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 "base/arena_bit_vector.h"
#include "base/array_ref.h"
#include "base/bit_vector-inl.h"
#include "base/scoped_arena_allocator.h"
#include "base/scoped_arena_containers.h"
#include "escape.h"
#include "load_store_analysis.h"
#include "optimizing/optimizing_compiler_stats.h"
#include "reference_type_propagation.h"

/**
 * The general algorithm of load-store elimination (LSE).
 *
 * We use load-store analysis to collect a list of heap locations and perform
 * alias analysis of those heap locations. LSE then keeps track of a list of
 * heap values corresponding to the heap locations and stores that put those
 * values in these locations.
 *  - In phase 1, we visit basic blocks in reverse post order and for each basic
 *    block, visit instructions sequentially, recording heap values and looking
 *    for loads and stores to eliminate without relying on loop Phis.
 *  - In phase 2, we look for loads that can be replaced by creating loop Phis
 *    or using a loop-invariant value.
 *  - In phase 3, we determine which stores are dead and can be eliminated and
 *    based on that information we re-evaluate whether some kept stores are
 *    storing the same value as the value in the heap location; such stores are
 *    also marked for elimination.
 *  - In phase 4, we commit the changes, replacing loads marked for elimination
 *    in previous processing and removing stores not marked for keeping. We also
 *    remove allocations that are no longer needed.
 *
 * 1. Walk over blocks and their instructions.
 *
 * The initial set of heap values for a basic block is
 *  - For a loop header of an irreducible loop, all heap values are unknown.
 *  - For a loop header of a normal loop, all values unknown at the end of the
 *    preheader are initialized to unknown, other heap values are set to Phi
 *    placeholders as we cannot determine yet whether these values are known on
 *    all back-edges. We use Phi placeholders also for array heap locations with
 *    index defined inside the loop but this helps only when the value remains
 *    zero from the array allocation throughout the loop.
 *  - For other basic blocks, we merge incoming values from the end of all
 *    predecessors. If any incoming value is unknown, the start value for this
 *    block is also unknown. Otherwise, if all the incoming values are the same
 *    (including the case of a single predecessor), the incoming value is used.
 *    Otherwise, we use a Phi placeholder to indicate different incoming values.
 *    We record whether such Phi placeholder depends on a loop Phi placeholder.
 *
 * For each instruction in the block
 *  - If the instruction is a load from a heap location with a known value not
 *    dependent on a loop Phi placeholder, the load can be eliminated, either by
 *    using an existing instruction or by creating new Phi(s) instead. In order
 *    to maintain the validity of all heap locations during the optimization
 *    phase, we only record substitutes at this phase and the real elimination
 *    is delayed till the end of LSE. Loads that require a loop Phi placeholder
 *    replacement are recorded for processing later.
 *  - If the instruction is a store, it updates the heap value for the heap
 *    location with the stored value and records the store itself so that we can
 *    mark it for keeping if the value becomes observable. Heap values are
 *    invalidated for heap locations that may alias with the store instruction's
 *    heap location and their recorded stores are marked for keeping as they are
 *    now potentially observable. The store instruction can be eliminated unless
 *    the value stored is later needed e.g. by a load from the same/aliased heap
 *    location or the heap location persists at method return/deoptimization.
 *  - A store that stores the same value as the heap value is eliminated.
 *  - For newly instantiated instances, their heap values are initialized to
 *    language defined default values.
 *  - Finalizable objects are considered as persisting at method
 *    return/deoptimization.
 *  - Some instructions such as invokes are treated as loading and invalidating
 *    all the heap values, depending on the instruction's side effects.
 *  - SIMD graphs (with VecLoad and VecStore instructions) are also handled. Any
 *    partial overlap access among ArrayGet/ArraySet/VecLoad/Store is seen as
 *    alias and no load/store is eliminated in such case.
 *  - Currently this LSE algorithm doesn't handle graph with try-catch, due to
 *    the special block merging structure.
 *
 * The time complexity of the initial phase has several components. The total
 * time for the initialization of heap values for all blocks is
 *    O(heap_locations * edges)
 * and the time complexity for simple instruction processing is
 *    O(instructions).
 * See the description of phase 3 for additional complexity due to matching of
 * existing Phis for replacing loads.
 *
 * 2. Process loads that depend on loop Phi placeholders.
 *
 * We go over these loads to determine whether they can be eliminated. We look
 * for the set of all Phi placeholders that feed the load and depend on a loop
 * Phi placeholder and, if we find no unknown value, we construct the necessary
 * Phi(s) or, if all other inputs are identical, i.e. the location does not
 * change in the loop, just use that input. If we do find an unknown input, this
 * must be from a loop back-edge and we replace the loop Phi placeholder with
 * unknown value and re-process loads and stores that previously depended on
 * loop Phi placeholders. This shall find at least one load of an unknown value
 * which is now known to be unreplaceable or a new unknown value on a back-edge
 * and we repeat this process until each load is either marked for replacement
 * or found to be unreplaceable. As we mark at least one additional loop Phi
 * placeholder as unreplacable in each iteration, this process shall terminate.
 *
 * The depth-first search for Phi placeholders in FindLoopPhisToMaterialize()
 * is limited by the number of Phi placeholders and their dependencies we need
 * to search with worst-case time complexity
 *    O(phi_placeholder_dependencies) .
 * The dependencies are usually just the Phi placeholders' potential inputs,
 * but if we use TryReplacingLoopPhiPlaceholderWithDefault() for default value
 * replacement search, there are additional dependencies to consider, see below.
 *
 * In the successful case (no unknown inputs found) we use the Floyd-Warshall
 * algorithm to determine transitive closures for each found Phi placeholder,
 * and then match or materialize Phis from the smallest transitive closure,
 * so that we can determine if such subset has a single other input. This has
 * time complexity
 *    O(phi_placeholders_found^3) .
 * Note that successful TryReplacingLoopPhiPlaceholderWithDefault() does not
 * contribute to this as such Phi placeholders are replaced immediately.
 * The total time of all such successful cases has time complexity
 *    O(phi_placeholders^3)
 * because the found sets are disjoint and `Sum(n_i^3) <= Sum(n_i)^3`. Similar
 * argument applies to the searches used to find all successful cases, so their
 * total contribution is also just an insignificant
 *    O(phi_placeholder_dependencies) .
 * The materialization of Phis has an insignificant total time complexity
 *    O(phi_placeholders * edges) .
 *
 * If we find an unknown input, we re-process heap values and loads with a time
 * complexity that's the same as the phase 1 in the worst case. Adding this to
 * the depth-first search time complexity yields
 *    O(phi_placeholder_dependencies + heap_locations * edges + instructions)
 * for a single iteration. We can ignore the middle term as it's proprotional
 * to the number of Phi placeholder inputs included in the first term. Using
 * the upper limit of number of such iterations, the total time complexity is
 *    O((phi_placeholder_dependencies + instructions) * phi_placeholders) .
 *
 * The upper bound of Phi placeholder inputs is
 *    heap_locations * edges
 * but if we use TryReplacingLoopPhiPlaceholderWithDefault(), the dependencies
 * include other heap locations in predecessor blocks with the upper bound of
 *    heap_locations^2 * edges .
 * Using the estimate
 *    edges <= blocks^2
 * and
 *    phi_placeholders <= heap_locations * blocks ,
 * the worst-case time complexity of the
 *    O(phi_placeholder_dependencies * phi_placeholders)
 * term from unknown input cases is actually
 *    O(heap_locations^3 * blocks^3) ,
 * exactly as the estimate for the Floyd-Warshall parts of successful cases.
 * Adding the other term from the unknown input cases (to account for the case
 * with significantly more instructions than blocks and heap locations), the
 * phase 2 time complexity is
 *    O(heap_locations^3 * blocks^3 + heap_locations * blocks * instructions) .
 *
 * See the description of phase 3 for additional complexity due to matching of
 * existing Phis for replacing loads.
 *
 * 3. Determine which stores to keep and which to eliminate.
 *
 * During instruction processing in phase 1 and re-processing in phase 2, we are
 * keeping a record of the stores and Phi placeholders that become observable
 * and now propagate the observable Phi placeholders to all actual stores that
 * feed them. Having determined observable stores, we look for stores that just
 * overwrite the old value with the same. Since ignoring non-observable stores
 * actually changes the old values in heap locations, we need to recalculate
 * Phi placeholder replacements but we proceed similarly to the previous phase.
 * We look for the set of all Phis that feed the old value replaced by the store
 * (but ignoring whether they depend on a loop Phi) and, if we find no unknown
 * value, we try to match existing Phis (we do not create new Phis anymore) or,
 * if all other inputs are identical, i.e. the location does not change in the
 * loop, just use that input. If this succeeds and the old value is identical to
 * the value we're storing, such store shall be eliminated.
 *
 * The work is similar to the phase 2, except that we're not re-processing loads
 * and stores anymore, so the time complexity of phase 3 is
 *    O(heap_locations^3 * blocks^3) .
 *
 * There is additional complexity in matching existing Phis shared between the
 * phases 1, 2 and 3. We are never trying to match two or more Phis at the same
 * time (this could be difficult and slow), so each matching attempt is just
 * looking at Phis in the block (both old Phis and newly created Phis) and their
 * inputs. As we create at most `heap_locations` Phis in each block, the upper
 * bound on the number of Phis we look at is
 *    heap_locations * (old_phis + heap_locations)
 * and the worst-case time complexity is
 *    O(heap_locations^2 * edges + heap_locations * old_phis * edges) .
 * The first term is lower than one term in phase 2, so the relevant part is
 *    O(heap_locations * old_phis * edges) .
 *
 * 4. Replace loads and remove unnecessary stores and singleton allocations.
 *
 * A special type of objects called singletons are instantiated in the method
 * and have a single name, i.e. no aliases. Singletons have exclusive heap
 * locations since they have no aliases. Singletons are helpful in narrowing
 * down the life span of a heap location such that they do not always need to
 * participate in merging heap values. Allocation of a singleton can be
 * eliminated if that singleton is not used and does not persist at method
 * return/deoptimization.
 *
 * The time complexity of this phase is
 *    O(instructions + instruction_uses) .
 *
 * FIXME: The time complexity described above assumes that the
 * HeapLocationCollector finds a heap location for an instruction in O(1)
 * time but it is currently O(heap_locations); this can be fixed by adding
 * a hash map to the HeapLocationCollector.
 */

namespace art {

// Use HGraphDelegateVisitor for which all VisitInvokeXXX() delegate to VisitInvoke().
class LSEVisitor final : private HGraphDelegateVisitor {
 public:
  LSEVisitor(HGraph* graph,
             const HeapLocationCollector& heap_location_collector,
             OptimizingCompilerStats* stats);

  void Run();

 private:
  class PhiPlaceholder {
   public:
    PhiPlaceholder(uint32_t block_id, uint32_t heap_location)
        : block_id_(block_id),
          heap_location_(dchecked_integral_cast<uint32_t>(heap_location)) {}

    uint32_t GetBlockId() const {
      return block_id_;
    }

    size_t GetHeapLocation() const {
      return heap_location_;
    }

   private:
    uint32_t block_id_;
    uint32_t heap_location_;
  };

  class Value {
   public:
    enum class Type {
      kInvalid,
      kUnknown,
      kDefault,
      kInstruction,
      kNeedsNonLoopPhi,
      kNeedsLoopPhi,
    };

    static Value Invalid() {
      Value value;
      value.type_ = Type::kInvalid;
      value.instruction_ = nullptr;
      return value;
    }

    // An unknown heap value. Loads with such a value in the heap location cannot be eliminated.
    // A heap location can be set to an unknown heap value when:
    // - it is coming from outside the method,
    // - it is killed due to aliasing, or side effects, or merging with an unknown value.
    static Value Unknown() {
      Value value;
      value.type_ = Type::kUnknown;
      value.instruction_ = nullptr;
      return value;
    }

