compiler/optimizing/load_store_elimination.cc

/*
 * Copyright (C) 2015 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.
 */

#include "load_store_elimination.h"
#include "side_effects_analysis.h"

#include <iostream>

namespace art {

class ReferenceInfo;

// A cap for the number of heap locations to prevent pathological time/space consumption.
// The number of heap locations for most of the methods stays below this threshold.
constexpr size_t kMaxNumberOfHeapLocations = 32;

// A ReferenceInfo contains additional info about a reference such as
// whether it's a singleton, returned, etc.
class ReferenceInfo : public ArenaObject<kArenaAllocMisc> {
 public:
  ReferenceInfo(HInstruction* reference, size_t pos) : reference_(reference), position_(pos) {
    is_singleton_ = true;
    is_singleton_and_not_returned_ = true;
    if (!reference_->IsNewInstance() && !reference_->IsNewArray()) {
      // For references not allocated in the method, don't assume anything.
      is_singleton_ = false;
      is_singleton_and_not_returned_ = false;
      return;
    }

    // Visit all uses to determine if this reference can spread into the heap,
    // a method call, etc.
    for (HUseIterator<HInstruction*> use_it(reference_->GetUses());
         !use_it.Done();
         use_it.Advance()) {
      HInstruction* use = use_it.Current()->GetUser();
      DCHECK(!use->IsNullCheck()) << "NullCheck should have been eliminated";
      if (use->IsBoundType()) {
        // BoundType shouldn't normally be necessary for a NewInstance.
        // Just be conservative for the uncommon cases.
        is_singleton_ = false;
        is_singleton_and_not_returned_ = false;
        return;
      }
      if (use->IsPhi() || use->IsSelect() || use->IsInvoke() ||
          (use->IsInstanceFieldSet() && (reference_ == use->InputAt(1))) ||
          (use->IsUnresolvedInstanceFieldSet() && (reference_ == use->InputAt(1))) ||
          (use->IsStaticFieldSet() && (reference_ == use->InputAt(1))) ||
          (use->IsUnresolvedStaticFieldSet() && (reference_ == use->InputAt(0))) ||
          (use->IsArraySet() && (reference_ == use->InputAt(2)))) {
        // reference_ is merged to HPhi/HSelect, passed to a callee, or stored to heap.
        // reference_ isn't the only name that can refer to its value anymore.
        is_singleton_ = false;
        is_singleton_and_not_returned_ = false;
        return;
      }
      if (use->IsReturn()) {
        is_singleton_and_not_returned_ = false;
      }
    }
  }

  HInstruction* GetReference() const {
    return reference_;
  }

  size_t GetPosition() const {
    return position_;
  }

  // Returns true if reference_ is the only name that can refer to its value during
  // the lifetime of the method. So it's guaranteed to not have any alias in
  // the method (including its callees).
  bool IsSingleton() const {
    return is_singleton_;
  }

  // Returns true if reference_ is a singleton and not returned to the caller.
  // The allocation and stores into reference_ may be eliminated for such cases.
  bool IsSingletonAndNotReturned() const {
    return is_singleton_and_not_returned_;
  }

 private:
  HInstruction* const reference_;
  const size_t position_;     // position in HeapLocationCollector's ref_info_array_.
  bool is_singleton_;         // can only be referred to by a single name in the method.
  bool is_singleton_and_not_returned_;  // reference_ is singleton and not returned to caller.

  DISALLOW_COPY_AND_ASSIGN(ReferenceInfo);
};

// A heap location is a reference-offset/index pair that a value can be loaded from
// or stored to.
class HeapLocation : public ArenaObject<kArenaAllocMisc> {
 public:
  static constexpr size_t kInvalidFieldOffset = -1;

  // TODO: more fine-grained array types.
  static constexpr int16_t kDeclaringClassDefIndexForArrays = -1;

  HeapLocation(ReferenceInfo* ref_info,
               size_t offset,
               HInstruction* index,
               int16_t declaring_class_def_index)
      : ref_info_(ref_info),
        offset_(offset),
        index_(index),
        declaring_class_def_index_(declaring_class_def_index),
        value_killed_by_loop_side_effects_(true) {
    DCHECK(ref_info != nullptr);
    DCHECK((offset == kInvalidFieldOffset && index != nullptr) ||
           (offset != kInvalidFieldOffset && index == nullptr));
    if (ref_info->IsSingleton() && !IsArrayElement()) {
      // Assume this location's value cannot be killed by loop side effects
      // until proven otherwise.
      value_killed_by_loop_side_effects_ = false;
    }
  }

  ReferenceInfo* GetReferenceInfo() const { return ref_info_; }
  size_t GetOffset() const { return offset_; }
  HInstruction* GetIndex() const { return index_; }

  // Returns the definition of declaring class' dex index.
  // It's kDeclaringClassDefIndexForArrays for an array element.
  int16_t GetDeclaringClassDefIndex() const {
    return declaring_class_def_index_;
  }

  bool IsArrayElement() const {
    return index_ != nullptr;
  }

  bool IsValueKilledByLoopSideEffects() const {
    return value_killed_by_loop_side_effects_;
  }

  void SetValueKilledByLoopSideEffects(bool val) {
    value_killed_by_loop_side_effects_ = val;
  }

 private:
  ReferenceInfo* const ref_info_;      // reference for instance/static field or array access.
  const size_t offset_;                // offset of static/instance field.
  HInstruction* const index_;          // index of an array element.
  const int16_t declaring_class_def_index_;  // declaring class's def's dex index.
  bool value_killed_by_loop_side_effects_;   // value of this location may be killed by loop
                                             // side effects because this location is stored
                                             // into inside a loop.

