compiler/backend/mid-tier-register-allocator.cc

// Copyright 2020 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "src/compiler/backend/mid-tier-register-allocator.h"

#include "src/base/bits.h"
#include "src/base/logging.h"
#include "src/base/macros.h"
#include "src/base/optional.h"
#include "src/codegen/machine-type.h"
#include "src/codegen/register-configuration.h"
#include "src/codegen/tick-counter.h"
#include "src/common/globals.h"
#include "src/compiler/backend/instruction.h"
#include "src/compiler/linkage.h"
#include "src/logging/counters.h"
#include "src/utils/bit-vector.h"
#include "src/zone/zone-containers.h"

namespace v8 {
namespace internal {
namespace compiler {

class RegisterState;
class DeferredBlocksRegion;

// BlockState stores details associated with a particular basic block.
class BlockState final {
 public:
  BlockState(int block_count, Zone* zone)
      : general_registers_in_state_(nullptr),
        double_registers_in_state_(nullptr),
        deferred_blocks_region_(nullptr),
        dominated_blocks_(block_count, zone),
        successors_phi_index_(-1),
        is_deferred_block_boundary_(false) {}

  // Returns the RegisterState that applies to the input of this block. Can be
  // |nullptr| if the no registers of |kind| have been allocated up to this
  // point.
  RegisterState* register_in_state(RegisterKind kind);
  void set_register_in_state(RegisterState* register_state, RegisterKind kind);

  // Returns a bitvector representing all the basic blocks that are dominated
  // by this basic block.
  BitVector* dominated_blocks() { return &dominated_blocks_; }

  // Set / get this block's index for successor's phi operations. Will return
  // -1 if this block has no successor's with phi operations.
  int successors_phi_index() const { return successors_phi_index_; }
  void set_successors_phi_index(int index) {
    DCHECK_EQ(successors_phi_index_, -1);
    successors_phi_index_ = index;
  }

  // If this block is deferred, this represents region of deferred blocks
  // that are directly reachable from this block.
  DeferredBlocksRegion* deferred_blocks_region() const {
    return deferred_blocks_region_;
  }
  void set_deferred_blocks_region(DeferredBlocksRegion* region) {
    DCHECK_NULL(deferred_blocks_region_);
    deferred_blocks_region_ = region;
  }

  // Returns true if this block represents either a transition from
  // non-deferred to deferred or vice versa.
  bool is_deferred_block_boundary() const {
    return is_deferred_block_boundary_;
  }
  void MarkAsDeferredBlockBoundary() { is_deferred_block_boundary_ = true; }

  MOVE_ONLY_NO_DEFAULT_CONSTRUCTOR(BlockState);

 private:
  RegisterState* general_registers_in_state_;
  RegisterState* double_registers_in_state_;

  DeferredBlocksRegion* deferred_blocks_region_;

  BitVector dominated_blocks_;
  int successors_phi_index_;
  bool is_deferred_block_boundary_;
};

RegisterState* BlockState::register_in_state(RegisterKind kind) {
  switch (kind) {
    case RegisterKind::kGeneral:
      return general_registers_in_state_;
    case RegisterKind::kDouble:
      return double_registers_in_state_;
  }
}

void BlockState::set_register_in_state(RegisterState* register_state,
                                       RegisterKind kind) {
  switch (kind) {
    case RegisterKind::kGeneral:
      DCHECK_NULL(general_registers_in_state_);
      general_registers_in_state_ = register_state;
      break;
    case RegisterKind::kDouble:
      DCHECK_NULL(double_registers_in_state_);
      double_registers_in_state_ = register_state;
      break;
  }
}

MidTierRegisterAllocationData::MidTierRegisterAllocationData(
    const RegisterConfiguration* config, Zone* zone, Frame* frame,
    InstructionSequence* code, TickCounter* tick_counter,
    const char* debug_name)
    : RegisterAllocationData(Type::kMidTier),
      allocation_zone_(zone),
      frame_(frame),
      code_(code),
      debug_name_(debug_name),
      config_(config),
      virtual_register_data_(code->VirtualRegisterCount(), allocation_zone()),
      block_states_(allocation_zone()),
      reference_map_instructions_(allocation_zone()),
      spilled_virtual_registers_(code->VirtualRegisterCount(),
                                 allocation_zone()),
      tick_counter_(tick_counter) {
  int basic_block_count = code->InstructionBlockCount();
  block_states_.reserve(basic_block_count);
  for (int i = 0; i < basic_block_count; i++) {
    block_states_.emplace_back(basic_block_count, allocation_zone());
  }
}

MoveOperands* MidTierRegisterAllocationData::AddGapMove(
    int instr_index, Instruction::GapPosition position,
    const InstructionOperand& from, const InstructionOperand& to) {
  Instruction* instr = code()->InstructionAt(instr_index);
  ParallelMove* moves = instr->GetOrCreateParallelMove(position, code_zone());
  return moves->AddMove(from, to);
}

MoveOperands* MidTierRegisterAllocationData::AddPendingOperandGapMove(
    int instr_index, Instruction::GapPosition position) {
  return AddGapMove(instr_index, position, PendingOperand(), PendingOperand());
}

MachineRepresentation MidTierRegisterAllocationData::RepresentationFor(
    int virtual_register) {
  if (virtual_register == InstructionOperand::kInvalidVirtualRegister) {
    return InstructionSequence::DefaultRepresentation();
  } else {
    DCHECK_LT(virtual_register, code()->VirtualRegisterCount());
    return code()->GetRepresentation(virtual_register);
  }
}

BlockState& MidTierRegisterAllocationData::block_state(RpoNumber rpo_number) {
  return block_states_[rpo_number.ToInt()];
}

const InstructionBlock* MidTierRegisterAllocationData::GetBlock(
    RpoNumber rpo_number) {
  return code()->InstructionBlockAt(rpo_number);
}

const InstructionBlock* MidTierRegisterAllocationData::GetBlock(
    int instr_index) {
  return code()->InstructionAt(instr_index)->block();
}

const BitVector* MidTierRegisterAllocationData::GetBlocksDominatedBy(
    const InstructionBlock* block) {
  return block_state(block->rpo_number()).dominated_blocks();
}

// RegisterIndex represents a particular register of a given kind (depending
// on the RegisterKind of the allocator).
class RegisterIndex final {
 public:
  RegisterIndex() : index_(kInvalidIndex) {}
  explicit RegisterIndex(int index) : index_(index) {}
  static RegisterIndex Invalid() { return RegisterIndex(); }

  bool is_valid() const { return index_ != kInvalidIndex; }

  int ToInt() const {
    DCHECK(is_valid());
    return index_;
  }

  uintptr_t ToBit(MachineRepresentation rep) const {
    if (kSimpleFPAliasing || rep != MachineRepresentation::kSimd128) {
      return 1ull << ToInt();
    } else {
      DCHECK_EQ(rep, MachineRepresentation::kSimd128);
      return 3ull << ToInt();
    }
  }

  bool operator==(const RegisterIndex& rhs) const {
    return index_ == rhs.index_;
  }
  bool operator!=(const RegisterIndex& rhs) const {
    return index_ != rhs.index_;
  }

  class Iterator {
   public:
    explicit Iterator(int index) : index_(index) {}

    bool operator!=(const Iterator& rhs) const { return index_ != rhs.index_; }
    void operator++() { index_++; }
    RegisterIndex operator*() const { return RegisterIndex(index_); }

   private:
    int index_;
  };

 private:
  static const int kInvalidIndex = -1;
  int8_t index_;
};

// A Range from [start, end] of instructions, inclusive of start and end.
class Range {
 public:
  Range() : start_(kMaxInt), end_(0) {}
  Range(int start, int end) : start_(start), end_(end) {}

  void AddInstr(int index) {
    start_ = std::min(start_, index);
    end_ = std::max(end_, index);
  }

  void AddRange(const Range& other) {
    start_ = std::min(start_, other.start_);
    end_ = std::max(end_, other.end_);
  }

  // Returns true if index is greater than start and less than or equal to end.
  bool Contains(int index) { return index >= start_ && index <= end_; }

  int start() const { return start_; }
  int end() const { return end_; }

 private:
  int start_;
  int end_;
};

// Represents a connected region of deferred basic blocks.
class DeferredBlocksRegion final {
 public:
  explicit DeferredBlocksRegion(Zone* zone, int number_of_blocks)
      : spilled_vregs_(zone), blocks_covered_(number_of_blocks, zone) {}

  void AddBlock(RpoNumber block, MidTierRegisterAllocationData* data) {
    DCHECK(data->GetBlock(block)->IsDeferred());
    blocks_covered_.Add(block.ToInt());
    data->block_state(block).set_deferred_blocks_region(this);
  }

  // Adds |vreg| to the list of variables to potentially defer their output to
  // a spill slot until we enter this deferred block region.
  void DeferSpillOutputUntilEntry(int vreg) { spilled_vregs_.insert(vreg); }

  ZoneSet<int>::iterator begin() const { return spilled_vregs_.begin(); }
  ZoneSet<int>::iterator end() const { return spilled_vregs_.end(); }

  const BitVector* blocks_covered() const { return &blocks_covered_; }

 private:
  ZoneSet<int> spilled_vregs_;
  BitVector blocks_covered_;
};

// VirtualRegisterData stores data specific to a particular virtual register,
// and tracks spilled operands for that virtual register.
class VirtualRegisterData final {
 public:
  VirtualRegisterData() = default;

  // Define VirtualRegisterData with the type of output that produces this
  // virtual register.
  void DefineAsUnallocatedOperand(int virtual_register, int instr_index,
                                  bool is_deferred_block,
                                  bool is_exceptional_call_output);
  void DefineAsFixedSpillOperand(AllocatedOperand* operand,
                                 int virtual_register, int instr_index,
                                 bool is_deferred_block,
                                 bool is_exceptional_call_output);
  void DefineAsConstantOperand(ConstantOperand* operand, int instr_index,
                               bool is_deferred_block);
  void DefineAsPhi(int virtual_register, int instr_index,
                   bool is_deferred_block);

  // Spill an operand that is assigned to this virtual register.
  void SpillOperand(InstructionOperand* operand, int instr_index,
                    MidTierRegisterAllocationData* data);

  // Emit gap moves to / from the spill slot.
  void EmitGapMoveToInputFromSpillSlot(AllocatedOperand to_operand,
                                       int instr_index,
                                       MidTierRegisterAllocationData* data);
  void EmitGapMoveFromOutputToSpillSlot(AllocatedOperand from_operand,
                                        const InstructionBlock* current_block,
                                        int instr_index,
                                        MidTierRegisterAllocationData* data);
  void EmitGapMoveToSpillSlot(AllocatedOperand from_operand, int instr_index,
                              MidTierRegisterAllocationData* data);

  // Adds pending spills for deferred-blocks.
  void AddDeferredSpillUse(int instr_index,
                           MidTierRegisterAllocationData* data);
  void AddDeferredSpillOutput(AllocatedOperand allocated_op, int instr_index,
                              MidTierRegisterAllocationData* data);

  // Accessors for spill operand, which may still be pending allocation.
  bool HasSpillOperand() const { return spill_operand_ != nullptr; }
  InstructionOperand* spill_operand() const {
    DCHECK(HasSpillOperand());
    return spill_operand_;
  }

  bool HasPendingSpillOperand() const {
    return HasSpillOperand() && spill_operand_->IsPending();
  }
  bool HasAllocatedSpillOperand() const {
    return HasSpillOperand() && spill_operand_->IsAllocated();
  }
  bool HasConstantSpillOperand() const {
    DCHECK_EQ(is_constant(), HasSpillOperand() && spill_operand_->IsConstant());
    return is_constant();
  }

  // Returns true if the virtual register should be spilled when it is output.
  bool NeedsSpillAtOutput() const { return needs_spill_at_output_; }
  void MarkAsNeedsSpillAtOutput() {
    if (is_constant()) return;
    needs_spill_at_output_ = true;
    if (HasSpillRange()) spill_range()->ClearDeferredBlockSpills();
  }

  // Returns true if the virtual register should be spilled at entry to deferred
  // blocks in which it is spilled (to avoid spilling on output on
  // non-deferred blocks).
  bool NeedsSpillAtDeferredBlocks() const;
  void EmitDeferredSpillOutputs(MidTierRegisterAllocationData* data);

  bool IsSpilledAt(int instr_index, MidTierRegisterAllocationData* data) {
    DCHECK_GE(instr_index, output_instr_index());
    if (NeedsSpillAtOutput() || HasConstantSpillOperand()) return true;
    if (HasSpillOperand() && data->GetBlock(instr_index)->IsDeferred()) {
      return true;
    }
    return false;
  }

  // Allocates pending spill operands to the |allocated| spill slot.
  void AllocatePendingSpillOperand(const AllocatedOperand& allocated);

  int vreg() const { return vreg_; }
  int output_instr_index() const { return output_instr_index_; }
  bool is_constant() const { return is_constant_; }
  bool is_phi() const { return is_phi_; }
  bool is_defined_in_deferred_block() const {
    return is_defined_in_deferred_block_;
  }
  bool is_exceptional_call_output() const {
    return is_exceptional_call_output_;
  }

  struct DeferredSpillSlotOutput {
   public:
    explicit DeferredSpillSlotOutput(int instr, AllocatedOperand op,
                                     const BitVector* blocks)
        : instr_index(instr), operand(op), live_blocks(blocks) {}

    int instr_index;
    AllocatedOperand operand;
    const BitVector* live_blocks;
  };

  // Represents the range of instructions for which this virtual register needs
  // to be spilled on the stack.
  class SpillRange : public ZoneObject {
   public:
    // Defines a spill range for an output operand.
    SpillRange(int definition_instr_index,
               const InstructionBlock* definition_block,
               MidTierRegisterAllocationData* data)
        : live_range_(definition_instr_index, definition_instr_index),
          live_blocks_(data->GetBlocksDominatedBy(definition_block)),
          deferred_spill_outputs_(nullptr) {}

    // Defines a spill range for a Phi variable.
    SpillRange(const InstructionBlock* phi_block,
               MidTierRegisterAllocationData* data)
        : live_range_(phi_block->first_instruction_index(),
                      phi_block->first_instruction_index()),
          live_blocks_(data->GetBlocksDominatedBy(phi_block)),
          deferred_spill_outputs_(nullptr) {
      // For phis, add the gap move instructions in the predecssor blocks to
      // the live range.
      for (RpoNumber pred_rpo : phi_block->predecessors()) {
        const InstructionBlock* block = data->GetBlock(pred_rpo);
        live_range_.AddInstr(block->last_instruction_index());
      }
    }

    SpillRange(const SpillRange&) = delete;
    SpillRange& operator=(const SpillRange&) = delete;

    bool IsLiveAt(int instr_index, InstructionBlock* block) {
      if (!live_range_.Contains(instr_index)) return false;

      int block_rpo = block->rpo_number().ToInt();
      if (!live_blocks_->Contains(block_rpo)) return false;

      if (!HasDeferredBlockSpills()) {
        return true;
      } else {
        // If this spill range is only output for deferred block, then the spill
        // slot will only be live for the deferred blocks, not all blocks that
        // the virtual register is live.
        for (auto deferred_spill_output : *deferred_spill_outputs()) {
          if (deferred_spill_output.live_blocks->Contains(block_rpo)) {
            return true;
          }
        }
        return false;
      }
    }

    void ExtendRangeTo(int instr_index) { live_range_.AddInstr(instr_index); }

    void AddDeferredSpillOutput(AllocatedOperand allocated_op, int instr_index,
                                MidTierRegisterAllocationData* data) {
      if (deferred_spill_outputs_ == nullptr) {
        Zone* zone = data->allocation_zone();
        deferred_spill_outputs_ =
            zone->New<ZoneVector<DeferredSpillSlotOutput>>(zone);
      }
      const InstructionBlock* block = data->GetBlock(instr_index);
      DCHECK_EQ(block->first_instruction_index(), instr_index);
      BlockState& block_state = data->block_state(block->rpo_number());
      const BitVector* deferred_blocks =
          block_state.deferred_blocks_region()->blocks_covered();
      deferred_spill_outputs_->emplace_back(instr_index, allocated_op,
                                            deferred_blocks);
    }

    void ClearDeferredBlockSpills() { deferred_spill_outputs_ = nullptr; }
    bool HasDeferredBlockSpills() const {
      return deferred_spill_outputs_ != nullptr;
    }
    const ZoneVector<DeferredSpillSlotOutput>* deferred_spill_outputs() const {
      DCHECK(HasDeferredBlockSpills());
      return deferred_spill_outputs_;
    }

