compiler/optimizing/ssa_liveness_analysis.h

/*
 * Copyright (C) 2014 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#ifndef ART_COMPILER_OPTIMIZING_SSA_LIVENESS_ANALYSIS_H_
#define ART_COMPILER_OPTIMIZING_SSA_LIVENESS_ANALYSIS_H_

#include <iostream>

#include "base/array_ref.h"
#include "base/bit_vector.h"
#include "base/intrusive_forward_list.h"
#include "base/iteration_range.h"
#include "base/macros.h"
#include "base/scoped_arena_allocator.h"
#include "base/scoped_arena_containers.h"
#include "nodes.h"

namespace art HIDDEN {

class CodeGenerator;
class SsaLivenessAnalysis;

static constexpr int kNoRegister = -1;

// Constants describing positions assigned to various data for an instruction.
static constexpr size_t kLivenessPositionsPerInstruction = 4u;
static constexpr size_t kLivenessPositionsForTemp = kLivenessPositionsPerInstruction - 1u;
static constexpr size_t kLivenessPositionsToBlock = kLivenessPositionsPerInstruction - 1u;
static constexpr size_t kLivenessPositionOfNormalUse = 1u;  // Inside instruction.
static constexpr size_t kLivenessPositionOfFixedOutput = kLivenessPositionsPerInstruction - 1u;
static constexpr size_t kLivenessPositionForMoveAfter = kLivenessPositionsPerInstruction - 1u;

class BlockInfo : public ArenaObject<kArenaAllocSsaLiveness> {
 public:
  BlockInfo(ScopedArenaAllocator* allocator, size_t number_of_ssa_values);

 private:
  BitVectorView<size_t> live_in_;
  BitVectorView<size_t> live_out_;
  BitVectorView<size_t> kill_;

  friend class SsaLivenessAnalysis;

  DISALLOW_COPY_AND_ASSIGN(BlockInfo);
};

/**
 * A live range contains the start and end of a range where an instruction or a temporary
 * is live.
 */
class LiveRange final : public ArenaObject<kArenaAllocSsaLiveness> {
 public:
  LiveRange(size_t start, size_t end, LiveRange* next) : start_(start), end_(end), next_(next) {
    DCHECK_LT(start, end);
    DCHECK(next_ == nullptr || next_->GetStart() > GetEnd());
  }

  size_t GetStart() const { return start_; }
  size_t GetEnd() const { return end_; }
  LiveRange* GetNext() const { return next_; }

  bool IntersectsWith(const LiveRange& other) const {
    return (start_ >= other.start_ && start_ < other.end_)
        || (other.start_ >= start_ && other.start_ < end_);
  }

  bool IsBefore(const LiveRange& other) const {
    return end_ <= other.start_;
  }

  void Dump(std::ostream& stream) const {
    stream << "[" << start_ << "," << end_ << ")";
  }

  LiveRange* Dup(ScopedArenaAllocator* allocator) const {
    return new (allocator) LiveRange(
        start_, end_, next_ == nullptr ? nullptr : next_->Dup(allocator));
  }

  LiveRange* GetLastRange() {
    return next_ == nullptr ? this : next_->GetLastRange();
  }

 private:
  size_t start_;
  size_t end_;
  LiveRange* next_;

  friend class LiveInterval;

  DISALLOW_COPY_AND_ASSIGN(LiveRange);
};

/**
 * A use position represents a live interval use at a given position.
 */
class UsePosition : public ArenaObject<kArenaAllocSsaLiveness>,
                    public IntrusiveForwardListNode<UsePosition> {
 public:
  UsePosition(HInstruction* user, size_t input_index, size_t position)
      : user_(user),
        input_index_(input_index),
        position_(position) {
  }

  explicit UsePosition(size_t position)
      : user_(nullptr),
        input_index_(kNoInput),
        position_(dchecked_integral_cast<uint32_t>(position)) {
  }

  size_t GetPosition() const { return position_; }

  HInstruction* GetUser() const { return user_; }

  bool IsSynthesized() const { return user_ == nullptr; }

  size_t GetInputIndex() const { return input_index_; }

  void Dump(std::ostream& stream) const {
    stream << position_;
  }

  HLoopInformation* GetLoopInformation() const {
    return user_->GetBlock()->GetLoopInformation();
  }

  UsePosition* Clone(ScopedArenaAllocator* allocator) const {
    return new (allocator) UsePosition(user_, input_index_, position_);
  }

  bool RequiresRegister() const {
    if (IsSynthesized()) return false;
    Location location = GetUser()->GetLocations()->InAt(GetInputIndex());
    return location.IsUnallocated() && location.RequiresRegisterKind();
  }

 private:
  static constexpr uint32_t kNoInput = static_cast<uint32_t>(-1);

  HInstruction* const user_;
  const size_t input_index_;
  const size_t position_;

  DISALLOW_COPY_AND_ASSIGN(UsePosition);
};
using UsePositionList = IntrusiveForwardList<UsePosition>;