    // Default heap value after an allocation.
    // A heap location can be set to that value right after an allocation.
    static Value Default() {
      Value value;
      value.type_ = Type::kDefault;
      value.instruction_ = nullptr;
      return value;
    }

    static Value ForInstruction(HInstruction* instruction) {
      Value value;
      value.type_ = Type::kInstruction;
      value.instruction_ = instruction;
      return value;
    }

    static Value ForNonLoopPhiPlaceholder(const PhiPlaceholder* phi_placeholder) {
      Value value;
      value.type_ = Type::kNeedsNonLoopPhi;
      value.phi_placeholder_ = phi_placeholder;
      return value;
    }

    static Value ForLoopPhiPlaceholder(const PhiPlaceholder* phi_placeholder) {
      Value value;
      value.type_ = Type::kNeedsLoopPhi;
      value.phi_placeholder_ = phi_placeholder;
      return value;
    }

    static Value ForPhiPlaceholder(const PhiPlaceholder* phi_placeholder, bool needs_loop_phi) {
      return needs_loop_phi ? ForLoopPhiPlaceholder(phi_placeholder)
                            : ForNonLoopPhiPlaceholder(phi_placeholder);
    }

    bool IsValid() const {
      return !IsInvalid();
    }

    bool IsInvalid() const {
      return type_ == Type::kInvalid;
    }

    bool IsUnknown() const {
      return type_ == Type::kUnknown;
    }

    bool IsDefault() const {
      return type_ == Type::kDefault;
    }

    bool IsInstruction() const {
      return type_ == Type::kInstruction;
    }

    bool NeedsNonLoopPhi() const {
      return type_ == Type::kNeedsNonLoopPhi;
    }

    bool NeedsLoopPhi() const {
      return type_ == Type::kNeedsLoopPhi;
    }

    bool NeedsPhi() const {
      return NeedsNonLoopPhi() || NeedsLoopPhi();
    }

    HInstruction* GetInstruction() const {
      DCHECK(IsInstruction());
      return instruction_;
    }

    const PhiPlaceholder* GetPhiPlaceholder() const {
      DCHECK(NeedsPhi());
      return phi_placeholder_;
    }

    bool Equals(Value other) const {
      // Only valid values can be compared.
      DCHECK(IsValid());
      DCHECK(other.IsValid());
      if (type_ != other.type_) {
        // Default values are equal to zero bit pattern instructions.
        return (IsDefault() && other.IsInstruction() && IsZeroBitPattern(other.GetInstruction())) ||
               (other.IsDefault() && IsInstruction() && IsZeroBitPattern(GetInstruction()));
      } else {
        // Note: Two unknown values are considered different.
        return IsDefault() ||
               (IsInstruction() && GetInstruction() == other.GetInstruction()) ||
               (NeedsPhi() && GetPhiPlaceholder() == other.GetPhiPlaceholder());
      }
    }

    bool Equals(HInstruction* instruction) const {
      return Equals(ForInstruction(instruction));
    }

   private:
    Type type_;
    union {
      HInstruction* instruction_;
      const PhiPlaceholder* phi_placeholder_;
    };
  };

  // Get Phi placeholder index for access to `phi_placeholder_replacements_`
  // and "visited" bit vectors during depth-first searches.
  size_t PhiPlaceholderIndex(const PhiPlaceholder* phi_placeholder) const {
    return static_cast<size_t>(phi_placeholder - phi_placeholders_.data());
  }

  size_t PhiPlaceholderIndex(Value phi_placeholder) const {
    return PhiPlaceholderIndex(phi_placeholder.GetPhiPlaceholder());
  }

  const PhiPlaceholder* GetPhiPlaceholder(uint32_t block_id, size_t idx) const {
    size_t phi_placeholders_begin = phi_placeholders_begin_for_block_[block_id];
    const PhiPlaceholder* phi_placeholder = &phi_placeholders_[phi_placeholders_begin + idx];
    DCHECK_EQ(phi_placeholder->GetBlockId(), block_id);
    DCHECK_EQ(phi_placeholder->GetHeapLocation(), idx);
    return phi_placeholder;
  }

  Value Replacement(Value value) const {
    DCHECK(value.NeedsPhi());
    Value replacement = phi_placeholder_replacements_[PhiPlaceholderIndex(value)];
    DCHECK(replacement.IsUnknown() || replacement.IsInstruction());
    DCHECK(replacement.IsUnknown() ||
           FindSubstitute(replacement.GetInstruction()) == replacement.GetInstruction());
    return replacement;
  }

  Value ReplacementOrValue(Value value) const {
    if (value.NeedsPhi() && phi_placeholder_replacements_[PhiPlaceholderIndex(value)].IsValid()) {
      return Replacement(value);
    } else {
      DCHECK(!value.IsInstruction() ||
             FindSubstitute(value.GetInstruction()) == value.GetInstruction());
      return value;
    }
  }

  static ScopedArenaVector<PhiPlaceholder> CreatePhiPlaceholders(
      HGraph* graph,
      const HeapLocationCollector& heap_location_collector,
      ScopedArenaAllocator* allocator);
  static ScopedArenaVector<size_t> CreatePhiPlaceholdersBeginForBlock(
      HGraph* graph,
      const HeapLocationCollector& heap_location_collector,
      ScopedArenaAllocator* allocator);

  // The record of a heap value and instruction(s) that feed that value.
  struct ValueRecord {
    Value value;
    Value stored_by;
  };

  HTypeConversion* FindOrAddTypeConversionIfNecessary(HInstruction* instruction,
                                                      HInstruction* value,
                                                      DataType::Type expected_type) {
    // Should never add type conversion into boolean value.
    if (expected_type == DataType::Type::kBool ||
        DataType::IsTypeConversionImplicit(value->GetType(), expected_type) ||
        // TODO: This prevents type conversion of default values but we can still insert
        // type conversion of other constants and there is no constant folding pass after LSE.
        IsZeroBitPattern(value)) {
      return nullptr;
    }

    // Check if there is already a suitable TypeConversion we can reuse.
    for (const HUseListNode<HInstruction*>& use : value->GetUses()) {
      if (use.GetUser()->IsTypeConversion() &&
          use.GetUser()->GetType() == expected_type &&
          // TODO: We could move the TypeConversion to a common dominator
          // if it does not cross irreducible loop header.
          use.GetUser()->GetBlock()->Dominates(instruction->GetBlock()) &&
          // Don't share across irreducible loop headers.
          // TODO: can be more fine-grained than this by testing each dominator.
          (use.GetUser()->GetBlock() == instruction->GetBlock() ||
           !GetGraph()->HasIrreducibleLoops())) {
        if (use.GetUser()->GetBlock() == instruction->GetBlock() &&
            use.GetUser()->GetBlock()->GetInstructions().FoundBefore(instruction, use.GetUser())) {
          // Move the TypeConversion before the instruction.
          use.GetUser()->MoveBefore(instruction);
        }
        DCHECK(use.GetUser()->StrictlyDominates(instruction));
        return use.GetUser()->AsTypeConversion();
      }
    }

    // We must create a new TypeConversion instruction.
    HTypeConversion* type_conversion = new (GetGraph()->GetAllocator()) HTypeConversion(
          expected_type, value, instruction->GetDexPc());
    instruction->GetBlock()->InsertInstructionBefore(type_conversion, instruction);
    return type_conversion;
  }

  // Find an instruction's substitute if it's a removed load.
  // Return the same instruction if it should not be removed.
  HInstruction* FindSubstitute(HInstruction* instruction) const {
    size_t id = static_cast<size_t>(instruction->GetId());
    if (id >= substitute_instructions_for_loads_.size()) {
      DCHECK(!IsLoad(instruction));  // New Phi (may not be in the graph yet) or default value.
      return instruction;
    }
    HInstruction* substitute = substitute_instructions_for_loads_[id];
    DCHECK(substitute == nullptr || IsLoad(instruction));
    return (substitute != nullptr) ? substitute : instruction;
  }

  void AddRemovedLoad(HInstruction* load, HInstruction* heap_value) {
    DCHECK(IsLoad(load));
    DCHECK_EQ(FindSubstitute(load), load);
    DCHECK_EQ(FindSubstitute(heap_value), heap_value) <<
        "Unexpected heap_value that has a substitute " << heap_value->DebugName();

    // The load expects to load the heap value as type load->GetType().
    // However the tracked heap value may not be of that type. An explicit
    // type conversion may be needed.
    // There are actually three types involved here:
    // (1) tracked heap value's type (type A)
    // (2) heap location (field or element)'s type (type B)
    // (3) load's type (type C)
    // We guarantee that type A stored as type B and then fetched out as
    // type C is the same as casting from type A to type C directly, since
    // type B and type C will have the same size which is guaranteed in
    // HInstanceFieldGet/HStaticFieldGet/HArrayGet/HVecLoad's SetType().
    // So we only need one type conversion from type A to type C.
    HTypeConversion* type_conversion = FindOrAddTypeConversionIfNecessary(
        load, heap_value, load->GetType());

    substitute_instructions_for_loads_[load->GetId()] =
        type_conversion != nullptr ? type_conversion : heap_value;
  }

  static bool IsLoad(HInstruction* instruction) {
    // Unresolved load is not treated as a load.
    return instruction->IsInstanceFieldGet() ||
           instruction->IsStaticFieldGet() ||
           instruction->IsVecLoad() ||
           instruction->IsArrayGet();
  }

  static bool IsStore(HInstruction* instruction) {
    // Unresolved store is not treated as a store.
    return instruction->IsInstanceFieldSet() ||
           instruction->IsArraySet() ||
           instruction->IsVecStore() ||
           instruction->IsStaticFieldSet();
  }

  // Check if it is allowed to use default values or Phis for the specified load.
  static bool IsDefaultOrPhiAllowedForLoad(HInstruction* instruction) {
    DCHECK(IsLoad(instruction));
    // Using defaults for VecLoads requires to create additional vector operations.
    // As there are some issues with scheduling vector operations it is better to avoid creating
    // them.
    return !instruction->IsVecOperation();
  }

  // Keep the store referenced by the instruction, or all stores that feed a Phi placeholder.
  // This is necessary if the stored heap value can be observed.
  void KeepStores(Value value) {
    if (value.IsUnknown()) {
      return;
    }
    if (value.NeedsPhi()) {
      phi_placeholders_to_search_for_kept_stores_.SetBit(PhiPlaceholderIndex(value));
    } else {
      HInstruction* instruction = value.GetInstruction();
      DCHECK(IsStore(instruction));
      kept_stores_.SetBit(instruction->GetId());
    }
  }

  // If a heap location X may alias with heap location at `loc_index`
  // and heap_values of that heap location X holds a store, keep that store.
  // It's needed for a dependent load that's not eliminated since any store
  // that may put value into the load's heap location needs to be kept.
  void KeepStoresIfAliasedToLocation(ScopedArenaVector<ValueRecord>& heap_values,
                                     size_t loc_index) {
    for (size_t i = 0u, size = heap_values.size(); i != size; ++i) {
      if (i == loc_index) {
        // We use this function when reading a location with unknown value and
        // therefore we cannot know what exact store wrote that unknown value.
        // But we can have a phi placeholder here marking multiple stores to keep.
        DCHECK(!heap_values[i].stored_by.IsInstruction());
        KeepStores(heap_values[i].stored_by);
        heap_values[i].stored_by = Value::Unknown();
      } else if (heap_location_collector_.MayAlias(i, loc_index)) {
        KeepStores(heap_values[i].stored_by);
        heap_values[i].stored_by = Value::Unknown();
      }
    }
  }

  // `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(DataType::Type type) {
    switch (type) {
      case DataType::Type::kReference:
        return GetGraph()->GetNullConstant();
      case DataType::Type::kBool:
      case DataType::Type::kUint8:
      case DataType::Type::kInt8:
      case DataType::Type::kUint16:
      case DataType::Type::kInt16:
      case DataType::Type::kInt32:
        return GetGraph()->GetIntConstant(0);
      case DataType::Type::kInt64:
        return GetGraph()->GetLongConstant(0);
      case DataType::Type::kFloat32:
        return GetGraph()->GetFloatConstant(0);
      case DataType::Type::kFloat64:
        return GetGraph()->GetDoubleConstant(0);
      default:
        UNREACHABLE();
    }
  }

  bool CanValueBeKeptIfSameAsNew(Value value,
                                 HInstruction* new_value,
                                 HInstruction* new_value_set_instr) {
    // For field/array set location operations, if the value is the same as the new_value
    // it can be kept even if aliasing happens. All aliased operations will access the same memory
    // range.
    // For vector values, this is not true. For example:
    //  packed_data = [0xA, 0xB, 0xC, 0xD];            <-- Different values in each lane.
    //  VecStore array[i  ,i+1,i+2,i+3] = packed_data;
    //  VecStore array[i+1,i+2,i+3,i+4] = packed_data; <-- We are here (partial overlap).
    //  VecLoad  vx = array[i,i+1,i+2,i+3];            <-- Cannot be eliminated because the value
    //                                                     here is not packed_data anymore.
    //
    // TODO: to allow such 'same value' optimization on vector data,
    // LSA needs to report more fine-grain MAY alias information:
    // (1) May alias due to two vector data partial overlap.
    //     e.g. a[i..i+3] and a[i+1,..,i+4].
    // (2) May alias due to two vector data may complete overlap each other.
    //     e.g. a[i..i+3] and b[i..i+3].
    // (3) May alias but the exact relationship between two locations is unknown.
    //     e.g. a[i..i+3] and b[j..j+3], where values of a,b,i,j are all unknown.
    // This 'same value' optimization can apply only on case (2).
    if (new_value_set_instr->IsVecOperation()) {
      return false;
    }

    return value.Equals(new_value);
  }

  Value PrepareLoopValue(HBasicBlock* block, size_t idx);
  Value PrepareLoopStoredBy(HBasicBlock* block, size_t idx);
  void PrepareLoopRecords(HBasicBlock* block);
  Value MergePredecessorValues(HBasicBlock* block, size_t idx);
  void MergePredecessorRecords(HBasicBlock* block);

  void MaterializeNonLoopPhis(const PhiPlaceholder* phi_placeholder, DataType::Type type);

  void VisitGetLocation(HInstruction* instruction, size_t idx);
  void VisitSetLocation(HInstruction* instruction, size_t idx, HInstruction* value);

  void VisitBasicBlock(HBasicBlock* block) override;

  enum class Phase {
    kLoadElimination,
    kStoreElimination
  };