    Range& live_range() { return live_range_; }

   private:
    Range live_range_;
    const BitVector* live_blocks_;
    ZoneVector<DeferredSpillSlotOutput>* deferred_spill_outputs_;
  };

  bool HasSpillRange() const { return spill_range_ != nullptr; }
  SpillRange* spill_range() const {
    DCHECK(HasSpillRange());
    return spill_range_;
  }

 private:
  void Initialize(int virtual_register, InstructionOperand* spill_operand,
                  int instr_index, bool is_phi, bool is_constant,
                  bool is_defined_in_deferred_block,
                  bool is_exceptional_call_output);

  void AddSpillUse(int instr_index, MidTierRegisterAllocationData* data);
  void AddPendingSpillOperand(PendingOperand* pending_operand);
  void EnsureSpillRange(MidTierRegisterAllocationData* data);
  bool CouldSpillOnEntryToDeferred(const InstructionBlock* block);

  InstructionOperand* spill_operand_;
  SpillRange* spill_range_;
  int output_instr_index_;

  int vreg_;
  bool is_phi_ : 1;
  bool is_constant_ : 1;
  bool is_defined_in_deferred_block_ : 1;
  bool needs_spill_at_output_ : 1;
  bool is_exceptional_call_output_ : 1;
};

VirtualRegisterData& MidTierRegisterAllocationData::VirtualRegisterDataFor(
    int virtual_register) {
  DCHECK_GE(virtual_register, 0);
  DCHECK_LT(virtual_register, virtual_register_data_.size());
  return virtual_register_data_[virtual_register];
}

void VirtualRegisterData::Initialize(int virtual_register,
                                     InstructionOperand* spill_operand,
                                     int instr_index, bool is_phi,
                                     bool is_constant,
                                     bool is_defined_in_deferred_block,
                                     bool is_exceptional_call_output) {
  vreg_ = virtual_register;
  spill_operand_ = spill_operand;
  spill_range_ = nullptr;
  output_instr_index_ = instr_index;
  is_phi_ = is_phi;
  is_constant_ = is_constant;
  is_defined_in_deferred_block_ = is_defined_in_deferred_block;
  needs_spill_at_output_ = !is_constant_ && spill_operand_ != nullptr;
  is_exceptional_call_output_ = is_exceptional_call_output;
}

void VirtualRegisterData::DefineAsConstantOperand(ConstantOperand* operand,
                                                  int instr_index,
                                                  bool is_deferred_block) {
  Initialize(operand->virtual_register(), operand, instr_index, false, true,
             is_deferred_block, false);
}

void VirtualRegisterData::DefineAsFixedSpillOperand(
    AllocatedOperand* operand, int virtual_register, int instr_index,
    bool is_deferred_block, bool is_exceptional_call_output) {
  Initialize(virtual_register, operand, instr_index, false, false,
             is_deferred_block, is_exceptional_call_output);
}

void VirtualRegisterData::DefineAsUnallocatedOperand(
    int virtual_register, int instr_index, bool is_deferred_block,
    bool is_exceptional_call_output) {
  Initialize(virtual_register, nullptr, instr_index, false, false,
             is_deferred_block, is_exceptional_call_output);
}

void VirtualRegisterData::DefineAsPhi(int virtual_register, int instr_index,
                                      bool is_deferred_block) {
  Initialize(virtual_register, nullptr, instr_index, true, false,
             is_deferred_block, false);
}

void VirtualRegisterData::EnsureSpillRange(
    MidTierRegisterAllocationData* data) {
  DCHECK(!is_constant());
  if (HasSpillRange()) return;

  const InstructionBlock* definition_block =
      data->GetBlock(output_instr_index_);
  if (is_phi()) {
    // Define a spill slot that is defined for the phi's range.
    spill_range_ =
        data->allocation_zone()->New<SpillRange>(definition_block, data);
  } else {
    if (is_exceptional_call_output()) {
      // If this virtual register is output by a call which has an exception
      // catch handler, then the output will only be live in the IfSuccess
      // successor block, not the IfException side, so make the definition block
      // the IfSuccess successor block explicitly.
      DCHECK_EQ(output_instr_index_,
                definition_block->last_instruction_index() - 1);
      DCHECK_EQ(definition_block->SuccessorCount(), 2);
      DCHECK(data->GetBlock(definition_block->successors()[1])->IsHandler());
      definition_block = data->GetBlock(definition_block->successors()[0]);
    }
    // The spill slot will be defined after the instruction that outputs it.
    spill_range_ = data->allocation_zone()->New<SpillRange>(
        output_instr_index_ + 1, definition_block, data);
  }
  data->spilled_virtual_registers().Add(vreg());
}

void VirtualRegisterData::AddSpillUse(int instr_index,
                                      MidTierRegisterAllocationData* data) {
  if (is_constant()) return;

  EnsureSpillRange(data);
  spill_range_->ExtendRangeTo(instr_index);

  const InstructionBlock* block = data->GetBlock(instr_index);
  if (CouldSpillOnEntryToDeferred(block)) {
    data->block_state(block->rpo_number())
        .deferred_blocks_region()
        ->DeferSpillOutputUntilEntry(vreg());
  } else {
    MarkAsNeedsSpillAtOutput();
  }
}

void VirtualRegisterData::AddDeferredSpillUse(
    int instr_index, MidTierRegisterAllocationData* data) {
  DCHECK(data->GetBlock(instr_index)->IsDeferred());
  DCHECK(!is_defined_in_deferred_block());
  AddSpillUse(instr_index, data);
}

bool VirtualRegisterData::CouldSpillOnEntryToDeferred(
    const InstructionBlock* block) {
  return !NeedsSpillAtOutput() && block->IsDeferred() &&
         !is_defined_in_deferred_block() && !is_constant();
}

void VirtualRegisterData::AddDeferredSpillOutput(
    AllocatedOperand allocated_op, int instr_index,
    MidTierRegisterAllocationData* data) {
  DCHECK(!NeedsSpillAtOutput());
  spill_range_->AddDeferredSpillOutput(allocated_op, instr_index, data);
}

void VirtualRegisterData::SpillOperand(InstructionOperand* operand,
                                       int instr_index,
                                       MidTierRegisterAllocationData* data) {
  AddSpillUse(instr_index, data);
  if (HasAllocatedSpillOperand() || HasConstantSpillOperand()) {
    InstructionOperand::ReplaceWith(operand, spill_operand());
  } else {
    PendingOperand pending_op;
    InstructionOperand::ReplaceWith(operand, &pending_op);
    AddPendingSpillOperand(PendingOperand::cast(operand));
  }
}

bool VirtualRegisterData::NeedsSpillAtDeferredBlocks() const {
  return HasSpillRange() && spill_range()->HasDeferredBlockSpills();
}

void VirtualRegisterData::EmitDeferredSpillOutputs(
    MidTierRegisterAllocationData* data) {
  DCHECK(NeedsSpillAtDeferredBlocks());
  for (auto deferred_spill : *spill_range()->deferred_spill_outputs()) {
    EmitGapMoveToSpillSlot(deferred_spill.operand, deferred_spill.instr_index,
                           data);
  }
}

void VirtualRegisterData::EmitGapMoveToInputFromSpillSlot(
    AllocatedOperand to_operand, int instr_index,
    MidTierRegisterAllocationData* data) {
  AddSpillUse(instr_index, data);
  DCHECK(!to_operand.IsPending());
  if (HasAllocatedSpillOperand() || HasConstantSpillOperand()) {
    data->AddGapMove(instr_index, Instruction::END, *spill_operand(),
                     to_operand);
  } else {
    MoveOperands* move_ops =
        data->AddPendingOperandGapMove(instr_index, Instruction::END);
    AddPendingSpillOperand(PendingOperand::cast(&move_ops->source()));
    InstructionOperand::ReplaceWith(&move_ops->destination(), &to_operand);
  }
}

void VirtualRegisterData::EmitGapMoveToSpillSlot(
    AllocatedOperand from_operand, int instr_index,
    MidTierRegisterAllocationData* data) {
  AddSpillUse(instr_index, data);
  if (HasAllocatedSpillOperand() || HasConstantSpillOperand()) {
    data->AddGapMove(instr_index, Instruction::START, from_operand,
                     *spill_operand());
  } else {
    MoveOperands* move_ops =
        data->AddPendingOperandGapMove(instr_index, Instruction::START);
    InstructionOperand::ReplaceWith(&move_ops->source(), &from_operand);
    AddPendingSpillOperand(PendingOperand::cast(&move_ops->destination()));
  }
}

void VirtualRegisterData::EmitGapMoveFromOutputToSpillSlot(
    AllocatedOperand from_operand, const InstructionBlock* current_block,
    int instr_index, MidTierRegisterAllocationData* data) {
  DCHECK_EQ(data->GetBlock(instr_index), current_block);
  if (instr_index == current_block->last_instruction_index()) {
    // Add gap move to the first instruction of every successor block.
    for (const RpoNumber& succ : current_block->successors()) {
      const InstructionBlock* successor = data->GetBlock(succ);
      DCHECK_EQ(1, successor->PredecessorCount());
      EmitGapMoveToSpillSlot(from_operand, successor->first_instruction_index(),
                             data);
    }
  } else {
    // Add gap move to the next instruction.
    EmitGapMoveToSpillSlot(from_operand, instr_index + 1, data);
  }
}

void VirtualRegisterData::AddPendingSpillOperand(PendingOperand* pending_op) {
  DCHECK(HasSpillRange());
  DCHECK_NULL(pending_op->next());
  if (HasSpillOperand()) {
    pending_op->set_next(PendingOperand::cast(spill_operand()));
  }
  spill_operand_ = pending_op;
}

void VirtualRegisterData::AllocatePendingSpillOperand(
    const AllocatedOperand& allocated) {
  DCHECK(!HasAllocatedSpillOperand() && !HasConstantSpillOperand());
  PendingOperand* current = PendingOperand::cast(spill_operand_);
  while (current) {
    PendingOperand* next = current->next();
    InstructionOperand::ReplaceWith(current, &allocated);
    current = next;
  }
}

// RegisterState represents the state of the |kind| registers at a particular
// point in program execution. The RegisterState can be cloned or merged with
// other RegisterStates to model branches and merges in program control flow.
class RegisterState final : public ZoneObject {
 public:
  static RegisterState* New(RegisterKind kind, int num_allocatable_registers,
                            Zone* zone) {
    return zone->New<RegisterState>(kind, num_allocatable_registers, zone);
  }

  RegisterState(RegisterKind kind, int num_allocatable_registers, Zone* zone);
  RegisterState(const RegisterState& other) V8_NOEXCEPT;

  bool IsAllocated(RegisterIndex reg);
  bool IsShared(RegisterIndex reg);
  int VirtualRegisterForRegister(RegisterIndex reg);

  // Commit the |reg| with the |allocated| operand.
  void Commit(RegisterIndex reg, AllocatedOperand allocated,
              InstructionOperand* operand, MidTierRegisterAllocationData* data);

  // Spill the contents of |reg| for an instruction in |current_block| using
  // the |allocated| operand to commit the spill gap move.
  void Spill(RegisterIndex reg, AllocatedOperand allocated,
             const InstructionBlock* current_block,
             MidTierRegisterAllocationData* data);

  // Add a pending spill of the contents of |reg| at the exit point of a
  // deferred block at |instr_index| using |allocated| operand to commit the
  // spill gap move, if the register never gets spilled in a non-deferred block.
  void SpillForDeferred(RegisterIndex reg, AllocatedOperand allocated,
                        int instr_index, MidTierRegisterAllocationData* data);

  // Add a pending gap move from |reg| to |virtual_register|'s spill at the
  // entry point of a deferred block at |instr_index|, if the |virtual_register|
  // never spilled in a non-deferred block.
  void MoveToSpillSlotOnDeferred(RegisterIndex reg, int virtual_register,
                                 int instr_index,
                                 MidTierRegisterAllocationData* data);

  // Allocate |reg| to |virtual_register| for the instruction at |instr_index|.
  // If the register is later spilled, a gap move will be added immediately
  // before |instr_index| to move |virtual_register| into this register.
  void AllocateUse(RegisterIndex reg, int virtual_register,
                   InstructionOperand* operand, int instr_index,
                   MidTierRegisterAllocationData* data);

  // Allocate |reg| as a pending use of |virtual_register| for |operand| in the
  // instruction at |instr_index|. If |virtual_register| later gets committed to
  // this register, then |operand| will be too, otherwise |operand| will be
  // replaced with |virtual_register|'s spill operand.
  void AllocatePendingUse(RegisterIndex reg, int virtual_register,
                          InstructionOperand* operand, int instr_index);

  // Mark that the register is holding a phi operand that is yet to be allocated
  // by the source block in the gap just before the last instruction in the
  // source block.
  void UseForPhiGapMove(RegisterIndex reg);
  bool IsPhiGapMove(RegisterIndex reg);

  // Returns true if |reg| only has pending uses allocated to it.
  bool HasPendingUsesOnly(RegisterIndex reg);

  // Clone this RegisterState for a successor block.
  RegisterState* Clone();

  // Copy register details for |reg| from |source| to |this| RegisterState.
  void CopyFrom(RegisterIndex reg, RegisterState* source);

  // Returns true if the register details for |reg| are equal in |source| and
  // |this| RegisterStates.
  bool Equals(RegisterIndex reg, RegisterState* source);

  // Signals that the registers in this state are going to be shared across
  // |shared_use_count| blocks.
  void AddSharedUses(int shared_use_count);

  // When merging multiple block's RegisterState into the successor block with
  // |this| RegisterState, commit |reg| as being merged from a given predecessor
  // block.
  void CommitAtMerge(RegisterIndex reg);

  // Resets |reg| if it has register data that was shared with other basic
  // blocks and was spilled in those blocks.
  void ResetIfSpilledWhileShared(RegisterIndex reg);

  // Enable range-based for on allocatable register indices.
  RegisterIndex::Iterator begin() const { return RegisterIndex::Iterator(0); }
  RegisterIndex::Iterator end() const {
    return RegisterIndex::Iterator(num_allocatable_registers());
  }

 private:
  // Represents a particular register and details of what virtual_register it is
  // currently holding, and how it should be updated if committed or spilled.
  class Register final : public ZoneObject {
   public:
    Register();
    void Reset();

    // Operations for committing, spilling and allocating uses of the register.
    void Commit(AllocatedOperand allocated_operand,
                MidTierRegisterAllocationData* data);
    void Spill(AllocatedOperand allocated_op,
               const InstructionBlock* current_block,
               MidTierRegisterAllocationData* data);
    void Use(int virtual_register, int instr_index);
    void PendingUse(InstructionOperand* operand, int virtual_register,
                    int instr_index);
    void SpillForDeferred(AllocatedOperand allocated, int instr_index,
                          MidTierRegisterAllocationData* data);
    void MoveToSpillSlotOnDeferred(int virtual_register, int instr_index,
                                   MidTierRegisterAllocationData* data);

    // Mark register as holding a phi.
    void MarkAsPhiMove();
    bool is_phi_gap_move() const { return is_phi_gap_move_; }