/**
 * An environment use position represents a live interval for environment use at a given position.
 */
class EnvUsePosition : public ArenaObject<kArenaAllocSsaLiveness>,
                       public IntrusiveForwardListNode<EnvUsePosition> {
 public:
  EnvUsePosition(HEnvironment* environment,
                 size_t input_index,
                 size_t position)
      : environment_(environment),
        input_index_(input_index),
        position_(position) {
    DCHECK(environment != nullptr);
  }

  size_t GetPosition() const { return position_; }

  HEnvironment* GetEnvironment() const { return environment_; }
  size_t GetInputIndex() const { return input_index_; }

  void Dump(std::ostream& stream) const {
    stream << position_;
  }

  EnvUsePosition* Clone(ScopedArenaAllocator* allocator) const {
    return new (allocator) EnvUsePosition(environment_, input_index_, position_);
  }

 private:
  HEnvironment* const environment_;
  const size_t input_index_;
  const size_t position_;

  DISALLOW_COPY_AND_ASSIGN(EnvUsePosition);
};
using EnvUsePositionList = IntrusiveForwardList<EnvUsePosition>;

template <typename Iterator>
inline Iterator FindUseAtOrAfterPosition(Iterator first, Iterator last, size_t position) {
  using value_type = const typename Iterator::value_type;
  static_assert(std::is_same<value_type, const UsePosition>::value ||
                    std::is_same<value_type, const EnvUsePosition>::value,
                "Expecting value type UsePosition or EnvUsePosition.");
  Iterator ret = std::find_if(
      first, last, [position](const value_type& use) { return use.GetPosition() >= position; });
  // Check that the processed range is sorted. Do not check the rest of the range to avoid
  // increasing the complexity of callers from O(n) to O(n^2).
  DCHECK(std::is_sorted(
      first,
      ret,
      [](const value_type& lhs, const value_type& rhs) {
          return lhs.GetPosition() < rhs.GetPosition();
      }));
  return ret;
}

template <typename Iterator>
inline IterationRange<Iterator> FindMatchingUseRange(Iterator first,
                                                     Iterator last,
                                                     size_t position_begin,
                                                     size_t position_end) {
  Iterator begin = FindUseAtOrAfterPosition(first, last, position_begin);
  Iterator end = FindUseAtOrAfterPosition(begin, last, position_end);
  return MakeIterationRange(begin, end);
}

class SafepointPosition : public ArenaObject<kArenaAllocSsaLiveness>,
                          public IntrusiveForwardListNode<SafepointPosition> {
 public:
  explicit SafepointPosition(HInstruction* instruction)
      : instruction_(instruction) {}

  static size_t ComputePosition(HInstruction* instruction) {
    // We special case instructions emitted at use site, as their
    // safepoint position needs to be at their use.
    if (instruction->IsEmittedAtUseSite()) {
      // Currently only applies to implicit null checks, which are emitted
      // at the next instruction.
      DCHECK(instruction->IsNullCheck()) << instruction->DebugName();
      return instruction->GetLifetimePosition() + kLivenessPositionsPerInstruction;
    } else {
      return instruction->GetLifetimePosition();
    }
  }

  size_t GetPosition() const {
    return ComputePosition(instruction_);
  }

  LocationSummary* GetLocations() const {
    return instruction_->GetLocations();
  }

  HInstruction* GetInstruction() const {
    return instruction_;
  }

 private:
  HInstruction* const instruction_;

  DISALLOW_COPY_AND_ASSIGN(SafepointPosition);
};

using SafepointPositionList = IntrusiveForwardList<SafepointPosition>;

/**
 * An interval is a list of disjoint live ranges where an instruction is live.
 * Each instruction that has uses gets an interval.
 */
class LiveInterval : public ArenaObject<kArenaAllocSsaLiveness> {
 public:
  static LiveInterval* MakeInterval(ScopedArenaAllocator* allocator,
                                    DataType::Type type,
                                    HInstruction* instruction = nullptr) {
    return new (allocator) LiveInterval(allocator, type, instruction);
  }

  static LiveInterval* MakeFixedInterval(ScopedArenaAllocator* allocator,
                                         int reg,
                                         DataType::Type type) {
    return new (allocator) LiveInterval(allocator, type, nullptr, true, reg);
  }

  static LiveInterval* MakeTempInterval(ScopedArenaAllocator* allocator,
                                        DataType::Type type,
                                        size_t temp_index,
                                        size_t position) {
    int8_t checked_index = dchecked_integral_cast<int8_t>(temp_index);
    LiveInterval* temp = new (allocator) LiveInterval(allocator,
                                                      type,
                                                      /*defined_by*/ nullptr,
                                                      /*is_fixed=*/ false,
                                                      /*reg=*/ kNoRegister,
                                                      checked_index);
    temp->AddRange(position, position + kLivenessPositionsForTemp);
    return temp;
  }

  bool IsTemp() const {
    static_assert(kNoTempIndex < 0);
    return temp_index_ >= 0;
  }

  size_t GetTempIndex() const {
    DCHECK(IsTemp());
    return dchecked_integral_cast<size_t>(temp_index_);
  }

  bool IsFixed() const { return is_fixed_; }
  // This interval is the result of a split.
  bool IsSplit() const { return parent_ != this; }

