VM: Move redundancy elimination phases into a separate file.

runtime/vm/redundancy_elimination.cc

+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#include "vm/redundancy_elimination.h"
+
+#include "vm/bit_vector.h"
+#include "vm/flow_graph.h"
+#include "vm/hash_map.h"
+#include "vm/il_printer.h"
+#include "vm/intermediate_language.h"
+#include "vm/stack_frame.h"
+
+namespace dart {
+
+
+DEFINE_FLAG(bool, dead_store_elimination, true, "Eliminate dead stores");
+DEFINE_FLAG(bool, load_cse, true, "Use redundant load elimination.");
+DEFINE_FLAG(bool, trace_load_optimization, false,
+    "Print live sets for load optimization pass.");
+
+DECLARE_FLAG(bool, fields_may_be_reset);
+DECLARE_FLAG(bool, precompilation);
+DECLARE_FLAG(bool, trace_optimization);
+
+// Quick access to the current zone.
+#define Z (zone())
+
+
+class CSEInstructionMap : public ValueObject {
+ public:
+  // Right now CSE and LICM track a single effect: possible externalization of
+  // strings.
+  // Other effects like modifications of fields are tracked in a separate load
+  // forwarding pass via Alias structure.
+  COMPILE_ASSERT(EffectSet::kLastEffect == 1);
+
+  CSEInstructionMap() : independent_(), dependent_() { }
+  explicit CSEInstructionMap(const CSEInstructionMap& other)
+      : ValueObject(),
+        independent_(other.independent_),
+        dependent_(other.dependent_) {
+  }
+
+  void RemoveAffected(EffectSet effects) {
+    if (!effects.IsNone()) {
+      dependent_.Clear();
+    }
+  }
+
+  Instruction* Lookup(Instruction* other) const {
+    return GetMapFor(other)->Lookup(other);
+  }
+
+  void Insert(Instruction* instr) {
+    return GetMapFor(instr)->Insert(instr);
+  }
+
+ private:
+  typedef DirectChainedHashMap<PointerKeyValueTrait<Instruction> >  Map;
+
+  Map* GetMapFor(Instruction* instr) {
+    return instr->Dependencies().IsNone() ? &independent_ : &dependent_;
+  }
+
+  const Map* GetMapFor(Instruction* instr) const {
+    return instr->Dependencies().IsNone() ? &independent_ : &dependent_;
+  }
+
+  // All computations that are not affected by any side-effect.
+  // Majority of computations are not affected by anything and will be in
+  // this map.
+  Map independent_;
+
+  // All computations that are affected by side effect.
+  Map dependent_;
+};
+
+
+// Place describes an abstract location (e.g. field) that IR can load
+// from or store to.
+//
+// Places are also used to describe wild-card locations also known as aliases,
+// that essentially represent sets of places that alias each other. Places A
+// and B are said to alias each other if store into A can affect load from B.
+//
+// We distinguish the following aliases:
+//
+//   - for fields
+//     - *.f, *.@offs - field inside some object;
+//     - X.f, X.@offs - field inside an allocated object X;
+//   - for indexed accesses
+//     - *[*] - non-constant index inside some object;
+//     - *[C] - constant index inside some object;
+//     - X[*] - non-constant index inside an allocated object X;
+//     - X[C] - constant index inside an allocated object X.
+//
+// Constant indexed places are divided into two subcategories:
+//
+//   - Access to homogeneous array-like objects: Array, ImmutableArray,
+//     OneByteString, TwoByteString. These objects can only be accessed
+//     on element by element basis with all elements having the same size.
+//     This means X[C] aliases X[K] if and only if C === K.
+//   - TypedData accesses. TypedData allow to read one of the primitive
+//     data types at the given byte offset. When TypedData is accessed through
+//     index operator on a typed array or a typed array view it is guaranteed
+//     that the byte offset is always aligned by the element size. We write
+//     these accesses as X[C|S], where C is constant byte offset and S is size
+//     of the data type. Obviously X[C|S] and X[K|U] alias if and only if either
+//     C = RoundDown(K, S) or K = RoundDown(C, U).
+//     Note that not all accesses to typed data are aligned: e.g. ByteData
+//     allows unanaligned access through it's get*/set* methods.
+//     Check in Place::SetIndex ensures that we never create a place X[C|S]
+//     such that C is not aligned by S.
+//
+// Separating allocations from other objects improves precision of the
+// load forwarding pass because of the following two properties:
+//
+//   - if X can be proven to have no aliases itself (i.e. there is no other SSA
+//     variable that points to X) then no place inside X can be aliased with any
+//     wildcard dependent place (*.f, *.@offs, *[*], *[C]);
+//   - given allocations X and Y no place inside X can be aliased with any place
+//     inside Y even if any of them or both escape.
+//
+// It important to realize that single place can belong to multiple aliases.
+// For example place X.f with aliased allocation X belongs both to X.f and *.f
+// aliases. Likewise X[C] with non-aliased allocation X belongs to X[C] and X[*]
+// aliases.
+//
+class Place : public ValueObject {
+ public:
+  enum Kind {
+    kNone,
+
+    // Field location. For instance fields is represented as a pair of a Field
+    // object and an instance (SSA definition) that is being accessed.
+    // For static fields instance is NULL.
+    kField,
+
+    // VMField location. Represented as a pair of an instance (SSA definition)
+    // being accessed and offset to the field.
+    kVMField,
+
+    // Indexed location with a non-constant index.
+    kIndexed,
+
+    // Indexed location with a constant index.
+    kConstantIndexed,
+  };
+
+  // Size of the element accessed by constant index. Size is only important
+  // for TypedData because those accesses can alias even when constant indexes
+  // are not the same: X[0|4] aliases X[0|2] and X[2|2].
+  enum ElementSize {
+    // If indexed access is not a TypedData access then element size is not
+    // important because there is only a single possible access size depending
+    // on the receiver - X[C] aliases X[K] if and only if C == K.
+    // This is the size set for Array, ImmutableArray, OneByteString and
+    // TwoByteString accesses.
+    kNoSize,
+
+    // 1 byte (Int8List, Uint8List, Uint8ClampedList).
+    kInt8,
+
+    // 2 bytes (Int16List, Uint16List).
+    kInt16,
+
+    // 4 bytes (Int32List, Uint32List, Float32List).
+    kInt32,
+
+    // 8 bytes (Int64List, Uint64List, Float64List).
+    kInt64,
+
+    // 16 bytes (Int32x4List, Float32x4List, Float64x2List).
+    kInt128,
+
+    kLargestElementSize = kInt128,
+  };
+
+  Place(const Place& other)
+      : ValueObject(),
+        flags_(other.flags_),
+        instance_(other.instance_),
+        raw_selector_(other.raw_selector_),
+        id_(other.id_) {
+  }
+
+  // Construct a place from instruction if instruction accesses any place.
+  // Otherwise constructs kNone place.