    // The register has deferred block spills, that will be emitted if the
    // register is committed without having been spilled in a non-deferred block
    void AddDeferredBlockSpill(int instr_index, bool on_exit, Zone* zone);
    bool has_deferred_block_spills() const {
      return deferred_block_spills_.has_value();
    }

    // Operations related to dealing with a Register that is shared across
    // multiple basic blocks.
    void CommitAtMerge();
    void AddSharedUses(int shared_use_count);
    bool is_shared() const { return is_shared_; }
    bool was_spilled_while_shared() const {
      return is_shared() && !is_allocated();
    }

    bool is_allocated() const {
      return virtual_register_ != InstructionOperand::kInvalidVirtualRegister;
    }

    // The current virtual register held by this register.
    int virtual_register() const { return virtual_register_; }

    // The instruction index for the last use of the current in-progress
    // allocation of this register in the instruction stream. Used both
    // as the instruction too add a gap move if |needs_gap_move_on_spill| and
    // the intruction which the virtual register's spill range should be
    // extended too if the register is spilled.
    int last_use_instr_index() const { return last_use_instr_index_; }

    // Returns true if a gap move should be added if the register is spilled.
    bool needs_gap_move_on_spill() const { return needs_gap_move_on_spill_; }

    // Returns a threaded list of the operands that have pending uses of this
    // register and will be resolved either to the register, or a spill slot
    // depending on whether this register is spilled or committed.
    PendingOperand* pending_uses() const { return pending_uses_; }

   private:
    struct DeferredBlockSpill {
      DeferredBlockSpill(int instr, bool on_exit)
          : instr_index(instr), on_deferred_exit(on_exit) {}

      int instr_index;
      bool on_deferred_exit;
    };

    void SpillPendingUses(MidTierRegisterAllocationData* data);
    void SpillPhiGapMove(AllocatedOperand allocated_op,
                         const InstructionBlock* block,
                         MidTierRegisterAllocationData* data);

    bool needs_gap_move_on_spill_;
    bool is_shared_;
    bool is_phi_gap_move_;
    int last_use_instr_index_;

    int num_commits_required_;
    int virtual_register_;
    PendingOperand* pending_uses_;
    base::Optional<ZoneVector<DeferredBlockSpill>> deferred_block_spills_;
  };

  void ResetDataFor(RegisterIndex reg);

  bool HasRegisterData(RegisterIndex reg);
  void EnsureRegisterData(RegisterIndex reg);

  int num_allocatable_registers() const {
    return static_cast<int>(register_data_.size());
  }
  Register& reg_data(RegisterIndex reg);
  Zone* zone() const { return zone_; }

  ZoneVector<Register*> register_data_;
  Zone* zone_;
};

RegisterState::Register::Register() { Reset(); }

void RegisterState::Register::Reset() {
  is_shared_ = false;
  is_phi_gap_move_ = false;
  needs_gap_move_on_spill_ = false;
  last_use_instr_index_ = -1;
  num_commits_required_ = 0;
  virtual_register_ = InstructionOperand::kInvalidVirtualRegister;
  pending_uses_ = nullptr;
  deferred_block_spills_.reset();
}

void RegisterState::Register::Use(int virtual_register, int instr_index) {
  // A register can have many pending uses, but should only ever have a single
  // non-pending use, since any subsiquent use will commit the preceeding use
  // first.
  DCHECK(!is_allocated());
  needs_gap_move_on_spill_ = true;
  virtual_register_ = virtual_register;
  last_use_instr_index_ = instr_index;
  num_commits_required_ = 1;
}

void RegisterState::Register::PendingUse(InstructionOperand* operand,
                                         int virtual_register,
                                         int instr_index) {
  if (!is_allocated()) {
    virtual_register_ = virtual_register;
    last_use_instr_index_ = instr_index;
    num_commits_required_ = 1;
  }
  DCHECK_EQ(virtual_register_, virtual_register);

  PendingOperand pending_op(pending_uses());
  InstructionOperand::ReplaceWith(operand, &pending_op);
  pending_uses_ = PendingOperand::cast(operand);
}

void RegisterState::Register::MarkAsPhiMove() {
  DCHECK(is_allocated());
  is_phi_gap_move_ = true;
}

void RegisterState::Register::AddDeferredBlockSpill(int instr_index,
                                                    bool on_exit, Zone* zone) {
  DCHECK(is_allocated());
  if (!deferred_block_spills_) {
    deferred_block_spills_.emplace(zone);
  }
  deferred_block_spills_->emplace_back(instr_index, on_exit);
}

void RegisterState::Register::AddSharedUses(int shared_use_count) {
  is_shared_ = true;
  num_commits_required_ += shared_use_count;
}

void RegisterState::Register::CommitAtMerge() {
  DCHECK(is_shared());
  DCHECK(is_allocated());
  --num_commits_required_;
  // We should still have commits required that will be resolved in the merge
  // block.
  DCHECK_GT(num_commits_required_, 0);
}

void RegisterState::Register::Commit(AllocatedOperand allocated_op,
                                     MidTierRegisterAllocationData* data) {
  DCHECK(is_allocated());
  DCHECK_GT(num_commits_required_, 0);

  if (--num_commits_required_ == 0) {
    // Allocate all pending uses to |allocated_op| if this commit is non-shared,
    // or if it is the final commit required on a register data shared across
    // blocks.
    PendingOperand* pending_use = pending_uses();
    while (pending_use) {
      PendingOperand* next = pending_use->next();
      InstructionOperand::ReplaceWith(pending_use, &allocated_op);
      pending_use = next;
    }
    pending_uses_ = nullptr;

    VirtualRegisterData& vreg_data =
        data->VirtualRegisterDataFor(virtual_register());

    // If there are deferred block gap moves pending, emit them now that the
    // register has been committed.
    if (has_deferred_block_spills()) {
      for (DeferredBlockSpill& spill : *deferred_block_spills_) {
        if (spill.on_deferred_exit) {
          vreg_data.EmitGapMoveToInputFromSpillSlot(allocated_op,
                                                    spill.instr_index, data);
        } else if (!vreg_data.NeedsSpillAtOutput()) {
          vreg_data.AddDeferredSpillOutput(allocated_op, spill.instr_index,
                                           data);
        }
      }
    }

    // If this register was used as a phi gap move, then it being commited
    // is the point at which we have output the Phi.
    if (is_phi_gap_move() && vreg_data.NeedsSpillAtDeferredBlocks()) {
      vreg_data.EmitDeferredSpillOutputs(data);
    }
  }
  DCHECK_IMPLIES(num_commits_required_ > 0, is_shared());
}

void RegisterState::Register::Spill(AllocatedOperand allocated_op,
                                    const InstructionBlock* current_block,
                                    MidTierRegisterAllocationData* data) {
  VirtualRegisterData& vreg_data =
      data->VirtualRegisterDataFor(virtual_register());
  SpillPendingUses(data);
  if (is_phi_gap_move()) {
    SpillPhiGapMove(allocated_op, current_block, data);
  }
  if (needs_gap_move_on_spill()) {
    vreg_data.EmitGapMoveToInputFromSpillSlot(allocated_op,
                                              last_use_instr_index(), data);
  }
  if (has_deferred_block_spills() || !current_block->IsDeferred()) {
    vreg_data.MarkAsNeedsSpillAtOutput();
  }
  virtual_register_ = InstructionOperand::kInvalidVirtualRegister;
}

void RegisterState::Register::SpillPhiGapMove(
    AllocatedOperand allocated_op, const InstructionBlock* current_block,
    MidTierRegisterAllocationData* data) {
  DCHECK_EQ(current_block->SuccessorCount(), 1);
  const InstructionBlock* phi_block =
      data->GetBlock(current_block->successors()[0]);

  // Add gap moves to the spilled phi for all blocks we previously allocated
  // assuming the the phi was in a register.
  VirtualRegisterData& vreg_data =
      data->VirtualRegisterDataFor(virtual_register());
  for (RpoNumber predecessor : phi_block->predecessors()) {
    // If the predecessor has a lower rpo number than the current block, then
    // we have already processed it, so add the required gap move.
    if (predecessor > current_block->rpo_number()) {
      const InstructionBlock* predecessor_block = data->GetBlock(predecessor);
      vreg_data.EmitGapMoveToSpillSlot(
          allocated_op, predecessor_block->last_instruction_index(), data);
    }
  }
}

void RegisterState::Register::SpillPendingUses(
    MidTierRegisterAllocationData* data) {
  VirtualRegisterData& vreg_data =
      data->VirtualRegisterDataFor(virtual_register());
  PendingOperand* pending_use = pending_uses();
  while (pending_use) {
    // Spill all the pending operands associated with this register.
    PendingOperand* next = pending_use->next();
    vreg_data.SpillOperand(pending_use, last_use_instr_index(), data);
    pending_use = next;
  }
  pending_uses_ = nullptr;
}

void RegisterState::Register::SpillForDeferred(
    AllocatedOperand allocated, int instr_index,
    MidTierRegisterAllocationData* data) {
  DCHECK(is_allocated());
  DCHECK(is_shared());
  // Add a pending deferred spill, then commit the register (with the commit
  // being fullfilled by the deferred spill if the register is fully commited).
  data->VirtualRegisterDataFor(virtual_register())
      .AddDeferredSpillUse(instr_index, data);
  AddDeferredBlockSpill(instr_index, true, data->allocation_zone());
  Commit(allocated, data);
}

void RegisterState::Register::MoveToSpillSlotOnDeferred(
    int virtual_register, int instr_index,
    MidTierRegisterAllocationData* data) {
  if (!is_allocated()) {
    virtual_register_ = virtual_register;
    last_use_instr_index_ = instr_index;
    num_commits_required_ = 1;
  }
  AddDeferredBlockSpill(instr_index, false, data->allocation_zone());
}

RegisterState::RegisterState(RegisterKind kind, int num_allocatable_registers,
                             Zone* zone)
    : register_data_(num_allocatable_registers, zone), zone_(zone) {}

RegisterState::RegisterState(const RegisterState& other) V8_NOEXCEPT
    : register_data_(other.register_data_.begin(), other.register_data_.end(),
                     other.zone_),
      zone_(other.zone_) {}

int RegisterState::VirtualRegisterForRegister(RegisterIndex reg) {
  if (IsAllocated(reg)) {
    return reg_data(reg).virtual_register();
  } else {
    return InstructionOperand::kInvalidVirtualRegister;
  }
}

bool RegisterState::IsPhiGapMove(RegisterIndex reg) {
  DCHECK(IsAllocated(reg));
  return reg_data(reg).is_phi_gap_move();
}

void RegisterState::Commit(RegisterIndex reg, AllocatedOperand allocated,
                           InstructionOperand* operand,
                           MidTierRegisterAllocationData* data) {
  InstructionOperand::ReplaceWith(operand, &allocated);
  if (IsAllocated(reg)) {
    reg_data(reg).Commit(allocated, data);
    ResetDataFor(reg);
  }
}

void RegisterState::Spill(RegisterIndex reg, AllocatedOperand allocated,
                          const InstructionBlock* current_block,
                          MidTierRegisterAllocationData* data) {
  DCHECK(IsAllocated(reg));
  reg_data(reg).Spill(allocated, current_block, data);
  ResetDataFor(reg);
}

void RegisterState::SpillForDeferred(RegisterIndex reg,
                                     AllocatedOperand allocated,
                                     int instr_index,
                                     MidTierRegisterAllocationData* data) {
  DCHECK(IsAllocated(reg));
  reg_data(reg).SpillForDeferred(allocated, instr_index, data);
  ResetDataFor(reg);
}

void RegisterState::MoveToSpillSlotOnDeferred(
    RegisterIndex reg, int virtual_register, int instr_index,
    MidTierRegisterAllocationData* data) {
  EnsureRegisterData(reg);
  reg_data(reg).MoveToSpillSlotOnDeferred(virtual_register, instr_index, data);
}

void RegisterState::AllocateUse(RegisterIndex reg, int virtual_register,
                                InstructionOperand* operand, int instr_index,
                                MidTierRegisterAllocationData* data) {
  EnsureRegisterData(reg);
  reg_data(reg).Use(virtual_register, instr_index);
}

void RegisterState::AllocatePendingUse(RegisterIndex reg, int virtual_register,
                                       InstructionOperand* operand,
                                       int instr_index) {
  EnsureRegisterData(reg);
  reg_data(reg).PendingUse(operand, virtual_register, instr_index);
}

void RegisterState::UseForPhiGapMove(RegisterIndex reg) {
  DCHECK(IsAllocated(reg));
  reg_data(reg).MarkAsPhiMove();
}

RegisterState::Register& RegisterState::reg_data(RegisterIndex reg) {
  DCHECK(HasRegisterData(reg));
  return *register_data_[reg.ToInt()];
}

bool RegisterState::IsShared(RegisterIndex reg) {
  return HasRegisterData(reg) && reg_data(reg).is_shared();
}

bool RegisterState::IsAllocated(RegisterIndex reg) {
  return HasRegisterData(reg) && reg_data(reg).is_allocated();
}

bool RegisterState::HasPendingUsesOnly(RegisterIndex reg) {
  DCHECK(IsAllocated(reg));
  return !reg_data(reg).needs_gap_move_on_spill();
}

void RegisterState::ResetDataFor(RegisterIndex reg) {
  DCHECK(HasRegisterData(reg));
  if (reg_data(reg).is_shared()) {
    register_data_[reg.ToInt()] = nullptr;
  } else {
    reg_data(reg).Reset();
  }
}

bool RegisterState::HasRegisterData(RegisterIndex reg) {
  DCHECK_LT(reg.ToInt(), register_data_.size());
  return register_data_[reg.ToInt()] != nullptr;
}

void RegisterState::EnsureRegisterData(RegisterIndex reg) {
  if (!HasRegisterData(reg)) {
    register_data_[reg.ToInt()] = zone()->New<RegisterState::Register>();
  }
}

void RegisterState::ResetIfSpilledWhileShared(RegisterIndex reg) {
  if (HasRegisterData(reg) && reg_data(reg).was_spilled_while_shared()) {
    ResetDataFor(reg);
  }
}

void RegisterState::CommitAtMerge(RegisterIndex reg) {
  DCHECK(IsAllocated(reg));
  reg_data(reg).CommitAtMerge();
}

void RegisterState::CopyFrom(RegisterIndex reg, RegisterState* source) {
  register_data_[reg.ToInt()] = source->register_data_[reg.ToInt()];
}

bool RegisterState::Equals(RegisterIndex reg, RegisterState* other) {
  return register_data_[reg.ToInt()] == other->register_data_[reg.ToInt()];
}

void RegisterState::AddSharedUses(int shared_use_count) {
  for (RegisterIndex reg : *this) {
    if (HasRegisterData(reg)) {
      reg_data(reg).AddSharedUses(shared_use_count);
    }
  }
}

RegisterState* RegisterState::Clone() {
  return zone_->New<RegisterState>(*this);
}

class RegisterBitVector {
 public:
  RegisterBitVector() : bits_(0) {}

  bool Contains(RegisterIndex reg, MachineRepresentation rep) const {
    return bits_ & reg.ToBit(rep);
  }

  RegisterIndex GetFirstSet() const {
    return RegisterIndex(base::bits::CountTrailingZeros(bits_));
  }

  RegisterIndex GetFirstCleared(int max_reg) const {
    int reg_index = base::bits::CountTrailingZeros(~bits_);
    if (reg_index < max_reg) {
      return RegisterIndex(reg_index);
    } else {
      return RegisterIndex::Invalid();
    }
  }

  void Add(RegisterIndex reg, MachineRepresentation rep) {
    bits_ |= reg.ToBit(rep);
  }

  void Clear(RegisterIndex reg, MachineRepresentation rep) {
    bits_ &= ~reg.ToBit(rep);
  }