  // Record use of an input. The use will be recorded as an environment use if
  // `environment` is not null and as register use otherwise. If `actual_user`
  // is specified, the use will be recorded at `actual_user`'s lifetime position.
  void AddUse(HInstruction* instruction,
              HEnvironment* environment,
              size_t input_index,
              HInstruction* actual_user = nullptr);

  void AddPhiUse(HInstruction* instruction, size_t input_index, HBasicBlock* block) {
    DCHECK(instruction->IsPhi());
    if (block->IsInLoop()) {
      AddBackEdgeUses(*block);
    }
    UsePosition* new_use =
        new (allocator_) UsePosition(instruction, input_index, block->GetLifetimeEnd());
    uses_.push_front(*new_use);
  }

  ALWAYS_INLINE void AddRange(size_t start, size_t end) {
    if (first_range_ == nullptr) {
      first_range_ = last_range_ = range_search_start_ =
          new (allocator_) LiveRange(start, end, first_range_);
    } else if (first_range_->GetStart() == end) {
      // There is a use in the following block.
      first_range_->start_ = start;
    } else if (first_range_->GetStart() == start && first_range_->GetEnd() == end) {
      DCHECK(is_fixed_);
    } else {
      DCHECK_GT(first_range_->GetStart(), end);
      // There is a hole in the interval. Create a new range.
      first_range_ = range_search_start_ = new (allocator_) LiveRange(start, end, first_range_);
    }
  }

  void AddLoopRange(size_t start, size_t end) {
    DCHECK(first_range_ != nullptr);
    DCHECK_LE(start, first_range_->GetStart());
    // Find the range that covers the positions after the loop.
    LiveRange* after_loop = first_range_;
    LiveRange* last_in_loop = nullptr;
    while (after_loop != nullptr && after_loop->GetEnd() < end) {
      DCHECK_LE(start, after_loop->GetStart());
      last_in_loop = after_loop;
      after_loop = after_loop->GetNext();
    }
    if (after_loop == nullptr) {
      // Uses are only in the loop.
      first_range_ = last_range_ = range_search_start_ =
          new (allocator_) LiveRange(start, end, nullptr);
    } else if (after_loop->GetStart() <= end) {
      first_range_ = range_search_start_ = after_loop;
      // There are uses after the loop.
      first_range_->start_ = start;
    } else {
      // The use after the loop is after a lifetime hole.
      DCHECK(last_in_loop != nullptr);
      first_range_ = range_search_start_ = last_in_loop;
      first_range_->start_ = start;
      first_range_->end_ = end;
    }
  }

  bool HasSpillSlot() const {
    static_assert(kNoSpillSlot == -1);
    return spill_slot_or_hint_ >= 0;
  }
  void SetSpillSlot(int slot) {
    DCHECK(!IsFixed());
    DCHECK(!IsTemp());
    spill_slot_or_hint_ = slot;
    DCHECK(HasSpillSlot());
  }
  int GetSpillSlot() const { return spill_slot_or_hint_; }

  bool HasSpillSlotOrHint() const {
    return spill_slot_or_hint_ != kNoSpillSlot;
  }
  bool HasSpillSlotHint() const {
    return spill_slot_or_hint_ < kNoSpillSlot;
  }
  void SetSpillSlotHint(int hint) {
    static_assert(kNoSpillSlot == -1);
    DCHECK(!HasSpillSlotOrHint());
    DCHECK_GE(hint, 0);
    spill_slot_or_hint_ = -2 - hint;
  }
  int GetSpillSlotHint() const {
    DCHECK(HasSpillSlotOrHint());
    return HasSpillSlot() ? GetSpillSlot() : -(spill_slot_or_hint_ + 2);
  }

  void SetFrom(size_t from) {
    if (first_range_ != nullptr) {
      first_range_->start_ = from;
    } else {
      // Instruction without uses.
      DCHECK(uses_.empty());
      DCHECK(from == defined_by_->GetLifetimePosition());
      // TODO: The `kLivenessPositionsPerInstruction` below looks like a bug for calls coming
      // from `RegisterAllocatorLinearScan::CheckForFixedOutput()` as the new range reaches
      // into the next instruction. However, those call always take the `first_range_ != nullptr`
      // path above. We should use another, simpler function for that.
      first_range_ = last_range_ = range_search_start_ =
          new (allocator_) LiveRange(from, from + kLivenessPositionsPerInstruction, nullptr);
    }
  }

  LiveInterval* GetParent() const { return parent_; }

  // Returns whether this interval is the parent interval, that is, the interval
  // that starts where the HInstruction is defined.
  bool IsParent() const { return parent_ == this; }

  LiveRange* GetFirstRange() const { return first_range_; }
  LiveRange* GetLastRange() const { return last_range_; }

  int GetRegister() const { return register_; }
  void SetRegister(int reg) { register_ = reg; }
  void ClearRegister() { register_ = kNoRegister; }
  bool HasRegister() const { return register_ != kNoRegister; }

  bool IsDeadAt(size_t position) const {
    return GetEnd() <= position;
  }

  bool IsDefinedAt(size_t position) const {
    return GetStart() <= position && !IsDeadAt(position);
  }