+  Place(Instruction* instr, bool* is_load, bool* is_store)
+      : flags_(0),
+        instance_(NULL),
+        raw_selector_(0),
+        id_(0) {
+    switch (instr->tag()) {
+      case Instruction::kLoadField: {
+        LoadFieldInstr* load_field = instr->AsLoadField();
+        set_representation(load_field->representation());
+        instance_ = load_field->instance()->definition()->OriginalDefinition();
+        if (load_field->field() != NULL) {
+          set_kind(kField);
+          field_ = load_field->field();
+        } else {
+          set_kind(kVMField);
+          offset_in_bytes_ = load_field->offset_in_bytes();
+        }
+        *is_load = true;
+        break;
+      }
+
+      case Instruction::kStoreInstanceField: {
+        StoreInstanceFieldInstr* store =
+            instr->AsStoreInstanceField();
+        set_representation(store->RequiredInputRepresentation(
+            StoreInstanceFieldInstr::kValuePos));
+        instance_ = store->instance()->definition()->OriginalDefinition();
+        if (!store->field().IsNull()) {
+          set_kind(kField);
+          field_ = &store->field();
+        } else {
+          set_kind(kVMField);
+          offset_in_bytes_ = store->offset_in_bytes();
+        }
+        *is_store = true;
+        break;
+      }
+
+      case Instruction::kLoadStaticField:
+        set_kind(kField);
+        set_representation(instr->AsLoadStaticField()->representation());
+        field_ = &instr->AsLoadStaticField()->StaticField();
+        *is_load = true;
+        break;
+
+      case Instruction::kStoreStaticField:
+        set_kind(kField);
+        set_representation(instr->AsStoreStaticField()->
+            RequiredInputRepresentation(StoreStaticFieldInstr::kValuePos));
+        field_ = &instr->AsStoreStaticField()->field();
+        *is_store = true;
+        break;
+
+      case Instruction::kLoadIndexed: {
+        LoadIndexedInstr* load_indexed = instr->AsLoadIndexed();
+        set_representation(load_indexed->representation());
+        instance_ = load_indexed->array()->definition()->OriginalDefinition();
+        SetIndex(load_indexed->index()->definition(),
+                 load_indexed->index_scale(),
+                 load_indexed->class_id());
+        *is_load = true;
+        break;
+      }
+
+      case Instruction::kStoreIndexed: {
+        StoreIndexedInstr* store_indexed = instr->AsStoreIndexed();
+        set_representation(store_indexed->
+            RequiredInputRepresentation(StoreIndexedInstr::kValuePos));
+        instance_ = store_indexed->array()->definition()->OriginalDefinition();
+        SetIndex(store_indexed->index()->definition(),
+                 store_indexed->index_scale(),
+                 store_indexed->class_id());
+        *is_store = true;
+        break;
+      }
+
+      default:
+        break;
+    }
+  }
+
+  // Create object representing *[*] alias.
+  static Place* CreateAnyInstanceAnyIndexAlias(Zone* zone,
+                                               intptr_t id) {
+    return Wrap(zone, Place(
+        EncodeFlags(kIndexed, kNoRepresentation, kNoSize),
+        NULL,
+        0), id);
+  }
+
+  // Return least generic alias for this place. Given that aliases are
+  // essentially sets of places we define least generic alias as a smallest
+  // alias that contains this place.
+  //
+  // We obtain such alias by a simple transformation:
+  //
+  //    - for places that depend on an instance X.f, X.@offs, X[i], X[C]
+  //      we drop X if X is not an allocation because in this case X does not
+  //      posess an identity obtaining aliases *.f, *.@offs, *[i] and *[C]
+  //      respectively;
+  //    - for non-constant indexed places X[i] we drop information about the
+  //      index obtaining alias X[*].
+  //    - we drop information about representation, but keep element size
+  //      if any.
+  //
+  Place ToAlias() const {
+    return Place(
+        RepresentationBits::update(kNoRepresentation, flags_),
+        (DependsOnInstance() && IsAllocation(instance())) ? instance() : NULL,
+        (kind() == kIndexed) ? 0 : raw_selector_);
+  }
+
+  bool DependsOnInstance() const {
+    switch (kind()) {
+      case kField:
+      case kVMField:
+      case kIndexed:
+      case kConstantIndexed:
+        return true;
+
+      case kNone:
+        return false;
+    }
+
+    UNREACHABLE();
+    return false;
+  }
+
+  // Given instance dependent alias X.f, X.@offs, X[C], X[*] return
+  // wild-card dependent alias *.f, *.@offs, *[C] or *[*] respectively.
+  Place CopyWithoutInstance() const {
+    ASSERT(DependsOnInstance());
+    return Place(flags_, NULL, raw_selector_);
+  }
+
+  // Given alias X[C] or *[C] return X[*] and *[*] respectively.
+  Place CopyWithoutIndex() const {
+    ASSERT(kind() == kConstantIndexed);
+    return Place(EncodeFlags(kIndexed, kNoRepresentation, kNoSize),
+                 instance_,
+                 0);
+  }
+
+  // Given alias X[ByteOffs|S] and a larger element size S', return
+  // alias X[RoundDown(ByteOffs, S')|S'] - this is the byte offset of a larger
+  // typed array element that contains this typed array element.
+  // In other words this method computes the only possible place with the given
+  // size that can alias this place (due to alignment restrictions).
+  // For example for X[9|kInt8] and target size kInt32 we would return
+  // X[8|kInt32].
+  Place ToLargerElement(ElementSize to) const {
+    ASSERT(kind() == kConstantIndexed);
+    ASSERT(element_size() != kNoSize);
+    ASSERT(element_size() < to);
+    return Place(ElementSizeBits::update(to, flags_),
+                 instance_,
+                 RoundByteOffset(to, index_constant_));
+  }
+
+
+  intptr_t id() const { return id_; }
+
+  Kind kind() const { return KindBits::decode(flags_); }
+
+  Representation representation() const {
+    return RepresentationBits::decode(flags_);
+  }
+
+  Definition* instance() const {
+    ASSERT(DependsOnInstance());
+    return instance_;
+  }
+
+  void set_instance(Definition* def) {
+    ASSERT(DependsOnInstance());
+    instance_ = def->OriginalDefinition();
+  }
+
+  const Field& field() const {
+    ASSERT(kind() == kField);
+    return *field_;
+  }
+
+  intptr_t offset_in_bytes() const {
+    ASSERT(kind() == kVMField);
+    return offset_in_bytes_;
+  }
+
+  Definition* index() const {
+    ASSERT(kind() == kIndexed);
+    return index_;
+  }
+
+  ElementSize element_size() const {
+    return ElementSizeBits::decode(flags_);
+  }
+
+  intptr_t index_constant() const {
+    ASSERT(kind() == kConstantIndexed);
+    return index_constant_;
+  }
+
+  static const char* DefinitionName(Definition* def) {
+    if (def == NULL) {
+      return "*";
+    } else {
+      return Thread::Current()->zone()->PrintToString(
+            "v%" Pd, def->ssa_temp_index());
+    }
+  }
+
+  const char* ToCString() const {
+    switch (kind()) {
+      case kNone:
+        return "<none>";
+
+      case kField: {
+        const char* field_name = String::Handle(field().name()).ToCString();
+        if (field().is_static()) {
+          return Thread::Current()->zone()->PrintToString(
+              "<%s>", field_name);
+        } else {
+          return Thread::Current()->zone()->PrintToString(
+              "<%s.%s>", DefinitionName(instance()), field_name);
+        }
+      }
+
+      case kVMField:
+        return Thread::Current()->zone()->PrintToString(
+            "<%s.@%" Pd ">",
+            DefinitionName(instance()),
+            offset_in_bytes());
+
+      case kIndexed:
+        return Thread::Current()->zone()->PrintToString(
+            "<%s[%s]>",
+            DefinitionName(instance()),
+            DefinitionName(index()));
+
+      case kConstantIndexed:
+        if (element_size() == kNoSize) {
+          return Thread::Current()->zone()->PrintToString(
+              "<%s[%" Pd "]>",
+              DefinitionName(instance()),
+              index_constant());
+        } else {
+          return Thread::Current()->zone()->PrintToString(
+              "<%s[%" Pd "|%" Pd "]>",
+              DefinitionName(instance()),
+              index_constant(),
+              ElementSizeMultiplier(element_size()));
+        }
+    }
+    UNREACHABLE();
+    return "<?>";
+  }
+
+  // Fields that are considered immutable by load optimization.
+  // Handle static finals as non-final with precompilation because
+  // they may be reset to uninitialized after compilation.