  DISALLOW_COPY_AND_ASSIGN(HeapLocation);
};

static HInstruction* HuntForOriginalReference(HInstruction* ref) {
  DCHECK(ref != nullptr);
  while (ref->IsNullCheck() || ref->IsBoundType()) {
    ref = ref->InputAt(0);
  }
  return ref;
}

// A HeapLocationCollector collects all relevant heap locations and keeps
// an aliasing matrix for all locations.
class HeapLocationCollector : public HGraphVisitor {
 public:
  static constexpr size_t kHeapLocationNotFound = -1;
  // Start with a single uint32_t word. That's enough bits for pair-wise
  // aliasing matrix of 8 heap locations.
  static constexpr uint32_t kInitialAliasingMatrixBitVectorSize = 32;

  explicit HeapLocationCollector(HGraph* graph)
      : HGraphVisitor(graph),
        ref_info_array_(graph->GetArena()->Adapter(kArenaAllocLSE)),
        heap_locations_(graph->GetArena()->Adapter(kArenaAllocLSE)),
        aliasing_matrix_(graph->GetArena(), kInitialAliasingMatrixBitVectorSize, true),
        has_heap_stores_(false),
        has_volatile_(false),
        has_monitor_operations_(false),
        may_deoptimize_(false) {}

  size_t GetNumberOfHeapLocations() const {
    return heap_locations_.size();
  }

  HeapLocation* GetHeapLocation(size_t index) const {
    return heap_locations_[index];
  }

  ReferenceInfo* FindReferenceInfoOf(HInstruction* ref) const {
    for (size_t i = 0; i < ref_info_array_.size(); i++) {
      ReferenceInfo* ref_info = ref_info_array_[i];
      if (ref_info->GetReference() == ref) {
        DCHECK_EQ(i, ref_info->GetPosition());
        return ref_info;
      }
    }
    return nullptr;
  }

  bool HasHeapStores() const {
    return has_heap_stores_;
  }

  bool HasVolatile() const {
    return has_volatile_;
  }

  bool HasMonitorOps() const {
    return has_monitor_operations_;
  }

  // Returns whether this method may be deoptimized.
  // Currently we don't have meta data support for deoptimizing
  // a method that eliminates allocations/stores.
  bool MayDeoptimize() const {
    return may_deoptimize_;
  }

  // Find and return the heap location index in heap_locations_.
  size_t FindHeapLocationIndex(ReferenceInfo* ref_info,
                               size_t offset,
                               HInstruction* index,
                               int16_t declaring_class_def_index) const {
    for (size_t i = 0; i < heap_locations_.size(); i++) {
      HeapLocation* loc = heap_locations_[i];
      if (loc->GetReferenceInfo() == ref_info &&
          loc->GetOffset() == offset &&
          loc->GetIndex() == index &&
          loc->GetDeclaringClassDefIndex() == declaring_class_def_index) {
        return i;
      }
    }
    return kHeapLocationNotFound;
  }

  // Returns true if heap_locations_[index1] and heap_locations_[index2] may alias.
  bool MayAlias(size_t index1, size_t index2) const {
    if (index1 < index2) {
      return aliasing_matrix_.IsBitSet(AliasingMatrixPosition(index1, index2));
    } else if (index1 > index2) {
      return aliasing_matrix_.IsBitSet(AliasingMatrixPosition(index2, index1));
    } else {
      DCHECK(false) << "index1 and index2 are expected to be different";
      return true;
    }
  }

  void BuildAliasingMatrix() {
    const size_t number_of_locations = heap_locations_.size();
    if (number_of_locations == 0) {
      return;
    }
    size_t pos = 0;
    // Compute aliasing info between every pair of different heap locations.
    // Save the result in a matrix represented as a BitVector.
    for (size_t i = 0; i < number_of_locations - 1; i++) {
      for (size_t j = i + 1; j < number_of_locations; j++) {
        if (ComputeMayAlias(i, j)) {
          aliasing_matrix_.SetBit(CheckedAliasingMatrixPosition(i, j, pos));
        }
        pos++;
      }
    }
  }

 private:
  // An allocation cannot alias with a name which already exists at the point
  // of the allocation, such as a parameter or a load happening before the allocation.
  bool MayAliasWithPreexistenceChecking(ReferenceInfo* ref_info1, ReferenceInfo* ref_info2) const {
    if (ref_info1->GetReference()->IsNewInstance() || ref_info1->GetReference()->IsNewArray()) {
      // Any reference that can alias with the allocation must appear after it in the block/in
      // the block's successors. In reverse post order, those instructions will be visited after
      // the allocation.
      return ref_info2->GetPosition() >= ref_info1->GetPosition();
    }
    return true;
  }

  bool CanReferencesAlias(ReferenceInfo* ref_info1, ReferenceInfo* ref_info2) const {
    if (ref_info1 == ref_info2) {
      return true;
    } else if (ref_info1->IsSingleton()) {
      return false;
    } else if (ref_info2->IsSingleton()) {
      return false;
    } else if (!MayAliasWithPreexistenceChecking(ref_info1, ref_info2) ||
        !MayAliasWithPreexistenceChecking(ref_info2, ref_info1)) {
      return false;
    }
    return true;
  }