  // Returns true if the interval contains a LiveRange covering `position`.
  // The range at or immediately after the current position of linear scan
  // is cached for better performance. If `position` can be smaller than
  // that, CoversSlow should be used instead.
  bool Covers(size_t position) {
    LiveRange* candidate = FindRangeAtOrAfter(position, range_search_start_);
    range_search_start_ = candidate;
    return (candidate != nullptr && candidate->GetStart() <= position);
  }

  // Same as Covers but always tests all ranges.
  bool CoversSlow(size_t position) const {
    LiveRange* candidate = FindRangeAtOrAfter(position, first_range_);
    return candidate != nullptr && candidate->GetStart() <= position;
  }

  // Returns the first intersection of this interval with `current`, which
  // must be the interval currently being allocated by linear scan.
  size_t FirstIntersectionWith(LiveInterval* current) const {
    // Find the first range after the start of `current`. We use the search
    // cache to improve performance.
    DCHECK(GetStart() <= current->GetStart() || IsFixed());
    LiveRange* other_range = current->first_range_;
    LiveRange* my_range = FindRangeAtOrAfter(other_range->GetStart(), range_search_start_);
    if (my_range == nullptr) {
      return kNoLifetime;
    }

    // Advance both intervals and find the first matching range start in
    // this interval.
    do {
      if (my_range->IsBefore(*other_range)) {
        my_range = my_range->GetNext();
        if (my_range == nullptr) {
          return kNoLifetime;
        }
      } else if (other_range->IsBefore(*my_range)) {
        other_range = other_range->GetNext();
        if (other_range == nullptr) {
          return kNoLifetime;
        }
      } else {
        DCHECK(my_range->IntersectsWith(*other_range));
        return std::max(my_range->GetStart(), other_range->GetStart());
      }
    } while (true);
  }

  size_t GetStart() const {
    return first_range_->GetStart();
  }

  size_t GetEnd() const {
    return last_range_->GetEnd();
  }

  size_t GetLength() const {
    return GetEnd() - GetStart();
  }

  size_t FirstRegisterUseAfter(size_t position) const {
    DCHECK(!IsTemp());
    if (IsDefiningPosition(position) && DefinitionRequiresRegister()) {
      return position;
    }

    size_t end = GetEnd();
    for (const UsePosition& use : GetUses()) {
      size_t use_position = use.GetPosition();
      if (use_position > end) {
        break;
      }
      if (use_position > position) {
        if (use.RequiresRegister()) {
          return use_position;
        }
      }
    }
    return kNoLifetime;
  }

  // Returns the location of the first register use for this live interval,
  // including a register definition if applicable.
  size_t FirstRegisterUse() const {
    size_t start = GetStart();
    return IsTemp() ? start : FirstRegisterUseAfter(start);
  }

  // Whether the interval requires a register rather than a stack location.
  // If needed for performance, this could be cached.
  bool RequiresRegister() const {
    return !HasRegister() && FirstRegisterUse() != kNoLifetime;
  }

  size_t FirstUseAfter(size_t position) const {
    DCHECK(!IsTemp());
    if (IsDefiningPosition(position)) {
      DCHECK(defined_by_->GetLocations()->Out().IsValid());
      return position;
    }

    size_t end = GetEnd();
    for (const UsePosition& use : GetUses()) {
      size_t use_position = use.GetPosition();
      if (use_position > end) {
        break;
      }
      if (use_position > position) {
        return use_position;
      }
    }
    return kNoLifetime;
  }

  const UsePositionList& GetUses() const {
    return parent_->uses_;
  }

  const EnvUsePositionList& GetEnvironmentUses() const {
    return parent_->env_uses_;
  }

  DataType::Type GetType() const {
    return type_;
  }

  HInstruction* GetDefinedBy() const {
    return defined_by_;
  }

  bool HasWillCallSafepoint() const {
    return std::any_of(
        safepoints_.begin(),
        safepoints_.end(),
        [](const SafepointPosition& safepoint) { return safepoint.GetLocations()->WillCall(); });
  }

  /**
   * Split this interval at `position`. This interval is changed to:
   * [start ... position).
   *
   * The new interval covers:
   * [position ... end)
   */
  LiveInterval* SplitAt(size_t position);

  bool StartsBeforeOrAt(LiveInterval* other) const {
    return GetStart() <= other->GetStart();
  }

  bool StartsAfter(LiveInterval* other) const {
    return GetStart() > other->GetStart();
  }

  void Dump(std::ostream& stream) const;

  // Same as Dump, but adds context such as the instruction defining this interval, and
  // the register currently assigned to this interval.
  void DumpWithContext(std::ostream& stream, const CodeGenerator& codegen) const;

  LiveInterval* GetNextSibling() const { return next_sibling_; }
  LiveInterval* GetLastSibling() {
    LiveInterval* result = this;
    while (result->next_sibling_ != nullptr) {
      result = result->next_sibling_;
    }
    return result;
  }

  // Returns the first register hint that is at least free before
  // the value contained in `free_until`. If none is found, returns
  // `kNoRegister`.
  int FindFirstRegisterHint(ArrayRef<size_t> free_until,
                            ArrayRef<HInstruction* const> instructions_from_positions) const;