  bool TryReplacingLoopPhiPlaceholderWithDefault(
      const PhiPlaceholder* phi_placeholder,
      DataType::Type type,
      /*inout*/ArenaBitVector* phi_placeholders_to_materialize);
  bool TryReplacingLoopPhiPlaceholderWithSingleInput(
      const PhiPlaceholder* phi_placeholder,
      /*inout*/ArenaBitVector* phi_placeholders_to_materialize);
  const PhiPlaceholder* FindLoopPhisToMaterialize(
      const PhiPlaceholder* phi_placeholder,
      /*out*/ArenaBitVector* phi_placeholders_to_materialize,
      DataType::Type type,
      bool can_use_default_or_phi);
  bool MaterializeLoopPhis(const ScopedArenaVector<size_t>& phi_placeholder_indexes,
                           DataType::Type type,
                           Phase phase);
  bool MaterializeLoopPhis(const ArenaBitVector& phi_placeholders_to_materialize,
                           DataType::Type type,
                           Phase phase);
  const PhiPlaceholder* TryToMaterializeLoopPhis(const PhiPlaceholder* phi_placeholder,
                                                 HInstruction* load);
  void ProcessLoopPhiWithUnknownInput(const PhiPlaceholder* loop_phi_with_unknown_input);
  void ProcessLoadsRequiringLoopPhis();

  void SearchPhiPlaceholdersForKeptStores();
  void UpdateValueRecordForStoreElimination(/*inout*/ValueRecord* value_record);
  void FindOldValueForPhiPlaceholder(const PhiPlaceholder* phi_placeholder, DataType::Type type);
  void FindStoresWritingOldValues();

  void VisitInstanceFieldGet(HInstanceFieldGet* instruction) override {
    HInstruction* object = instruction->InputAt(0);
    const FieldInfo& field = instruction->GetFieldInfo();
    VisitGetLocation(instruction, heap_location_collector_.GetFieldHeapLocation(object, &field));
  }

  void VisitInstanceFieldSet(HInstanceFieldSet* instruction) override {
    HInstruction* object = instruction->InputAt(0);
    const FieldInfo& field = instruction->GetFieldInfo();
    HInstruction* value = instruction->InputAt(1);
    size_t idx = heap_location_collector_.GetFieldHeapLocation(object, &field);
    VisitSetLocation(instruction, idx, value);
  }

  void VisitStaticFieldGet(HStaticFieldGet* instruction) override {
    HInstruction* cls = instruction->InputAt(0);
    const FieldInfo& field = instruction->GetFieldInfo();
    VisitGetLocation(instruction, heap_location_collector_.GetFieldHeapLocation(cls, &field));
  }

  void VisitStaticFieldSet(HStaticFieldSet* instruction) override {
    HInstruction* cls = instruction->InputAt(0);
    const FieldInfo& field = instruction->GetFieldInfo();
    HInstruction* value = instruction->InputAt(1);
    size_t idx = heap_location_collector_.GetFieldHeapLocation(cls, &field);
    VisitSetLocation(instruction, idx, value);
  }

  void VisitArrayGet(HArrayGet* instruction) override {
    VisitGetLocation(instruction, heap_location_collector_.GetArrayHeapLocation(instruction));
  }

  void VisitArraySet(HArraySet* instruction) override {
    size_t idx = heap_location_collector_.GetArrayHeapLocation(instruction);
    VisitSetLocation(instruction, idx, instruction->GetValue());
  }

  void VisitVecLoad(HVecLoad* instruction) override {
    VisitGetLocation(instruction, heap_location_collector_.GetArrayHeapLocation(instruction));
  }

  void VisitVecStore(HVecStore* instruction) override {
    size_t idx = heap_location_collector_.GetArrayHeapLocation(instruction);
    VisitSetLocation(instruction, idx, instruction->GetValue());
  }

  void VisitDeoptimize(HDeoptimize* instruction) override {
    ScopedArenaVector<ValueRecord>& heap_values =
        heap_values_for_[instruction->GetBlock()->GetBlockId()];
    for (size_t i = 0u, size = heap_values.size(); i != size; ++i) {
      Value* stored_by = &heap_values[i].stored_by;
      if (stored_by->IsUnknown()) {
        continue;
      }
      // Stores are generally observeable after deoptimization, except
      // for singletons that don't escape in the deoptimization environment.
      bool observable = true;
      ReferenceInfo* info = heap_location_collector_.GetHeapLocation(i)->GetReferenceInfo();
      if (info->IsSingleton()) {
        HInstruction* reference = info->GetReference();
        // Finalizable objects always escape.
        if (!reference->IsNewInstance() || !reference->AsNewInstance()->IsFinalizable()) {
          // Check whether the reference for a store is used by an environment local of
          // the HDeoptimize. If not, the singleton is not observed after deoptimization.
          const HUseList<HEnvironment*>& env_uses = reference->GetEnvUses();
          observable = std::any_of(
              env_uses.begin(),
              env_uses.end(),
              [instruction](const HUseListNode<HEnvironment*>& use) {
                return use.GetUser()->GetHolder() == instruction;
              });
        }
      }
      if (observable) {
        KeepStores(*stored_by);
        *stored_by = Value::Unknown();
      }
    }
  }

  // Keep necessary stores before exiting a method via return/throw.
  void HandleExit(HBasicBlock* block) {
    ScopedArenaVector<ValueRecord>& heap_values = heap_values_for_[block->GetBlockId()];
    for (size_t i = 0u, size = heap_values.size(); i != size; ++i) {
      ReferenceInfo* ref_info = heap_location_collector_.GetHeapLocation(i)->GetReferenceInfo();
      if (!ref_info->IsSingletonAndRemovable()) {
        KeepStores(heap_values[i].stored_by);
        heap_values[i].stored_by = Value::Unknown();
      }
    }
  }

  void VisitReturn(HReturn* instruction) override {
    HandleExit(instruction->GetBlock());
  }

  void VisitReturnVoid(HReturnVoid* return_void) override {
    HandleExit(return_void->GetBlock());
  }

  void VisitThrow(HThrow* throw_instruction) override {
    HandleExit(throw_instruction->GetBlock());
  }

  void HandleInvoke(HInstruction* instruction) {
    SideEffects side_effects = instruction->GetSideEffects();
    ScopedArenaVector<ValueRecord>& heap_values =
        heap_values_for_[instruction->GetBlock()->GetBlockId()];
    for (size_t i = 0u, size = heap_values.size(); i != size; ++i) {
      ReferenceInfo* ref_info = heap_location_collector_.GetHeapLocation(i)->GetReferenceInfo();
      if (ref_info->IsSingleton()) {
        // Singleton references cannot be seen by the callee.
      } else {
        if (side_effects.DoesAnyRead() || side_effects.DoesAnyWrite()) {
          // Previous stores may become visible (read) and/or impossible for LSE to track (write).
          KeepStores(heap_values[i].stored_by);
          heap_values[i].stored_by = Value::Unknown();
        }
        if (side_effects.DoesAnyWrite()) {
          // The value may be clobbered.
          heap_values[i].value = Value::Unknown();
        }
      }
    }
  }

  void VisitInvoke(HInvoke* invoke) override {
    HandleInvoke(invoke);
  }

  void VisitClinitCheck(HClinitCheck* clinit) override {
    // Class initialization check can result in class initializer calling arbitrary methods.
    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 (ref_info->IsSingletonAndRemovable() && !new_instance->NeedsChecks()) {
      DCHECK(!new_instance->IsFinalizable());
      // new_instance can potentially be eliminated.
      singleton_new_instances_.push_back(new_instance);
    }
    ScopedArenaVector<ValueRecord>& heap_values =
        heap_values_for_[new_instance->GetBlock()->GetBlockId()];
    for (size_t i = 0u, size = heap_values.size(); i != 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].value = Value::Default();
        heap_values[i].stored_by = Value::Unknown();
      }
    }
  }

  void VisitNewArray(HNewArray* new_array) override {
    ReferenceInfo* ref_info = heap_location_collector_.FindReferenceInfoOf(new_array);
    if (ref_info == nullptr) {
      // new_array isn't used for array accesses. No need to process it.
      return;
    }
    if (ref_info->IsSingletonAndRemovable()) {
      if (new_array->GetLength()->IsIntConstant() &&
          new_array->GetLength()->AsIntConstant()->GetValue() >= 0) {
        // new_array can potentially be eliminated.
        singleton_new_instances_.push_back(new_array);
      } else {
        // new_array may throw NegativeArraySizeException. Keep it.
      }
    }
    ScopedArenaVector<ValueRecord>& heap_values =
        heap_values_for_[new_array->GetBlock()->GetBlockId()];
    for (size_t i = 0u, size = heap_values.size(); i != size; ++i) {
      HeapLocation* location = heap_location_collector_.GetHeapLocation(i);
      HInstruction* ref = location->GetReferenceInfo()->GetReference();
      if (ref == new_array && location->GetIndex() != nullptr) {
        // Array elements are set to default heap values.
        heap_values[i].value = Value::Default();
        heap_values[i].stored_by = Value::Unknown();
      }
    }
  }

  const HeapLocationCollector& heap_location_collector_;

  // Use local allocator for allocating memory.
  ScopedArenaAllocator allocator_;

  // Phi placeholders used for keeping track of values and stores for multiple predecessors.
  ScopedArenaVector<PhiPlaceholder> phi_placeholders_;

  // The start of the Phi placeholders in the `phi_placeholders_`
  // for each block with multiple predecessors.
  ScopedArenaVector<size_t> phi_placeholders_begin_for_block_;

  // One array of heap value records for each block.
  ScopedArenaVector<ScopedArenaVector<ValueRecord>> heap_values_for_;

  // We record loads and stores for re-processing when we find a loop Phi placeholder
  // with unknown value from a predecessor, and also for removing stores that are
  // found to be dead, i.e. not marked in `kept_stores_` at the end.
  struct LoadStoreRecord {
    HInstruction* load_or_store;
    size_t heap_location_index;
  };
  ScopedArenaVector<LoadStoreRecord> loads_and_stores_;

  // We record the substitute instructions for loads that should be
  // eliminated but may be used by heap locations. They'll be removed
  // in the end. These are indexed by the load's id.
  ScopedArenaVector<HInstruction*> substitute_instructions_for_loads_;

  // Record stores to keep in a bit vector indexed by instruction ID.
  ArenaBitVector kept_stores_;
  // When we need to keep all stores that feed a Phi placeholder, we just record the
  // index of that placeholder for processing after graph traversal.
  ArenaBitVector phi_placeholders_to_search_for_kept_stores_;

  // Loads that would require a loop Phi to replace are recorded for processing
  // later as we do not have enough information from back-edges to determine if
  // a suitable Phi can be found or created when we visit these loads.
  ScopedArenaHashMap<HInstruction*, ValueRecord> loads_requiring_loop_phi_;

  // For stores, record the old value records that were replaced and the stored values.
  struct StoreRecord {
    ValueRecord old_value_record;
    HInstruction* stored_value;
  };
  ScopedArenaHashMap<HInstruction*, StoreRecord> store_records_;

  // Replacements for Phi placeholders.
  // The unknown heap value is used to mark Phi placeholders that cannot be replaced.
  ScopedArenaVector<Value> phi_placeholder_replacements_;

  ScopedArenaVector<HInstruction*> singleton_new_instances_;

  DISALLOW_COPY_AND_ASSIGN(LSEVisitor);
};

ScopedArenaVector<LSEVisitor::PhiPlaceholder> LSEVisitor::CreatePhiPlaceholders(
    HGraph* graph,
    const HeapLocationCollector& heap_location_collector,
    ScopedArenaAllocator* allocator) {
  size_t num_phi_placeholders = 0u;
  size_t num_heap_locations = heap_location_collector.GetNumberOfHeapLocations();
  for (HBasicBlock* block : graph->GetReversePostOrder()) {
    if (block->GetPredecessors().size() >= 2u) {
      num_phi_placeholders += num_heap_locations;
    }
  }
  ScopedArenaVector<PhiPlaceholder> phi_placeholders(allocator->Adapter(kArenaAllocLSE));
  phi_placeholders.reserve(num_phi_placeholders);
  for (HBasicBlock* block : graph->GetReversePostOrder()) {
    if (block->GetPredecessors().size() >= 2u) {
      // Create Phi placeholders referencing the block by the block ID.
      DCHECK_LE(num_heap_locations, phi_placeholders.capacity() - phi_placeholders.size());
      uint32_t block_id = block->GetBlockId();
      for (size_t idx = 0; idx != num_heap_locations; ++idx) {
        phi_placeholders.push_back(PhiPlaceholder(block_id, idx));
      }
    }
  }
  return phi_placeholders;
}

ScopedArenaVector<size_t> LSEVisitor::CreatePhiPlaceholdersBeginForBlock(
    HGraph* graph,
    const HeapLocationCollector& heap_location_collector,
    ScopedArenaAllocator* allocator) {
  size_t num_phi_placeholders = 0u;
  size_t num_heap_locations = heap_location_collector.GetNumberOfHeapLocations();
  ScopedArenaVector<size_t> phi_placeholders_begin_for_block(graph->GetBlocks().size(),
                                                             allocator->Adapter(kArenaAllocLSE));
  for (HBasicBlock* block : graph->GetReversePostOrder()) {
    if (block->GetPredecessors().size() >= 2u) {
      phi_placeholders_begin_for_block[block->GetBlockId()] = num_phi_placeholders;
      num_phi_placeholders += num_heap_locations;
    }
  }
  return phi_placeholders_begin_for_block;
}