  // `index1` and `index2` are indices in the array of collected heap locations.
  // Returns the position in the bit vector that tracks whether the two heap
  // locations may alias.
  size_t AliasingMatrixPosition(size_t index1, size_t index2) const {
    DCHECK(index2 > index1);
    const size_t number_of_locations = heap_locations_.size();
    // It's (num_of_locations - 1) + ... + (num_of_locations - index1) + (index2 - index1 - 1).
    return (number_of_locations * index1 - (1 + index1) * index1 / 2 + (index2 - index1 - 1));
  }

  // An additional position is passed in to make sure the calculated position is correct.
  size_t CheckedAliasingMatrixPosition(size_t index1, size_t index2, size_t position) {
    size_t calculated_position = AliasingMatrixPosition(index1, index2);
    DCHECK_EQ(calculated_position, position);
    return calculated_position;
  }

  // Compute if two locations may alias to each other.
  bool ComputeMayAlias(size_t index1, size_t index2) const {
    HeapLocation* loc1 = heap_locations_[index1];
    HeapLocation* loc2 = heap_locations_[index2];
    if (loc1->GetOffset() != loc2->GetOffset()) {
      // Either two different instance fields, or one is an instance
      // field and the other is an array element.
      return false;
    }
    if (loc1->GetDeclaringClassDefIndex() != loc2->GetDeclaringClassDefIndex()) {
      // Different types.
      return false;
    }
    if (!CanReferencesAlias(loc1->GetReferenceInfo(), loc2->GetReferenceInfo())) {
      return false;
    }
    if (loc1->IsArrayElement() && loc2->IsArrayElement()) {
      HInstruction* array_index1 = loc1->GetIndex();
      HInstruction* array_index2 = loc2->GetIndex();
      DCHECK(array_index1 != nullptr);
      DCHECK(array_index2 != nullptr);
      if (array_index1->IsIntConstant() &&
          array_index2->IsIntConstant() &&
          array_index1->AsIntConstant()->GetValue() != array_index2->AsIntConstant()->GetValue()) {
        // Different constant indices do not alias.
        return false;
      }
    }
    return true;
  }

  ReferenceInfo* GetOrCreateReferenceInfo(HInstruction* instruction) {
    ReferenceInfo* ref_info = FindReferenceInfoOf(instruction);
    if (ref_info == nullptr) {
      size_t pos = ref_info_array_.size();
      ref_info = new (GetGraph()->GetArena()) ReferenceInfo(instruction, pos);
      ref_info_array_.push_back(ref_info);
    }
    return ref_info;
  }

  void CreateReferenceInfoForReferenceType(HInstruction* instruction) {
    if (instruction->GetType() != Primitive::kPrimNot) {
      return;
    }
    DCHECK(FindReferenceInfoOf(instruction) == nullptr);
    GetOrCreateReferenceInfo(instruction);
  }

  HeapLocation* GetOrCreateHeapLocation(HInstruction* ref,
                                        size_t offset,
                                        HInstruction* index,
                                        int16_t declaring_class_def_index) {
    HInstruction* original_ref = HuntForOriginalReference(ref);
    ReferenceInfo* ref_info = GetOrCreateReferenceInfo(original_ref);
    size_t heap_location_idx = FindHeapLocationIndex(
        ref_info, offset, index, declaring_class_def_index);
    if (heap_location_idx == kHeapLocationNotFound) {
      HeapLocation* heap_loc = new (GetGraph()->GetArena())
          HeapLocation(ref_info, offset, index, declaring_class_def_index);
      heap_locations_.push_back(heap_loc);
      return heap_loc;
    }
    return heap_locations_[heap_location_idx];
  }

  HeapLocation* VisitFieldAccess(HInstruction* ref, const FieldInfo& field_info) {
    if (field_info.IsVolatile()) {
      has_volatile_ = true;
    }
    const uint16_t declaring_class_def_index = field_info.GetDeclaringClassDefIndex();
    const size_t offset = field_info.GetFieldOffset().SizeValue();
    return GetOrCreateHeapLocation(ref, offset, nullptr, declaring_class_def_index);
  }

  void VisitArrayAccess(HInstruction* array, HInstruction* index) {
    GetOrCreateHeapLocation(array, HeapLocation::kInvalidFieldOffset,
        index, HeapLocation::kDeclaringClassDefIndexForArrays);
  }

  void VisitInstanceFieldGet(HInstanceFieldGet* instruction) OVERRIDE {
    VisitFieldAccess(instruction->InputAt(0), instruction->GetFieldInfo());
    CreateReferenceInfoForReferenceType(instruction);
  }

  void VisitInstanceFieldSet(HInstanceFieldSet* instruction) OVERRIDE {
    HeapLocation* location = VisitFieldAccess(instruction->InputAt(0), instruction->GetFieldInfo());
    has_heap_stores_ = true;
    if (instruction->GetBlock()->GetLoopInformation() != nullptr) {
      location->SetValueKilledByLoopSideEffects(true);
    }
  }

  void VisitStaticFieldGet(HStaticFieldGet* instruction) OVERRIDE {
    VisitFieldAccess(instruction->InputAt(0), instruction->GetFieldInfo());
    CreateReferenceInfoForReferenceType(instruction);
  }

  void VisitStaticFieldSet(HStaticFieldSet* instruction) OVERRIDE {
    VisitFieldAccess(instruction->InputAt(0), instruction->GetFieldInfo());
    has_heap_stores_ = true;
  }