+  bool IsImmutableField() const {
+    return (kind() == kField)
+        && field().is_final()
+        && (!field().is_static() || !FLAG_fields_may_be_reset);
+  }
+
+  intptr_t Hashcode() const {
+    return (flags_ * 63 + reinterpret_cast<intptr_t>(instance_)) * 31 +
+        FieldHashcode();
+  }
+
+  bool Equals(const Place* other) const {
+    return (flags_ == other->flags_) &&
+        (instance_ == other->instance_) &&
+        SameField(other);
+  }
+
+  // Create a zone allocated copy of this place and assign given id to it.
+  static Place* Wrap(Zone* zone, const Place& place, intptr_t id);
+
+  static bool IsAllocation(Definition* defn) {
+    return (defn != NULL) &&
+        (defn->IsAllocateObject() ||
+         defn->IsCreateArray() ||
+         defn->IsAllocateUninitializedContext() ||
+         (defn->IsStaticCall() &&
+          defn->AsStaticCall()->IsRecognizedFactory()));
+  }
+
+ private:
+  Place(uword flags, Definition* instance, intptr_t selector)
+      : flags_(flags),
+        instance_(instance),
+        raw_selector_(selector),
+        id_(0) {
+  }
+
+  bool SameField(const Place* other) const {
+    return (kind() == kField) ? (field().raw() == other->field().raw())
+                              : (offset_in_bytes_ == other->offset_in_bytes_);
+  }
+
+  intptr_t FieldHashcode() const {
+    return (kind() == kField) ? reinterpret_cast<intptr_t>(field().raw())
+                              : offset_in_bytes_;
+  }
+
+  void set_representation(Representation rep) {
+    flags_ = RepresentationBits::update(rep, flags_);
+  }
+
+  void set_kind(Kind kind) {
+    flags_ = KindBits::update(kind, flags_);
+  }
+
+  void set_element_size(ElementSize scale) {
+    flags_ = ElementSizeBits::update(scale, flags_);
+  }
+
+  void SetIndex(Definition* index, intptr_t scale, intptr_t class_id) {
+    ConstantInstr* index_constant = index->AsConstant();
+    if ((index_constant != NULL) && index_constant->value().IsSmi()) {
+      const intptr_t index_value = Smi::Cast(index_constant->value()).Value();
+      const ElementSize size = ElementSizeFor(class_id);
+      const bool is_typed_data = (size != kNoSize);
+
+      // If we are writing into the typed data scale the index to
+      // get byte offset. Otherwise ignore the scale.
+      if (!is_typed_data) {
+        scale = 1;
+      }
+
+      // Guard against potential multiplication overflow and negative indices.
+      if ((0 <= index_value) && (index_value < (kMaxInt32 / scale))) {
+        const intptr_t scaled_index = index_value * scale;
+
+        // Guard against unaligned byte offsets.
+        if (!is_typed_data ||
+            Utils::IsAligned(scaled_index, ElementSizeMultiplier(size))) {
+          set_kind(kConstantIndexed);
+          set_element_size(size);
+          index_constant_ = scaled_index;
+          return;
+        }
+      }
+
+      // Fallthrough: create generic _[*] place.
+    }
+
+    set_kind(kIndexed);
+    index_ = index;
+  }
+
+  static uword EncodeFlags(Kind kind, Representation rep, ElementSize scale) {
+    ASSERT((kind == kConstantIndexed) || (scale == kNoSize));
+    return KindBits::encode(kind) |
+        RepresentationBits::encode(rep) |
+        ElementSizeBits::encode(scale);
+  }
+
+  static ElementSize ElementSizeFor(intptr_t class_id) {
+    switch (class_id) {
+      case kArrayCid:
+      case kImmutableArrayCid:
+      case kOneByteStringCid:
+      case kTwoByteStringCid:
+        // Object arrays and strings do not allow accessing them through
+        // different types. No need to attach scale.
+        return kNoSize;
+
+      case kTypedDataInt8ArrayCid:
+      case kTypedDataUint8ArrayCid:
+      case kTypedDataUint8ClampedArrayCid:
+      case kExternalTypedDataUint8ArrayCid:
+      case kExternalTypedDataUint8ClampedArrayCid:
+        return kInt8;
+
+      case kTypedDataInt16ArrayCid:
+      case kTypedDataUint16ArrayCid:
+        return kInt16;
+
+      case kTypedDataInt32ArrayCid:
+      case kTypedDataUint32ArrayCid:
+      case kTypedDataFloat32ArrayCid:
+        return kInt32;
+
+      case kTypedDataInt64ArrayCid:
+      case kTypedDataUint64ArrayCid:
+      case kTypedDataFloat64ArrayCid:
+        return kInt64;
+
+      case kTypedDataInt32x4ArrayCid:
+      case kTypedDataFloat32x4ArrayCid:
+      case kTypedDataFloat64x2ArrayCid:
+        return kInt128;
+
+      default:
+        UNREACHABLE();
+        return kNoSize;
+    }
+  }
+
+  static intptr_t ElementSizeMultiplier(ElementSize size) {
+    return 1 << (static_cast<intptr_t>(size) - static_cast<intptr_t>(kInt8));
+  }
+
+  static intptr_t RoundByteOffset(ElementSize size, intptr_t offset) {
+    return offset & ~(ElementSizeMultiplier(size) - 1);
+  }
+
+  class KindBits : public BitField<uword, Kind, 0, 3> {};
+  class RepresentationBits :
+      public BitField<uword, Representation, KindBits::kNextBit, 11> {};
+  class ElementSizeBits :
+      public BitField<uword, ElementSize, RepresentationBits::kNextBit, 3> {};
+
+  uword flags_;
+  Definition* instance_;
+  union {
+    intptr_t raw_selector_;
+    const Field* field_;
+    intptr_t offset_in_bytes_;
+    intptr_t index_constant_;
+    Definition* index_;
+  };
+
+  intptr_t id_;
+};
+
+
+class ZonePlace : public ZoneAllocated {
+ public:
+  explicit ZonePlace(const Place& place) : place_(place) { }
+
+  Place* place() { return &place_; }
+
+ private:
+  Place place_;
+};
+
+
+Place* Place::Wrap(Zone* zone, const Place& place, intptr_t id) {
+  Place* wrapped = (new(zone) ZonePlace(place))->place();
+  wrapped->id_ = id;
+  return wrapped;
+}
+
+
+// Correspondence between places connected through outgoing phi moves on the
+// edge that targets join.
+class PhiPlaceMoves : public ZoneAllocated {
+ public:
+  // Record a move from the place with id |from| to the place with id |to| at
+  // the given block.
+  void CreateOutgoingMove(Zone* zone,
+                          BlockEntryInstr* block, intptr_t from, intptr_t to) {
+    const intptr_t block_num = block->preorder_number();
+    while (moves_.length() <= block_num) {
+      moves_.Add(NULL);
+    }
+
+    if (moves_[block_num] == NULL) {
+      moves_[block_num] = new(zone) ZoneGrowableArray<Move>(5);
+    }
+
+    moves_[block_num]->Add(Move(from, to));
+  }
+
+  class Move {
+   public:
+    Move(intptr_t from, intptr_t to) : from_(from), to_(to) { }
+
+    intptr_t from() const { return from_; }
+    intptr_t to() const { return to_; }
+
+   private:
+    intptr_t from_;
+    intptr_t to_;
+  };
+
+  typedef const ZoneGrowableArray<Move>* MovesList;
+
+  MovesList GetOutgoingMoves(BlockEntryInstr* block) const {
+    const intptr_t block_num = block->preorder_number();
+    return (block_num < moves_.length()) ?