  RegisterBitVector Union(const RegisterBitVector& other) {
    return RegisterBitVector(bits_ | other.bits_);
  }

  void Reset() { bits_ = 0; }
  bool IsEmpty() const { return bits_ == 0; }

 private:
  explicit RegisterBitVector(uintptr_t bits) : bits_(bits) {}

  static_assert(RegisterConfiguration::kMaxRegisters <= sizeof(uintptr_t) * 8,
                "Maximum registers must fit in uintptr_t bitmap");
  uintptr_t bits_;
};

// A SinglePassRegisterAllocator is a fast register allocator that does a single
// pass through the instruction stream without performing any live-range
// analysis beforehand. It deals with a single RegisterKind, either general or
// double registers, with the MidTierRegisterAllocator choosing the correct
// SinglePassRegisterAllocator based on a values representation.
class SinglePassRegisterAllocator final {
 public:
  SinglePassRegisterAllocator(RegisterKind kind,
                              MidTierRegisterAllocationData* data);

  // Convert to / from a register code and a register index.
  RegisterIndex FromRegCode(int reg_code, MachineRepresentation rep) const;
  int ToRegCode(RegisterIndex index, MachineRepresentation rep) const;

  // Allocation routines used to allocate a particular operand to either a
  // register or a spill slot.
  void AllocateConstantOutput(ConstantOperand* operand);
  void AllocateOutput(UnallocatedOperand* operand, int instr_index);
  void AllocateInput(UnallocatedOperand* operand, int instr_index);
  void AllocateSameInputOutput(UnallocatedOperand* output,
                               UnallocatedOperand* input, int instr_index);
  void AllocateGapMoveInput(UnallocatedOperand* operand, int instr_index);
  void AllocateTemp(UnallocatedOperand* operand, int instr_index);
  void AllocatePhi(int virtual_register, const InstructionBlock* block);
  void AllocatePhiGapMove(int to_vreg, int from_vreg, int instr_index);

  // Reserve any fixed registers for the operands on an instruction before doing
  // allocation on the operands.
  void ReserveFixedInputRegister(const UnallocatedOperand* operand,
                                 int instr_index);
  void ReserveFixedTempRegister(const UnallocatedOperand* operand,
                                int instr_index);
  void ReserveFixedOutputRegister(const UnallocatedOperand* operand,
                                  int instr_index);

  // Spills all registers that are currently holding data, for example, due to
  // an instruction that clobbers all registers.
  void SpillAllRegisters();

  // Inform the allocator that we are starting / ending a block or ending
  // allocation for the current instruction.
  void StartBlock(const InstructionBlock* block);
  void EndBlock(const InstructionBlock* block);
  void EndInstruction();

  void UpdateForDeferredBlock(int instr_index);
  void AllocateDeferredBlockSpillOutput(int instr_index,
                                        RpoNumber deferred_block,
                                        int virtual_register);

  RegisterKind kind() const { return kind_; }
  BitVector* assigned_registers() const { return assigned_registers_; }

 private:
  enum class UsePosition {
    // Operand used at start of instruction.
    kStart,
    // Operand used at end of instruction.
    kEnd,
    // Operand is used at both the start and end of instruction.
    kAll,
    // Operand is not used in the instruction (used when initializing register
    // state on block entry).
    kNone,
  };

  // The allocator is initialized without any RegisterState by default to avoid
  // having to allocate per-block allocator state for functions that don't
  // allocate registers of a particular type. All allocation functions should
  // call EnsureRegisterState to allocate a RegisterState if necessary.
  void EnsureRegisterState();

  // Clone the register state from |successor| into the current register state.
  void CloneStateFrom(RpoNumber successor);

  // Merge the register state of |successors| into the current register state.
  void MergeStateFrom(const InstructionBlock::Successors& successors);

  // Spill a register in a previously processed successor block when merging
  // state into the current block.
  void SpillRegisterAtMerge(RegisterState* reg_state, RegisterIndex reg);

  // Introduce a gap move to move |virtual_register| from reg |from| to reg |to|
  // on entry to a |successor| block.
  void MoveRegisterOnMerge(RegisterIndex from, RegisterIndex to,
                           int virtual_register, RpoNumber successor,
                           RegisterState* succ_state);

  // Update the virtual register data with the data in register_state()
  void UpdateVirtualRegisterState();

  // Returns true if |virtual_register| is defined after use position |pos| at
  // |instr_index|.
  bool DefinedAfter(int virtual_register, int instr_index, UsePosition pos);

  // Allocate |reg| to |virtual_register| for |operand| of the instruction at
  // |instr_index|. The register will be reserved for this use for the specified
  // |pos| use position.
  void AllocateUse(RegisterIndex reg, int virtual_register,
                   InstructionOperand* operand, int instr_index,
                   UsePosition pos);

  // Allocate |reg| to |virtual_register| as a pending use (i.e., only if the
  // register is not subsequently spilled) for |operand| of the instruction at
  // |instr_index|.
  void AllocatePendingUse(RegisterIndex reg, int virtual_register,
                          InstructionOperand* operand, int instr_index);

  // Allocate |operand| to |reg| and add a gap move to move |virtual_register|
  // to this register for the instruction at |instr_index|. |reg| will be
  // reserved for this use for the specified |pos| use position.
  void AllocateUseWithMove(RegisterIndex reg, int virtual_register,
                           UnallocatedOperand* operand, int instr_index,
                           UsePosition pos);

  void CommitRegister(RegisterIndex reg, int virtual_register,
                      InstructionOperand* operand, UsePosition pos);
  void SpillRegister(RegisterIndex reg);
  void SpillRegisterForVirtualRegister(int virtual_register);

  // Pre-emptively spill the register at the exit of deferred blocks such that
  // uses of this register in non-deferred blocks don't need to be spilled.
  void SpillRegisterForDeferred(RegisterIndex reg, int instr_index);

  // Returns an AllocatedOperand corresponding to the use of |reg| for
  // |virtual_register|.
  AllocatedOperand AllocatedOperandForReg(RegisterIndex reg,
                                          int virtual_register);

  void ReserveFixedRegister(const UnallocatedOperand* operand, int instr_index,
                            UsePosition pos);
  RegisterIndex AllocateOutput(UnallocatedOperand* operand, int instr_index,
                               UsePosition pos);
  void EmitGapMoveFromOutput(InstructionOperand from, InstructionOperand to,
                             int instr_index);

  // Helper functions to choose the best register for a given operand.
  V8_INLINE RegisterIndex
  ChooseRegisterFor(VirtualRegisterData& virtual_register, int instr_index,
                    UsePosition pos, bool must_use_register);
  V8_INLINE RegisterIndex ChooseRegisterFor(MachineRepresentation rep,
                                            UsePosition pos,
                                            bool must_use_register);
  V8_INLINE RegisterIndex ChooseFreeRegister(MachineRepresentation rep,
                                             UsePosition pos);
  V8_INLINE RegisterIndex ChooseFreeRegister(
      const RegisterBitVector& allocated_regs, MachineRepresentation rep);
  V8_INLINE RegisterIndex ChooseRegisterToSpill(MachineRepresentation rep,
                                                UsePosition pos);

  // Assign, free and mark use's of |reg| for a |virtual_register| at use
  // position |pos|.
  V8_INLINE void AssignRegister(RegisterIndex reg, int virtual_register,
                                UsePosition pos);
  V8_INLINE void FreeRegister(RegisterIndex reg, int virtual_register);
  V8_INLINE void MarkRegisterUse(RegisterIndex reg, MachineRepresentation rep,
                                 UsePosition pos);
  V8_INLINE RegisterBitVector InUseBitmap(UsePosition pos);
  V8_INLINE bool IsValidForRep(RegisterIndex reg, MachineRepresentation rep);

  // Return the register allocated to |virtual_register|, if any.
  RegisterIndex RegisterForVirtualRegister(int virtual_register);
  // Return the virtual register being held by |reg|, or kInvalidVirtualRegister
  // if |reg| is unallocated.
  int VirtualRegisterForRegister(RegisterIndex reg);

  // Returns true if |reg| is unallocated or holds |virtual_register|.
  bool IsFreeOrSameVirtualRegister(RegisterIndex reg, int virtual_register);
  // Returns true if |virtual_register| is unallocated or is allocated to |reg|.
  bool VirtualRegisterIsUnallocatedOrInReg(int virtual_register,
                                           RegisterIndex reg);

  // Returns a RegisterBitVector representing the allocated registers in
  // reg_state.
  RegisterBitVector GetAllocatedRegBitVector(RegisterState* reg_state);

  // Check the consistency of reg->vreg and vreg->reg mappings if a debug build.
  void CheckConsistency();

  bool HasRegisterState() const { return register_state_; }
  RegisterState* register_state() const {
    DCHECK(HasRegisterState());
    return register_state_;
  }

  VirtualRegisterData& VirtualRegisterDataFor(int virtual_register) const {
    return data()->VirtualRegisterDataFor(virtual_register);
  }

  MachineRepresentation RepresentationFor(int virtual_register) const {
    return data()->RepresentationFor(virtual_register);
  }

  int num_allocatable_registers() const { return num_allocatable_registers_; }
  const InstructionBlock* current_block() const { return current_block_; }
  MidTierRegisterAllocationData* data() const { return data_; }

  // Virtual register to register mapping.
  ZoneVector<RegisterIndex> virtual_register_to_reg_;

  // Current register state during allocation.
  RegisterState* register_state_;

  // The current block being processed.
  const InstructionBlock* current_block_;

  const RegisterKind kind_;
  const int num_allocatable_registers_;
  ZoneVector<RegisterIndex> reg_code_to_index_;
  const int* index_to_reg_code_;
  BitVector* assigned_registers_;

  MidTierRegisterAllocationData* data_;

  RegisterBitVector in_use_at_instr_start_bits_;
  RegisterBitVector in_use_at_instr_end_bits_;
  RegisterBitVector allocated_registers_bits_;

  // These fields are only used when kSimpleFPAliasing == false.
  base::Optional<ZoneVector<RegisterIndex>> float32_reg_code_to_index_;
  base::Optional<ZoneVector<int>> index_to_float32_reg_code_;
  base::Optional<ZoneVector<RegisterIndex>> simd128_reg_code_to_index_;
  base::Optional<ZoneVector<int>> index_to_simd128_reg_code_;
};

SinglePassRegisterAllocator::SinglePassRegisterAllocator(
    RegisterKind kind, MidTierRegisterAllocationData* data)
    : virtual_register_to_reg_(data->code()->VirtualRegisterCount(),
                               data->allocation_zone()),
      register_state_(nullptr),
      current_block_(nullptr),
      kind_(kind),
      num_allocatable_registers_(
          GetAllocatableRegisterCount(data->config(), kind)),
      reg_code_to_index_(GetRegisterCount(data->config(), kind),
                         data->allocation_zone()),
      index_to_reg_code_(GetAllocatableRegisterCodes(data->config(), kind)),
      assigned_registers_(data->code_zone()->New<BitVector>(
          GetRegisterCount(data->config(), kind), data->code_zone())),
      data_(data),
      in_use_at_instr_start_bits_(),
      in_use_at_instr_end_bits_(),
      allocated_registers_bits_() {
  for (int i = 0; i < num_allocatable_registers_; i++) {
    int reg_code = index_to_reg_code_[i];
    reg_code_to_index_[reg_code] = RegisterIndex(i);
  }

  // If the architecture has non-simple FP aliasing, initialize float and
  // simd128 specific register details.
  if (!kSimpleFPAliasing && kind == RegisterKind::kDouble) {
    const RegisterConfiguration* config = data->config();

    //  Float registers.
    float32_reg_code_to_index_.emplace(config->num_float_registers(),
                                       data->allocation_zone());
    index_to_float32_reg_code_.emplace(num_allocatable_registers_, -1,
                                       data->allocation_zone());
    for (int i = 0; i < config->num_allocatable_float_registers(); i++) {
      int reg_code = config->allocatable_float_codes()[i];
      // Only add even float register codes to avoid overlapping multiple float
      // registers on each RegisterIndex.
      if (reg_code % 2 != 0) continue;
      int double_reg_base_code;
      CHECK_EQ(1, config->GetAliases(MachineRepresentation::kFloat32, reg_code,
                                     MachineRepresentation::kFloat64,
                                     &double_reg_base_code));
      RegisterIndex double_reg(reg_code_to_index_[double_reg_base_code]);
      float32_reg_code_to_index_->at(reg_code) = double_reg;
      index_to_float32_reg_code_->at(double_reg.ToInt()) = reg_code;
    }

    //  Simd128 registers.
    simd128_reg_code_to_index_.emplace(config->num_simd128_registers(),
                                       data->allocation_zone());
    index_to_simd128_reg_code_.emplace(num_allocatable_registers_, -1,
                                       data->allocation_zone());
    for (int i = 0; i < config->num_allocatable_simd128_registers(); i++) {
      int reg_code = config->allocatable_simd128_codes()[i];
      int double_reg_base_code;
      CHECK_EQ(2, config->GetAliases(MachineRepresentation::kSimd128, reg_code,
                                     MachineRepresentation::kFloat64,
                                     &double_reg_base_code));
      RegisterIndex double_reg(reg_code_to_index_[double_reg_base_code]);
      simd128_reg_code_to_index_->at(reg_code) = double_reg;
      index_to_simd128_reg_code_->at(double_reg.ToInt()) = reg_code;
    }
  }
}

int SinglePassRegisterAllocator::VirtualRegisterForRegister(RegisterIndex reg) {
  return register_state()->VirtualRegisterForRegister(reg);
}

RegisterIndex SinglePassRegisterAllocator::RegisterForVirtualRegister(
    int virtual_register) {
  DCHECK_NE(virtual_register, InstructionOperand::kInvalidVirtualRegister);
  return virtual_register_to_reg_[virtual_register];
}

void SinglePassRegisterAllocator::UpdateForDeferredBlock(int instr_index) {
  if (!HasRegisterState()) return;
  for (RegisterIndex reg : *register_state()) {
    SpillRegisterForDeferred(reg, instr_index);
  }
}

void SinglePassRegisterAllocator::EndInstruction() {
  in_use_at_instr_end_bits_.Reset();
  in_use_at_instr_start_bits_.Reset();
}

void SinglePassRegisterAllocator::StartBlock(const InstructionBlock* block) {
  DCHECK(!HasRegisterState());
  DCHECK_NULL(current_block_);
  DCHECK(in_use_at_instr_start_bits_.IsEmpty());
  DCHECK(in_use_at_instr_end_bits_.IsEmpty());
  DCHECK(allocated_registers_bits_.IsEmpty());

  // Update the current block we are processing.
  current_block_ = block;

  if (block->SuccessorCount() == 1) {
    // If we have only a single successor, we can directly clone our state
    // from that successor.
    CloneStateFrom(block->successors()[0]);
  } else if (block->SuccessorCount() > 1) {
    // If we have multiple successors, merge the state from all the successors
    // into our block.
    MergeStateFrom(block->successors());
  }
}

void SinglePassRegisterAllocator::EndBlock(const InstructionBlock* block) {
  DCHECK(in_use_at_instr_start_bits_.IsEmpty());
  DCHECK(in_use_at_instr_end_bits_.IsEmpty());

  // If we didn't allocate any registers of this kind, or we have reached the
  // start, nothing to do here.
  if (!HasRegisterState() || block->PredecessorCount() == 0) {
    current_block_ = nullptr;
    return;
  }

  if (block->PredecessorCount() > 1) {
    register_state()->AddSharedUses(
        static_cast<int>(block->PredecessorCount()) - 1);
  }

  BlockState& block_state = data()->block_state(block->rpo_number());
  block_state.set_register_in_state(register_state(), kind());