  // We intentionally don't collect HUnresolvedInstanceField/HUnresolvedStaticField accesses
  // since we cannot accurately track the fields.

  void VisitArrayGet(HArrayGet* instruction) OVERRIDE {
    VisitArrayAccess(instruction->InputAt(0), instruction->InputAt(1));
    CreateReferenceInfoForReferenceType(instruction);
  }

  void VisitArraySet(HArraySet* instruction) OVERRIDE {
    VisitArrayAccess(instruction->InputAt(0), instruction->InputAt(1));
    has_heap_stores_ = true;
  }

  void VisitNewInstance(HNewInstance* new_instance) OVERRIDE {
    // Any references appearing in the ref_info_array_ so far cannot alias with new_instance.
    CreateReferenceInfoForReferenceType(new_instance);
  }

  void VisitInvokeStaticOrDirect(HInvokeStaticOrDirect* instruction) OVERRIDE {
    CreateReferenceInfoForReferenceType(instruction);
  }

  void VisitInvokeVirtual(HInvokeVirtual* instruction) OVERRIDE {
    CreateReferenceInfoForReferenceType(instruction);
  }

  void VisitInvokeInterface(HInvokeInterface* instruction) OVERRIDE {
    CreateReferenceInfoForReferenceType(instruction);
  }

  void VisitParameterValue(HParameterValue* instruction) OVERRIDE {
    CreateReferenceInfoForReferenceType(instruction);
  }

  void VisitSelect(HSelect* instruction) OVERRIDE {
    CreateReferenceInfoForReferenceType(instruction);
  }

  void VisitDeoptimize(HDeoptimize* instruction ATTRIBUTE_UNUSED) OVERRIDE {
    may_deoptimize_ = true;
  }

  void VisitMonitorOperation(HMonitorOperation* monitor ATTRIBUTE_UNUSED) OVERRIDE {
    has_monitor_operations_ = true;
  }

  ArenaVector<ReferenceInfo*> ref_info_array_;   // All references used for heap accesses.
  ArenaVector<HeapLocation*> heap_locations_;    // All heap locations.
  ArenaBitVector aliasing_matrix_;    // aliasing info between each pair of locations.
  bool has_heap_stores_;    // If there is no heap stores, LSE acts as GVN with better
                            // alias analysis and won't be as effective.
  bool has_volatile_;       // If there are volatile field accesses.
  bool has_monitor_operations_;    // If there are monitor operations.
  bool may_deoptimize_;

  DISALLOW_COPY_AND_ASSIGN(HeapLocationCollector);
};

// An unknown heap value. Loads with such a value in the heap location cannot be eliminated.
// A heap location can be set to kUnknownHeapValue when:
// - initially set a value.
// - killed due to aliasing, merging, invocation, or loop side effects.
static HInstruction* const kUnknownHeapValue =
    reinterpret_cast<HInstruction*>(static_cast<uintptr_t>(-1));

// Default heap value after an allocation.
// A heap location can be set to that value right after an allocation.
static HInstruction* const kDefaultHeapValue =
    reinterpret_cast<HInstruction*>(static_cast<uintptr_t>(-2));

class LSEVisitor : public HGraphVisitor {
 public:
  LSEVisitor(HGraph* graph,
             const HeapLocationCollector& heap_locations_collector,
             const SideEffectsAnalysis& side_effects)
      : HGraphVisitor(graph),
        heap_location_collector_(heap_locations_collector),
        side_effects_(side_effects),
        heap_values_for_(graph->GetBlocks().size(),
                         ArenaVector<HInstruction*>(heap_locations_collector.
                                                        GetNumberOfHeapLocations(),
                                                    kUnknownHeapValue,
                                                    graph->GetArena()->Adapter(kArenaAllocLSE)),
                         graph->GetArena()->Adapter(kArenaAllocLSE)),
        removed_loads_(graph->GetArena()->Adapter(kArenaAllocLSE)),
        substitute_instructions_for_loads_(graph->GetArena()->Adapter(kArenaAllocLSE)),
        possibly_removed_stores_(graph->GetArena()->Adapter(kArenaAllocLSE)),
        singleton_new_instances_(graph->GetArena()->Adapter(kArenaAllocLSE)) {
  }

  void VisitBasicBlock(HBasicBlock* block) OVERRIDE {
    // Populate the heap_values array for this block.
    // TODO: try to reuse the heap_values array from one predecessor if possible.
    if (block->IsLoopHeader()) {
      HandleLoopSideEffects(block);
    } else {
      MergePredecessorValues(block);
    }
    HGraphVisitor::VisitBasicBlock(block);
  }

  // Remove recorded instructions that should be eliminated.
  void RemoveInstructions() {
    size_t size = removed_loads_.size();
    DCHECK_EQ(size, substitute_instructions_for_loads_.size());
    for (size_t i = 0; i < size; i++) {
      HInstruction* load = removed_loads_[i];
      DCHECK(load != nullptr);
      DCHECK(load->IsInstanceFieldGet() ||
             load->IsStaticFieldGet() ||
             load->IsArrayGet());
      HInstruction* substitute = substitute_instructions_for_loads_[i];
      DCHECK(substitute != nullptr);
      // Keep tracing substitute till one that's not removed.
      HInstruction* sub_sub = FindSubstitute(substitute);
      while (sub_sub != substitute) {
        substitute = sub_sub;
        sub_sub = FindSubstitute(substitute);
      }
      load->ReplaceWith(substitute);
      load->GetBlock()->RemoveInstruction(load);
    }