+        moves_[block_num] : NULL;
+  }
+
+ private:
+  GrowableArray<ZoneGrowableArray<Move>* > moves_;
+};
+
+
+// A map from aliases to a set of places sharing the alias. Additionally
+// carries a set of places that can be aliased by side-effects, essentially
+// those that are affected by calls.
+class AliasedSet : public ZoneAllocated {
+ public:
+  AliasedSet(Zone* zone,
+             DirectChainedHashMap<PointerKeyValueTrait<Place> >* places_map,
+             ZoneGrowableArray<Place*>* places,
+             PhiPlaceMoves* phi_moves)
+      : zone_(zone),
+        places_map_(places_map),
+        places_(*places),
+        phi_moves_(phi_moves),
+        aliases_(5),
+        aliases_map_(),
+        typed_data_access_sizes_(),
+        representatives_(),
+        killed_(),
+        aliased_by_effects_(new(zone) BitVector(zone, places->length())) {
+    InsertAlias(Place::CreateAnyInstanceAnyIndexAlias(zone_,
+        kAnyInstanceAnyIndexAlias));
+    for (intptr_t i = 0; i < places_.length(); i++) {
+      AddRepresentative(places_[i]);
+    }
+    ComputeKillSets();
+  }
+
+  intptr_t LookupAliasId(const Place& alias) {
+    const Place* result = aliases_map_.Lookup(&alias);
+    return (result != NULL) ? result->id() : static_cast<intptr_t>(kNoAlias);
+  }
+
+  BitVector* GetKilledSet(intptr_t alias) {
+    return (alias < killed_.length()) ? killed_[alias] : NULL;
+  }
+
+  intptr_t max_place_id() const { return places().length(); }
+  bool IsEmpty() const { return max_place_id() == 0; }
+
+  BitVector* aliased_by_effects() const { return aliased_by_effects_; }
+
+  const ZoneGrowableArray<Place*>& places() const {
+    return places_;
+  }
+
+  Place* LookupCanonical(Place* place) const {
+    return places_map_->Lookup(place);
+  }
+
+  void PrintSet(BitVector* set) {
+    bool comma = false;
+    for (BitVector::Iterator it(set);
+         !it.Done();
+         it.Advance()) {
+      if (comma) {
+        THR_Print(", ");
+      }
+      THR_Print("%s", places_[it.Current()]->ToCString());
+      comma = true;
+    }
+  }
+
+  const PhiPlaceMoves* phi_moves() const { return phi_moves_; }
+
+  void RollbackAliasedIdentites() {
+    for (intptr_t i = 0; i < identity_rollback_.length(); ++i) {
+      identity_rollback_[i]->SetIdentity(AliasIdentity::Unknown());
+    }
+  }
+
+  // Returns false if the result of an allocation instruction can't be aliased
+  // by another SSA variable and true otherwise.
+  bool CanBeAliased(Definition* alloc) {
+    if (!Place::IsAllocation(alloc)) {
+      return true;
+    }
+
+    if (alloc->Identity().IsUnknown()) {
+      ComputeAliasing(alloc);
+    }
+
+    return !alloc->Identity().IsNotAliased();
+  }
+
+  enum {
+    kNoAlias = 0
+  };
+
+ private:
+  enum {
+    // Artificial alias that is used to collect all representatives of the
+    // *[C], X[C] aliases for arbitrary C.
+    kAnyConstantIndexedAlias = 1,
+
+    // Artificial alias that is used to collect all representatives of
+    // *[C] alias for arbitrary C.
+    kUnknownInstanceConstantIndexedAlias = 2,
+
+    // Artificial alias that is used to collect all representatives of
+    // X[*] alias for all X.
+    kAnyAllocationIndexedAlias = 3,
+
+    // *[*] alias.
+    kAnyInstanceAnyIndexAlias = 4
+  };
+
+  // Compute least generic alias for the place and assign alias id to it.
+  void AddRepresentative(Place* place) {
+    if (!place->IsImmutableField()) {
+      const Place* alias = CanonicalizeAlias(place->ToAlias());
+      EnsureSet(&representatives_, alias->id())->Add(place->id());
+
+      // Update cumulative representative sets that are used during
+      // killed sets computation.
+      if (alias->kind() == Place::kConstantIndexed) {
+        if (CanBeAliased(alias->instance())) {
+          EnsureSet(&representatives_, kAnyConstantIndexedAlias)->
+              Add(place->id());
+        }
+
+        if (alias->instance() == NULL) {
+          EnsureSet(&representatives_, kUnknownInstanceConstantIndexedAlias)->
+              Add(place->id());
+        }
+
+        // Collect all element sizes used to access TypedData arrays in
+        // the function. This is used to skip sizes without representatives
+        // when computing kill sets.
+        if (alias->element_size() != Place::kNoSize) {
+          typed_data_access_sizes_.Add(alias->element_size());
+        }
+      } else if ((alias->kind() == Place::kIndexed) &&
+                 CanBeAliased(place->instance())) {
+        EnsureSet(&representatives_, kAnyAllocationIndexedAlias)->
+            Add(place->id());
+      }
+
+      if (!IsIndependentFromEffects(place)) {
+        aliased_by_effects_->Add(place->id());
+      }
+    }
+  }
+
+  void ComputeKillSets() {
+    for (intptr_t i = 0; i < aliases_.length(); ++i) {
+      const Place* alias = aliases_[i];
+      // Add all representatives to the kill set.
+      AddAllRepresentatives(alias->id(), alias->id());
+      ComputeKillSet(alias);
+    }
+
+    if (FLAG_trace_load_optimization) {
+      THR_Print("Aliases KILL sets:\n");
+      for (intptr_t i = 0; i < aliases_.length(); ++i) {
+        const Place* alias = aliases_[i];
+        BitVector* kill = GetKilledSet(alias->id());
+
+        THR_Print("%s: ", alias->ToCString());
+        if (kill != NULL) {
+          PrintSet(kill);
+        }
+        THR_Print("\n");
+      }
+    }
+  }
+
+  void InsertAlias(const Place* alias) {
+    aliases_map_.Insert(alias);
+    aliases_.Add(alias);
+  }
+
+  const Place* CanonicalizeAlias(const Place& alias) {
+    const Place* canonical = aliases_map_.Lookup(&alias);
+    if (canonical == NULL) {
+      canonical = Place::Wrap(zone_,
+                              alias,
+                              kAnyInstanceAnyIndexAlias + aliases_.length());
+      InsertAlias(canonical);
+    }
+    ASSERT(aliases_map_.Lookup(&alias) == canonical);
+    return canonical;
+  }
+
+  BitVector* GetRepresentativesSet(intptr_t alias) {
+    return (alias < representatives_.length()) ? representatives_[alias] : NULL;
+  }
+
+  BitVector* EnsureSet(GrowableArray<BitVector*>* sets,
+                       intptr_t alias) {
+    while (sets->length() <= alias) {
+      sets->Add(NULL);
+    }
+
+    BitVector* set = (*sets)[alias];
+    if (set == NULL) {
+      (*sets)[alias] = set = new(zone_) BitVector(zone_, max_place_id());
+    }
+    return set;
+  }
+
+  void AddAllRepresentatives(const Place* to, intptr_t from) {
+    AddAllRepresentatives(to->id(), from);
+  }
+
+  void AddAllRepresentatives(intptr_t to, intptr_t from) {
+    BitVector* from_set = GetRepresentativesSet(from);
+    if (from_set != NULL) {
+      EnsureSet(&killed_, to)->AddAll(from_set);
+    }
+  }
+
+  void CrossAlias(const Place* to, const Place& from) {
+    const intptr_t from_id = LookupAliasId(from);
+    if (from_id == kNoAlias) {
+      return;
+    }
+    CrossAlias(to, from_id);
+  }
+
+  void CrossAlias(const Place* to, intptr_t from) {
+    AddAllRepresentatives(to->id(), from);
+    AddAllRepresentatives(from, to->id());
+  }
+
+  // When computing kill sets we let less generic alias insert its
+  // representatives into more generic alias'es kill set. For example
+  // when visiting alias X[*] instead of searching for all aliases X[C]
+  // and inserting their representatives into kill set for X[*] we update
+  // kill set for X[*] each time we visit new X[C] for some C.