  // Remove virtual register to register mappings and clear register state.
  // We will update the register state when starting the next block.
  while (!allocated_registers_bits_.IsEmpty()) {
    RegisterIndex reg = allocated_registers_bits_.GetFirstSet();
    FreeRegister(reg, VirtualRegisterForRegister(reg));
  }
  current_block_ = nullptr;
  register_state_ = nullptr;
}

void SinglePassRegisterAllocator::CloneStateFrom(RpoNumber successor) {
  BlockState& block_state = data()->block_state(successor);
  RegisterState* successor_registers = block_state.register_in_state(kind());
  if (successor_registers != nullptr) {
    if (data()->GetBlock(successor)->PredecessorCount() == 1) {
      // Avoids cloning for successors where we are the only predecessor.
      register_state_ = successor_registers;
    } else {
      register_state_ = successor_registers->Clone();
    }
    UpdateVirtualRegisterState();
  }
}

void SinglePassRegisterAllocator::MergeStateFrom(
    const InstructionBlock::Successors& successors) {
  for (RpoNumber successor : successors) {
    BlockState& block_state = data()->block_state(successor);
    RegisterState* successor_registers = block_state.register_in_state(kind());
    if (successor_registers == nullptr) {
      continue;
    }

    if (register_state_ == nullptr) {
      // If we haven't merged any register state yet, just use successor's
      // register directly.
      register_state_ = successor_registers;
      UpdateVirtualRegisterState();
    } else {
      // Otherwise try to merge our state with the existing state.
      RegisterBitVector processed_regs;
      RegisterBitVector succ_allocated_regs =
          GetAllocatedRegBitVector(successor_registers);
      for (RegisterIndex reg : *successor_registers) {
        // If |reg| isn't allocated in successor registers, nothing to do.
        if (!successor_registers->IsAllocated(reg)) continue;

        int virtual_register =
            successor_registers->VirtualRegisterForRegister(reg);
        MachineRepresentation rep = RepresentationFor(virtual_register);

        // If we have already processed |reg|, e.g., adding gap move to that
        // register, then we can continue.
        if (processed_regs.Contains(reg, rep)) continue;
        processed_regs.Add(reg, rep);

        if (register_state()->IsAllocated(reg)) {
          if (successor_registers->Equals(reg, register_state())) {
            // Both match, keep the merged register data.
            register_state()->CommitAtMerge(reg);
          } else {
            // Try to find a new register for this successor register in the
            // merge block, and add a gap move on entry of the successor block.
            RegisterIndex new_reg =
                RegisterForVirtualRegister(virtual_register);
            if (!new_reg.is_valid()) {
              new_reg = ChooseFreeRegister(
                  allocated_registers_bits_.Union(succ_allocated_regs), rep);
            } else if (new_reg != reg) {
              // Spill the |new_reg| in the successor block to be able to use it
              // for this gap move. It would be spilled anyway since it contains
              // a different virtual register than the merge block.
              SpillRegisterAtMerge(successor_registers, new_reg);
            }

            if (new_reg.is_valid()) {
              MoveRegisterOnMerge(new_reg, reg, virtual_register, successor,
                                  successor_registers);
              processed_regs.Add(new_reg, rep);
            } else {
              SpillRegisterAtMerge(successor_registers, reg);
            }
          }
        } else {
          DCHECK(successor_registers->IsAllocated(reg));
          if (RegisterForVirtualRegister(virtual_register).is_valid()) {
            // If we already hold the virtual register in a different register
            // then spill this register in the sucessor block to avoid
            // invalidating the 1:1 vreg<->reg mapping.
            // TODO(rmcilroy): Add a gap move to avoid spilling.
            SpillRegisterAtMerge(successor_registers, reg);
          } else {
            // Register is free in our current register state, so merge the
            // successor block's register details into it.
            register_state()->CopyFrom(reg, successor_registers);
            AssignRegister(reg, virtual_register, UsePosition::kNone);
          }
        }
      }
    }
  }
}

RegisterBitVector SinglePassRegisterAllocator::GetAllocatedRegBitVector(
    RegisterState* reg_state) {
  RegisterBitVector allocated_regs;
  for (RegisterIndex reg : *reg_state) {
    if (reg_state->IsAllocated(reg)) {
      int virtual_register = reg_state->VirtualRegisterForRegister(reg);
      allocated_regs.Add(reg, RepresentationFor(virtual_register));
    }
  }
  return allocated_regs;
}

void SinglePassRegisterAllocator::SpillRegisterAtMerge(RegisterState* reg_state,
                                                       RegisterIndex reg) {
  DCHECK_NE(reg_state, register_state());
  if (reg_state->IsAllocated(reg)) {
    int virtual_register = reg_state->VirtualRegisterForRegister(reg);
    AllocatedOperand allocated = AllocatedOperandForReg(reg, virtual_register);
    reg_state->Spill(reg, allocated, current_block(), data());
  }
}

void SinglePassRegisterAllocator::MoveRegisterOnMerge(
    RegisterIndex from, RegisterIndex to, int virtual_register,
    RpoNumber successor, RegisterState* succ_state) {
  int instr_index = data()->GetBlock(successor)->first_instruction_index();
  MoveOperands* move =
      data()->AddPendingOperandGapMove(instr_index, Instruction::START);
  succ_state->Commit(to, AllocatedOperandForReg(to, virtual_register),
                     &move->destination(), data());
  AllocatePendingUse(from, virtual_register, &move->source(), instr_index);
}

void SinglePassRegisterAllocator::UpdateVirtualRegisterState() {
  // Update to the new register state and update vreg_to_register map and
  // resetting any shared registers that were spilled by another block.
  DCHECK(HasRegisterState());
  for (RegisterIndex reg : *register_state()) {
    register_state()->ResetIfSpilledWhileShared(reg);
    int virtual_register = VirtualRegisterForRegister(reg);
    if (virtual_register != InstructionOperand::kInvalidVirtualRegister) {
      AssignRegister(reg, virtual_register, UsePosition::kNone);
    }
  }
  CheckConsistency();
}

void SinglePassRegisterAllocator::CheckConsistency() {
#ifdef DEBUG
  for (int virtual_register = 0;
       virtual_register < data()->code()->VirtualRegisterCount();
       virtual_register++) {
    RegisterIndex reg = RegisterForVirtualRegister(virtual_register);
    if (reg.is_valid()) {
      CHECK_EQ(virtual_register, VirtualRegisterForRegister(reg));
      CHECK(allocated_registers_bits_.Contains(
          reg, RepresentationFor(virtual_register)));
    }
  }

  for (RegisterIndex reg : *register_state()) {
    int virtual_register = VirtualRegisterForRegister(reg);
    if (virtual_register != InstructionOperand::kInvalidVirtualRegister) {
      CHECK_EQ(reg, RegisterForVirtualRegister(virtual_register));
      CHECK(allocated_registers_bits_.Contains(
          reg, RepresentationFor(virtual_register)));
    }
  }
#endif
}

RegisterIndex SinglePassRegisterAllocator::FromRegCode(
    int reg_code, MachineRepresentation rep) const {
  if (!kSimpleFPAliasing && kind() == RegisterKind::kDouble) {
    if (rep == MachineRepresentation::kFloat32) {
      return RegisterIndex(float32_reg_code_to_index_->at(reg_code));
    } else if (rep == MachineRepresentation::kSimd128) {
      return RegisterIndex(simd128_reg_code_to_index_->at(reg_code));
    }
    DCHECK_EQ(rep, MachineRepresentation::kFloat64);
  }

  return RegisterIndex(reg_code_to_index_[reg_code]);
}

int SinglePassRegisterAllocator::ToRegCode(RegisterIndex reg,
                                           MachineRepresentation rep) const {
  if (!kSimpleFPAliasing && kind() == RegisterKind::kDouble) {
    if (rep == MachineRepresentation::kFloat32) {
      DCHECK_NE(-1, index_to_float32_reg_code_->at(reg.ToInt()));
      return index_to_float32_reg_code_->at(reg.ToInt());
    } else if (rep == MachineRepresentation::kSimd128) {
      DCHECK_NE(-1, index_to_simd128_reg_code_->at(reg.ToInt()));
      return index_to_simd128_reg_code_->at(reg.ToInt());
    }
    DCHECK_EQ(rep, MachineRepresentation::kFloat64);
  }
  return index_to_reg_code_[reg.ToInt()];
}

bool SinglePassRegisterAllocator::VirtualRegisterIsUnallocatedOrInReg(
    int virtual_register, RegisterIndex reg) {
  RegisterIndex existing_reg = RegisterForVirtualRegister(virtual_register);
  return !existing_reg.is_valid() || existing_reg == reg;
}

bool SinglePassRegisterAllocator::IsFreeOrSameVirtualRegister(
    RegisterIndex reg, int virtual_register) {
  int allocated_vreg = VirtualRegisterForRegister(reg);
  return allocated_vreg == InstructionOperand::kInvalidVirtualRegister ||
         allocated_vreg == virtual_register;
}

void SinglePassRegisterAllocator::EmitGapMoveFromOutput(InstructionOperand from,
                                                        InstructionOperand to,
                                                        int instr_index) {
  DCHECK(from.IsAllocated());
  DCHECK(to.IsAllocated());
  const InstructionBlock* block = current_block();
  DCHECK_EQ(data()->GetBlock(instr_index), block);
  if (instr_index == block->last_instruction_index()) {
    // Add gap move to the first instruction of every successor block.
    for (const RpoNumber succ : block->successors()) {
      const InstructionBlock* successor = data()->GetBlock(succ);
      DCHECK_EQ(1, successor->PredecessorCount());
      data()->AddGapMove(successor->first_instruction_index(),
                         Instruction::START, from, to);
    }
  } else {
    data()->AddGapMove(instr_index + 1, Instruction::START, from, to);
  }
}

void SinglePassRegisterAllocator::AssignRegister(RegisterIndex reg,
                                                 int virtual_register,
                                                 UsePosition pos) {
  MachineRepresentation rep = RepresentationFor(virtual_register);
  assigned_registers()->Add(ToRegCode(reg, rep));
  allocated_registers_bits_.Add(reg, rep);
  MarkRegisterUse(reg, rep, pos);
  if (virtual_register != InstructionOperand::kInvalidVirtualRegister) {
    virtual_register_to_reg_[virtual_register] = reg;
  }
}

void SinglePassRegisterAllocator::MarkRegisterUse(RegisterIndex reg,
                                                  MachineRepresentation rep,
                                                  UsePosition pos) {
  if (pos == UsePosition::kStart || pos == UsePosition::kAll) {
    in_use_at_instr_start_bits_.Add(reg, rep);
  }
  if (pos == UsePosition::kEnd || pos == UsePosition::kAll) {
    in_use_at_instr_end_bits_.Add(reg, rep);
  }
}

void SinglePassRegisterAllocator::FreeRegister(RegisterIndex reg,
                                               int virtual_register) {
  allocated_registers_bits_.Clear(reg, RepresentationFor(virtual_register));
  if (virtual_register != InstructionOperand::kInvalidVirtualRegister) {
    virtual_register_to_reg_[virtual_register] = RegisterIndex::Invalid();
  }
}

RegisterIndex SinglePassRegisterAllocator::ChooseRegisterFor(
    VirtualRegisterData& virtual_register, int instr_index, UsePosition pos,
    bool must_use_register) {
  // If register is already allocated to the virtual register, use that.
  RegisterIndex reg = RegisterForVirtualRegister(virtual_register.vreg());

  // If we don't need a register, only try to allocate one if the virtual
  // register hasn't yet been spilled, to try to avoid spilling it.
  if (!reg.is_valid() && (must_use_register ||
                          !virtual_register.IsSpilledAt(instr_index, data()))) {
    reg = ChooseRegisterFor(RepresentationFor(virtual_register.vreg()), pos,
                            must_use_register);
  }
  return reg;
}

RegisterIndex SinglePassRegisterAllocator::ChooseRegisterFor(
    MachineRepresentation rep, UsePosition pos, bool must_use_register) {
  RegisterIndex reg = ChooseFreeRegister(rep, pos);
  if (!reg.is_valid() && must_use_register) {
    reg = ChooseRegisterToSpill(rep, pos);
    SpillRegister(reg);
  }
  return reg;
}

RegisterBitVector SinglePassRegisterAllocator::InUseBitmap(UsePosition pos) {
  switch (pos) {
    case UsePosition::kStart:
      return in_use_at_instr_start_bits_;
    case UsePosition::kEnd:
      return in_use_at_instr_end_bits_;
    case UsePosition::kAll:
      return in_use_at_instr_start_bits_.Union(in_use_at_instr_end_bits_);
    case UsePosition::kNone:
      UNREACHABLE();
  }
}

bool SinglePassRegisterAllocator::IsValidForRep(RegisterIndex reg,
                                                MachineRepresentation rep) {
  if (kSimpleFPAliasing || kind() == RegisterKind::kGeneral) {
    return true;
  } else {
    switch (rep) {
      case MachineRepresentation::kFloat32:
        return index_to_float32_reg_code_->at(reg.ToInt()) != -1;
      case MachineRepresentation::kFloat64:
        return true;
      case MachineRepresentation::kSimd128:
        return index_to_simd128_reg_code_->at(reg.ToInt()) != -1;
      default:
        UNREACHABLE();
    }
  }
}

RegisterIndex SinglePassRegisterAllocator::ChooseFreeRegister(
    MachineRepresentation rep, UsePosition pos) {
  // Take the first free, non-blocked register, if available.
  // TODO(rmcilroy): Consider a better heuristic.
  RegisterBitVector allocated_or_in_use =
      InUseBitmap(pos).Union(allocated_registers_bits_);
  return ChooseFreeRegister(allocated_or_in_use, rep);
}

RegisterIndex SinglePassRegisterAllocator::ChooseFreeRegister(
    const RegisterBitVector& allocated_regs, MachineRepresentation rep) {
  RegisterIndex chosen_reg = RegisterIndex::Invalid();
  if (kSimpleFPAliasing || kind() == RegisterKind::kGeneral) {
    chosen_reg = allocated_regs.GetFirstCleared(num_allocatable_registers());
  } else {
    // If we don't have simple fp aliasing, we need to check each register
    // individually to get one with the required representation.
    for (RegisterIndex reg : *register_state()) {
      if (IsValidForRep(reg, rep) && !allocated_regs.Contains(reg, rep)) {
        chosen_reg = reg;
        break;
      }
    }
  }

  DCHECK_IMPLIES(chosen_reg.is_valid(), IsValidForRep(chosen_reg, rep));
  return chosen_reg;
}

RegisterIndex SinglePassRegisterAllocator::ChooseRegisterToSpill(
    MachineRepresentation rep, UsePosition pos) {
  RegisterBitVector in_use = InUseBitmap(pos);

  // Choose a register that will need to be spilled. Preferentially choose:
  //  - A register with only pending uses, to avoid having to add a gap move for
  //    a non-pending use.
  //  - A register holding a virtual register that has already been spilled, to
  //    avoid adding a new gap move to spill the virtual register when it is
  //    output.
  //  - Prefer the register holding the virtual register with the earliest
  //    definition point, since it is more likely to be spilled anyway.
  RegisterIndex chosen_reg;
  int earliest_definition = kMaxInt;
  bool pending_only_use = false;
  bool already_spilled = false;
  for (RegisterIndex reg : *register_state()) {
    // Skip if register is in use, or not valid for representation.
    if (!IsValidForRep(reg, rep) || in_use.Contains(reg, rep)) continue;

    VirtualRegisterData& vreg_data =
        VirtualRegisterDataFor(VirtualRegisterForRegister(reg));
    if ((!pending_only_use && register_state()->HasPendingUsesOnly(reg)) ||
        (!already_spilled && vreg_data.HasSpillOperand()) ||
        vreg_data.output_instr_index() < earliest_definition) {
      chosen_reg = reg;
      earliest_definition = vreg_data.output_instr_index();
      pending_only_use = register_state()->HasPendingUsesOnly(reg);
      already_spilled = vreg_data.HasSpillOperand();
    }
  }