    // At this point, stores in possibly_removed_stores_ can be safely removed.
    size = possibly_removed_stores_.size();
    for (size_t i = 0; i < size; i++) {
      HInstruction* store = possibly_removed_stores_[i];
      DCHECK(store->IsInstanceFieldSet() || store->IsStaticFieldSet() || store->IsArraySet());
      store->GetBlock()->RemoveInstruction(store);
    }

    // TODO: remove unnecessary allocations.
    // Eliminate instructions in singleton_new_instances_ that:
    // - don't have uses,
    // - don't have finalizers,
    // - are instantiable and accessible,
    // - have no/separate clinit check.
  }

 private:
  // If heap_values[index] is an instance field store, need to keep the store.
  // This is necessary if a heap value is killed due to merging, or loop side
  // effects (which is essentially merging also), since a load later from the
  // location won't be eliminated.
  void KeepIfIsStore(HInstruction* heap_value) {
    if (heap_value == kDefaultHeapValue ||
        heap_value == kUnknownHeapValue ||
        !heap_value->IsInstanceFieldSet()) {
      return;
    }
    auto idx = std::find(possibly_removed_stores_.begin(),
        possibly_removed_stores_.end(), heap_value);
    if (idx != possibly_removed_stores_.end()) {
      // Make sure the store is kept.
      possibly_removed_stores_.erase(idx);
    }
  }

  void HandleLoopSideEffects(HBasicBlock* block) {
    DCHECK(block->IsLoopHeader());
    int block_id = block->GetBlockId();
    ArenaVector<HInstruction*>& heap_values = heap_values_for_[block_id];

    // Don't eliminate loads in irreducible loops. This is safe for singletons, because
    // they are always used by the non-eliminated loop-phi.
    if (block->GetLoopInformation()->IsIrreducible()) {
      if (kIsDebugBuild) {
        for (size_t i = 0; i < heap_values.size(); i++) {
          DCHECK_EQ(heap_values[i], kUnknownHeapValue);
        }
      }
      return;
    }

    HBasicBlock* pre_header = block->GetLoopInformation()->GetPreHeader();
    ArenaVector<HInstruction*>& pre_header_heap_values =
        heap_values_for_[pre_header->GetBlockId()];

    // Inherit the values from pre-header.
    for (size_t i = 0; i < heap_values.size(); i++) {
      heap_values[i] = pre_header_heap_values[i];
    }

    // We do a single pass in reverse post order. For loops, use the side effects as a hint
    // to see if the heap values should be killed.
    if (side_effects_.GetLoopEffects(block).DoesAnyWrite()) {
      for (size_t i = 0; i < heap_values.size(); i++) {
        HeapLocation* location = heap_location_collector_.GetHeapLocation(i);
        ReferenceInfo* ref_info = location->GetReferenceInfo();
        if (!ref_info->IsSingleton() || location->IsValueKilledByLoopSideEffects()) {
          // heap value is killed by loop side effects (stored into directly, or due to
          // aliasing).
          KeepIfIsStore(pre_header_heap_values[i]);
          heap_values[i] = kUnknownHeapValue;
        } else {
          // A singleton's field that's not stored into inside a loop is invariant throughout
          // the loop.
        }
      }
    }
  }

  void MergePredecessorValues(HBasicBlock* block) {
    const ArenaVector<HBasicBlock*>& predecessors = block->GetPredecessors();
    if (predecessors.size() == 0) {
      return;
    }
    ArenaVector<HInstruction*>& heap_values = heap_values_for_[block->GetBlockId()];
    for (size_t i = 0; i < heap_values.size(); i++) {
      HInstruction* pred0_value = heap_values_for_[predecessors[0]->GetBlockId()][i];
      heap_values[i] = pred0_value;
      if (pred0_value != kUnknownHeapValue) {
        for (size_t j = 1; j < predecessors.size(); j++) {
          HInstruction* pred_value = heap_values_for_[predecessors[j]->GetBlockId()][i];
          if (pred_value != pred0_value) {
            heap_values[i] = kUnknownHeapValue;
            break;
          }
        }
      }

      if (heap_values[i] == kUnknownHeapValue) {
        // Keep the last store in each predecessor since future loads cannot be eliminated.
        for (size_t j = 0; j < predecessors.size(); j++) {
          ArenaVector<HInstruction*>& pred_values = heap_values_for_[predecessors[j]->GetBlockId()];
          KeepIfIsStore(pred_values[i]);
        }
      }
    }
  }

  // `instruction` is being removed. Try to see if the null check on it
  // can be removed. This can happen if the same value is set in two branches
  // but not in dominators. Such as:
  //   int[] a = foo();
  //   if () {
  //     a[0] = 2;
  //   } else {
  //     a[0] = 2;
  //   }
  //   // a[0] can now be replaced with constant 2, and the null check on it can be removed.
  void TryRemovingNullCheck(HInstruction* instruction) {
    HInstruction* prev = instruction->GetPrevious();
    if ((prev != nullptr) && prev->IsNullCheck() && (prev == instruction->InputAt(0))) {
      // Previous instruction is a null check for this instruction. Remove the null check.
      prev->ReplaceWith(prev->InputAt(0));
      prev->GetBlock()->RemoveInstruction(prev);
    }
  }