  // If there is enough at the definition site to find a register (for example
  // it uses the same input as the first input), returns the register as a hint.
  // Returns kNoRegister otherwise.
  int FindHintAtDefinition() const;

  // Returns the number of required spilling slots (measured as a multiple of the
  // Dex virtual register size `kVRegSize`).
  size_t NumberOfSpillSlotsNeeded() const;

  bool IsFloatingPoint() const {
    return type_ == DataType::Type::kFloat32 || type_ == DataType::Type::kFloat64;
  }

  // Converts the location of the interval to a `Location` object.
  Location ToLocation() const;

  // Returns the location of the interval following its siblings at `position`.
  Location GetLocationAt(size_t position);

  // Finds the sibling that is defined at `position`.
  LiveInterval* GetSiblingAt(size_t position);

  // Returns whether `other` and `this` share the same kind of register.
  bool SameRegisterKind(Location other) const;
  bool SameRegisterKind(const LiveInterval& other) const {
    return IsFloatingPoint() == other.IsFloatingPoint();
  }

  bool HasHighInterval() const {
    return IsLowInterval();
  }

  bool HasLowInterval() const {
    return IsHighInterval();
  }

  LiveInterval* GetLowInterval() const {
    DCHECK(HasLowInterval());
    return high_or_low_interval_;
  }

  LiveInterval* GetHighInterval() const {
    DCHECK(HasHighInterval());
    return high_or_low_interval_;
  }

  bool IsHighInterval() const {
    return GetParent()->is_high_interval_;
  }

  bool IsLowInterval() const {
    return !IsHighInterval() && (GetParent()->high_or_low_interval_ != nullptr);
  }

  void SetLowInterval(LiveInterval* low) {
    DCHECK(IsHighInterval());
    high_or_low_interval_ = low;
  }

  void SetHighInterval(LiveInterval* high) {
    DCHECK(IsLowInterval());
    high_or_low_interval_ = high;
  }

  void AddHighTempInterval() {
    DCHECK(IsParent());
    DCHECK(!HasHighInterval());
    DCHECK(!HasLowInterval());
    DCHECK(IsTemp());
    LiveInterval* high = new (allocator_) LiveInterval(allocator_,
                                                       type_,
                                                       /*defined_by=*/ nullptr,
                                                       /*is_fixed=*/ false,
                                                       /*reg=*/ kNoRegister,
                                                       temp_index_,
                                                       /*is_high_interval=*/ true);
    DCHECK(first_range_ != nullptr);
    DCHECK(first_range_->GetNext() == nullptr);
    high->AddRange(GetStart(), GetStart() + kLivenessPositionsForTemp);
    DCHECK(uses_.empty());
    DCHECK(env_uses_.empty());
    high_or_low_interval_ = high;
    high->high_or_low_interval_ = this;
  }

  void AddHighInterval() {
    DCHECK(IsParent());
    DCHECK(!HasHighInterval());
    DCHECK(!HasLowInterval());
    DCHECK(!IsTemp());
    LiveInterval* high = new (allocator_) LiveInterval(allocator_,
                                                       type_,
                                                       defined_by_,
                                                       /*is_fixed=*/ false,
                                                       /*reg=*/ kNoRegister,
                                                       kNoTempIndex,
                                                       /*is_high_interval=*/ true);
    if (first_range_ != nullptr) {
      high->first_range_ = first_range_->Dup(allocator_);
      high->last_range_ = high->first_range_->GetLastRange();
      high->range_search_start_ = high->first_range_;
    }
    auto pos = high->uses_.before_begin();
    for (const UsePosition& use : uses_) {
      UsePosition* new_use = use.Clone(allocator_);
      pos = high->uses_.insert_after(pos, *new_use);
    }
    auto env_pos = high->env_uses_.before_begin();
    for (const EnvUsePosition& env_use : env_uses_) {
      EnvUsePosition* new_env_use = env_use.Clone(allocator_);
      env_pos = high->env_uses_.insert_after(env_pos, *new_env_use);
    }
    high_or_low_interval_ = high;
    high->high_or_low_interval_ = this;
  }

  // Returns whether an interval, when it is non-split, is using
  // the same register of one of its input. This function should
  // be used only for DCHECKs.
  bool IsUsingInputRegister() const {
    if (defined_by_ != nullptr && !IsSplit()) {
      for (const HInstruction* input : defined_by_->GetInputs()) {
        LiveInterval* interval = input->GetLiveInterval();

        // Find the interval that covers `defined_by`_. Calls to this function
        // are made outside the linear scan, hence we need to use CoversSlow.
        while (interval != nullptr && !interval->CoversSlow(defined_by_->GetLifetimePosition())) {
          interval = interval->GetNextSibling();
        }

        // Check if both intervals have the same register of the same kind.
        if (interval != nullptr
            && interval->SameRegisterKind(*this)
            && interval->GetRegister() == GetRegister()) {
          return true;
        }
      }
    }
    return false;
  }