+  // There is an exception however: if both aliases are parametric like *[C]
+  // and X[*] which cross alias when X is an aliased allocation then we use
+  // artificial aliases that contain all possible representatives for the given
+  // alias for any value of the parameter to compute resulting kill set.
+  void ComputeKillSet(const Place* alias) {
+    switch (alias->kind()) {
+      case Place::kIndexed:  // Either *[*] or X[*] alias.
+        if (alias->instance() == NULL) {
+          // *[*] aliases with X[*], X[C], *[C].
+          AddAllRepresentatives(alias, kAnyConstantIndexedAlias);
+          AddAllRepresentatives(alias, kAnyAllocationIndexedAlias);
+        } else if (CanBeAliased(alias->instance())) {
+          // X[*] aliases with X[C].
+          // If X can be aliased then X[*] also aliases with *[C], *[*].
+          CrossAlias(alias, kAnyInstanceAnyIndexAlias);
+          AddAllRepresentatives(alias, kUnknownInstanceConstantIndexedAlias);
+        }
+        break;
+
+      case Place::kConstantIndexed:  // Either X[C] or *[C] alias.
+        if (alias->element_size() != Place::kNoSize) {
+          const bool has_aliased_instance =
+              (alias->instance() != NULL) && CanBeAliased(alias->instance());
+
+          // If this is a TypedData access then X[C|S] aliases larger elements
+          // covering this one X[RoundDown(C, S')|S'] for all S' > S and
+          // all smaller elements being covered by this one X[C'|S'] for
+          // some S' < S and all C' such that C = RoundDown(C', S).
+          // In the loop below it's enough to only propagate aliasing to
+          // larger aliases because propagation is symmetric: smaller aliases
+          // (if there are any) would update kill set for this alias when they
+          // are visited.
+          for (intptr_t i = static_cast<intptr_t>(alias->element_size()) + 1;
+               i <= Place::kLargestElementSize;
+               i++) {
+            // Skip element sizes that a guaranteed to have no representatives.
+            if (!typed_data_access_sizes_.Contains(alias->element_size())) {
+              continue;
+            }
+
+            // X[C|S] aliases with X[RoundDown(C, S')|S'] and likewise
+            // *[C|S] aliases with *[RoundDown(C, S')|S'].
+            const Place larger_alias =
+                alias->ToLargerElement(static_cast<Place::ElementSize>(i));
+            CrossAlias(alias, larger_alias);
+            if (has_aliased_instance) {
+              // If X is an aliased instance then X[C|S] aliases
+              // with *[RoundDown(C, S')|S'].
+              CrossAlias(alias, larger_alias.CopyWithoutInstance());
+            }
+          }
+        }
+
+        if (alias->instance() == NULL) {
+          // *[C] aliases with X[C], X[*], *[*].
+          AddAllRepresentatives(alias, kAnyAllocationIndexedAlias);
+          CrossAlias(alias, kAnyInstanceAnyIndexAlias);
+        } else {
+          // X[C] aliases with X[*].
+          // If X can be aliased then X[C] also aliases with *[C], *[*].
+          CrossAlias(alias, alias->CopyWithoutIndex());
+          if (CanBeAliased(alias->instance())) {
+            CrossAlias(alias, alias->CopyWithoutInstance());
+            CrossAlias(alias, kAnyInstanceAnyIndexAlias);
+          }
+        }
+        break;
+
+      case Place::kField:
+      case Place::kVMField:
+        if (CanBeAliased(alias->instance())) {
+          // X.f or X.@offs alias with *.f and *.@offs respectively.
+          CrossAlias(alias, alias->CopyWithoutInstance());
+        }
+        break;
+
+      case Place::kNone:
+        UNREACHABLE();
+    }
+  }
+
+  // Returns true if the given load is unaffected by external side-effects.
+  // This essentially means that no stores to the same location can
+  // occur in other functions.
+  bool IsIndependentFromEffects(Place* place) {
+    if (place->IsImmutableField()) {
+      // Note that we can't use LoadField's is_immutable attribute here because
+      // some VM-fields (those that have no corresponding Field object and
+      // accessed through offset alone) can share offset but have different
+      // immutability properties.
+      // One example is the length property of growable and fixed size list. If
+      // loads of these two properties occur in the same function for the same
+      // receiver then they will get the same expression number. However
+      // immutability of the length of fixed size list does not mean that
+      // growable list also has immutable property. Thus we will make a
+      // conservative assumption for the VM-properties.
+      // TODO(vegorov): disambiguate immutable and non-immutable VM-fields with
+      // the same offset e.g. through recognized kind.
+      return true;
+    }
+
+    return ((place->kind() == Place::kField) ||
+         (place->kind() == Place::kVMField)) &&
+        !CanBeAliased(place->instance());
+  }
+
+  // Returns true if there are direct loads from the given place.
+  bool HasLoadsFromPlace(Definition* defn, const Place* place) {
+    ASSERT((place->kind() == Place::kField) ||
+           (place->kind() == Place::kVMField));
+
+    for (Value* use = defn->input_use_list();
+         use != NULL;
+         use = use->next_use()) {
+      Instruction* instr = use->instruction();
+      if ((instr->IsRedefinition() ||
+           instr->IsAssertAssignable()) &&
+          HasLoadsFromPlace(instr->AsDefinition(), place)) {
+        return true;
+      }
+      bool is_load = false, is_store;
+      Place load_place(instr, &is_load, &is_store);
+
+      if (is_load && load_place.Equals(place)) {
+        return true;
+      }
+    }
+
+    return false;
+  }
+
+  // Check if any use of the definition can create an alias.
+  // Can add more objects into aliasing_worklist_.
+  bool AnyUseCreatesAlias(Definition* defn) {
+    for (Value* use = defn->input_use_list();
+         use != NULL;
+         use = use->next_use()) {
+      Instruction* instr = use->instruction();
+      if (instr->IsPushArgument() ||
+          (instr->IsStoreIndexed()
+           && (use->use_index() == StoreIndexedInstr::kValuePos)) ||
+          instr->IsStoreStaticField() ||
+          instr->IsPhi()) {
+        return true;
+      } else if ((instr->IsAssertAssignable() || instr->IsRedefinition()) &&
+                 AnyUseCreatesAlias(instr->AsDefinition())) {
+        return true;
+      } else if ((instr->IsStoreInstanceField()
+           && (use->use_index() != StoreInstanceFieldInstr::kInstancePos))) {
+        ASSERT(use->use_index() == StoreInstanceFieldInstr::kValuePos);
+        // If we store this value into an object that is not aliased itself
+        // and we never load again then the store does not create an alias.
+        StoreInstanceFieldInstr* store = instr->AsStoreInstanceField();
+        Definition* instance =
+            store->instance()->definition()->OriginalDefinition();
+        if (Place::IsAllocation(instance) &&
+            !instance->Identity().IsAliased()) {
+          bool is_load, is_store;
+          Place store_place(instr, &is_load, &is_store);
+
+          if (!HasLoadsFromPlace(instance, &store_place)) {
+            // No loads found that match this store. If it is yet unknown if
+            // the object is not aliased then optimistically assume this but
+            // add it to the worklist to check its uses transitively.