  HInstruction* GetDefaultValue(Primitive::Type type) {
    switch (type) {
      case Primitive::kPrimNot:
        return GetGraph()->GetNullConstant();
      case Primitive::kPrimBoolean:
      case Primitive::kPrimByte:
      case Primitive::kPrimChar:
      case Primitive::kPrimShort:
      case Primitive::kPrimInt:
        return GetGraph()->GetIntConstant(0);
      case Primitive::kPrimLong:
        return GetGraph()->GetLongConstant(0);
      case Primitive::kPrimFloat:
        return GetGraph()->GetFloatConstant(0);
      case Primitive::kPrimDouble:
        return GetGraph()->GetDoubleConstant(0);
      default:
        UNREACHABLE();
    }
  }

  void VisitGetLocation(HInstruction* instruction,
                        HInstruction* ref,
                        size_t offset,
                        HInstruction* index,
                        int16_t declaring_class_def_index) {
    HInstruction* original_ref = HuntForOriginalReference(ref);
    ReferenceInfo* ref_info = heap_location_collector_.FindReferenceInfoOf(original_ref);
    size_t idx = heap_location_collector_.FindHeapLocationIndex(
        ref_info, offset, index, declaring_class_def_index);
    DCHECK_NE(idx, HeapLocationCollector::kHeapLocationNotFound);
    ArenaVector<HInstruction*>& heap_values =
        heap_values_for_[instruction->GetBlock()->GetBlockId()];
    HInstruction* heap_value = heap_values[idx];
    if (heap_value == kDefaultHeapValue) {
      HInstruction* constant = GetDefaultValue(instruction->GetType());
      removed_loads_.push_back(instruction);
      substitute_instructions_for_loads_.push_back(constant);
      heap_values[idx] = constant;
      return;
    }
    if (heap_value != kUnknownHeapValue && heap_value->IsInstanceFieldSet()) {
      HInstruction* store = heap_value;
      // This load must be from a singleton since it's from the same field
      // that a "removed" store puts the value. That store must be to a singleton's field.
      DCHECK(ref_info->IsSingleton());
      // Get the real heap value of the store.
      heap_value = store->InputAt(1);
    }
    if (heap_value == kUnknownHeapValue) {
      // Load isn't eliminated. Put the load as the value into the HeapLocation.
      // This acts like GVN but with better aliasing analysis.
      heap_values[idx] = instruction;
    } else {
      if (Primitive::PrimitiveKind(heap_value->GetType())
              != Primitive::PrimitiveKind(instruction->GetType())) {
        // The only situation where the same heap location has different type is when
        // we do an array get from a null constant. In order to stay properly typed
        // we do not merge the array gets.
        if (kIsDebugBuild) {
          DCHECK(heap_value->IsArrayGet()) << heap_value->DebugName();
          DCHECK(instruction->IsArrayGet()) << instruction->DebugName();
          HInstruction* array = instruction->AsArrayGet()->GetArray();
          DCHECK(array->IsNullCheck()) << array->DebugName();
          DCHECK(array->InputAt(0)->IsNullConstant()) << array->InputAt(0)->DebugName();
          array = heap_value->AsArrayGet()->GetArray();
          DCHECK(array->IsNullCheck()) << array->DebugName();
          DCHECK(array->InputAt(0)->IsNullConstant()) << array->InputAt(0)->DebugName();
        }
        return;
      }
      removed_loads_.push_back(instruction);
      substitute_instructions_for_loads_.push_back(heap_value);
      TryRemovingNullCheck(instruction);
    }
  }

  bool Equal(HInstruction* heap_value, HInstruction* value) {
    if (heap_value == value) {
      return true;
    }
    if (heap_value == kDefaultHeapValue && GetDefaultValue(value->GetType()) == value) {
      return true;
    }
    return false;
  }

  void VisitSetLocation(HInstruction* instruction,
                        HInstruction* ref,
                        size_t offset,
                        HInstruction* index,
                        int16_t declaring_class_def_index,
                        HInstruction* value) {
    HInstruction* original_ref = HuntForOriginalReference(ref);
    ReferenceInfo* ref_info = heap_location_collector_.FindReferenceInfoOf(original_ref);
    size_t idx = heap_location_collector_.FindHeapLocationIndex(
        ref_info, offset, index, declaring_class_def_index);
    DCHECK_NE(idx, HeapLocationCollector::kHeapLocationNotFound);
    ArenaVector<HInstruction*>& heap_values =
        heap_values_for_[instruction->GetBlock()->GetBlockId()];
    HInstruction* heap_value = heap_values[idx];
    bool same_value = false;
    bool possibly_redundant = false;
    if (Equal(heap_value, value)) {
      // Store into the heap location with the same value.
      same_value = true;
    } else if (index != nullptr) {
      // For array element, don't eliminate stores since it can be easily aliased
      // with non-constant index.
    } else if (!heap_location_collector_.MayDeoptimize() &&
               ref_info->IsSingletonAndNotReturned()) {
      // Store into a field of a singleton that's not returned. The value cannot be
      // killed due to aliasing/invocation. It can be redundant since future loads can
      // directly get the value set by this instruction. The value can still be killed due to
      // merging or loop side effects. Stores whose values are killed due to merging/loop side
      // effects later will be removed from possibly_removed_stores_ when that is detected.
      possibly_redundant = true;
      HNewInstance* new_instance = ref_info->GetReference()->AsNewInstance();
      DCHECK(new_instance != nullptr);
      if (new_instance->IsFinalizable()) {
        // Finalizable objects escape globally. Need to keep the store.
        possibly_redundant = false;
      } else {
        HLoopInformation* loop_info = instruction->GetBlock()->GetLoopInformation();
        if (loop_info != nullptr) {
          // instruction is a store in the loop so the loop must does write.
          DCHECK(side_effects_.GetLoopEffects(loop_info->GetHeader()).DoesAnyWrite());
          // If it's a singleton, IsValueKilledByLoopSideEffects() must be true.
          DCHECK(!ref_info->IsSingleton() ||
                 heap_location_collector_.GetHeapLocation(idx)->IsValueKilledByLoopSideEffects());