LSEVisitor::LSEVisitor(HGraph* graph,
                       const HeapLocationCollector& heap_location_collector,
                       OptimizingCompilerStats* stats)
    : HGraphDelegateVisitor(graph, stats),
      heap_location_collector_(heap_location_collector),
      allocator_(graph->GetArenaStack()),
      phi_placeholders_(CreatePhiPlaceholders(graph, heap_location_collector, &allocator_)),
      phi_placeholders_begin_for_block_(
          CreatePhiPlaceholdersBeginForBlock(graph, heap_location_collector, &allocator_)),
      heap_values_for_(graph->GetBlocks().size(),
                       ScopedArenaVector<ValueRecord>(allocator_.Adapter(kArenaAllocLSE)),
                       allocator_.Adapter(kArenaAllocLSE)),
      loads_and_stores_(allocator_.Adapter(kArenaAllocLSE)),
      // We may add new instructions (default values, Phis) but we're not adding loads
      // or stores, so we shall not need to resize following vector and BitVector.
      substitute_instructions_for_loads_(graph->GetCurrentInstructionId(),
                                         nullptr,
                                         allocator_.Adapter(kArenaAllocLSE)),
      kept_stores_(&allocator_,
                   /*start_bits=*/ graph->GetCurrentInstructionId(),
                   /*expandable=*/ false,
                   kArenaAllocLSE),
      phi_placeholders_to_search_for_kept_stores_(&allocator_,
                                                  phi_placeholders_.size(),
                                                  /*expandable=*/ false,
                                                  kArenaAllocLSE),
      loads_requiring_loop_phi_(allocator_.Adapter(kArenaAllocLSE)),
      store_records_(allocator_.Adapter(kArenaAllocLSE)),
      phi_placeholder_replacements_(phi_placeholders_.size(),
                                    Value::Invalid(),
                                    allocator_.Adapter(kArenaAllocLSE)),
      singleton_new_instances_(allocator_.Adapter(kArenaAllocLSE)) {
  // Clear bit vectors.
  phi_placeholders_to_search_for_kept_stores_.ClearAllBits();
  kept_stores_.ClearAllBits();
}

LSEVisitor::Value LSEVisitor::PrepareLoopValue(HBasicBlock* block, size_t idx) {
  // If the pre-header value is known (which implies that the reference dominates this
  // block), use a Phi placeholder for the value in the loop header. If all predecessors
  // are later found to have a known value, we can replace loads from this location,
  // either with the pre-header value or with a new Phi. For array locations, the index
  // may be defined inside the loop but the only known value in that case should be the
  // default value or a Phi placeholder that can be replaced only with the default value.
  HLoopInformation* loop_info = block->GetLoopInformation();
  uint32_t pre_header_block_id = loop_info->GetPreHeader()->GetBlockId();
  Value pre_header_value = ReplacementOrValue(heap_values_for_[pre_header_block_id][idx].value);
  if (pre_header_value.IsUnknown()) {
    return Value::Unknown();
  }
  if (kIsDebugBuild) {
    // Check that the reference indeed dominates this loop.
    HeapLocation* location = heap_location_collector_.GetHeapLocation(idx);
    HInstruction* ref = location->GetReferenceInfo()->GetReference();
    CHECK(ref->GetBlock() != block && ref->GetBlock()->Dominates(block));
    // Check that the index, if defined inside the loop, tracks a default value
    // or a Phi placeholder requiring a loop Phi.
    HInstruction* index = location->GetIndex();
    if (index != nullptr && loop_info->Contains(*index->GetBlock())) {
      CHECK(pre_header_value.NeedsLoopPhi() || pre_header_value.Equals(Value::Default()));
    }
  }
  const PhiPlaceholder* phi_placeholder = GetPhiPlaceholder(block->GetBlockId(), idx);
  return ReplacementOrValue(Value::ForLoopPhiPlaceholder(phi_placeholder));
}

LSEVisitor::Value LSEVisitor::PrepareLoopStoredBy(HBasicBlock* block, size_t idx) {
  // Use the Phi placeholder for `stored_by` to make sure all incoming stores are kept
  // if the value in the location escapes. This is not applicable to singletons that are
  // defined inside the loop as they shall be dead in the loop header.
  ReferenceInfo* ref_info = heap_location_collector_.GetHeapLocation(idx)->GetReferenceInfo();
  if (ref_info->IsSingleton() &&
      block->GetLoopInformation()->Contains(*ref_info->GetReference()->GetBlock())) {
    return Value::Unknown();
  }
  const PhiPlaceholder* phi_placeholder = GetPhiPlaceholder(block->GetBlockId(), idx);
  return Value::ForLoopPhiPlaceholder(phi_placeholder);
}

void LSEVisitor::PrepareLoopRecords(HBasicBlock* block) {
  DCHECK(block->IsLoopHeader());
  int block_id = block->GetBlockId();
  HBasicBlock* pre_header = block->GetLoopInformation()->GetPreHeader();
  ScopedArenaVector<ValueRecord>& pre_header_heap_values =
      heap_values_for_[pre_header->GetBlockId()];
  size_t num_heap_locations = heap_location_collector_.GetNumberOfHeapLocations();
  DCHECK_EQ(num_heap_locations, pre_header_heap_values.size());
  ScopedArenaVector<ValueRecord>& heap_values = heap_values_for_[block_id];
  DCHECK(heap_values.empty());

  // Don't eliminate loads in irreducible loops.
  if (block->GetLoopInformation()->IsIrreducible()) {
    heap_values.resize(num_heap_locations,
                       { /*value=*/ Value::Unknown(), /*stored_by=*/ Value::Unknown() });
    // Also keep the stores before the loop header, including in blocks that were not visited yet.
    for (size_t idx = 0u; idx != num_heap_locations; ++idx) {
      KeepStores(Value::ForLoopPhiPlaceholder(GetPhiPlaceholder(block->GetBlockId(), idx)));
    }
    return;
  }

  // Fill `heap_values` based on values from pre-header.
  heap_values.reserve(num_heap_locations);
  for (size_t idx = 0u; idx != num_heap_locations; ++idx) {
    heap_values.push_back({ PrepareLoopValue(block, idx), PrepareLoopStoredBy(block, idx) });
  }
}

LSEVisitor::Value LSEVisitor::MergePredecessorValues(HBasicBlock* block, size_t idx) {
  ArrayRef<HBasicBlock* const> predecessors(block->GetPredecessors());
  DCHECK(!predecessors.empty());
  Value merged_value =
      ReplacementOrValue(heap_values_for_[predecessors[0]->GetBlockId()][idx].value);
  for (size_t i = 1u, size = predecessors.size(); i != size; ++i) {
    if (merged_value.IsUnknown()) {
      break;
    }
    Value pred_value =
        ReplacementOrValue(heap_values_for_[predecessors[i]->GetBlockId()][idx].value);
    if (pred_value.IsUnknown()) {
      merged_value = Value::Unknown();
    } else if (!pred_value.Equals(merged_value)) {
      // There are conflicting known values. We may still be able to replace loads with a Phi.
      const PhiPlaceholder* phi_placeholder = GetPhiPlaceholder(block->GetBlockId(), idx);
      // Propagate the need for a new loop Phi from all predecessors.
      bool needs_loop_phi = merged_value.NeedsLoopPhi() || pred_value.NeedsLoopPhi();
      merged_value = ReplacementOrValue(Value::ForPhiPlaceholder(phi_placeholder, needs_loop_phi));
    }
  }
  return merged_value;
}

void LSEVisitor::MergePredecessorRecords(HBasicBlock* block) {
  if (block->IsExitBlock()) {
    // Exit block doesn't really merge values since the control flow ends in
    // its predecessors. Each predecessor needs to make sure stores are kept
    // if necessary.
    return;
  }

  ScopedArenaVector<ValueRecord>& heap_values = heap_values_for_[block->GetBlockId()];
  DCHECK(heap_values.empty());
  size_t num_heap_locations = heap_location_collector_.GetNumberOfHeapLocations();
  if (block->GetPredecessors().empty()) {
    DCHECK(block->IsEntryBlock());
    heap_values.resize(num_heap_locations,
                       { /*value=*/ Value::Unknown(), /*stored_by=*/ Value::Unknown() });
    return;
  }

  heap_values.reserve(num_heap_locations);
  for (size_t idx = 0u; idx != num_heap_locations; ++idx) {
    Value merged_value = MergePredecessorValues(block, idx);
    if (kIsDebugBuild) {
      if (merged_value.NeedsPhi()) {
        uint32_t block_id = merged_value.GetPhiPlaceholder()->GetBlockId();
        CHECK(GetGraph()->GetBlocks()[block_id]->Dominates(block));
      } else if (merged_value.IsInstruction()) {
        CHECK(merged_value.GetInstruction()->GetBlock()->Dominates(block));
      }
    }
    ArrayRef<HBasicBlock* const> predecessors(block->GetPredecessors());
    Value merged_stored_by = heap_values_for_[predecessors[0]->GetBlockId()][idx].stored_by;
    for (size_t predecessor_idx = 1u; predecessor_idx != predecessors.size(); ++predecessor_idx) {
      uint32_t predecessor_block_id = predecessors[predecessor_idx]->GetBlockId();
      Value stored_by = heap_values_for_[predecessor_block_id][idx].stored_by;
      if ((!stored_by.IsUnknown() || !merged_stored_by.IsUnknown()) &&
          !merged_stored_by.Equals(stored_by)) {
        // Use the Phi placeholder to track that we need to keep stores from all predecessors.
        const PhiPlaceholder* phi_placeholder = GetPhiPlaceholder(block->GetBlockId(), idx);
        merged_stored_by = Value::ForNonLoopPhiPlaceholder(phi_placeholder);
        break;
      }
    }
    heap_values.push_back({ merged_value, merged_stored_by });
  }
}

static HInstruction* FindOrConstructNonLoopPhi(
    HBasicBlock* block,
    const ScopedArenaVector<HInstruction*>& phi_inputs,
    DataType::Type type) {
  for (HInstructionIterator phi_it(block->GetPhis()); !phi_it.Done(); phi_it.Advance()) {
    HInstruction* phi = phi_it.Current();
    DCHECK_EQ(phi->InputCount(), phi_inputs.size());
    auto cmp = [](HInstruction* lhs, const HUserRecord<HInstruction*>& rhs) {
      return lhs == rhs.GetInstruction();
    };
    if (std::equal(phi_inputs.begin(), phi_inputs.end(), phi->GetInputRecords().begin(), cmp)) {
      return phi;
    }
  }
  ArenaAllocator* allocator = block->GetGraph()->GetAllocator();
  HPhi* phi = new (allocator) HPhi(allocator, kNoRegNumber, phi_inputs.size(), type);
  for (size_t i = 0, size = phi_inputs.size(); i != size; ++i) {
    DCHECK_NE(phi_inputs[i]->GetType(), DataType::Type::kVoid) << phi_inputs[i]->DebugName();
    phi->SetRawInputAt(i, phi_inputs[i]);
  }
  block->AddPhi(phi);
  if (type == DataType::Type::kReference) {
    // Update reference type information. Pass invalid handles, these are not used for Phis.
    ReferenceTypePropagation rtp_fixup(block->GetGraph(),
                                       Handle<mirror::ClassLoader>(),
                                       Handle<mirror::DexCache>(),
                                       /* is_first_run= */ false);
    rtp_fixup.Visit(phi);
  }
  return phi;
}

void LSEVisitor::MaterializeNonLoopPhis(const PhiPlaceholder* phi_placeholder,
                                        DataType::Type type) {
  DCHECK(phi_placeholder_replacements_[PhiPlaceholderIndex(phi_placeholder)].IsInvalid());
  const ArenaVector<HBasicBlock*>& blocks = GetGraph()->GetBlocks();
  size_t idx = phi_placeholder->GetHeapLocation();

  // Use local allocator to reduce peak memory usage.
  ScopedArenaAllocator allocator(allocator_.GetArenaStack());
  // Reuse the same vector for collecting phi inputs.
  ScopedArenaVector<HInstruction*> phi_inputs(allocator.Adapter(kArenaAllocLSE));

  ScopedArenaVector<const PhiPlaceholder*> work_queue(allocator.Adapter(kArenaAllocLSE));
  work_queue.push_back(phi_placeholder);
  while (!work_queue.empty()) {
    const PhiPlaceholder* current_phi_placeholder = work_queue.back();
    if (phi_placeholder_replacements_[PhiPlaceholderIndex(current_phi_placeholder)].IsValid()) {
      // This Phi placeholder was pushed to the `work_queue` followed by another Phi placeholder
      // that directly or indirectly depends on it, so it was already processed as part of the
      // other Phi placeholder's dependencies before this one got back to the top of the stack.
      work_queue.pop_back();
      continue;
    }
    uint32_t current_block_id = current_phi_placeholder->GetBlockId();
    HBasicBlock* current_block = blocks[current_block_id];
    DCHECK_GE(current_block->GetPredecessors().size(), 2u);