  // There should always be an unblocked register available.
  DCHECK(chosen_reg.is_valid());
  DCHECK(IsValidForRep(chosen_reg, rep));
  return chosen_reg;
}

void SinglePassRegisterAllocator::CommitRegister(RegisterIndex reg,
                                                 int virtual_register,
                                                 InstructionOperand* operand,
                                                 UsePosition pos) {
  // Committing the output operation, and mark the register use in this
  // instruction, then mark it as free going forward.
  AllocatedOperand allocated = AllocatedOperandForReg(reg, virtual_register);
  register_state()->Commit(reg, allocated, operand, data());
  MarkRegisterUse(reg, RepresentationFor(virtual_register), pos);
  FreeRegister(reg, virtual_register);
  CheckConsistency();
}

void SinglePassRegisterAllocator::SpillRegister(RegisterIndex reg) {
  if (!register_state()->IsAllocated(reg)) return;

  // Spill the register and free register.
  int virtual_register = VirtualRegisterForRegister(reg);
  AllocatedOperand allocated = AllocatedOperandForReg(reg, virtual_register);
  register_state()->Spill(reg, allocated, current_block(), data());
  FreeRegister(reg, virtual_register);
}

void SinglePassRegisterAllocator::SpillAllRegisters() {
  if (!HasRegisterState()) return;

  for (RegisterIndex reg : *register_state()) {
    SpillRegister(reg);
  }
}

void SinglePassRegisterAllocator::SpillRegisterForVirtualRegister(
    int virtual_register) {
  DCHECK_NE(virtual_register, InstructionOperand::kInvalidVirtualRegister);
  RegisterIndex reg = RegisterForVirtualRegister(virtual_register);
  if (reg.is_valid()) {
    SpillRegister(reg);
  }
}

void SinglePassRegisterAllocator::SpillRegisterForDeferred(RegisterIndex reg,
                                                           int instr_index) {
  // Committing the output operation, and mark the register use in this
  // instruction, then mark it as free going forward.
  if (register_state()->IsAllocated(reg) && register_state()->IsShared(reg)) {
    int virtual_register = VirtualRegisterForRegister(reg);
    AllocatedOperand allocated = AllocatedOperandForReg(reg, virtual_register);
    register_state()->SpillForDeferred(reg, allocated, instr_index, data());
    FreeRegister(reg, virtual_register);
  }
  CheckConsistency();
}

void SinglePassRegisterAllocator::AllocateDeferredBlockSpillOutput(
    int instr_index, RpoNumber deferred_block, int virtual_register) {
  DCHECK(data()->GetBlock(deferred_block)->IsDeferred());
  VirtualRegisterData& vreg_data =
      data()->VirtualRegisterDataFor(virtual_register);
  if (!vreg_data.NeedsSpillAtOutput() &&
      !DefinedAfter(virtual_register, instr_index, UsePosition::kEnd)) {
    // If a register has been assigned to the virtual register, and the virtual
    // register still doesn't need to be spilled at it's output, and add a
    // pending move to output the virtual register to it's spill slot on entry
    // of the deferred block (to avoid spilling on in non-deferred code).
    // TODO(rmcilroy): Consider assigning a register even if the virtual
    // register isn't yet assigned - currently doing this regresses performance.
    RegisterIndex reg = RegisterForVirtualRegister(virtual_register);
    if (reg.is_valid()) {
      int deferred_block_start =
          data()->GetBlock(deferred_block)->first_instruction_index();
      register_state()->MoveToSpillSlotOnDeferred(reg, virtual_register,
                                                  deferred_block_start, data());
      return;
    } else {
      vreg_data.MarkAsNeedsSpillAtOutput();
    }
  }
}

AllocatedOperand SinglePassRegisterAllocator::AllocatedOperandForReg(
    RegisterIndex reg, int virtual_register) {
  MachineRepresentation rep = RepresentationFor(virtual_register);
  return AllocatedOperand(AllocatedOperand::REGISTER, rep, ToRegCode(reg, rep));
}

void SinglePassRegisterAllocator::AllocateUse(RegisterIndex reg,
                                              int virtual_register,
                                              InstructionOperand* operand,
                                              int instr_index,
                                              UsePosition pos) {
  DCHECK_NE(virtual_register, InstructionOperand::kInvalidVirtualRegister);
  DCHECK(IsFreeOrSameVirtualRegister(reg, virtual_register));

  AllocatedOperand allocated = AllocatedOperandForReg(reg, virtual_register);
  register_state()->Commit(reg, allocated, operand, data());
  register_state()->AllocateUse(reg, virtual_register, operand, instr_index,
                                data());
  AssignRegister(reg, virtual_register, pos);
  CheckConsistency();
}

void SinglePassRegisterAllocator::AllocatePendingUse(
    RegisterIndex reg, int virtual_register, InstructionOperand* operand,
    int instr_index) {
  DCHECK_NE(virtual_register, InstructionOperand::kInvalidVirtualRegister);
  DCHECK(IsFreeOrSameVirtualRegister(reg, virtual_register));

  register_state()->AllocatePendingUse(reg, virtual_register, operand,
                                       instr_index);
  // Since this is a pending use and the operand doesn't need to use a register,
  // allocate with UsePosition::kNone to avoid blocking it's use by other
  // operands in this instruction.
  AssignRegister(reg, virtual_register, UsePosition::kNone);
  CheckConsistency();
}

void SinglePassRegisterAllocator::AllocateUseWithMove(
    RegisterIndex reg, int virtual_register, UnallocatedOperand* operand,
    int instr_index, UsePosition pos) {
  AllocatedOperand to = AllocatedOperandForReg(reg, virtual_register);
  UnallocatedOperand from = UnallocatedOperand(
      UnallocatedOperand::REGISTER_OR_SLOT, virtual_register);
  data()->AddGapMove(instr_index, Instruction::END, from, to);
  InstructionOperand::ReplaceWith(operand, &to);
  MarkRegisterUse(reg, RepresentationFor(virtual_register), pos);
  CheckConsistency();
}

void SinglePassRegisterAllocator::AllocateInput(UnallocatedOperand* operand,
                                                int instr_index) {
  EnsureRegisterState();
  int virtual_register = operand->virtual_register();
  MachineRepresentation rep = RepresentationFor(virtual_register);
  VirtualRegisterData& vreg_data = VirtualRegisterDataFor(virtual_register);

  // Spill slot policy operands.
  if (operand->HasFixedSlotPolicy()) {
    // If the operand is from a fixed slot, allocate it to that fixed slot,
    // then add a gap move from an unconstrained copy of that input operand,
    // and spill the gap move's input operand.
    // TODO(rmcilroy): We could allocate a register for the gap move however
    // we would need to wait until we've done all the allocations for the
    // instruction since the allocation needs to reflect the state before
    // the instruction (at the gap move). For now spilling is fine since
    // fixed slot inputs are uncommon.
    UnallocatedOperand input_copy(UnallocatedOperand::REGISTER_OR_SLOT,
                                  virtual_register);
    AllocatedOperand allocated = AllocatedOperand(
        AllocatedOperand::STACK_SLOT, rep, operand->fixed_slot_index());
    InstructionOperand::ReplaceWith(operand, &allocated);
    MoveOperands* move_op =
        data()->AddGapMove(instr_index, Instruction::END, input_copy, *operand);
    vreg_data.SpillOperand(&move_op->source(), instr_index, data());
    return;
  } else if (operand->HasSlotPolicy()) {
    vreg_data.SpillOperand(operand, instr_index, data());
    return;
  }

  // Otherwise try to allocate a register for the operation.
  UsePosition pos =
      operand->IsUsedAtStart() ? UsePosition::kStart : UsePosition::kAll;
  if (operand->HasFixedRegisterPolicy() ||
      operand->HasFixedFPRegisterPolicy()) {
    // With a fixed register operand, we must use that register.
    RegisterIndex reg = FromRegCode(operand->fixed_register_index(), rep);
    if (!VirtualRegisterIsUnallocatedOrInReg(virtual_register, reg)) {
      // If the virtual register is already in a different register, then just
      // add a gap move from that register to the fixed register.
      AllocateUseWithMove(reg, virtual_register, operand, instr_index, pos);
    } else {
      // Otherwise allocate a use of the fixed register for |virtual_register|.
      AllocateUse(reg, virtual_register, operand, instr_index, pos);
    }
  } else {
    bool must_use_register = operand->HasRegisterPolicy() ||
                             (vreg_data.is_constant() &&
                              !operand->HasRegisterOrSlotOrConstantPolicy());
    RegisterIndex reg =
        ChooseRegisterFor(vreg_data, instr_index, pos, must_use_register);

    if (reg.is_valid()) {
      if (must_use_register) {
        AllocateUse(reg, virtual_register, operand, instr_index, pos);
      } else {
        AllocatePendingUse(reg, virtual_register, operand, instr_index);
      }
    } else {
      vreg_data.SpillOperand(operand, instr_index, data());
    }
  }
}

void SinglePassRegisterAllocator::AllocateGapMoveInput(
    UnallocatedOperand* operand, int instr_index) {
  EnsureRegisterState();
  int virtual_register = operand->virtual_register();
  VirtualRegisterData& vreg_data = VirtualRegisterDataFor(virtual_register);

  // Gap move inputs should be unconstrained.
  DCHECK(operand->HasRegisterOrSlotPolicy());
  RegisterIndex reg =
      ChooseRegisterFor(vreg_data, instr_index, UsePosition::kStart, false);
  if (reg.is_valid()) {
    AllocatePendingUse(reg, virtual_register, operand, instr_index);
  } else {
    vreg_data.SpillOperand(operand, instr_index, data());
  }
}

void SinglePassRegisterAllocator::AllocateConstantOutput(
    ConstantOperand* operand) {
  EnsureRegisterState();
  // If the constant is allocated to a register, spill it now to add the
  // necessary gap moves from the constant operand to the register.
  int virtual_register = operand->virtual_register();
  SpillRegisterForVirtualRegister(virtual_register);
}

void SinglePassRegisterAllocator::AllocateOutput(UnallocatedOperand* operand,
                                                 int instr_index) {
  AllocateOutput(operand, instr_index, UsePosition::kEnd);
}

RegisterIndex SinglePassRegisterAllocator::AllocateOutput(
    UnallocatedOperand* operand, int instr_index, UsePosition pos) {
  EnsureRegisterState();
  int virtual_register = operand->virtual_register();
  VirtualRegisterData& vreg_data = VirtualRegisterDataFor(virtual_register);

  RegisterIndex reg;
  if (operand->HasSlotPolicy() || operand->HasFixedSlotPolicy()) {
    // We can't allocate a register for output given the policy, so make sure
    // to spill the register holding this virtual register if any.
    SpillRegisterForVirtualRegister(virtual_register);
    reg = RegisterIndex::Invalid();
  } else if (operand->HasFixedPolicy()) {
    reg = FromRegCode(operand->fixed_register_index(),
                      RepresentationFor(virtual_register));
  } else {
    reg = ChooseRegisterFor(vreg_data, instr_index, pos,
                            operand->HasRegisterPolicy());
  }

  // TODO(rmcilroy): support secondary storage.
  if (!reg.is_valid()) {
    vreg_data.SpillOperand(operand, instr_index, data());
  } else {
    InstructionOperand move_output_to;
    if (!VirtualRegisterIsUnallocatedOrInReg(virtual_register, reg)) {
      // If the |virtual register| was in a different register (e.g., due to
      // the output having a fixed register), then commit its use in that
      // register here, and move it from the output operand below.
      RegisterIndex existing_reg = RegisterForVirtualRegister(virtual_register);
      // Don't mark |existing_reg| as used in this instruction, since it is used
      // in the (already allocated) following instruction's gap-move.
      CommitRegister(existing_reg, virtual_register, &move_output_to,
                     UsePosition::kNone);
    }
    CommitRegister(reg, virtual_register, operand, pos);
    if (move_output_to.IsAllocated()) {
      // Emit a move from output to the register that the |virtual_register| was
      // allocated to.
      EmitGapMoveFromOutput(*operand, move_output_to, instr_index);
    }
    if (vreg_data.NeedsSpillAtOutput()) {
      vreg_data.EmitGapMoveFromOutputToSpillSlot(
          *AllocatedOperand::cast(operand), current_block(), instr_index,
          data());
    } else if (vreg_data.NeedsSpillAtDeferredBlocks()) {
      vreg_data.EmitDeferredSpillOutputs(data());
    }
  }

  return reg;
}

void SinglePassRegisterAllocator::AllocateSameInputOutput(
    UnallocatedOperand* output, UnallocatedOperand* input, int instr_index) {
  EnsureRegisterState();
  int input_vreg = input->virtual_register();
  int output_vreg = output->virtual_register();

  // The input operand has the details of the register constraints, so replace
  // the output operand with a copy of the input, with the output's vreg.
  UnallocatedOperand output_as_input(*input, output_vreg);
  InstructionOperand::ReplaceWith(output, &output_as_input);
  RegisterIndex reg = AllocateOutput(output, instr_index, UsePosition::kAll);

  if (reg.is_valid()) {
    // Replace the input operand with an unallocated fixed register policy for
    // the same register.
    UnallocatedOperand::ExtendedPolicy policy =
        kind() == RegisterKind::kGeneral
            ? UnallocatedOperand::FIXED_REGISTER
            : UnallocatedOperand::FIXED_FP_REGISTER;
    MachineRepresentation rep = RepresentationFor(input_vreg);
    UnallocatedOperand fixed_input(policy, ToRegCode(reg, rep), input_vreg);
    InstructionOperand::ReplaceWith(input, &fixed_input);
  } else {
    // Output was spilled. Due to the SameAsInput allocation policy, we need to
    // make the input operand the same as the output, i.e., the output virtual
    // register's spill slot. As such, spill this input operand using the output
    // virtual register's spill slot, then add a gap-move to move the input
    // value into this spill slot.
    VirtualRegisterData& output_vreg_data = VirtualRegisterDataFor(output_vreg);
    output_vreg_data.SpillOperand(input, instr_index, data());

    // Add an unconstrained gap move for the input virtual register.
    UnallocatedOperand unconstrained_input(UnallocatedOperand::REGISTER_OR_SLOT,
                                           input_vreg);
    MoveOperands* move_ops = data()->AddGapMove(
        instr_index, Instruction::END, unconstrained_input, PendingOperand());
    output_vreg_data.SpillOperand(&move_ops->destination(), instr_index,
                                  data());
  }
}

void SinglePassRegisterAllocator::AllocateTemp(UnallocatedOperand* operand,
                                               int instr_index) {
  EnsureRegisterState();
  int virtual_register = operand->virtual_register();
  RegisterIndex reg;
  DCHECK(!operand->HasFixedSlotPolicy());
  if (operand->HasSlotPolicy()) {
    reg = RegisterIndex::Invalid();
  } else if (operand->HasFixedRegisterPolicy() ||
             operand->HasFixedFPRegisterPolicy()) {
    reg = FromRegCode(operand->fixed_register_index(),
                      RepresentationFor(virtual_register));
  } else {
    reg = ChooseRegisterFor(RepresentationFor(virtual_register),
                            UsePosition::kAll, operand->HasRegisterPolicy());
  }

  if (reg.is_valid()) {
    DCHECK(virtual_register == InstructionOperand::kInvalidVirtualRegister ||
           VirtualRegisterIsUnallocatedOrInReg(virtual_register, reg));
    CommitRegister(reg, virtual_register, operand, UsePosition::kAll);
  } else {
    VirtualRegisterData& vreg_data = VirtualRegisterDataFor(virtual_register);
    vreg_data.SpillOperand(operand, instr_index, data());
  }
}

bool SinglePassRegisterAllocator::DefinedAfter(int virtual_register,
                                               int instr_index,
                                               UsePosition pos) {
  if (virtual_register == InstructionOperand::kInvalidVirtualRegister)
    return false;
  int defined_at =
      VirtualRegisterDataFor(virtual_register).output_instr_index();
  return defined_at > instr_index ||
         (defined_at == instr_index && pos == UsePosition::kStart);
}

void SinglePassRegisterAllocator::ReserveFixedInputRegister(
    const UnallocatedOperand* operand, int instr_index) {
  ReserveFixedRegister(
      operand, instr_index,
      operand->IsUsedAtStart() ? UsePosition::kStart : UsePosition::kAll);
}

void SinglePassRegisterAllocator::ReserveFixedTempRegister(
    const UnallocatedOperand* operand, int instr_index) {
  ReserveFixedRegister(operand, instr_index, UsePosition::kAll);
}

void SinglePassRegisterAllocator::ReserveFixedOutputRegister(
    const UnallocatedOperand* operand, int instr_index) {
  ReserveFixedRegister(operand, instr_index, UsePosition::kEnd);
}

void SinglePassRegisterAllocator::ReserveFixedRegister(
    const UnallocatedOperand* operand, int instr_index, UsePosition pos) {
  EnsureRegisterState();
  int virtual_register = operand->virtual_register();
  MachineRepresentation rep = RepresentationFor(virtual_register);
  RegisterIndex reg = FromRegCode(operand->fixed_register_index(), rep);
  if (!IsFreeOrSameVirtualRegister(reg, virtual_register) &&
      !DefinedAfter(virtual_register, instr_index, pos)) {
    // If register is in-use by a different virtual register, spill it now.
    // TODO(rmcilroy): Consider moving to a unconstrained register instead of
    // spilling.
    SpillRegister(reg);
  }
  MarkRegisterUse(reg, rep, pos);
}

void SinglePassRegisterAllocator::AllocatePhiGapMove(int to_vreg, int from_vreg,
                                                     int instr_index) {
  EnsureRegisterState();
  RegisterIndex from_register = RegisterForVirtualRegister(from_vreg);
  RegisterIndex to_register = RegisterForVirtualRegister(to_vreg);