          if (loop_info->IsDefinedOutOfTheLoop(original_ref)) {
            DCHECK(original_ref->GetBlock()->Dominates(loop_info->GetPreHeader()));
            // Keep the store since its value may be needed at the loop header.
            possibly_redundant = false;
          } else {
            // The singleton is created inside the loop. Value stored to it isn't needed at
            // the loop header. This is true for outer loops also.
          }
        }
      }
    }
    if (same_value || possibly_redundant) {
      possibly_removed_stores_.push_back(instruction);
    }

    if (!same_value) {
      if (possibly_redundant) {
        DCHECK(instruction->IsInstanceFieldSet());
        // Put the store as the heap value. If the value is loaded from heap
        // by a load later, this store isn't really redundant.
        heap_values[idx] = instruction;
      } else {
        heap_values[idx] = value;
      }
    }
    // This store may kill values in other heap locations due to aliasing.
    for (size_t i = 0; i < heap_values.size(); i++) {
      if (i == idx) {
        continue;
      }
      if (heap_values[i] == value) {
        // Same value should be kept even if aliasing happens.
        continue;
      }
      if (heap_values[i] == kUnknownHeapValue) {
        // Value is already unknown, no need for aliasing check.
        continue;
      }
      if (heap_location_collector_.MayAlias(i, idx)) {
        // Kill heap locations that may alias.
        heap_values[i] = kUnknownHeapValue;
      }
    }
  }

  void VisitInstanceFieldGet(HInstanceFieldGet* instruction) OVERRIDE {
    HInstruction* obj = instruction->InputAt(0);
    size_t offset = instruction->GetFieldInfo().GetFieldOffset().SizeValue();
    int16_t declaring_class_def_index = instruction->GetFieldInfo().GetDeclaringClassDefIndex();
    VisitGetLocation(instruction, obj, offset, nullptr, declaring_class_def_index);
  }

  void VisitInstanceFieldSet(HInstanceFieldSet* instruction) OVERRIDE {
    HInstruction* obj = instruction->InputAt(0);
    size_t offset = instruction->GetFieldInfo().GetFieldOffset().SizeValue();
    int16_t declaring_class_def_index = instruction->GetFieldInfo().GetDeclaringClassDefIndex();
    HInstruction* value = instruction->InputAt(1);
    VisitSetLocation(instruction, obj, offset, nullptr, declaring_class_def_index, value);
  }

  void VisitStaticFieldGet(HStaticFieldGet* instruction) OVERRIDE {
    HInstruction* cls = instruction->InputAt(0);
    size_t offset = instruction->GetFieldInfo().GetFieldOffset().SizeValue();
    int16_t declaring_class_def_index = instruction->GetFieldInfo().GetDeclaringClassDefIndex();
    VisitGetLocation(instruction, cls, offset, nullptr, declaring_class_def_index);
  }

  void VisitStaticFieldSet(HStaticFieldSet* instruction) OVERRIDE {
    HInstruction* cls = instruction->InputAt(0);
    size_t offset = instruction->GetFieldInfo().GetFieldOffset().SizeValue();
    int16_t declaring_class_def_index = instruction->GetFieldInfo().GetDeclaringClassDefIndex();
    HInstruction* value = instruction->InputAt(1);
    VisitSetLocation(instruction, cls, offset, nullptr, declaring_class_def_index, value);
  }

  void VisitArrayGet(HArrayGet* instruction) OVERRIDE {
    HInstruction* array = instruction->InputAt(0);
    HInstruction* index = instruction->InputAt(1);
    VisitGetLocation(instruction,
                     array,
                     HeapLocation::kInvalidFieldOffset,
                     index,
                     HeapLocation::kDeclaringClassDefIndexForArrays);
  }

  void VisitArraySet(HArraySet* instruction) OVERRIDE {
    HInstruction* array = instruction->InputAt(0);
    HInstruction* index = instruction->InputAt(1);
    HInstruction* value = instruction->InputAt(2);
    VisitSetLocation(instruction,
                     array,
                     HeapLocation::kInvalidFieldOffset,
                     index,
                     HeapLocation::kDeclaringClassDefIndexForArrays,
                     value);
  }

  void HandleInvoke(HInstruction* invoke) {
    ArenaVector<HInstruction*>& heap_values =
        heap_values_for_[invoke->GetBlock()->GetBlockId()];
    for (size_t i = 0; i < heap_values.size(); i++) {
      ReferenceInfo* ref_info = heap_location_collector_.GetHeapLocation(i)->GetReferenceInfo();
      if (ref_info->IsSingleton()) {
        // Singleton references cannot be seen by the callee.
      } else {
        heap_values[i] = kUnknownHeapValue;
      }
    }
  }