    // Non-loop Phis cannot depend on a loop Phi, so we should not see any loop header here.
    // And the only way for such merged value to reach a different heap location is through
    // a load at which point we materialize the Phi. Therefore all non-loop Phi placeholders
    // seen here are tied to one heap location.
    DCHECK(!current_block->IsLoopHeader());
    DCHECK_EQ(current_phi_placeholder->GetHeapLocation(), idx);

    phi_inputs.clear();
    for (HBasicBlock* predecessor : current_block->GetPredecessors()) {
      Value pred_value = ReplacementOrValue(heap_values_for_[predecessor->GetBlockId()][idx].value);
      DCHECK(!pred_value.IsUnknown());
      if (pred_value.NeedsNonLoopPhi()) {
        // We need to process the Phi placeholder first.
        work_queue.push_back(pred_value.GetPhiPlaceholder());
      } else if (pred_value.IsDefault()) {
        phi_inputs.push_back(GetDefaultValue(type));
      } else {
        phi_inputs.push_back(pred_value.GetInstruction());
      }
    }
    if (phi_inputs.size() == current_block->GetPredecessors().size()) {
      // All inputs are available. Find or construct the Phi replacement.
      phi_placeholder_replacements_[PhiPlaceholderIndex(current_phi_placeholder)] =
          Value::ForInstruction(FindOrConstructNonLoopPhi(current_block, phi_inputs, type));
      // Remove the block from the queue.
      DCHECK_EQ(current_phi_placeholder, work_queue.back());
      work_queue.pop_back();
    }
  }
}

void LSEVisitor::VisitGetLocation(HInstruction* instruction, size_t idx) {
  DCHECK_NE(idx, HeapLocationCollector::kHeapLocationNotFound);
  uint32_t block_id = instruction->GetBlock()->GetBlockId();
  ScopedArenaVector<ValueRecord>& heap_values = heap_values_for_[block_id];
  ValueRecord& record = heap_values[idx];
  DCHECK(record.value.IsUnknown() || record.value.Equals(ReplacementOrValue(record.value)));
  loads_and_stores_.push_back({ instruction, idx });
  if ((record.value.IsDefault() || record.value.NeedsNonLoopPhi()) &&
      !IsDefaultOrPhiAllowedForLoad(instruction)) {
    record.value = Value::Unknown();
  }
  if (record.value.IsDefault()) {
    KeepStores(record.stored_by);
    HInstruction* constant = GetDefaultValue(instruction->GetType());
    AddRemovedLoad(instruction, constant);
    record.value = Value::ForInstruction(constant);
  } else if (record.value.IsUnknown()) {
    // Load isn't eliminated. Put the load as the value into the HeapLocation.
    // This acts like GVN but with better aliasing analysis.
    record.value = Value::ForInstruction(instruction);
    KeepStoresIfAliasedToLocation(heap_values, idx);
  } else if (record.value.NeedsLoopPhi()) {
    // We do not know yet if the value is known for all back edges. Record for future processing.
    loads_requiring_loop_phi_.insert(std::make_pair(instruction, record));
  } else {
    // This load can be eliminated but we may need to construct non-loop Phis.
    if (record.value.NeedsNonLoopPhi()) {
      MaterializeNonLoopPhis(record.value.GetPhiPlaceholder(), instruction->GetType());
      record.value = Replacement(record.value);
    }
    HInstruction* heap_value = FindSubstitute(record.value.GetInstruction());
    AddRemovedLoad(instruction, heap_value);
    TryRemovingNullCheck(instruction);
  }
}

void LSEVisitor::VisitSetLocation(HInstruction* instruction, size_t idx, HInstruction* value) {
  DCHECK_NE(idx, HeapLocationCollector::kHeapLocationNotFound);
  DCHECK(!IsStore(value)) << value->DebugName();
  // value may already have a substitute.
  value = FindSubstitute(value);
  HBasicBlock* block = instruction->GetBlock();
  ScopedArenaVector<ValueRecord>& heap_values = heap_values_for_[block->GetBlockId()];
  ValueRecord& record = heap_values[idx];
  DCHECK(!record.value.IsInstruction() ||
         FindSubstitute(record.value.GetInstruction()) == record.value.GetInstruction());

  if (record.value.Equals(value)) {
    // Store into the heap location with the same value.
    // This store can be eliminated right away.
    block->RemoveInstruction(instruction);
    return;
  }

  store_records_.insert(std::make_pair(instruction, StoreRecord{record, value}));
  loads_and_stores_.push_back({ instruction, idx });

  // If the `record.stored_by` specified a store from this block, it shall be removed
  // at the end, except for throwing ArraySet; it cannot be marked for keeping in
  // `kept_stores_` anymore after we update the `record.stored_by` below.
  DCHECK(!record.stored_by.IsInstruction() ||
         record.stored_by.GetInstruction()->GetBlock() != block ||
         record.stored_by.GetInstruction()->CanThrow() ||
         !kept_stores_.IsBitSet(record.stored_by.GetInstruction()->GetId()));

  if (instruction->CanThrow()) {
    // Previous stores can become visible.
    HandleExit(instruction->GetBlock());
    // We cannot remove a possibly throwing store.
    // After marking it as kept, it does not matter if we track it in `stored_by` or not.
    kept_stores_.SetBit(instruction->GetId());
  }

  // Update the record.
  auto it = loads_requiring_loop_phi_.find(value);
  if (it != loads_requiring_loop_phi_.end()) {
    // Propapate the Phi placeholder to the record.
    record.value = it->second.value;
    DCHECK(record.value.NeedsLoopPhi());
  } else {
    record.value = Value::ForInstruction(value);
  }
  // Track the store in the value record. If the value is loaded or needed after
  // return/deoptimization later, this store isn't really redundant.
  record.stored_by = Value::ForInstruction(instruction);

  // This store may kill values in other heap locations due to aliasing.
  for (size_t i = 0u, size = heap_values.size(); i != size; ++i) {
    if (i == idx ||
        heap_values[i].value.IsUnknown() ||
        CanValueBeKeptIfSameAsNew(heap_values[i].value, value, instruction) ||
        !heap_location_collector_.MayAlias(i, idx)) {
      continue;
    }
    // Kill heap locations that may alias and keep previous stores to these locations.
    KeepStores(heap_values[i].stored_by);
    heap_values[i].stored_by = Value::Unknown();
    heap_values[i].value = Value::Unknown();
  }
}

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

bool LSEVisitor::TryReplacingLoopPhiPlaceholderWithDefault(
    const PhiPlaceholder* phi_placeholder,
    DataType::Type type,
    /*inout*/ArenaBitVector* phi_placeholders_to_materialize) {
  // Use local allocator to reduce peak memory usage.
  ScopedArenaAllocator allocator(allocator_.GetArenaStack());
  ArenaBitVector visited(&allocator,
                         /*start_bits=*/ phi_placeholders_.size(),
                         /*expandable=*/ false,
                         kArenaAllocLSE);
  visited.ClearAllBits();
  ScopedArenaVector<const PhiPlaceholder*> work_queue(allocator.Adapter(kArenaAllocLSE));

  // Use depth first search to check if any non-Phi input is unknown.
  const ArenaVector<HBasicBlock*>& blocks = GetGraph()->GetBlocks();
  size_t num_heap_locations = heap_location_collector_.GetNumberOfHeapLocations();
  visited.SetBit(PhiPlaceholderIndex(phi_placeholder));
  work_queue.push_back(phi_placeholder);
  while (!work_queue.empty()) {
    const PhiPlaceholder* current_phi_placeholder = work_queue.back();
    work_queue.pop_back();
    HBasicBlock* block = blocks[current_phi_placeholder->GetBlockId()];
    DCHECK_GE(block->GetPredecessors().size(), 2u);
    size_t idx = current_phi_placeholder->GetHeapLocation();
    for (HBasicBlock* predecessor : block->GetPredecessors()) {
      Value value = ReplacementOrValue(heap_values_for_[predecessor->GetBlockId()][idx].value);
      if (value.NeedsPhi()) {
        // Visit the predecessor Phi placeholder if it's not visited yet.
        if (!visited.IsBitSet(PhiPlaceholderIndex(value))) {
          visited.SetBit(PhiPlaceholderIndex(value));
          work_queue.push_back(value.GetPhiPlaceholder());
        }
      } else if (!value.Equals(Value::Default())) {
        return false;  // Report failure.
      }
    }
    if (block->IsLoopHeader()) {
      // For back-edges we need to check all locations that write to the same array,
      // even those that LSA declares non-aliasing, such as `a[i]` and `a[i + 1]`
      // as they may actually refer to the same locations for different iterations.
      for (size_t i = 0; i != num_heap_locations; ++i) {
        if (i == idx ||
            heap_location_collector_.GetHeapLocation(i)->GetReferenceInfo() !=
                heap_location_collector_.GetHeapLocation(idx)->GetReferenceInfo()) {
          continue;
        }
        for (HBasicBlock* predecessor : block->GetPredecessors()) {
          // Check if there were any writes to this location.
          // Note: We could simply process the values but due to the vector operation
          // carve-out (see `IsDefaultOrPhiAllowedForLoad()`), a vector load can cause
          // the value to change and not be equal to default. To work around this and
          // allow replacing the non-vector load of loop-invariant default values
          // anyway, skip over paths that do not have any writes.
          ValueRecord record = heap_values_for_[predecessor->GetBlockId()][i];
          while (record.stored_by.NeedsLoopPhi() &&
                 blocks[record.stored_by.GetPhiPlaceholder()->GetBlockId()]->IsLoopHeader()) {
            HLoopInformation* loop_info =
                blocks[record.stored_by.GetPhiPlaceholder()->GetBlockId()]->GetLoopInformation();
            record = heap_values_for_[loop_info->GetPreHeader()->GetBlockId()][i];
          }
          Value value = ReplacementOrValue(record.value);
          if (value.NeedsPhi()) {
            // Visit the predecessor Phi placeholder if it's not visited yet.
            if (!visited.IsBitSet(PhiPlaceholderIndex(value))) {
              visited.SetBit(PhiPlaceholderIndex(value));
              work_queue.push_back(value.GetPhiPlaceholder());
            }
          } else if (!value.Equals(Value::Default())) {
            return false;  // Report failure.
          }
        }
      }
    }
  }

  // Record replacement and report success.
  HInstruction* replacement = GetDefaultValue(type);
  for (uint32_t phi_placeholder_index : visited.Indexes()) {
    DCHECK(phi_placeholder_replacements_[phi_placeholder_index].IsInvalid());
    phi_placeholder_replacements_[phi_placeholder_index] = Value::ForInstruction(replacement);
  }
  phi_placeholders_to_materialize->Subtract(&visited);
  return true;
}

bool LSEVisitor::TryReplacingLoopPhiPlaceholderWithSingleInput(
    const PhiPlaceholder* phi_placeholder,
    /*inout*/ArenaBitVector* phi_placeholders_to_materialize) {
  // Use local allocator to reduce peak memory usage.
  ScopedArenaAllocator allocator(allocator_.GetArenaStack());
  ArenaBitVector visited(&allocator,
                         /*start_bits=*/ phi_placeholders_.size(),
                         /*expandable=*/ false,
                         kArenaAllocLSE);
  visited.ClearAllBits();
  ScopedArenaVector<const PhiPlaceholder*> work_queue(allocator.Adapter(kArenaAllocLSE));

  // Use depth first search to check if any non-Phi input is unknown.
  HInstruction* replacement = nullptr;
  const ArenaVector<HBasicBlock*>& blocks = GetGraph()->GetBlocks();
  visited.SetBit(PhiPlaceholderIndex(phi_placeholder));
  work_queue.push_back(phi_placeholder);
  while (!work_queue.empty()) {
    const PhiPlaceholder* current_phi_placeholder = work_queue.back();
    work_queue.pop_back();
    HBasicBlock* current_block = blocks[current_phi_placeholder->GetBlockId()];
    DCHECK_GE(current_block->GetPredecessors().size(), 2u);
    size_t idx = current_phi_placeholder->GetHeapLocation();
    for (HBasicBlock* predecessor : current_block->GetPredecessors()) {
      Value value = ReplacementOrValue(heap_values_for_[predecessor->GetBlockId()][idx].value);
      if (value.NeedsPhi()) {
        // Visit the predecessor Phi placeholder if it's not visited yet.
        if (!visited.IsBitSet(PhiPlaceholderIndex(value))) {
          visited.SetBit(PhiPlaceholderIndex(value));
          work_queue.push_back(value.GetPhiPlaceholder());
        }
      } else {
        if (!value.IsInstruction() ||
            (replacement != nullptr && replacement != value.GetInstruction())) {
          return false;  // Report failure.
        }
        replacement = value.GetInstruction();
      }
    }
  }

  // Record replacement and report success.
  DCHECK(replacement != nullptr);
  for (uint32_t phi_placeholder_index : visited.Indexes()) {
    DCHECK(phi_placeholder_replacements_[phi_placeholder_index].IsInvalid());
    phi_placeholder_replacements_[phi_placeholder_index] = Value::ForInstruction(replacement);
  }
  phi_placeholders_to_materialize->Subtract(&visited);
  return true;
}

const LSEVisitor::PhiPlaceholder* LSEVisitor::FindLoopPhisToMaterialize(
    const PhiPlaceholder* phi_placeholder,
    /*inout*/ArenaBitVector* phi_placeholders_to_materialize,
    DataType::Type type,
    bool can_use_default_or_phi) {
  DCHECK(phi_placeholder_replacements_[PhiPlaceholderIndex(phi_placeholder)].IsInvalid());