+            if (instance->Identity().IsUnknown()) {
+              instance->SetIdentity(AliasIdentity::NotAliased());
+              aliasing_worklist_.Add(instance);
+            }
+            continue;
+          }
+        }
+        return true;
+      }
+    }
+    return false;
+  }
+
+  // Mark any value stored into the given object as potentially aliased.
+  void MarkStoredValuesEscaping(Definition* defn) {
+    // Find all stores into this object.
+    for (Value* use = defn->input_use_list();
+         use != NULL;
+         use = use->next_use()) {
+      if (use->instruction()->IsRedefinition() ||
+          use->instruction()->IsAssertAssignable()) {
+        MarkStoredValuesEscaping(use->instruction()->AsDefinition());
+        continue;
+      }
+      if ((use->use_index() == StoreInstanceFieldInstr::kInstancePos) &&
+          use->instruction()->IsStoreInstanceField()) {
+        StoreInstanceFieldInstr* store =
+            use->instruction()->AsStoreInstanceField();
+        Definition* value = store->value()->definition()->OriginalDefinition();
+        if (value->Identity().IsNotAliased()) {
+          value->SetIdentity(AliasIdentity::Aliased());
+          identity_rollback_.Add(value);
+
+          // Add to worklist to propagate the mark transitively.
+          aliasing_worklist_.Add(value);
+        }
+      }
+    }
+  }
+
+  // Determine if the given definition can't be aliased.
+  void ComputeAliasing(Definition* alloc) {
+    ASSERT(Place::IsAllocation(alloc));
+    ASSERT(alloc->Identity().IsUnknown());
+    ASSERT(aliasing_worklist_.is_empty());
+
+    alloc->SetIdentity(AliasIdentity::NotAliased());
+    aliasing_worklist_.Add(alloc);
+
+    while (!aliasing_worklist_.is_empty()) {
+      Definition* defn = aliasing_worklist_.RemoveLast();
+      ASSERT(Place::IsAllocation(defn));
+      // If the definition in the worklist was optimistically marked as
+      // not-aliased check that optimistic assumption still holds: check if
+      // any of its uses can create an alias.
+      if (!defn->Identity().IsAliased() && AnyUseCreatesAlias(defn)) {
+        defn->SetIdentity(AliasIdentity::Aliased());
+        identity_rollback_.Add(defn);
+      }
+
+      // If the allocation site is marked as aliased conservatively mark
+      // any values stored into the object aliased too.
+      if (defn->Identity().IsAliased()) {
+        MarkStoredValuesEscaping(defn);
+      }
+    }
+  }
+
+  Zone* zone_;
+
+  DirectChainedHashMap<PointerKeyValueTrait<Place> >* places_map_;
+
+  const ZoneGrowableArray<Place*>& places_;
+
+  const PhiPlaceMoves* phi_moves_;
+
+  // A list of all seen aliases and a map that allows looking up canonical
+  // alias object.
+  GrowableArray<const Place*> aliases_;
+  DirectChainedHashMap<PointerKeyValueTrait<const Place> > aliases_map_;
+
+  SmallSet<Place::ElementSize> typed_data_access_sizes_;
+
+  // Maps alias id to set of ids of places representing the alias.
+  // Place represents an alias if this alias is least generic alias for
+  // the place.
+  // (see ToAlias for the definition of least generic alias).
+  GrowableArray<BitVector*> representatives_;
+
+  // Maps alias id to set of ids of places aliased.
+  GrowableArray<BitVector*> killed_;
+
+  // Set of ids of places that can be affected by side-effects other than
+  // explicit stores (i.e. through calls).
+  BitVector* aliased_by_effects_;
+
+  // Worklist used during alias analysis.
+  GrowableArray<Definition*> aliasing_worklist_;
+
+  // List of definitions that had their identity set to Aliased. At the end
+  // of load optimization their identity will be rolled back to Unknown to
+  // avoid treating them as Aliased at later stages without checking first
+  // as optimizations can potentially eliminate instructions leading to
+  // aliasing.
+  GrowableArray<Definition*> identity_rollback_;
+};
+
+
+static Definition* GetStoredValue(Instruction* instr) {
+  if (instr->IsStoreIndexed()) {
+    return instr->AsStoreIndexed()->value()->definition();
+  }
+
+  StoreInstanceFieldInstr* store_instance_field = instr->AsStoreInstanceField();
+  if (store_instance_field != NULL) {
+    return store_instance_field->value()->definition();
+  }
+
+  StoreStaticFieldInstr* store_static_field = instr->AsStoreStaticField();
+  if (store_static_field != NULL) {
+    return store_static_field->value()->definition();
+  }
+
+  UNREACHABLE();  // Should only be called for supported store instructions.
+  return NULL;
+}
+
+
+static bool IsPhiDependentPlace(Place* place) {
+  return ((place->kind() == Place::kField) ||
+          (place->kind() == Place::kVMField)) &&
+        (place->instance() != NULL) &&
+        place->instance()->IsPhi();
+}
+
+
+// For each place that depends on a phi ensure that equivalent places
+// corresponding to phi input are numbered and record outgoing phi moves
+// for each block which establish correspondence between phi dependent place
+// and phi input's place that is flowing in.
+static PhiPlaceMoves* ComputePhiMoves(
+    DirectChainedHashMap<PointerKeyValueTrait<Place> >* map,
+    ZoneGrowableArray<Place*>* places) {
+  Thread* thread = Thread::Current();
+  Zone* zone = thread->zone();
+  PhiPlaceMoves* phi_moves = new(zone) PhiPlaceMoves();
+
+  for (intptr_t i = 0; i < places->length(); i++) {
+    Place* place = (*places)[i];
+
+    if (IsPhiDependentPlace(place)) {
+      PhiInstr* phi = place->instance()->AsPhi();
+      BlockEntryInstr* block = phi->GetBlock();
+
+      if (FLAG_trace_optimization) {
+        THR_Print("phi dependent place %s\n", place->ToCString());
+      }
+
+      Place input_place(*place);
+      for (intptr_t j = 0; j < phi->InputCount(); j++) {
+        input_place.set_instance(phi->InputAt(j)->definition());
+
+        Place* result = map->Lookup(&input_place);
+        if (result == NULL) {
+          result = Place::Wrap(zone, input_place, places->length());
+          map->Insert(result);
+          places->Add(result);
+          if (FLAG_trace_optimization) {
+            THR_Print("  adding place %s as %" Pd "\n",
+                      result->ToCString(),
+                      result->id());
+          }
+        }
+        phi_moves->CreateOutgoingMove(zone,
+                                      block->PredecessorAt(j),
+                                      result->id(),
+                                      place->id());
+      }
+    }
+  }
+
+  return phi_moves;
+}
+
+
+enum CSEMode {
+  kOptimizeLoads,
+  kOptimizeStores
+};
+
+
+static AliasedSet* NumberPlaces(
+    FlowGraph* graph,
+    DirectChainedHashMap<PointerKeyValueTrait<Place> >* map,
+    CSEMode mode) {
+  // Loads representing different expression ids will be collected and
+  // used to build per offset kill sets.