  // Returns whether an interval, when it is non-split, can safely use
  // the same register of one of its input. Note that this method requires
  // IsUsingInputRegister() to be true. This function should be used only
  // for DCHECKs.
  bool CanUseInputRegister() const {
    DCHECK(IsUsingInputRegister());
    if (defined_by_ != nullptr && !IsSplit()) {
      LocationSummary* locations = defined_by_->GetLocations();
      if (locations->OutputCanOverlapWithInputs()) {
        return false;
      }
      for (const HInstruction* input : defined_by_->GetInputs()) {
        LiveInterval* interval = input->GetLiveInterval();

        // Find the interval that covers `defined_by`_. Calls to this function
        // are made outside the linear scan, hence we need to use CoversSlow.
        while (interval != nullptr && !interval->CoversSlow(defined_by_->GetLifetimePosition())) {
          interval = interval->GetNextSibling();
        }

        if (interval != nullptr
            && interval->SameRegisterKind(*this)
            && interval->GetRegister() == GetRegister()) {
          // We found the input that has the same register. Check if it is live after
          // `defined_by_`.
          return !interval->CoversSlow(
              defined_by_->GetLifetimePosition() + kLivenessPositionOfNormalUse);
        }
      }
    }
    LOG(FATAL) << "Unreachable";
    UNREACHABLE();
  }

  SafepointPosition* CreateSafepointPosition(HInstruction* instruction) {
    return new (allocator_) SafepointPosition(instruction);
  }

  void SetSafepointPositions(SafepointPositionList&& list) {
    DCHECK(safepoints_.empty());
    safepoints_.swap(list);
  }

  const SafepointPositionList& GetSafepoints() const {
    return safepoints_;
  }

  // Resets the starting point for range-searching queries to the first range.
  // Intervals must be reset prior to starting a new linear scan over them.
  void ResetSearchCache() {
    range_search_start_ = first_range_;
  }

  bool DefinitionRequiresRegister() const {
    DCHECK(IsParent());
    LocationSummary* locations = defined_by_->GetLocations();
    Location location = locations->Out();
    // This interval is the first interval of the instruction. If the output
    // of the instruction requires a register, we return the position of that instruction
    // as the first register use.
    if (location.IsUnallocated()) {
      if ((location.GetPolicy() == Location::kRequiresRegister)
           || (location.GetPolicy() == Location::kSameAsFirstInput
               && (locations->InAt(0).IsRegister()
                   || locations->InAt(0).IsRegisterPair()
                   || locations->InAt(0).GetPolicy() == Location::kRequiresRegister))) {
        return true;
      } else if ((location.GetPolicy() == Location::kRequiresFpuRegister)
                 || (location.GetPolicy() == Location::kSameAsFirstInput
                     && (locations->InAt(0).IsFpuRegister()
                         || locations->InAt(0).IsFpuRegisterPair()
                         || locations->InAt(0).GetPolicy() == Location::kRequiresFpuRegister))) {
        return true;
      }
    } else if (location.IsRegister() || location.IsRegisterPair()) {
      return true;
    }
    return false;
  }

  void SetHintPhiInterval(LiveInterval* hint_phi_interval) {
    DCHECK(hint_phi_interval->GetDefinedBy() != nullptr);
    DCHECK(hint_phi_interval->GetDefinedBy()->IsPhi());
    hint_phi_interval_ = hint_phi_interval;
  }

  LiveInterval* GetHintPhiInterval() {
    return hint_phi_interval_;
  }

 private:
  LiveInterval(ScopedArenaAllocator* allocator,
               DataType::Type type,
               HInstruction* defined_by = nullptr,
               bool is_fixed = false,
               int reg = kNoRegister,
               int8_t temp_index = kNoTempIndex,
               bool is_high_interval = false)
      : allocator_(allocator),
        first_range_(nullptr),
        last_range_(nullptr),
        range_search_start_(nullptr),
        safepoints_(),
        uses_(),
        env_uses_(),
        next_sibling_(nullptr),
        parent_(this),
        defined_by_(defined_by),
        high_or_low_interval_(nullptr),
        hint_phi_interval_(nullptr),
        register_(reg),
        spill_slot_or_hint_(kNoSpillSlot),
        type_(type),
        temp_index_(temp_index),
        is_fixed_(is_fixed),
        is_high_interval_(is_high_interval) {}

  // Searches for a LiveRange that either covers the given position or is the
  // first next LiveRange. Returns null if no such LiveRange exists. Ranges
  // known to end before `position` can be skipped with `search_start`.
  LiveRange* FindRangeAtOrAfter(size_t position, LiveRange* search_start) const {
    if (kIsDebugBuild) {
      if (search_start != first_range_) {
        // If we are not searching the entire list of ranges, make sure we do
        // not skip the range we are searching for.
        if (search_start == nullptr) {
          DCHECK(IsDeadAt(position));
        } else if (search_start->GetStart() > position) {
          DCHECK_EQ(search_start, FindRangeAtOrAfter(position, first_range_));
        }
      }
    }