  void VisitInvokeStaticOrDirect(HInvokeStaticOrDirect* invoke) OVERRIDE {
    HandleInvoke(invoke);
  }

  void VisitInvokeVirtual(HInvokeVirtual* invoke) OVERRIDE {
    HandleInvoke(invoke);
  }

  void VisitInvokeInterface(HInvokeInterface* invoke) OVERRIDE {
    HandleInvoke(invoke);
  }

  void VisitInvokeUnresolved(HInvokeUnresolved* invoke) OVERRIDE {
    HandleInvoke(invoke);
  }

  void VisitClinitCheck(HClinitCheck* clinit) OVERRIDE {
    HandleInvoke(clinit);
  }

  void VisitUnresolvedInstanceFieldGet(HUnresolvedInstanceFieldGet* instruction) OVERRIDE {
    // Conservatively treat it as an invocation.
    HandleInvoke(instruction);
  }

  void VisitUnresolvedInstanceFieldSet(HUnresolvedInstanceFieldSet* instruction) OVERRIDE {
    // Conservatively treat it as an invocation.
    HandleInvoke(instruction);
  }

  void VisitUnresolvedStaticFieldGet(HUnresolvedStaticFieldGet* instruction) OVERRIDE {
    // Conservatively treat it as an invocation.
    HandleInvoke(instruction);
  }

  void VisitUnresolvedStaticFieldSet(HUnresolvedStaticFieldSet* instruction) OVERRIDE {
    // Conservatively treat it as an invocation.
    HandleInvoke(instruction);
  }

  void VisitNewInstance(HNewInstance* new_instance) OVERRIDE {
    ReferenceInfo* ref_info = heap_location_collector_.FindReferenceInfoOf(new_instance);
    if (ref_info == nullptr) {
      // new_instance isn't used for field accesses. No need to process it.
      return;
    }
    if (!heap_location_collector_.MayDeoptimize() &&
        ref_info->IsSingletonAndNotReturned() &&
        !new_instance->IsFinalizable() &&
        !new_instance->CanThrow()) {
      // TODO: add new_instance to singleton_new_instances_ and enable allocation elimination.
    }
    ArenaVector<HInstruction*>& heap_values =
        heap_values_for_[new_instance->GetBlock()->GetBlockId()];
    for (size_t i = 0; i < heap_values.size(); i++) {
      HInstruction* ref =
          heap_location_collector_.GetHeapLocation(i)->GetReferenceInfo()->GetReference();
      size_t offset = heap_location_collector_.GetHeapLocation(i)->GetOffset();
      if (ref == new_instance && offset >= mirror::kObjectHeaderSize) {
        // Instance fields except the header fields are set to default heap values.
        heap_values[i] = kDefaultHeapValue;
      }
    }
  }

  // Find an instruction's substitute if it should be removed.
  // Return the same instruction if it should not be removed.
  HInstruction* FindSubstitute(HInstruction* instruction) {
    size_t size = removed_loads_.size();
    for (size_t i = 0; i < size; i++) {
      if (removed_loads_[i] == instruction) {
        return substitute_instructions_for_loads_[i];
      }
    }
    return instruction;
  }

  const HeapLocationCollector& heap_location_collector_;
  const SideEffectsAnalysis& side_effects_;

  // One array of heap values for each block.
  ArenaVector<ArenaVector<HInstruction*>> heap_values_for_;

  // We record the instructions that should be eliminated but may be
  // used by heap locations. They'll be removed in the end.
  ArenaVector<HInstruction*> removed_loads_;
  ArenaVector<HInstruction*> substitute_instructions_for_loads_;

  // Stores in this list may be removed from the list later when it's
  // found that the store cannot be eliminated.
  ArenaVector<HInstruction*> possibly_removed_stores_;

  ArenaVector<HInstruction*> singleton_new_instances_;

  DISALLOW_COPY_AND_ASSIGN(LSEVisitor);
};

void LoadStoreElimination::Run() {
  if (graph_->IsDebuggable() || graph_->HasTryCatch()) {
    // Debugger may set heap values or trigger deoptimization of callers.
    // Try/catch support not implemented yet.
    // Skip this optimization.
    return;
  }
  HeapLocationCollector heap_location_collector(graph_);
  for (HReversePostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
    heap_location_collector.VisitBasicBlock(it.Current());
  }
  if (heap_location_collector.GetNumberOfHeapLocations() > kMaxNumberOfHeapLocations) {
    // Bail out if there are too many heap locations to deal with.
    return;
  }
  if (!heap_location_collector.HasHeapStores()) {
    // Without heap stores, this pass would act mostly as GVN on heap accesses.
    return;
  }
  if (heap_location_collector.HasVolatile() || heap_location_collector.HasMonitorOps()) {
    // Don't do load/store elimination if the method has volatile field accesses or
    // monitor operations, for now.
    // TODO: do it right.
    return;
  }
  heap_location_collector.BuildAliasingMatrix();
  LSEVisitor lse_visitor(graph_, heap_location_collector, side_effects_);
  for (HReversePostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
    lse_visitor.VisitBasicBlock(it.Current());
  }
  lse_visitor.RemoveInstructions();
}

}  // namespace art