+  Zone* zone = graph->zone();
+  ZoneGrowableArray<Place*>* places =
+      new(zone) ZoneGrowableArray<Place*>(10);
+
+  bool has_loads = false;
+  bool has_stores = false;
+  for (BlockIterator it = graph->reverse_postorder_iterator();
+       !it.Done();
+       it.Advance()) {
+    BlockEntryInstr* block = it.Current();
+
+    for (ForwardInstructionIterator instr_it(block);
+         !instr_it.Done();
+         instr_it.Advance()) {
+      Instruction* instr = instr_it.Current();
+      Place place(instr, &has_loads, &has_stores);
+      if (place.kind() == Place::kNone) {
+        continue;
+      }
+
+      Place* result = map->Lookup(&place);
+      if (result == NULL) {
+        result = Place::Wrap(zone, place, places->length());
+        map->Insert(result);
+        places->Add(result);
+
+        if (FLAG_trace_optimization) {
+          THR_Print("numbering %s as %" Pd "\n",
+                    result->ToCString(),
+                    result->id());
+        }
+      }
+
+      instr->set_place_id(result->id());
+    }
+  }
+
+  if ((mode == kOptimizeLoads) && !has_loads) {
+    return NULL;
+  }
+  if ((mode == kOptimizeStores) && !has_stores) {
+    return NULL;
+  }
+
+  PhiPlaceMoves* phi_moves = ComputePhiMoves(map, places);
+
+  // Build aliasing sets mapping aliases to loads.
+  return new(zone) AliasedSet(zone, map, places, phi_moves);
+}
+
+
+// Load instructions handled by load elimination.
+static bool IsLoadEliminationCandidate(Instruction* instr) {
+  return instr->IsLoadField()
+      || instr->IsLoadIndexed()
+      || instr->IsLoadStaticField();
+}
+
+
+static bool IsLoopInvariantLoad(ZoneGrowableArray<BitVector*>* sets,
+                                intptr_t loop_header_index,
+                                Instruction* instr) {
+  return IsLoadEliminationCandidate(instr) &&
+      (sets != NULL) &&
+      instr->HasPlaceId() &&
+      ((*sets)[loop_header_index] != NULL) &&
+      (*sets)[loop_header_index]->Contains(instr->place_id());
+}
+
+
+LICM::LICM(FlowGraph* flow_graph) : flow_graph_(flow_graph) {
+  ASSERT(flow_graph->is_licm_allowed());
+}
+
+
+void LICM::Hoist(ForwardInstructionIterator* it,
+                 BlockEntryInstr* pre_header,
+                 Instruction* current) {
+  if (current->IsCheckClass()) {
+    current->AsCheckClass()->set_licm_hoisted(true);
+  } else if (current->IsCheckSmi()) {
+    current->AsCheckSmi()->set_licm_hoisted(true);
+  } else if (current->IsCheckEitherNonSmi()) {
+    current->AsCheckEitherNonSmi()->set_licm_hoisted(true);
+  } else if (current->IsCheckArrayBound()) {
+    current->AsCheckArrayBound()->set_licm_hoisted(true);
+  }
+  if (FLAG_trace_optimization) {
+    THR_Print("Hoisting instruction %s:%" Pd " from B%" Pd " to B%" Pd "\n",
+              current->DebugName(),
+              current->GetDeoptId(),
+              current->GetBlock()->block_id(),
+              pre_header->block_id());
+  }
+  // Move the instruction out of the loop.
+  current->RemoveEnvironment();
+  if (it != NULL) {
+    it->RemoveCurrentFromGraph();
+  } else {
+    current->RemoveFromGraph();
+  }
+  GotoInstr* last = pre_header->last_instruction()->AsGoto();
+  // Using kind kEffect will not assign a fresh ssa temporary index.
+  flow_graph()->InsertBefore(last, current, last->env(), FlowGraph::kEffect);
+  current->CopyDeoptIdFrom(*last);
+}
+
+
+void LICM::TrySpecializeSmiPhi(PhiInstr* phi,
+                               BlockEntryInstr* header,
+                               BlockEntryInstr* pre_header) {
+  if (phi->Type()->ToCid() == kSmiCid) {
+    return;
+  }
+
+  // Check if there is only a single kDynamicCid input to the phi that
+  // comes from the pre-header.
+  const intptr_t kNotFound = -1;
+  intptr_t non_smi_input = kNotFound;
+  for (intptr_t i = 0; i < phi->InputCount(); ++i) {
+    Value* input = phi->InputAt(i);
+    if (input->Type()->ToCid() != kSmiCid) {
+      if ((non_smi_input != kNotFound) ||
+          (input->Type()->ToCid() != kDynamicCid)) {
+        // There are multiple kDynamicCid inputs or there is an input that is
+        // known to be non-smi.
+        return;
+      } else {
+        non_smi_input = i;
+      }
+    }
+  }
+
+  if ((non_smi_input == kNotFound) ||
+      (phi->block()->PredecessorAt(non_smi_input) != pre_header)) {
+    return;
+  }
+
+  CheckSmiInstr* check = NULL;
+  for (Value* use = phi->input_use_list();
+       (use != NULL) && (check == NULL);
+       use = use->next_use()) {
+    check = use->instruction()->AsCheckSmi();
+  }
+
+  if (check == NULL) {
+    return;
+  }
+
+  // Host CheckSmi instruction and make this phi smi one.
+  Hoist(NULL, pre_header, check);
+
+  // Replace value we are checking with phi's input.
+  check->value()->BindTo(phi->InputAt(non_smi_input)->definition());
+
+  phi->UpdateType(CompileType::FromCid(kSmiCid));
+}
+
+
+void LICM::OptimisticallySpecializeSmiPhis() {
+  if (!flow_graph()->function().allows_hoisting_check_class() ||
+      FLAG_precompilation) {
+    // Do not hoist any: Either deoptimized on a hoisted check,
+    // or compiling precompiled code where we can't do optimistic
+    // hoisting of checks.
+    return;
+  }
+
+  const ZoneGrowableArray<BlockEntryInstr*>& loop_headers =
+      flow_graph()->LoopHeaders();
+
+  for (intptr_t i = 0; i < loop_headers.length(); ++i) {
+    JoinEntryInstr* header = loop_headers[i]->AsJoinEntry();
+    // Skip loop that don't have a pre-header block.
+    BlockEntryInstr* pre_header = header->ImmediateDominator();
+    if (pre_header == NULL) continue;
+
+    for (PhiIterator it(header); !it.Done(); it.Advance()) {
+      TrySpecializeSmiPhi(it.Current(), header, pre_header);
+    }
+  }
+}
+
+
+void LICM::Optimize() {
+  if (!flow_graph()->function().allows_hoisting_check_class()) {
+    // Do not hoist any.
+    return;
+  }
+
+  const ZoneGrowableArray<BlockEntryInstr*>& loop_headers =
+      flow_graph()->LoopHeaders();
+
+  ZoneGrowableArray<BitVector*>* loop_invariant_loads =
+      flow_graph()->loop_invariant_loads();
+
+  BlockEffects* block_effects = flow_graph()->block_effects();
+
+  for (intptr_t i = 0; i < loop_headers.length(); ++i) {
+    BlockEntryInstr* header = loop_headers[i];
+    // Skip loop that don't have a pre-header block.
+    BlockEntryInstr* pre_header = header->ImmediateDominator();
+    if (pre_header == NULL) continue;
+
+    for (BitVector::Iterator loop_it(header->loop_info());
+         !loop_it.Done();
+         loop_it.Advance()) {
+      BlockEntryInstr* block = flow_graph()->preorder()[loop_it.Current()];
+      for (ForwardInstructionIterator it(block);
+           !it.Done();
+           it.Advance()) {
+        Instruction* current = it.Current();
+        if ((current->AllowsCSE() &&
+             block_effects->CanBeMovedTo(current, pre_header)) ||
+            IsLoopInvariantLoad(loop_invariant_loads, i, current)) {
+          bool inputs_loop_invariant = true;
+          for (int i = 0; i < current->InputCount(); ++i) {
+            Definition* input_def = current->InputAt(i)->definition();
+            if (!input_def->GetBlock()->Dominates(pre_header)) {
+              inputs_loop_invariant = false;
+              break;
+            }
+          }
+          if (inputs_loop_invariant &&
+              !current->IsAssertAssignable() &&
+              !current->IsAssertBoolean()) {
+            // TODO(fschneider): Enable hoisting of Assert-instructions
+            // if it safe to do.