    LiveRange* range;
    for (range = search_start;
         range != nullptr && range->GetEnd() <= position;
         range = range->GetNext()) {
      continue;
    }
    return range;
  }

  bool IsDefiningPosition(size_t position) const {
    return IsParent() && (position == GetStart());
  }

  bool HasSynthesizeUseAt(size_t position) const {
    for (const UsePosition& use : GetUses()) {
      size_t use_position = use.GetPosition();
      if ((use_position == position) && use.IsSynthesized()) {
        return true;
      }
      if (use_position > position) break;
    }
    return false;
  }

  void AddBackEdgeUses(const HBasicBlock& block_at_use);

  ScopedArenaAllocator* const allocator_;

  // Ranges of this interval. We need a quick access to the last range to test
  // for liveness (see `IsDeadAt`).
  LiveRange* first_range_;
  LiveRange* last_range_;

  // The first range at or after the current position of a linear scan. It is
  // used to optimize range-searching queries.
  LiveRange* range_search_start_;

  // Safepoints where this interval is live.
  SafepointPositionList safepoints_;

  // Uses of this interval. Only the parent interval keeps these lists.
  UsePositionList uses_;
  EnvUsePositionList env_uses_;

  // Live interval that is the result of a split.
  LiveInterval* next_sibling_;

  // The first interval from which split intervals come from.
  LiveInterval* parent_;

  // The instruction represented by this interval.
  HInstruction* const defined_by_;

  // If this interval needs a register pair, the high or low equivalent.
  // `is_high_interval_` tells whether this holds the low or the high.
  LiveInterval* high_or_low_interval_;

  // If the last use of the instruction is a Phi, keep a record of that Phi's interval
  // for hints, except if the Phi is a loop Phi in an irreducible loop.
  LiveInterval* hint_phi_interval_;

  // The register allocated to this interval.
  int register_;

  // The spill slot allocated to this interval, or a spill slot hint, `kNoSpillSlot` if neither.
  //
  // Values >= 0 represent an actual spill slot, -1 is reserved for `kNoSpillSlot`
  // and values <= -2 encode a non-negative spill slot hint as `-2 - hint`.
  int spill_slot_or_hint_;

  // The instruction type this interval corresponds to.
  const DataType::Type type_;

  // The index of the temporary, `kNoTempIndex` if not a temporary.
  // Currently, we support only 32 core and 32 FP registers. We should never request more
  // temps than that, so `int8_t` is enough. (Even if we added another register type.)
  const int8_t temp_index_;

  // Whether the interval is for a fixed register.
  const bool is_fixed_;

  // Whether this interval is a synthesized interval for register pair.
  const bool is_high_interval_;

  static constexpr int kNoRegister = -1;
  static constexpr int kNoSpillSlot = -1;
  static constexpr int8_t kNoTempIndex = -1;

  friend class RegisterAllocatorTest;

  DISALLOW_COPY_AND_ASSIGN(LiveInterval);
};

/**
 * Analysis that computes the liveness of instructions:
 *
 * (a) Non-environment uses of an instruction always make
 *     the instruction live.
 * (b) Environment uses of an instruction whose type is object (that is, non-primitive), make the
 *     instruction live, unless the class has an @DeadReferenceSafe annotation.
 *     This avoids unexpected premature reference enqueuing or finalization, which could
 *     result in premature deletion of native objects.  In the presence of @DeadReferenceSafe,
 *     object references are treated like primitive types.
 * (c) When the graph has the debuggable property, environment uses
 *     of an instruction that has a primitive type make the instruction live.
 *     If the graph does not have the debuggable property, the environment
 *     use has no effect, and may get a 'none' value after register allocation.
 * (d) When compiling in OSR mode, all loops in the compiled method may be entered
 *     from the interpreter via SuspendCheck; such use in SuspendCheck makes the instruction
 *     live.
 *
 * (b), (c) and (d) are implemented through SsaLivenessAnalysis::ShouldBeLiveForEnvironment.
 */
class SsaLivenessAnalysis : public ValueObject {
 public:
  SsaLivenessAnalysis(HGraph* graph, CodeGenerator* codegen, ScopedArenaAllocator* allocator)
      : graph_(graph),
        codegen_(codegen),
        allocator_(allocator),
        block_infos_(graph->GetBlocks().size(),
                     nullptr,
                     allocator_->Adapter(kArenaAllocSsaLiveness)),
        instructions_from_ssa_index_(allocator_->Adapter(kArenaAllocSsaLiveness)),
        instructions_from_lifetime_position_(allocator_->Adapter(kArenaAllocSsaLiveness)) {
  }

  void Analyze();

  BitVectorView<size_t> GetLiveInSet(const HBasicBlock& block) const {
    return block_infos_[block.GetBlockId()]->live_in_;
  }

  BitVectorView<size_t> GetLiveOutSet(const HBasicBlock& block) const {
    return block_infos_[block.GetBlockId()]->live_out_;
  }

  BitVectorView<size_t> GetKillSet(const HBasicBlock& block) const {
    return block_infos_[block.GetBlockId()]->kill_;
  }

  HInstruction* GetInstructionFromSsaIndex(size_t index) const {
    return instructions_from_ssa_index_[index];
  }

