted blocks for remaining lanes and process each in reverse
  // order, connecting each lane's Exiting block to the subsequent lane's entry.
  VPBlockBase *NextLaneEntry = Successor;
  unsigned NumLanes = VF.getFixedValue();
  for (int Lane = NumLanes - 1; Lane > 0; --Lane) {
    const auto &[CurrentLaneEntry, CurrentLaneExiting] =
        VPBlockUtils::cloneFrom(FirstLaneEntry);
    for (VPBlockBase *VPB : vp_depth_first_shallow(CurrentLaneEntry))
      VPB->setParent(ParentRegion);
    processLaneForReplicateRegion(Plan, IdxTy, Lane,
                                  cast<VPBasicBlock>(FirstLaneEntry),
                                  cast<VPBasicBlock>(CurrentLaneEntry));
    VPBlockUtils::connectBlocks(CurrentLaneExiting, NextLaneEntry);
    NextLaneEntry = CurrentLaneEntry;
  }
 
  // Connect Predecessor to FirstLaneEntry, and FirstLaneRegionExit to
  // NextLaneEntry which is the second lane region entry. The latter is
  // done last so that earlier clonings from FirstLaneEntry stop at
  // FirstLaneExiting.
  VPBlockUtils::connectBlocks(Predecessor, FirstLaneEntry);
  VPBlockUtils::connectBlocks(FirstLaneExiting, NextLaneEntry);
}
 
/// Collect and dissolve all replicate regions in the vector loop, replicating
/// their blocks and recipes for each lane of \p VF.
static void replicateReplicateRegionsByVF(VPlan &Plan, ElementCount VF,
                                          Type *IdxTy) {
  // Collect all replicate regions before modifying the CFG.
  SmallVector<VPRegionBlock *> ReplicateRegions;
  for (VPRegionBlock *Region : VPBlockUtils::blocksOnly<VPRegionBlock>(
           vp_depth_first_shallow(Plan.getVectorLoopRegion()->getEntry()))) {
    // Skip regions with live-outs when vectorizing as packing scalar results
    // back into vectors is not yet implemented.
    if (Region->isReplicator() &&
        (VF.isScalar() || Region->getExitingBasicBlock()->empty()))
      ReplicateRegions.push_back(Region);
  }
 
  assert((ReplicateRegions.empty() || !VF.isScalable()) &&
         "cannot replicate across scalable VFs");
 
  // Dissolve replicate regions by replicating their blocks for each lane.
  for (VPRegionBlock *Region : ReplicateRegions)
    dissolveReplicateRegion(Region, VF, Plan, IdxTy);
 
  VPlanTransforms::mergeBlocksIntoPredecessors(Plan);
}
 
void VPlanTransforms::replicateByVF(VPlan &Plan, ElementCount VF) {
  Type *IdxTy = IntegerType::get(
      Plan.getScalarHeader()->getIRBasicBlock()->getContext(), 32);
 
  if (Plan.hasScalarVFOnly()) {
    // When Plan is only unrolled by UF, replicating by VF amounts to dissolving
    // replicate regions.
    replicateReplicateRegionsByVF(Plan, VF, IdxTy);
    return;
  }
 
  // Visit all VPBBs outside the loop region and directly inside the top-level
  // loop region.
  auto VPBBsOutsideLoopRegion = VPBlockUtils::blocksOnly<VPBasicBlock>(
      vp_depth_first_shallow(Plan.getEntry()));
  auto VPBBsInsideLoopRegion = VPBlockUtils::blocksOnly<VPBasicBlock>(
      vp_depth_first_shallow(Plan.getVectorLoopRegion()->getEntry()));
  auto VPBBsToUnroll =
      concat<VPBasicBlock *>(VPBBsOutsideLoopRegion, VPBBsInsideLoopRegion);
  // A mapping of current VPValue definitions to collections of new VPValues
  // defined per lane. Serves to hook-up potential users of current VPValue
  // definition that are replicated-per-VF later.
  DenseMap<VPValue *, SmallVector<VPValue *>> Def2LaneDefs;
  // The removal of current recipes being replaced by new ones needs to be
  // delayed after Def2LaneDefs is no longer in use.
  SmallVector<VPRecipeBase *> ToRemove;
  for (VPBasicBlock *VPBB : VPBBsToUnroll) {
    for (VPRecipeBase &R : make_early_inc_range(*VPBB)) {
      if (!isa<VPInstruction, VPReplicateRecipe, VPScalarIVStepsRecipe>(&R) ||
          (isa<VPReplicateRecipe>(&R) &&
           cast<VPReplicateRecipe>(&R)->isSingleScalar()) ||
          (isa<VPInstruction>(&R) &&
           !cast<VPInstruction>(&R)->doesGeneratePerAllLanes() &&
           cast<VPInstruction>(&R)->getOpcode() != VPInstruction::Unpack))
        continue;
 
      auto *DefR = cast<VPSingleDefRecipe>(&R);
      VPBuilder Builder(DefR);
      if (DefR->getNumUsers() == 0) {
        // Create single-scalar version of DefR for all lanes.
        for (unsigned I = 0; I != VF.getKnownMinValue(); ++I)
          cloneForLane(Plan, Builder, IdxTy, DefR, VPLane(I), Def2LaneDefs);
        DefR->eraseFromParent();
        continue;
      }
      /// Create single-scalar version of DefR for all lanes.
      SmallVector<VPValue *> LaneDefs;
      for (unsigned I = 0; I != VF.getKnownMinValue(); ++I)
        LaneDefs.push_back(
            cloneForLane(Plan, Builder, IdxTy, DefR, VPLane(I), Def2LaneDefs));
 
      Def2LaneDefs[DefR] = LaneDefs;
      /// Users that only demand the first lane can use the definition for lane
      /// 0.
      DefR->replaceUsesWithIf(LaneDefs[0], [DefR](VPUser &U, unsigned) {
        return U.usesFirstLaneOnly(DefR);
      });
 
      // Update each build vector user that currently has DefR as its only
      // operand, to have all LaneDefs as its operands.
      for (VPUser *U : to_vector(DefR->users())) {
        auto *VPI = dyn_cast<VPInstruction>(U);
        if (!VPI || (VPI->getOpcode() != VPInstruction::BuildVector &&
                     VPI->getOpcode() != VPInstruction::BuildStructVector))
          continue;
        assert(VPI->getNumOperands() == 1 &&
               "Build(Struct)Vector must have a single operand before "
               "replicating by VF");
        VPI->setOperand(0, LaneDefs[0]);
        for (VPValue *LaneDef : drop_begin(LaneDefs))
          VPI->addOperand(LaneDef);
      }
      ToRemove.push_back(DefR);
    }
  }
  for (auto *R : reverse(ToRemove))
    R->eraseFromParent();
 
  replicateReplicateRegionsByVF(Plan, VF, IdxTy);
}