  // Use local allocator to reduce peak memory usage.
  ScopedArenaAllocator allocator(allocator_.GetArenaStack());
  ScopedArenaVector<const PhiPlaceholder*> work_queue(allocator.Adapter(kArenaAllocLSE));

  // Use depth first search to check if any non-Phi input is unknown.
  const ArenaVector<HBasicBlock*>& blocks = GetGraph()->GetBlocks();
  phi_placeholders_to_materialize->ClearAllBits();
  phi_placeholders_to_materialize->SetBit(PhiPlaceholderIndex(phi_placeholder));
  work_queue.push_back(phi_placeholder);
  while (!work_queue.empty()) {
    const PhiPlaceholder* current_phi_placeholder = work_queue.back();
    work_queue.pop_back();
    if (!phi_placeholders_to_materialize->IsBitSet(PhiPlaceholderIndex(current_phi_placeholder))) {
      // Replaced by `TryReplacingLoopPhiPlaceholderWith{Default,SingleInput}()`.
      DCHECK(phi_placeholder_replacements_[PhiPlaceholderIndex(current_phi_placeholder)].Equals(
                 Value::Default()));
      continue;
    }
    HBasicBlock* current_block = blocks[current_phi_placeholder->GetBlockId()];
    DCHECK_GE(current_block->GetPredecessors().size(), 2u);
    size_t idx = current_phi_placeholder->GetHeapLocation();
    if (current_block->IsLoopHeader()) {
      // If the index is defined inside the loop, it may reference different elements of the
      // array on each iteration. Since we do not track if all elements of an array are set
      // to the same value explicitly, the only known value in pre-header can be the default
      // value from NewArray or a Phi placeholder depending on a default value from some outer
      // loop pre-header. This Phi placeholder can be replaced only by the default value.
      HInstruction* index = heap_location_collector_.GetHeapLocation(idx)->GetIndex();
      if (index != nullptr && current_block->GetLoopInformation()->Contains(*index->GetBlock())) {
        if (can_use_default_or_phi &&
            TryReplacingLoopPhiPlaceholderWithDefault(current_phi_placeholder,
                                                      type,
                                                      phi_placeholders_to_materialize)) {
          continue;
        } else {
          return current_phi_placeholder;  // Report the loop Phi placeholder.
        }
      }
      // A similar situation arises with the index defined outside the loop if we cannot use
      // default values or Phis, i.e. for vector loads, as we can only replace the Phi
      // placeholder with a single instruction defined before the loop.
      if (!can_use_default_or_phi) {
        if (TryReplacingLoopPhiPlaceholderWithSingleInput(current_phi_placeholder,
                                                          phi_placeholders_to_materialize)) {
          continue;
        } else {
          return current_phi_placeholder;  // Report the loop Phi placeholder.
        }
      }
    }
    for (HBasicBlock* predecessor : current_block->GetPredecessors()) {
      Value value = ReplacementOrValue(heap_values_for_[predecessor->GetBlockId()][idx].value);
      if (value.IsUnknown()) {
        // We cannot create a Phi for this loop Phi placeholder.
        return current_phi_placeholder;  // Report the loop Phi placeholder.
      }
      if (value.NeedsLoopPhi()) {
        // Visit the predecessor Phi placeholder if it's not visited yet.
        if (!phi_placeholders_to_materialize->IsBitSet(PhiPlaceholderIndex(value))) {
          phi_placeholders_to_materialize->SetBit(PhiPlaceholderIndex(value));
          work_queue.push_back(value.GetPhiPlaceholder());
        }
      }
    }
  }

  // There are no unknown values feeding this Phi, so we can construct the Phis if needed.
  return nullptr;
}

bool LSEVisitor::MaterializeLoopPhis(const ScopedArenaVector<size_t>& phi_placeholder_indexes,
                                     DataType::Type type,
                                     Phase phase) {
  // Materialize all predecessors that do not need a loop Phi and determine if all inputs
  // other than loop Phis are the same.
  const ArenaVector<HBasicBlock*>& blocks = GetGraph()->GetBlocks();
  Value other_value = Value::Invalid();
  for (size_t phi_placeholder_index : phi_placeholder_indexes) {
    const PhiPlaceholder* phi_placeholder = &phi_placeholders_[phi_placeholder_index];
    HBasicBlock* block = blocks[phi_placeholder->GetBlockId()];
    DCHECK_GE(block->GetPredecessors().size(), 2u);
    size_t idx = phi_placeholder->GetHeapLocation();
    for (HBasicBlock* predecessor : block->GetPredecessors()) {
      Value value = ReplacementOrValue(heap_values_for_[predecessor->GetBlockId()][idx].value);
      if (value.NeedsNonLoopPhi()) {
        DCHECK(phase == Phase::kLoadElimination);
        MaterializeNonLoopPhis(value.GetPhiPlaceholder(), type);
        value = Replacement(value);
      }
      if (!value.NeedsLoopPhi()) {
        if (other_value.IsInvalid()) {
          // The first other value we found.
          other_value = value;
        } else if (!other_value.IsUnknown()) {
          // Check if the current `value` differs from the previous `other_value`.
          if (!value.Equals(other_value)) {
            other_value = Value::Unknown();
          }
        }
      }
    }
  }

  DCHECK(other_value.IsValid());
  if (!other_value.IsUnknown()) {
    HInstruction* replacement =
        (other_value.IsDefault()) ? GetDefaultValue(type) : other_value.GetInstruction();
    for (size_t phi_placeholder_index : phi_placeholder_indexes) {
      phi_placeholder_replacements_[phi_placeholder_index] = Value::ForInstruction(replacement);
    }
    return true;
  }

  // If we're materializing only a single Phi, try to match it with an existing Phi.
  // (Matching multiple Phis would need investigation. It may be prohibitively slow.)
  // This also covers the case when after replacing a previous set of Phi placeholders,
  // we continue with a Phi placeholder that does not really need a loop Phi anymore.
  if (phi_placeholder_indexes.size() == 1u) {
    const PhiPlaceholder* phi_placeholder = &phi_placeholders_[phi_placeholder_indexes[0]];
    size_t idx = phi_placeholder->GetHeapLocation();
    HBasicBlock* block = GetGraph()->GetBlocks()[phi_placeholder->GetBlockId()];
    ArrayRef<HBasicBlock* const> predecessors(block->GetPredecessors());
    for (HInstructionIterator phi_it(block->GetPhis()); !phi_it.Done(); phi_it.Advance()) {
      HInstruction* phi = phi_it.Current();
      DCHECK_EQ(phi->InputCount(), predecessors.size());
      ArrayRef<HUserRecord<HInstruction*>> phi_inputs = phi->GetInputRecords();
      auto cmp = [=](const HUserRecord<HInstruction*>& lhs, HBasicBlock* rhs) {
        Value value = ReplacementOrValue(heap_values_for_[rhs->GetBlockId()][idx].value);
        if (value.NeedsPhi()) {
          DCHECK(value.GetPhiPlaceholder() == phi_placeholder);
          return lhs.GetInstruction() == phi;
        } else {
          DCHECK(value.IsDefault() || value.IsInstruction());
          return value.Equals(lhs.GetInstruction());
        }
      };
      if (std::equal(phi_inputs.begin(), phi_inputs.end(), predecessors.begin(), cmp)) {
        phi_placeholder_replacements_[phi_placeholder_indexes[0]] = Value::ForInstruction(phi);
        return true;
      }
    }
  }

  if (phase == Phase::kStoreElimination) {
    // We're not creating Phis during the final store elimination phase.
    return false;
  }

  // There are different inputs to the Phi chain. Create the Phis.
  ArenaAllocator* allocator = GetGraph()->GetAllocator();
  for (size_t phi_placeholder_index : phi_placeholder_indexes) {
    const PhiPlaceholder* phi_placeholder = &phi_placeholders_[phi_placeholder_index];
    HBasicBlock* block = blocks[phi_placeholder->GetBlockId()];
    phi_placeholder_replacements_[phi_placeholder_index] = Value::ForInstruction(
        new (allocator) HPhi(allocator, kNoRegNumber, block->GetPredecessors().size(), type));
  }
  // Fill the Phi inputs.
  for (size_t phi_placeholder_index : phi_placeholder_indexes) {
    const PhiPlaceholder* phi_placeholder = &phi_placeholders_[phi_placeholder_index];
    HBasicBlock* block = blocks[phi_placeholder->GetBlockId()];
    size_t idx = phi_placeholder->GetHeapLocation();
    HInstruction* phi = phi_placeholder_replacements_[phi_placeholder_index].GetInstruction();
    for (size_t i = 0, size = block->GetPredecessors().size(); i != size; ++i) {
      HBasicBlock* predecessor = block->GetPredecessors()[i];
      Value value = ReplacementOrValue(heap_values_for_[predecessor->GetBlockId()][idx].value);
      HInstruction* input = value.IsDefault() ? GetDefaultValue(type) : value.GetInstruction();
      DCHECK_NE(input->GetType(), DataType::Type::kVoid);
      phi->SetRawInputAt(i, input);
    }
  }
  // Add the Phis to their blocks.
  for (size_t phi_placeholder_index : phi_placeholder_indexes) {
    const PhiPlaceholder* phi_placeholder = &phi_placeholders_[phi_placeholder_index];
    HBasicBlock* block = blocks[phi_placeholder->GetBlockId()];
    block->AddPhi(phi_placeholder_replacements_[phi_placeholder_index].GetInstruction()->AsPhi());
  }
  if (type == DataType::Type::kReference) {
    ScopedArenaAllocator local_allocator(allocator_.GetArenaStack());
    ScopedArenaVector<HInstruction*> phis(local_allocator.Adapter(kArenaAllocLSE));
    for (size_t phi_placeholder_index : phi_placeholder_indexes) {
      phis.push_back(phi_placeholder_replacements_[phi_placeholder_index].GetInstruction());
    }
    // Update reference type information. Pass invalid handles, these are not used for Phis.
    ReferenceTypePropagation rtp_fixup(GetGraph(),
                                       Handle<mirror::ClassLoader>(),
                                       Handle<mirror::DexCache>(),
                                       /* is_first_run= */ false);
    rtp_fixup.Visit(ArrayRef<HInstruction* const>(phis));
  }

  return true;
}

bool LSEVisitor::MaterializeLoopPhis(const ArenaBitVector& phi_placeholders_to_materialize,
                                     DataType::Type type,
                                     Phase phase) {
  // Use local allocator to reduce peak memory usage.
  ScopedArenaAllocator allocator(allocator_.GetArenaStack());

  // We want to recognize when a subset of these loop Phis that do not need other
  // loop Phis, i.e. a transitive closure, has only one other instruction as an input,
  // i.e. that instruction can be used instead of each Phi in the set. See for example
  // Main.testLoop{5,6,7,8}() in the test 530-checker-lse. To do that, we shall
  // materialize these loop Phis from the smallest transitive closure.