  ArrayRef<HInstruction* const> GetInstructionsFromSsaIndexes() const {
    return ArrayRef<HInstruction* const>(instructions_from_ssa_index_);
  }

  HInstruction* GetInstructionFromPosition(size_t index) const {
    return instructions_from_lifetime_position_[index];
  }

  ArrayRef<HInstruction* const> GetInstructionsFromPositions() const {
    return ArrayRef<HInstruction* const>(instructions_from_lifetime_position_);
  }

  static HBasicBlock* GetBlockFromPosition(
      size_t index, ArrayRef<HInstruction* const> instructions_from_positions) {
    HInstruction* instruction = instructions_from_positions[index];
    if (instruction == nullptr) {
      // If we are at a block boundary, get the block following.
      instruction = instructions_from_positions[index + 1];
    }
    return instruction->GetBlock();
  }

  static bool IsAtBlockBoundary(
      size_t index, ArrayRef<HInstruction* const> instructions_from_positions) {
    return instructions_from_positions[index] == nullptr;
  }

  HInstruction* GetTempUser(LiveInterval* temp) const {
    // A temporary shares the same lifetime start as the instruction that requires it.
    DCHECK(temp->IsTemp());
    HInstruction* user =
        GetInstructionFromPosition(temp->GetStart() / kLivenessPositionsPerInstruction);
    DCHECK(user != nullptr);
    DCHECK_EQ(temp->GetStart(), user->GetLifetimePosition());
    return user;
  }

  size_t GetNumberOfSsaValues() const {
    return instructions_from_ssa_index_.size();
  }

  static constexpr const char* kLivenessPassName = "liveness";

 private:
  // Give an SSA number to each instruction that defines a value used by another instruction,
  // and setup the lifetime information of each instruction and block.
  void NumberInstructions();

  // Compute live ranges of instructions, as well as live_in, live_out and kill sets.
  void ComputeLiveness();

  // Compute the live ranges of instructions, as well as the initial live_in, live_out and
  // kill sets, that do not take into account backward branches.
  void ComputeLiveRanges();

  // After computing the initial sets, this method does a fixed point
  // calculation over the live_in and live_out set to take into account
  // backwards branches.
  void ComputeLiveInAndLiveOutSets();

  // Update the live_in set of the block and returns whether it has changed.
  bool UpdateLiveIn(const HBasicBlock& block);

  // Update the live_out set of the block and returns whether it has changed.
  bool UpdateLiveOut(const HBasicBlock& block);

  static void ProcessEnvironment(HInstruction* instruction,
                                 HInstruction* actual_user,
                                 BitVectorView<size_t> live_in);
  static void RecursivelyProcessInputs(HInstruction* instruction,
                                       HInstruction* actual_user,
                                       BitVectorView<size_t> live_in);

  // Returns whether `instruction` in an HEnvironment held by `env_holder`
  // should be kept live by the HEnvironment.
  static bool ShouldBeLiveForEnvironment(HInstruction* env_holder, HInstruction* instruction) {
    DCHECK(instruction != nullptr);
    // A value that's not live in compiled code may still be needed in interpreter,
    // due to code motion, etc.
    if (env_holder->IsDeoptimize()) return true;
    // A value live at a throwing instruction in a try block may be copied by
    // the exception handler to its location at the top of the catch block.
    if (env_holder->CanThrowIntoCatchBlock()) return true;
    HGraph* graph = instruction->GetBlock()->GetGraph();
    if (graph->IsDebuggable()) return true;
    // When compiling in OSR mode, all loops in the compiled method may be entered
    // from the interpreter via SuspendCheck; thus we need to preserve the environment.
    if (env_holder->IsSuspendCheck() && graph->IsCompilingOsr()) return true;
    if (graph -> IsDeadReferenceSafe()) return false;
    return instruction->GetType() == DataType::Type::kReference;
  }

  void CheckNoLiveInIrreducibleLoop(const HBasicBlock& block) const {
    if (!block.IsLoopHeader() || !block.GetLoopInformation()->IsIrreducible()) {
      return;
    }
    DoCheckNoLiveInIrreducibleLoop(block);
  }

  void DoCheckNoLiveInIrreducibleLoop(const HBasicBlock& block) const;

  HGraph* const graph_;
  CodeGenerator* const codegen_;

  // Use a local ScopedArenaAllocator for allocating memory.
  // This allocator must remain alive while doing register allocation.
  ScopedArenaAllocator* const allocator_;

  ScopedArenaVector<BlockInfo*> block_infos_;

  // Temporary array used when computing live_in, live_out, and kill sets.
  ScopedArenaVector<HInstruction*> instructions_from_ssa_index_;

  // Compressed map from lifetime position to instruction (nullptr for block start).
  // Indexed by the lifetime position divided by `kLivenessPositionsPerInstruction`.
  ScopedArenaVector<HInstruction*> instructions_from_lifetime_position_;

  friend class RegisterAllocatorTest;

  DISALLOW_COPY_AND_ASSIGN(SsaLivenessAnalysis);
};

}  // namespace art

#endif  // ART_COMPILER_OPTIMIZING_SSA_LIVENESS_ANALYSIS_H_