  // If to_register isn't marked as a phi gap move, we can't use it as such.
  if (to_register.is_valid() && !register_state()->IsPhiGapMove(to_register)) {
    to_register = RegisterIndex::Invalid();
  }

  if (to_register.is_valid() && !from_register.is_valid()) {
    // If |to| virtual register is allocated to a register, and the |from|
    // virtual register isn't allocated, then commit this register and
    // re-allocate it to the |from| virtual register.
    InstructionOperand operand;
    CommitRegister(to_register, to_vreg, &operand, UsePosition::kAll);
    AllocateUse(to_register, from_vreg, &operand, instr_index,
                UsePosition::kAll);
  } else {
    // Otherwise add a gap move.
    MoveOperands* move =
        data()->AddPendingOperandGapMove(instr_index, Instruction::END);
    PendingOperand* to_operand = PendingOperand::cast(&move->destination());
    PendingOperand* from_operand = PendingOperand::cast(&move->source());

    // Commit the |to| side to either a register or the pending spills.
    if (to_register.is_valid()) {
      CommitRegister(to_register, to_vreg, to_operand, UsePosition::kAll);
    } else {
      VirtualRegisterDataFor(to_vreg).SpillOperand(to_operand, instr_index,
                                                   data());
    }

    // The from side is unconstrained.
    UnallocatedOperand unconstrained_input(UnallocatedOperand::REGISTER_OR_SLOT,
                                           from_vreg);
    InstructionOperand::ReplaceWith(from_operand, &unconstrained_input);
  }
}

void SinglePassRegisterAllocator::AllocatePhi(int virtual_register,
                                              const InstructionBlock* block) {
  VirtualRegisterData& vreg_data = VirtualRegisterDataFor(virtual_register);
  if (vreg_data.NeedsSpillAtOutput() || block->IsLoopHeader()) {
    // If the Phi needs to be spilled, just spill here directly so that all
    // gap moves into the Phi move into the spill slot.
    SpillRegisterForVirtualRegister(virtual_register);
  } else {
    RegisterIndex reg = RegisterForVirtualRegister(virtual_register);
    if (reg.is_valid()) {
      // If the register is valid, assign it as a phi gap move to be processed
      // at the successor blocks. If no register or spill slot was used then
      // the virtual register was never used.
      register_state()->UseForPhiGapMove(reg);
    }
  }
}

void SinglePassRegisterAllocator::EnsureRegisterState() {
  if (!HasRegisterState()) {
    register_state_ = RegisterState::New(kind(), num_allocatable_registers_,
                                         data()->allocation_zone());
  }
}

class MidTierOutputProcessor final {
 public:
  explicit MidTierOutputProcessor(MidTierRegisterAllocationData* data);

  void InitializeBlockState(const InstructionBlock* block);
  void DefineOutputs(const InstructionBlock* block);

 private:
  void PopulateDeferredBlockRegion(RpoNumber initial_block);

  VirtualRegisterData& VirtualRegisterDataFor(int virtual_register) const {
    return data()->VirtualRegisterDataFor(virtual_register);
  }
  MachineRepresentation RepresentationFor(int virtual_register) const {
    return data()->RepresentationFor(virtual_register);
  }

  bool IsDeferredBlockBoundary(const ZoneVector<RpoNumber>& blocks) {
    return blocks.size() == 1 && !data()->GetBlock(blocks[0])->IsDeferred();
  }

  MidTierRegisterAllocationData* data() const { return data_; }
  InstructionSequence* code() const { return data()->code(); }
  Zone* zone() const { return data()->allocation_zone(); }

  MidTierRegisterAllocationData* const data_;
  ZoneQueue<RpoNumber> deferred_blocks_worklist_;
  ZoneSet<RpoNumber> deferred_blocks_processed_;
};

MidTierOutputProcessor::MidTierOutputProcessor(
    MidTierRegisterAllocationData* data)
    : data_(data),
      deferred_blocks_worklist_(data->allocation_zone()),
      deferred_blocks_processed_(data->allocation_zone()) {}

void MidTierOutputProcessor::PopulateDeferredBlockRegion(
    RpoNumber initial_block) {
  DeferredBlocksRegion* deferred_blocks_region =
      zone()->New<DeferredBlocksRegion>(zone(),
                                        code()->InstructionBlockCount());
  DCHECK(deferred_blocks_worklist_.empty());
  deferred_blocks_worklist_.push(initial_block);
  deferred_blocks_processed_.insert(initial_block);
  while (!deferred_blocks_worklist_.empty()) {
    RpoNumber current = deferred_blocks_worklist_.front();
    deferred_blocks_worklist_.pop();
    deferred_blocks_region->AddBlock(current, data());

    const InstructionBlock* curr_block = data()->GetBlock(current);
    // Check for whether the predecessor blocks are still deferred.
    if (IsDeferredBlockBoundary(curr_block->predecessors())) {
      // If not, mark the predecessor as having a deferred successor.
      data()
          ->block_state(curr_block->predecessors()[0])
          .MarkAsDeferredBlockBoundary();
    } else {
      // Otherwise process predecessors.
      for (RpoNumber pred : curr_block->predecessors()) {
        if (deferred_blocks_processed_.count(pred) == 0) {
          deferred_blocks_worklist_.push(pred);
          deferred_blocks_processed_.insert(pred);
        }
      }
    }

    // Check for whether the successor blocks are still deferred.
    // Process any unprocessed successors if we aren't at a boundary.
    if (IsDeferredBlockBoundary(curr_block->successors())) {
      // If not, mark the predecessor as having a deferred successor.
      data()->block_state(current).MarkAsDeferredBlockBoundary();
    } else {
      // Otherwise process successors.
      for (RpoNumber succ : curr_block->successors()) {
        if (deferred_blocks_processed_.count(succ) == 0) {
          deferred_blocks_worklist_.push(succ);
          deferred_blocks_processed_.insert(succ);
        }
      }
    }
  }
}

void MidTierOutputProcessor::InitializeBlockState(
    const InstructionBlock* block) {
  // Update our predecessor blocks with their successors_phi_index if we have
  // phis.
  if (block->phis().size()) {
    for (int i = 0; i < static_cast<int>(block->PredecessorCount()); ++i) {
      data()->block_state(block->predecessors()[i]).set_successors_phi_index(i);
    }
  }

  BlockState& block_state = data()->block_state(block->rpo_number());

  if (block->IsDeferred() && !block_state.deferred_blocks_region()) {
    PopulateDeferredBlockRegion(block->rpo_number());
  }

  // Mark this block as dominating itself.
  block_state.dominated_blocks()->Add(block->rpo_number().ToInt());

  if (block->dominator().IsValid()) {
    // Add all the blocks this block dominates to its dominator.
    BlockState& dominator_block_state = data()->block_state(block->dominator());
    dominator_block_state.dominated_blocks()->Union(
        *block_state.dominated_blocks());
  } else {
    // Only the first block shouldn't have a dominator.
    DCHECK_EQ(block, code()->instruction_blocks().front());
  }
}

void MidTierOutputProcessor::DefineOutputs(const InstructionBlock* block) {
  int block_start = block->first_instruction_index();
  bool is_deferred = block->IsDeferred();

  for (int index = block->last_instruction_index(); index >= block_start;
       index--) {
    Instruction* instr = code()->InstructionAt(index);

    // For each instruction, define details of the output with the associated
    // virtual register data.
    for (size_t i = 0; i < instr->OutputCount(); i++) {
      InstructionOperand* output = instr->OutputAt(i);
      if (output->IsConstant()) {
        ConstantOperand* constant_operand = ConstantOperand::cast(output);
        int virtual_register = constant_operand->virtual_register();
        VirtualRegisterDataFor(virtual_register)
            .DefineAsConstantOperand(constant_operand, index, is_deferred);
      } else {
        DCHECK(output->IsUnallocated());
        UnallocatedOperand* unallocated_operand =
            UnallocatedOperand::cast(output);
        int virtual_register = unallocated_operand->virtual_register();
        bool is_exceptional_call_output =
            instr->IsCallWithDescriptorFlags() &&
            instr->HasCallDescriptorFlag(CallDescriptor::kHasExceptionHandler);
        if (unallocated_operand->HasFixedSlotPolicy()) {
          // If output has a fixed slot policy, allocate its spill operand now
          // so that the register allocator can use this knowledge.
          MachineRepresentation rep = RepresentationFor(virtual_register);
          AllocatedOperand* fixed_spill_operand =
              AllocatedOperand::New(zone(), AllocatedOperand::STACK_SLOT, rep,
                                    unallocated_operand->fixed_slot_index());
          VirtualRegisterDataFor(virtual_register)
              .DefineAsFixedSpillOperand(fixed_spill_operand, virtual_register,
                                         index, is_deferred,
                                         is_exceptional_call_output);
        } else {
          VirtualRegisterDataFor(virtual_register)
              .DefineAsUnallocatedOperand(virtual_register, index, is_deferred,
                                          is_exceptional_call_output);
        }
      }
    }

    // Mark any instructions that require reference maps for later reference map
    // processing.
    if (instr->HasReferenceMap()) {
      data()->reference_map_instructions().push_back(index);
    }
  }

  // Define phi output operands.
  for (PhiInstruction* phi : block->phis()) {
    int virtual_register = phi->virtual_register();
    VirtualRegisterDataFor(virtual_register)
        .DefineAsPhi(virtual_register, block->first_instruction_index(),
                     is_deferred);
  }
}

void DefineOutputs(MidTierRegisterAllocationData* data) {
  MidTierOutputProcessor processor(data);

  for (const InstructionBlock* block :
       base::Reversed(data->code()->instruction_blocks())) {
    data->tick_counter()->TickAndMaybeEnterSafepoint();

    processor.InitializeBlockState(block);
    processor.DefineOutputs(block);
  }
}

class MidTierRegisterAllocator final {
 public:
  explicit MidTierRegisterAllocator(MidTierRegisterAllocationData* data);
  MidTierRegisterAllocator(const MidTierRegisterAllocator&) = delete;
  MidTierRegisterAllocator& operator=(const MidTierRegisterAllocator&) = delete;

  void AllocateRegisters(const InstructionBlock* block);
  void UpdateSpillRangesForLoops();

  SinglePassRegisterAllocator& general_reg_allocator() {
    return general_reg_allocator_;
  }
  SinglePassRegisterAllocator& double_reg_allocator() {
    return double_reg_allocator_;
  }

 private:
  void AllocatePhis(const InstructionBlock* block);
  void AllocatePhiGapMoves(const InstructionBlock* block);

  bool IsFixedRegisterPolicy(const UnallocatedOperand* operand);
  void ReserveFixedRegisters(int instr_index);

  SinglePassRegisterAllocator& AllocatorFor(MachineRepresentation rep);
  SinglePassRegisterAllocator& AllocatorFor(const UnallocatedOperand* operand);
  SinglePassRegisterAllocator& AllocatorFor(const ConstantOperand* operand);

  VirtualRegisterData& VirtualRegisterDataFor(int virtual_register) const {
    return data()->VirtualRegisterDataFor(virtual_register);
  }
  MachineRepresentation RepresentationFor(int virtual_register) const {
    return data()->RepresentationFor(virtual_register);
  }
  MidTierRegisterAllocationData* data() const { return data_; }
  InstructionSequence* code() const { return data()->code(); }
  Zone* allocation_zone() const { return data()->allocation_zone(); }

  MidTierRegisterAllocationData* const data_;
  SinglePassRegisterAllocator general_reg_allocator_;
  SinglePassRegisterAllocator double_reg_allocator_;
};

MidTierRegisterAllocator::MidTierRegisterAllocator(
    MidTierRegisterAllocationData* data)
    : data_(data),
      general_reg_allocator_(RegisterKind::kGeneral, data),
      double_reg_allocator_(RegisterKind::kDouble, data) {}

void MidTierRegisterAllocator::AllocateRegisters(
    const InstructionBlock* block) {
  RpoNumber block_rpo = block->rpo_number();
  bool is_deferred_block_boundary =
      data()->block_state(block_rpo).is_deferred_block_boundary();

  general_reg_allocator().StartBlock(block);
  double_reg_allocator().StartBlock(block);

  // If the block is not deferred but has deferred successors, then try to
  // output spill slots for virtual_registers that are only spilled in the
  // deferred blocks at the start of those deferred blocks to avoid spilling
  // them at their output in non-deferred blocks.
  if (is_deferred_block_boundary && !block->IsDeferred()) {
    for (RpoNumber successor : block->successors()) {
      if (!data()->GetBlock(successor)->IsDeferred()) continue;
      DCHECK_GT(successor, block_rpo);
      for (int virtual_register :
           *data()->block_state(successor).deferred_blocks_region()) {
        USE(virtual_register);
        AllocatorFor(RepresentationFor(virtual_register))
            .AllocateDeferredBlockSpillOutput(block->last_instruction_index(),
                                              successor, virtual_register);
      }
    }
  }

  // Allocate registers for instructions in reverse, from the end of the block
  // to the start.
  int block_start = block->first_instruction_index();
  for (int instr_index = block->last_instruction_index();
       instr_index >= block_start; instr_index--) {
    Instruction* instr = code()->InstructionAt(instr_index);

    // Reserve any fixed register operands to prevent the register being
    // allocated to another operand.
    ReserveFixedRegisters(instr_index);

    // Allocate outputs.
    for (size_t i = 0; i < instr->OutputCount(); i++) {
      InstructionOperand* output = instr->OutputAt(i);
      DCHECK(!output->IsAllocated());
      if (output->IsConstant()) {
        ConstantOperand* constant_operand = ConstantOperand::cast(output);
        AllocatorFor(constant_operand).AllocateConstantOutput(constant_operand);
      } else {
        UnallocatedOperand* unallocated_output =
            UnallocatedOperand::cast(output);
        if (unallocated_output->HasSameAsInputPolicy()) {
          DCHECK_EQ(i, 0);
          UnallocatedOperand* unallocated_input =
              UnallocatedOperand::cast(instr->InputAt(0));
          DCHECK_EQ(AllocatorFor(unallocated_input).kind(),
                    AllocatorFor(unallocated_output).kind());
          AllocatorFor(unallocated_output)
              .AllocateSameInputOutput(unallocated_output, unallocated_input,
                                       instr_index);
        } else {
          AllocatorFor(unallocated_output)
              .AllocateOutput(unallocated_output, instr_index);
        }
      }
    }

    if (instr->ClobbersRegisters()) {
      general_reg_allocator().SpillAllRegisters();
    }
    if (instr->ClobbersDoubleRegisters()) {
      double_reg_allocator().SpillAllRegisters();
    }