  // Construct a matrix of loop phi placeholder dependencies. To reduce the memory usage,
  // assign new indexes to the Phi placeholders, making the matrix dense.
  ScopedArenaVector<size_t> matrix_indexes(phi_placeholders_.size(),
                                           static_cast<size_t>(-1),  // Invalid.
                                           allocator.Adapter(kArenaAllocLSE));
  ScopedArenaVector<size_t> phi_placeholder_indexes(allocator.Adapter(kArenaAllocLSE));
  size_t num_phi_placeholders = phi_placeholders_to_materialize.NumSetBits();
  phi_placeholder_indexes.reserve(num_phi_placeholders);
  for (uint32_t marker_index : phi_placeholders_to_materialize.Indexes()) {
    matrix_indexes[marker_index] = phi_placeholder_indexes.size();
    phi_placeholder_indexes.push_back(marker_index);
  }
  const ArenaVector<HBasicBlock*>& blocks = GetGraph()->GetBlocks();
  ScopedArenaVector<ArenaBitVector*> dependencies(allocator.Adapter(kArenaAllocLSE));
  dependencies.reserve(num_phi_placeholders);
  for (size_t matrix_index = 0; matrix_index != num_phi_placeholders; ++matrix_index) {
    static constexpr bool kExpandable = false;
    dependencies.push_back(
        ArenaBitVector::Create(&allocator, num_phi_placeholders, kExpandable, kArenaAllocLSE));
    ArenaBitVector* current_dependencies = dependencies.back();
    current_dependencies->ClearAllBits();
    current_dependencies->SetBit(matrix_index);  // Count the Phi placeholder as its own dependency.
    const PhiPlaceholder* current_phi_placeholder =
        &phi_placeholders_[phi_placeholder_indexes[matrix_index]];
    HBasicBlock* current_block = blocks[current_phi_placeholder->GetBlockId()];
    DCHECK_GE(current_block->GetPredecessors().size(), 2u);
    size_t idx = current_phi_placeholder->GetHeapLocation();
    for (HBasicBlock* predecessor : current_block->GetPredecessors()) {
      Value pred_value = ReplacementOrValue(heap_values_for_[predecessor->GetBlockId()][idx].value);
      if (pred_value.NeedsLoopPhi()) {
        size_t pred_value_index = PhiPlaceholderIndex(pred_value);
        DCHECK(phi_placeholder_replacements_[pred_value_index].IsInvalid());
        DCHECK_NE(matrix_indexes[pred_value_index], static_cast<size_t>(-1));
        current_dependencies->SetBit(matrix_indexes[PhiPlaceholderIndex(pred_value)]);
      }
    }
  }

  // Use the Floyd-Warshall algorithm to determine all transitive dependencies.
  for (size_t k = 0; k != num_phi_placeholders; ++k) {
    for (size_t i = 0; i != num_phi_placeholders; ++i) {
      for (size_t j = 0; j != num_phi_placeholders; ++j) {
        if (dependencies[i]->IsBitSet(k) && dependencies[k]->IsBitSet(j)) {
          dependencies[i]->SetBit(j);
        }
      }
    }
  }

  // Count the number of transitive dependencies for each replaceable Phi placeholder.
  ScopedArenaVector<size_t> num_dependencies(allocator.Adapter(kArenaAllocLSE));
  num_dependencies.reserve(num_phi_placeholders);
  for (size_t matrix_index = 0; matrix_index != num_phi_placeholders; ++matrix_index) {
    num_dependencies.push_back(dependencies[matrix_index]->NumSetBits());
  }

  // Pick a Phi placeholder with the smallest number of transitive dependencies and
  // materialize it and its dependencies. Repeat until we have materialized all.
  ScopedArenaVector<size_t> current_subset(allocator.Adapter(kArenaAllocLSE));
  current_subset.reserve(num_phi_placeholders);
  size_t remaining_phi_placeholders = num_phi_placeholders;
  while (remaining_phi_placeholders != 0u) {
    auto it = std::min_element(num_dependencies.begin(), num_dependencies.end());
    DCHECK_LE(*it, remaining_phi_placeholders);
    size_t current_matrix_index = std::distance(num_dependencies.begin(), it);
    ArenaBitVector* current_dependencies = dependencies[current_matrix_index];
    size_t current_num_dependencies = num_dependencies[current_matrix_index];
    current_subset.clear();
    for (uint32_t matrix_index : current_dependencies->Indexes()) {
      current_subset.push_back(phi_placeholder_indexes[matrix_index]);
    }
    if (!MaterializeLoopPhis(current_subset, type, phase)) {
      DCHECK(phase == Phase::kStoreElimination);
      // This is the final store elimination phase and we shall not be able to eliminate any
      // stores that depend on the current subset, so mark these Phi placeholders unreplaceable.
      for (uint32_t matrix_index = 0; matrix_index != num_phi_placeholders; ++matrix_index) {
        if (dependencies[matrix_index]->IsBitSet(current_matrix_index)) {
          DCHECK(phi_placeholder_replacements_[phi_placeholder_indexes[matrix_index]].IsInvalid());
          phi_placeholder_replacements_[phi_placeholder_indexes[matrix_index]] = Value::Unknown();
        }
      }
      return false;
    }
    for (uint32_t matrix_index = 0; matrix_index != num_phi_placeholders; ++matrix_index) {
      if (current_dependencies->IsBitSet(matrix_index)) {
        // Mark all dependencies as done by incrementing their `num_dependencies[.]`,
        // so that they shall never be the minimum again.
        num_dependencies[matrix_index] = num_phi_placeholders;
      } else if (dependencies[matrix_index]->IsBitSet(current_matrix_index)) {
        // Remove dependencies from other Phi placeholders.
        dependencies[matrix_index]->Subtract(current_dependencies);
        num_dependencies[matrix_index] -= current_num_dependencies;
      }
    }
    remaining_phi_placeholders -= current_num_dependencies;
  }
  return true;
}

const LSEVisitor::PhiPlaceholder* LSEVisitor::TryToMaterializeLoopPhis(
    const PhiPlaceholder* phi_placeholder,
    HInstruction* load) {
  DCHECK(phi_placeholder_replacements_[PhiPlaceholderIndex(phi_placeholder)].IsInvalid());

  // Use local allocator to reduce peak memory usage.
  ScopedArenaAllocator allocator(allocator_.GetArenaStack());

  // Find Phi placeholders to materialize.
  ArenaBitVector phi_placeholders_to_materialize(
      &allocator, phi_placeholders_.size(), /*expandable=*/ false, kArenaAllocLSE);
  phi_placeholders_to_materialize.ClearAllBits();
  DataType::Type type = load->GetType();
  bool can_use_default_or_phi = IsDefaultOrPhiAllowedForLoad(load);
  const PhiPlaceholder* loop_phi_with_unknown_input = FindLoopPhisToMaterialize(
      phi_placeholder, &phi_placeholders_to_materialize, type, can_use_default_or_phi);
  if (loop_phi_with_unknown_input != nullptr) {
    return loop_phi_with_unknown_input;  // Return failure.
  }

  bool success =
      MaterializeLoopPhis(phi_placeholders_to_materialize, type, Phase::kLoadElimination);
  DCHECK(success);

  // Report success.
  return nullptr;
}

// Re-process loads and stores in successors from the `loop_phi_with_unknown_input`. This may
// find one or more loads from `loads_requiring_loop_phi_` which cannot be replaced by Phis and
// propagate the load(s) as the new value(s) to successors; this may uncover new elimination
// opportunities. If we find no such load, we shall at least propagate an unknown value to some
// heap location that is needed by another loop Phi placeholder.
void LSEVisitor::ProcessLoopPhiWithUnknownInput(const PhiPlaceholder* loop_phi_with_unknown_input) {
  size_t loop_phi_with_unknown_input_index = PhiPlaceholderIndex(loop_phi_with_unknown_input);
  DCHECK(phi_placeholder_replacements_[loop_phi_with_unknown_input_index].IsInvalid());
  phi_placeholder_replacements_[loop_phi_with_unknown_input_index] = Value::Unknown();

  uint32_t block_id = loop_phi_with_unknown_input->GetBlockId();
  const ArenaVector<HBasicBlock*> reverse_post_order = GetGraph()->GetReversePostOrder();
  size_t reverse_post_order_index = 0;
  size_t reverse_post_order_size = reverse_post_order.size();
  size_t loads_and_stores_index = 0u;
  size_t loads_and_stores_size = loads_and_stores_.size();

  // Skip blocks and instructions before the block containing the loop phi with unknown input.
  DCHECK_NE(reverse_post_order_index, reverse_post_order_size);
  while (reverse_post_order[reverse_post_order_index]->GetBlockId() != block_id) {
    HBasicBlock* block = reverse_post_order[reverse_post_order_index];
    while (loads_and_stores_index != loads_and_stores_size &&
           loads_and_stores_[loads_and_stores_index].load_or_store->GetBlock() == block) {
      ++loads_and_stores_index;
    }
    ++reverse_post_order_index;
    DCHECK_NE(reverse_post_order_index, reverse_post_order_size);
  }

  // Use local allocator to reduce peak memory usage.
  ScopedArenaAllocator allocator(allocator_.GetArenaStack());
  // Reuse one temporary vector for all remaining blocks.
  size_t num_heap_locations = heap_location_collector_.GetNumberOfHeapLocations();
  ScopedArenaVector<Value> local_heap_values(allocator.Adapter(kArenaAllocLSE));

  auto get_initial_value = [this](HBasicBlock* block, size_t idx) {
    Value value;
    if (block->IsLoopHeader()) {
      if (block->GetLoopInformation()->IsIrreducible()) {
        value = Value::Unknown();
      } else {
        value = PrepareLoopValue(block, idx);
      }
    } else {
      value = MergePredecessorValues(block, idx);
    }
    DCHECK(value.IsUnknown() || ReplacementOrValue(value).Equals(value));
    return value;
  };

  // Process remaining blocks and instructions.
  bool found_unreplaceable_load = false;
  bool replaced_heap_value_with_unknown = false;
  for (; reverse_post_order_index != reverse_post_order_size; ++reverse_post_order_index) {
    HBasicBlock* block = reverse_post_order[reverse_post_order_index];
    if (block->IsExitBlock()) {
      continue;
    }

    // We shall reconstruct only the heap values that we need for processing loads and stores.
    local_heap_values.clear();
    local_heap_values.resize(num_heap_locations, Value::Invalid());

    for (; loads_and_stores_index != loads_and_stores_size; ++loads_and_stores_index) {
      HInstruction* load_or_store = loads_and_stores_[loads_and_stores_index].load_or_store;
      size_t idx = loads_and_stores_[loads_and_stores_index].heap_location_index;
      if (load_or_store->GetBlock() != block) {
        break;  // End of instructions from the current block.
      }
      bool is_store = load_or_store->GetSideEffects().DoesAnyWrite();
      DCHECK_EQ(is_store, IsStore(load_or_store));
      HInstruction* stored_value = nullptr;
      if (is_store) {
        auto it = store_records_.find(load_or_store);
        DCHECK(it != store_records_.end());
        stored_value = it->second.stored_value;
      }
      auto it = loads_requiring_loop_phi_.find(
          stored_value != nullptr ? stored_value : load_or_store);
      if (it == loads_requiring_loop_phi_.end()) {
        continue;  // This load or store never needed a loop Phi.
      }
      ValueRecord& record = it->second;
      if (is_store) {
        // Process the store by updating `local_heap_values[idx]`. The last update shall
        // be propagated to the `heap_values[idx].value` if it previously needed a loop Phi
        // at the end of the block.
        Value replacement = ReplacementOrValue(record.value);
        if (replacement.NeedsLoopPhi()) {
          // No replacement yet, use the Phi placeholder from the load.
          DCHECK(record.value.NeedsLoopPhi());
          local_heap_values[idx] = record.value;
        } else {
          // If the load fetched a known value, use it, otherwise use the load.
          local_heap_values[idx] = Value::ForInstruction(
              replacement.IsUnknown() ? stored_value : replacement.GetInstruction());
        }
      } else {
        // Process the load unless it has previously been marked unreplaceable.
        if (record.value.NeedsLoopPhi()) {
          if (local_heap_values[idx].IsInvalid()) {
            local_heap_values[idx] = get_initial_value(block, idx);
          }
          if (local_heap_values[idx].IsUnknown()) {
            // This load cannot be replaced. Keep stores that feed the Phi placeholder
            // (no aliasing since then, otherwise the Phi placeholder would not have been
            // propagated as a value to this load) and store the load as the new heap value.
            found_unreplaceable_load = true;
            KeepStores(record.value);
            record.value = Value::Unknown();
            local_heap_values[idx] = Value::ForInstruction(load_or_store);
          } else if (local_heap_values[idx].NeedsLoopPhi()) {
            // The load may still be replaced with a Phi later.
            DCHECK(local_heap_values[idx].Equals(record.value));
          } else {
            // This load can be eliminated but we may need to construct non-loop Phis.
            if (local_heap_values[idx].NeedsNonLoopPhi()) {
              MaterializeNonLoopPhis(local_heap_values[idx].GetPhiPlaceholder(),
                                     load_or_store->GetType());
              local_heap_values[idx] = Replacement(local_heap_values[idx]);
            }
            record.value = local_heap_values[idx];
            HInstruction* heap_value = local_heap_values[idx].GetInstruction();
            AddRemovedLoad(load_or_store, heap_value);
            TryRemovingNullCheck(load_or_store);
          }
        }
      }
    }

    // All heap values that previously needed a loop Phi at the end of the block
    // need to be updated for processing successors.
    ScopedArenaVector<ValueRecord>& heap_values = heap_values_for_[block->GetBlockId()];
    for (size_t idx = 0; idx != num_heap_locations; ++idx) {
      if (heap_values[idx].value.NeedsLoopPhi()) {
        if (local_heap_values[idx].IsValid()) {
          heap_values[idx].value = local_heap_values[idx];
        } else {
          heap_values[idx].value = get_initial_value(block, idx);
        }
        if (heap_values[idx].value.IsUnknown()) {
          replaced_heap_value_with_unknown = true;
        }
      }
    }
  }
  DCHECK(found_unreplaceable_load || replaced_heap_value_with_unknown);
}

void LSEVisitor::ProcessLoadsRequiringLoopPhis() {
  // Note: The vector operations carve-out (see `IsDefaultOrPhiAllowedForLoad()`) can possibly
  // make the result of the processing depend on the order in which we process these loads.
  // To make sure the result is deterministic, iterate over `loads_and_stores_` instead of the
  // `loads_requiring_loop_phi_` indexed by non-deterministic pointers.
  for (const LoadStoreRecord& load_store_record : loads_and_stores_) {
    auto it = loads_requiring_loop_phi_.find(load_store_record.load_or_store);
    if (it == loads_requiring_loop_phi_.end()) {
      continue;
    }
    HInstruction* load = it->first;
    ValueRecord& record = it->second;
    while (record.value.NeedsLoopPhi() &&
           phi_placeholder_replacements_[PhiPlaceholderIndex(record.value)].IsInvalid()) {
      const PhiPlaceholder* loop_phi_with_unknown_input =
          TryToMaterializeLoopPhis(record.value.GetPhiPlaceholder(), load);
      DCHECK_EQ(loop_phi_with_unknown_input != nullptr,
                phi_placeholder_replacements_[PhiPlaceholderIndex(record.value)].IsInvalid());
      if (loop_phi_with_unknown_input != nullptr) {
        ProcessLoopPhiWithUnknownInput(loop_phi_with_unknown_input);
      }
    }
    // The load could have been marked as unreplaceable (and stores marked for keeping)
    // or marked for replacement with an instruction in ProcessLoopPhiWithUnknownInput().
    DCHECK(record.value.IsUnknown() || record.value.IsInstruction() || record.value.NeedsLoopPhi());
    if (record.value.NeedsLoopPhi()) {
      record.value = Replacement(record.value);
      HInstruction* heap_value = record.value.GetInstruction();
      AddRemovedLoad(load, heap_value);
      TryRemovingNullCheck(load);
    }
  }
}

void LSEVisitor::SearchPhiPlaceholdersForKeptStores() {
  ScopedArenaVector<uint32_t> work_queue(allocator_.Adapter(kArenaAllocLSE));
  size_t start_size = phi_placeholders_to_search_for_kept_stores_.NumSetBits();
  work_queue.reserve(((start_size * 3u) + 1u) / 2u);  // Reserve 1.5x start size, rounded up.
  for (uint32_t index : phi_placeholders_to_search_for_kept_stores_.Indexes()) {
    work_queue.push_back(index);
  }
  const ArenaVector<HBasicBlock*>& blocks = GetGraph()->GetBlocks();
  while (!work_queue.empty()) {
    const PhiPlaceholder* phi_placeholder = &phi_placeholders_[work_queue.back()];
    work_queue.pop_back();
    size_t idx = phi_placeholder->GetHeapLocation();
    HBasicBlock* block = blocks[phi_placeholder->GetBlockId()];
    for (HBasicBlock* predecessor : block->GetPredecessors()) {
      ScopedArenaVector<ValueRecord>& heap_values = heap_values_for_[predecessor->GetBlockId()];
      // For loop back-edges we must also preserve all stores to locations that
      // may alias with the location `idx`.
      // TODO: Review whether we need to keep stores to aliased locations from pre-header.
      // TODO: Add tests cases around this.
      bool is_back_edge =
          block->IsLoopHeader() && predecessor != block->GetLoopInformation()->GetPreHeader();
      size_t start = is_back_edge ? 0u : idx;
      size_t end = is_back_edge ? heap_values.size() : idx + 1u;
      for (size_t i = start; i != end; ++i) {
        Value stored_by = heap_values[i].stored_by;
        auto may_alias = [this, block, idx](size_t i) {
            DCHECK_NE(i, idx);
            DCHECK(block->IsLoopHeader());
            if (heap_location_collector_.MayAlias(i, idx)) {
              return true;
            }
            // For array locations with index defined inside the loop, include
            // all other locations in the array, even those that LSA declares
            // non-aliasing, such as `a[i]` and `a[i + 1]`, as they may actually
            // refer to the same locations for different iterations. (LSA's
            // `ComputeMayAlias()` does not consider different loop iterations.)
            HeapLocation* heap_loc = heap_location_collector_.GetHeapLocation(idx);
            HeapLocation* other_loc = heap_location_collector_.GetHeapLocation(i);
            if (heap_loc->IsArray() &&
                other_loc->IsArray() &&
                heap_loc->GetReferenceInfo() == other_loc->GetReferenceInfo() &&
                block->GetLoopInformation()->Contains(*heap_loc->GetIndex()->GetBlock())) {
              // If one location has index defined inside and the other index defined outside
              // of the loop, LSA considers them aliasing and we take an early return above.
              DCHECK(block->GetLoopInformation()->Contains(*other_loc->GetIndex()->GetBlock()));
              return true;
            }
            return false;
        };
        if (!stored_by.IsUnknown() && (i == idx || may_alias(i))) {
          if (stored_by.NeedsPhi()) {
            size_t phi_placeholder_index = PhiPlaceholderIndex(stored_by);
            if (!phi_placeholders_to_search_for_kept_stores_.IsBitSet(phi_placeholder_index)) {
              phi_placeholders_to_search_for_kept_stores_.SetBit(phi_placeholder_index);
              work_queue.push_back(phi_placeholder_index);
            }
          } else {
            DCHECK(IsStore(stored_by.GetInstruction()));
            kept_stores_.SetBit(stored_by.GetInstruction()->GetId());
          }
        }
      }
    }
  }
}

void LSEVisitor::UpdateValueRecordForStoreElimination(/*inout*/ValueRecord* value_record) {
  while (value_record->stored_by.IsInstruction() &&
         !kept_stores_.IsBitSet(value_record->stored_by.GetInstruction()->GetId())) {
    auto it = store_records_.find(value_record->stored_by.GetInstruction());
    DCHECK(it != store_records_.end());
    *value_record = it->second.old_value_record;
  }
  if (value_record->stored_by.NeedsPhi() &&
      !phi_placeholders_to_search_for_kept_stores_.IsBitSet(
           PhiPlaceholderIndex(value_record->stored_by))) {
    // Some stores feeding this heap location may have been eliminated. Use the `stored_by`
    // Phi placeholder to recalculate the actual value.
    value_record->value = value_record->stored_by;
  }
  value_record->value = ReplacementOrValue(value_record->value);
  if (value_record->value.NeedsNonLoopPhi()) {
    // Treat all Phi placeholders as requiring loop Phis at this point.
    // We do not want MaterializeLoopPhis() to call MaterializeNonLoopPhis().
    value_record->value = Value::ForLoopPhiPlaceholder(value_record->value.GetPhiPlaceholder());
  }
}

void LSEVisitor::FindOldValueForPhiPlaceholder(const PhiPlaceholder* phi_placeholder,
                                               DataType::Type type) {
  DCHECK(phi_placeholder_replacements_[PhiPlaceholderIndex(phi_placeholder)].IsInvalid());

  // Use local allocator to reduce peak memory usage.
  ScopedArenaAllocator allocator(allocator_.GetArenaStack());
  ArenaBitVector visited(&allocator,
                         /*start_bits=*/ phi_placeholders_.size(),
                         /*expandable=*/ false,
                         kArenaAllocLSE);
  visited.ClearAllBits();

  // Find Phi placeholders to try and match against existing Phis or other replacement values.
  ArenaBitVector phi_placeholders_to_materialize(
      &allocator, phi_placeholders_.size(), /*expandable=*/ false, kArenaAllocLSE);
  phi_placeholders_to_materialize.ClearAllBits();
  const PhiPlaceholder* loop_phi_with_unknown_input = FindLoopPhisToMaterialize(
      phi_placeholder, &phi_placeholders_to_materialize, type, /*can_use_default_or_phi=*/ true);
  if (loop_phi_with_unknown_input != nullptr) {
    // Mark the unreplacable placeholder as well as the input Phi placeholder as unreplaceable.
    phi_placeholder_replacements_[PhiPlaceholderIndex(phi_placeholder)] = Value::Unknown();
    phi_placeholder_replacements_[PhiPlaceholderIndex(loop_phi_with_unknown_input)] =
        Value::Unknown();
    return;
  }

  bool success =
      MaterializeLoopPhis(phi_placeholders_to_materialize, type, Phase::kStoreElimination);
  DCHECK(phi_placeholder_replacements_[PhiPlaceholderIndex(phi_placeholder)].IsValid());
  DCHECK_EQ(phi_placeholder_replacements_[PhiPlaceholderIndex(phi_placeholder)].IsUnknown(),
            !success);
}

void LSEVisitor::FindStoresWritingOldValues() {
  // The Phi placeholder replacements have so far been used for eliminating loads,
  // tracking values that would be stored if all stores were kept. As we want to
  // compare actual old values after removing unmarked stores, prune the Phi
  // placeholder replacements that can be fed by values we may not actually store.
  // Replacements marked as unknown can be kept as they are fed by some unknown
  // value and would end up as unknown again if we recalculated them.
  for (size_t i = 0, size = phi_placeholder_replacements_.size(); i != size; ++i) {
    if (!phi_placeholder_replacements_[i].IsUnknown() &&
        !phi_placeholders_to_search_for_kept_stores_.IsBitSet(i)) {
      phi_placeholder_replacements_[i] = Value::Invalid();
    }
  }

  // Update heap values at end of blocks.
  for (HBasicBlock* block : GetGraph()->GetReversePostOrder()) {
    for (ValueRecord& value_record : heap_values_for_[block->GetBlockId()]) {
      UpdateValueRecordForStoreElimination(&value_record);
    }
  }

  // Use local allocator to reduce peak memory usage.
  ScopedArenaAllocator allocator(allocator_.GetArenaStack());
  // Mark the stores we want to eliminate in a separate bit vector.
  ArenaBitVector eliminated_stores(&allocator,
                                   /*start_bits=*/ GetGraph()->GetCurrentInstructionId(),
                                   /*expandable=*/ false,
                                   kArenaAllocLSE);
  eliminated_stores.ClearAllBits();

  for (auto& entry : store_records_) {
    HInstruction* store = entry.first;
    StoreRecord& store_record = entry.second;
    if (!kept_stores_.IsBitSet(store->GetId())) {
      continue;  // Ignore stores that are not kept.
    }
    UpdateValueRecordForStoreElimination(&store_record.old_value_record);
    if (store_record.old_value_record.value.NeedsPhi()) {
      DataType::Type type = store_record.stored_value->GetType();
      FindOldValueForPhiPlaceholder(store_record.old_value_record.value.GetPhiPlaceholder(), type);
      store_record.old_value_record.value = ReplacementOrValue(store_record.old_value_record.value);
    }
    DCHECK(!store_record.old_value_record.value.NeedsPhi());
    HInstruction* stored_value = FindSubstitute(store_record.stored_value);
    if (store_record.old_value_record.value.Equals(stored_value)) {
      eliminated_stores.SetBit(store->GetId());
    }
  }

  // Commit the stores to eliminate by removing them from `kept_stores_`.
  kept_stores_.Subtract(&eliminated_stores);
}

void LSEVisitor::Run() {
  // 1. Process blocks and instructions in reverse post order.
  for (HBasicBlock* block : GetGraph()->GetReversePostOrder()) {
    VisitBasicBlock(block);
  }

  // 2. Process loads that require loop Phis, trying to find/create replacements.
  ProcessLoadsRequiringLoopPhis();

  // 3. Determine which stores to keep and which to eliminate.

  // Finish marking stores for keeping.
  SearchPhiPlaceholdersForKeptStores();

  // Find stores that write the same value as is already present in the location.
  FindStoresWritingOldValues();

  // 4. Replace loads and remove unnecessary stores and singleton allocations.

  // Remove recorded load instructions that should be eliminated.
  for (const LoadStoreRecord& record : loads_and_stores_) {
    size_t id = dchecked_integral_cast<size_t>(record.load_or_store->GetId());
    HInstruction* substitute = substitute_instructions_for_loads_[id];
    if (substitute == nullptr) {
      continue;
    }
    HInstruction* load = record.load_or_store;
    DCHECK(load != nullptr);
    DCHECK(IsLoad(load));
    DCHECK(load->GetBlock() != nullptr) << load->DebugName() << "@" << load->GetDexPc();
    // We proactively retrieve the substitute for a removed load, so
    // a load that has a substitute should not be observed as a heap
    // location value.
    DCHECK_EQ(FindSubstitute(substitute), substitute);

    load->ReplaceWith(substitute);
    load->GetBlock()->RemoveInstruction(load);
  }

  // Remove all the stores we can.
  for (const LoadStoreRecord& record : loads_and_stores_) {
    bool is_store = record.load_or_store->GetSideEffects().DoesAnyWrite();
    DCHECK_EQ(is_store, IsStore(record.load_or_store));
    if (is_store && !kept_stores_.IsBitSet(record.load_or_store->GetId())) {
      record.load_or_store->GetBlock()->RemoveInstruction(record.load_or_store);
    }
  }

  // Eliminate singleton-classified instructions:
  //   * - Constructor fences (they never escape this thread).
  //   * - Allocations (if they are unused).
  for (HInstruction* new_instance : singleton_new_instances_) {
    size_t removed = HConstructorFence::RemoveConstructorFences(new_instance);
    MaybeRecordStat(stats_,
                    MethodCompilationStat::kConstructorFenceRemovedLSE,
                    removed);

    if (!new_instance->HasNonEnvironmentUses()) {
      new_instance->RemoveEnvironmentUsers();
      new_instance->GetBlock()->RemoveInstruction(new_instance);
    }
  }
}

bool 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 false;
  }
  ScopedArenaAllocator allocator(graph_->GetArenaStack());
  LoadStoreAnalysis lsa(graph_, &allocator);
  lsa.Run();
  const HeapLocationCollector& heap_location_collector = lsa.GetHeapLocationCollector();
  if (heap_location_collector.GetNumberOfHeapLocations() == 0) {
    // No HeapLocation information from LSA, skip this optimization.
    return false;
  }

  LSEVisitor lse_visitor(graph_, heap_location_collector, stats_);
  lse_visitor.Run();
  return true;
}

}  // namespace art