    // Allocate temporaries.
    for (size_t i = 0; i < instr->TempCount(); i++) {
      UnallocatedOperand* temp = UnallocatedOperand::cast(instr->TempAt(i));
      AllocatorFor(temp).AllocateTemp(temp, instr_index);
    }

    // Allocate inputs that are used across the whole instruction.
    for (size_t i = 0; i < instr->InputCount(); i++) {
      if (!instr->InputAt(i)->IsUnallocated()) continue;
      UnallocatedOperand* input = UnallocatedOperand::cast(instr->InputAt(i));
      if (input->IsUsedAtStart()) continue;
      AllocatorFor(input).AllocateInput(input, instr_index);
    }

    // Then allocate inputs that are only used at the start of the instruction.
    for (size_t i = 0; i < instr->InputCount(); i++) {
      if (!instr->InputAt(i)->IsUnallocated()) continue;
      UnallocatedOperand* input = UnallocatedOperand::cast(instr->InputAt(i));
      DCHECK(input->IsUsedAtStart());
      AllocatorFor(input).AllocateInput(input, instr_index);
    }

    // If we are allocating for the last instruction in the block, allocate any
    // phi gap move operations that are needed to resolve phis in our successor.
    if (instr_index == block->last_instruction_index()) {
      AllocatePhiGapMoves(block);

      // If this block is deferred but it's successor isn't, update the state to
      // limit spills to the deferred blocks where possible.
      if (is_deferred_block_boundary && block->IsDeferred()) {
        general_reg_allocator().UpdateForDeferredBlock(instr_index);
        double_reg_allocator().UpdateForDeferredBlock(instr_index);
      }
    }

    // Allocate any unallocated gap move inputs.
    ParallelMove* moves = instr->GetParallelMove(Instruction::END);
    if (moves != nullptr) {
      for (MoveOperands* move : *moves) {
        DCHECK(!move->destination().IsUnallocated());
        if (move->source().IsUnallocated()) {
          UnallocatedOperand* source =
              UnallocatedOperand::cast(&move->source());
          AllocatorFor(source).AllocateGapMoveInput(source, instr_index);
        }
      }
    }

    general_reg_allocator().EndInstruction();
    double_reg_allocator().EndInstruction();
  }

  // For now we spill all registers at a loop header.
  // TODO(rmcilroy): Add support for register allocations across loops.
  if (block->IsLoopHeader()) {
    general_reg_allocator().SpillAllRegisters();
    double_reg_allocator().SpillAllRegisters();
  }

  AllocatePhis(block);

  general_reg_allocator().EndBlock(block);
  double_reg_allocator().EndBlock(block);
}

SinglePassRegisterAllocator& MidTierRegisterAllocator::AllocatorFor(
    MachineRepresentation rep) {
  if (IsFloatingPoint(rep)) {
    return double_reg_allocator();
  } else {
    return general_reg_allocator();
  }
}

SinglePassRegisterAllocator& MidTierRegisterAllocator::AllocatorFor(
    const UnallocatedOperand* operand) {
  return AllocatorFor(RepresentationFor(operand->virtual_register()));
}

SinglePassRegisterAllocator& MidTierRegisterAllocator::AllocatorFor(
    const ConstantOperand* operand) {
  return AllocatorFor(RepresentationFor(operand->virtual_register()));
}

bool MidTierRegisterAllocator::IsFixedRegisterPolicy(
    const UnallocatedOperand* operand) {
  return operand->HasFixedRegisterPolicy() ||
         operand->HasFixedFPRegisterPolicy();
}

void MidTierRegisterAllocator::ReserveFixedRegisters(int instr_index) {
  Instruction* instr = code()->InstructionAt(instr_index);
  for (size_t i = 0; i < instr->OutputCount(); i++) {
    if (!instr->OutputAt(i)->IsUnallocated()) continue;
    const UnallocatedOperand* operand =
        UnallocatedOperand::cast(instr->OutputAt(i));
    if (operand->HasSameAsInputPolicy()) {
      // Input operand has the register constraints, use it here to reserve the
      // register for the output (it will be reserved for input below).
      operand = UnallocatedOperand::cast(instr->InputAt(i));
    }
    if (IsFixedRegisterPolicy(operand)) {
      AllocatorFor(operand).ReserveFixedOutputRegister(operand, instr_index);
    }
  }
  for (size_t i = 0; i < instr->TempCount(); i++) {
    if (!instr->TempAt(i)->IsUnallocated()) continue;
    const UnallocatedOperand* operand =
        UnallocatedOperand::cast(instr->TempAt(i));
    if (IsFixedRegisterPolicy(operand)) {
      AllocatorFor(operand).ReserveFixedTempRegister(operand, instr_index);
    }
  }
  for (size_t i = 0; i < instr->InputCount(); i++) {
    if (!instr->InputAt(i)->IsUnallocated()) continue;
    const UnallocatedOperand* operand =
        UnallocatedOperand::cast(instr->InputAt(i));
    if (IsFixedRegisterPolicy(operand)) {
      AllocatorFor(operand).ReserveFixedInputRegister(operand, instr_index);
    }
  }
}

void MidTierRegisterAllocator::AllocatePhiGapMoves(
    const InstructionBlock* block) {
  int successors_phi_index =
      data()->block_state(block->rpo_number()).successors_phi_index();

  // If successors_phi_index is -1 there are no phi's in the successor.
  if (successors_phi_index == -1) return;

  // The last instruction of a block with phis can't require reference maps
  // since we won't record phi gap moves that get spilled when populating the
  // reference maps
  int instr_index = block->last_instruction_index();
  DCHECK(!code()->InstructionAt(instr_index)->HasReferenceMap());

  // If there are phis, we only have a single successor due to edge-split form.
  DCHECK_EQ(block->SuccessorCount(), 1);
  const InstructionBlock* successor = data()->GetBlock(block->successors()[0]);

  for (PhiInstruction* phi : successor->phis()) {
    int to_vreg = phi->virtual_register();
    int from_vreg = phi->operands()[successors_phi_index];

    MachineRepresentation rep = RepresentationFor(to_vreg);
    AllocatorFor(rep).AllocatePhiGapMove(to_vreg, from_vreg, instr_index);
  }
}

void MidTierRegisterAllocator::AllocatePhis(const InstructionBlock* block) {
  for (PhiInstruction* phi : block->phis()) {
    int virtual_register = phi->virtual_register();
    MachineRepresentation rep = RepresentationFor(virtual_register);
    AllocatorFor(rep).AllocatePhi(virtual_register, block);
  }
}

void MidTierRegisterAllocator::UpdateSpillRangesForLoops() {
  // Extend the spill range of any spill that crosses a loop header to
  // the full loop.
  for (InstructionBlock* block : code()->instruction_blocks()) {
    if (block->IsLoopHeader()) {
      RpoNumber last_loop_block =
          RpoNumber::FromInt(block->loop_end().ToInt() - 1);
      int last_loop_instr =
          data()->GetBlock(last_loop_block)->last_instruction_index();
      // Extend spill range for all spilled values that are live on entry to the
      // loop header.
      BitVector::Iterator iterator(&data()->spilled_virtual_registers());
      for (; !iterator.Done(); iterator.Advance()) {
        const VirtualRegisterData& vreg_data =
            VirtualRegisterDataFor(iterator.Current());
        if (vreg_data.HasSpillRange() &&
            vreg_data.spill_range()->IsLiveAt(block->first_instruction_index(),
                                              block)) {
          vreg_data.spill_range()->ExtendRangeTo(last_loop_instr);
        }
      }
    }
  }
}

void AllocateRegisters(MidTierRegisterAllocationData* data) {
  MidTierRegisterAllocator allocator(data);
  for (InstructionBlock* block :
       base::Reversed(data->code()->instruction_blocks())) {
    data->tick_counter()->TickAndMaybeEnterSafepoint();
    allocator.AllocateRegisters(block);
  }

  allocator.UpdateSpillRangesForLoops();

  data->frame()->SetAllocatedRegisters(
      allocator.general_reg_allocator().assigned_registers());
  data->frame()->SetAllocatedDoubleRegisters(
      allocator.double_reg_allocator().assigned_registers());
}

// Spill slot allocator for mid-tier register allocation.
class MidTierSpillSlotAllocator final {
 public:
  explicit MidTierSpillSlotAllocator(MidTierRegisterAllocationData* data);
  MidTierSpillSlotAllocator(const MidTierSpillSlotAllocator&) = delete;
  MidTierSpillSlotAllocator& operator=(const MidTierSpillSlotAllocator&) =
      delete;

  void Allocate(VirtualRegisterData* virtual_register);

 private:
  class SpillSlot;

  void AdvanceTo(int instr_index);
  SpillSlot* GetFreeSpillSlot(int byte_width);

  MidTierRegisterAllocationData* data() const { return data_; }
  InstructionSequence* code() const { return data()->code(); }
  Frame* frame() const { return data()->frame(); }
  Zone* zone() const { return data()->allocation_zone(); }

  struct OrderByLastUse {
    bool operator()(const SpillSlot* a, const SpillSlot* b) const;
  };

  MidTierRegisterAllocationData* const data_;
  ZonePriorityQueue<SpillSlot*, OrderByLastUse> allocated_slots_;
  ZoneLinkedList<SpillSlot*> free_slots_;
  int position_;
};

class MidTierSpillSlotAllocator::SpillSlot : public ZoneObject {
 public:
  SpillSlot(int stack_slot, int byte_width)
      : stack_slot_(stack_slot), byte_width_(byte_width), range_() {}
  SpillSlot(const SpillSlot&) = delete;
  SpillSlot& operator=(const SpillSlot&) = delete;

  void AddRange(const Range& range) { range_.AddRange(range); }

  AllocatedOperand ToOperand(MachineRepresentation rep) const {
    return AllocatedOperand(AllocatedOperand::STACK_SLOT, rep, stack_slot_);
  }

  int byte_width() const { return byte_width_; }
  int last_use() const { return range_.end(); }

 private:
  int stack_slot_;
  int byte_width_;
  Range range_;
};

bool MidTierSpillSlotAllocator::OrderByLastUse::operator()(
    const SpillSlot* a, const SpillSlot* b) const {
  return a->last_use() > b->last_use();
}

MidTierSpillSlotAllocator::MidTierSpillSlotAllocator(
    MidTierRegisterAllocationData* data)
    : data_(data),
      allocated_slots_(data->allocation_zone()),
      free_slots_(data->allocation_zone()),
      position_(0) {}

void MidTierSpillSlotAllocator::AdvanceTo(int instr_index) {
  // Move any slots that are no longer in use to the free slots list.
  DCHECK_LE(position_, instr_index);
  while (!allocated_slots_.empty() &&
         instr_index > allocated_slots_.top()->last_use()) {
    free_slots_.push_front(allocated_slots_.top());
    allocated_slots_.pop();
  }
  position_ = instr_index;
}

MidTierSpillSlotAllocator::SpillSlot*
MidTierSpillSlotAllocator::GetFreeSpillSlot(int byte_width) {
  for (auto it = free_slots_.begin(); it != free_slots_.end(); ++it) {
    SpillSlot* slot = *it;
    if (slot->byte_width() == byte_width) {
      free_slots_.erase(it);
      return slot;
    }
  }
  return nullptr;
}

void MidTierSpillSlotAllocator::Allocate(
    VirtualRegisterData* virtual_register) {
  DCHECK(virtual_register->HasPendingSpillOperand());
  VirtualRegisterData::SpillRange* spill_range =
      virtual_register->spill_range();
  MachineRepresentation rep =
      data()->RepresentationFor(virtual_register->vreg());
  int byte_width = ByteWidthForStackSlot(rep);
  Range live_range = spill_range->live_range();

  AdvanceTo(live_range.start());

  // Try to re-use an existing free spill slot.
  SpillSlot* slot = GetFreeSpillSlot(byte_width);
  if (slot == nullptr) {
    // Otherwise allocate a new slot.
    int stack_slot_ = frame()->AllocateSpillSlot(byte_width);
    slot = zone()->New<SpillSlot>(stack_slot_, byte_width);
  }

  // Extend the range of the slot to include this spill range, and allocate the
  // pending spill operands with this slot.
  slot->AddRange(live_range);
  virtual_register->AllocatePendingSpillOperand(slot->ToOperand(rep));
  allocated_slots_.push(slot);
}

void AllocateSpillSlots(MidTierRegisterAllocationData* data) {
  ZoneVector<VirtualRegisterData*> spilled(data->allocation_zone());
  BitVector::Iterator iterator(&data->spilled_virtual_registers());
  for (; !iterator.Done(); iterator.Advance()) {
    VirtualRegisterData& vreg_data =
        data->VirtualRegisterDataFor(iterator.Current());
    if (vreg_data.HasPendingSpillOperand()) {
      spilled.push_back(&vreg_data);
    }
  }

  // Sort the spill ranges by order of their first use to enable linear
  // allocation of spill slots.
  std::sort(spilled.begin(), spilled.end(),
            [](const VirtualRegisterData* a, const VirtualRegisterData* b) {
              return a->spill_range()->live_range().start() <
                     b->spill_range()->live_range().start();
            });

  // Allocate a spill slot for each virtual register with a spill range.
  MidTierSpillSlotAllocator allocator(data);
  for (VirtualRegisterData* spill : spilled) {
    allocator.Allocate(spill);
  }
}

// Populates reference maps for mid-tier register allocation.
class MidTierReferenceMapPopulator final {
 public:
  explicit MidTierReferenceMapPopulator(MidTierRegisterAllocationData* data);
  MidTierReferenceMapPopulator(const MidTierReferenceMapPopulator&) = delete;
  MidTierReferenceMapPopulator& operator=(const MidTierReferenceMapPopulator&) =
      delete;

  void RecordReferences(const VirtualRegisterData& virtual_register);

 private:
  MidTierRegisterAllocationData* data() const { return data_; }
  InstructionSequence* code() const { return data()->code(); }

  MidTierRegisterAllocationData* const data_;
};

MidTierReferenceMapPopulator::MidTierReferenceMapPopulator(
    MidTierRegisterAllocationData* data)
    : data_(data) {}

void MidTierReferenceMapPopulator::RecordReferences(
    const VirtualRegisterData& virtual_register) {
  if (!virtual_register.HasAllocatedSpillOperand()) return;
  if (!code()->IsReference(virtual_register.vreg())) return;

  VirtualRegisterData::SpillRange* spill_range = virtual_register.spill_range();
  Range& live_range = spill_range->live_range();
  AllocatedOperand allocated =
      *AllocatedOperand::cast(virtual_register.spill_operand());
  for (int instr_index : data()->reference_map_instructions()) {
    if (instr_index > live_range.end() || instr_index < live_range.start())
      continue;
    Instruction* instr = data()->code()->InstructionAt(instr_index);
    DCHECK(instr->HasReferenceMap());

    if (spill_range->IsLiveAt(instr_index, instr->block())) {
      instr->reference_map()->RecordReference(allocated);
    }
  }
}

void PopulateReferenceMaps(MidTierRegisterAllocationData* data) {
  MidTierReferenceMapPopulator populator(data);
  BitVector::Iterator iterator(&data->spilled_virtual_registers());
  for (; !iterator.Done(); iterator.Advance()) {
    populator.RecordReferences(
        data->VirtualRegisterDataFor(iterator.Current()));
  }
}

}  // namespace compiler
}  // namespace internal
}  // namespace v8