+            Hoist(&it, pre_header, current);
+          }
+        }
+      }
+    }
+  }
+}
+
+
+class LoadOptimizer : public ValueObject {
+ public:
+  LoadOptimizer(FlowGraph* graph, AliasedSet* aliased_set)
+      : graph_(graph),
+        aliased_set_(aliased_set),
+        in_(graph_->preorder().length()),
+        out_(graph_->preorder().length()),
+        gen_(graph_->preorder().length()),
+        kill_(graph_->preorder().length()),
+        exposed_values_(graph_->preorder().length()),
+        out_values_(graph_->preorder().length()),
+        phis_(5),
+        worklist_(5),
+        congruency_worklist_(6),
+        in_worklist_(NULL),
+        forwarded_(false) {
+    const intptr_t num_blocks = graph_->preorder().length();
+    for (intptr_t i = 0; i < num_blocks; i++) {
+      out_.Add(NULL);
+      gen_.Add(new(Z) BitVector(Z, aliased_set_->max_place_id()));
+      kill_.Add(new(Z) BitVector(Z, aliased_set_->max_place_id()));
+      in_.Add(new(Z) BitVector(Z, aliased_set_->max_place_id()));
+
+      exposed_values_.Add(NULL);
+      out_values_.Add(NULL);
+    }
+  }
+
+  ~LoadOptimizer() {
+    aliased_set_->RollbackAliasedIdentites();
+  }
+
+  Isolate* isolate() const { return graph_->isolate(); }
+  Zone* zone() const { return graph_->zone(); }
+
+  static bool OptimizeGraph(FlowGraph* graph) {
+    ASSERT(FLAG_load_cse);
+    if (FLAG_trace_load_optimization) {
+      FlowGraphPrinter::PrintGraph("Before LoadOptimizer", graph);
+    }
+
+    DirectChainedHashMap<PointerKeyValueTrait<Place> > map;
+    AliasedSet* aliased_set = NumberPlaces(graph, &map, kOptimizeLoads);
+    if ((aliased_set != NULL) && !aliased_set->IsEmpty()) {
+      // If any loads were forwarded return true from Optimize to run load
+      // forwarding again. This will allow to forward chains of loads.
+      // This is especially important for context variables as they are built
+      // as loads from loaded context.
+      // TODO(vegorov): renumber newly discovered congruences during the
+      // forwarding to forward chains without running whole pass twice.
+      LoadOptimizer load_optimizer(graph, aliased_set);
+      return load_optimizer.Optimize();
+    }
+    return false;
+  }
+
+ private:
+  bool Optimize() {
+    ComputeInitialSets();
+    ComputeOutSets();
+    ComputeOutValues();
+    if (graph_->is_licm_allowed()) {
+      MarkLoopInvariantLoads();
+    }
+    ForwardLoads();
+    EmitPhis();
+
+    if (FLAG_trace_load_optimization) {
+      FlowGraphPrinter::PrintGraph("After LoadOptimizer", graph_);
+    }
+
+    return forwarded_;
+  }
+
+  // Compute sets of loads generated and killed by each block.
+  // Additionally compute upwards exposed and generated loads for each block.
+  // Exposed loads are those that can be replaced if a corresponding
+  // reaching load will be found.
+  // Loads that are locally redundant will be replaced as we go through
+  // instructions.
+  void ComputeInitialSets() {
+    for (BlockIterator block_it = graph_->reverse_postorder_iterator();
+         !block_it.Done();
+         block_it.Advance()) {
+      BlockEntryInstr* block = block_it.Current();
+      const intptr_t preorder_number = block->preorder_number();
+
+      BitVector* kill = kill_[preorder_number];
+      BitVector* gen = gen_[preorder_number];
+
+      ZoneGrowableArray<Definition*>* exposed_values = NULL;
+      ZoneGrowableArray<Definition*>* out_values = NULL;
+
+      for (ForwardInstructionIterator instr_it(block);
+           !instr_it.Done();
+           instr_it.Advance()) {
+        Instruction* instr = instr_it.Current();
+
+        bool is_load = false, is_store = false;
+        Place place(instr, &is_load, &is_store);
+
+        BitVector* killed = NULL;
+        if (is_store) {
+          const intptr_t alias_id =
+              aliased_set_->LookupAliasId(place.ToAlias());
+          if (alias_id != AliasedSet::kNoAlias) {
+            killed = aliased_set_->GetKilledSet(alias_id);
+          } else if (!place.IsImmutableField()) {
+            // We encountered unknown alias: this means intrablock load
+            // forwarding refined parameter of this store, for example
+            //
+            //     o   <- alloc()
+            //     a.f <- o
+            //     u   <- a.f
+            //     u.x <- null ;; this store alias is *.x
+            //
+            // after intrablock load forwarding
+            //
+            //     o   <- alloc()
+            //     a.f <- o
+            //     o.x <- null ;; this store alias is o.x
+            //
+            // In this case we fallback to using place id recorded in the
+            // instruction that still points to the old place with a more
+            // generic alias.
+            const intptr_t old_alias_id = aliased_set_->LookupAliasId(
+                aliased_set_->places()[instr->place_id()]->ToAlias());
+            killed = aliased_set_->GetKilledSet(old_alias_id);
+          }
+
+          if (killed != NULL) {
+            kill->AddAll(killed);
+            // There is no need to clear out_values when clearing GEN set
+            // because only those values that are in the GEN set
+            // will ever be used.
+            gen->RemoveAll(killed);
+          }
+
+          // Only forward stores to normal arrays, float64, and simd arrays
+          // to loads because other array stores (intXX/uintXX/float32)
+          // may implicitly convert the value stored.
+          StoreIndexedInstr* array_store = instr->AsStoreIndexed();
+          if ((array_store == NULL) ||
+              (array_store->class_id() == kArrayCid) ||
+              (array_store->class_id() == kTypedDataFloat64ArrayCid) ||
+              (array_store->class_id() == kTypedDataFloat32ArrayCid) ||
+              (array_store->class_id() == kTypedDataFloat32x4ArrayCid)) {
+            Place* canonical_place = aliased_set_->LookupCanonical(&place);
+            if (canonical_place != NULL) {
+              // Store has a corresponding numbered place that might have a
+              // load. Try forwarding stored value to it.
+              gen->Add(canonical_place->id());
+              if (out_values == NULL) out_values = CreateBlockOutValues();
+              (*out_values)[canonical_place->id()] = GetStoredValue(instr);
+            }
+          }
+
+          ASSERT(!instr->IsDefinition() ||
+                 !IsLoadEliminationCandidate(instr->AsDefinition()));
+          continue;
+        } else if (is_load) {
+          // Check if this load needs renumbering because of the intrablock
+          // load forwarding.
+          const Place* canonical = aliased_set_->LookupCanonical(&place);
+          if ((canonical != NULL) &&
+            (canonical->id() != instr->AsDefinition()->place_id())) {
+            instr->AsDefinition()->set_place_id(canonical->id());
+          }
+        }
+
+        // If instruction has effects then kill all loads affected.
+        if (!instr->Effects().IsNone()) {
+          kill->AddAll(aliased_set_->aliased_by_effects());
+          // There is no need to clear out_values when removing values from GEN
+          // set because only those values that are in the GEN set
+          // will ever be used.
+          gen->RemoveAll(aliased_set_->aliased_by_effects());
+          continue;
+        }
+
+        Definition* defn = instr->AsDefinition();
+        if (defn == NULL) {
+          continue;
+        }
+
+        // For object allocation forward initial values of the fields to
+        // subsequent loads. For skip final fields.  Final fields are
+        // initialized in constructor that potentially can be not inlined into
+        // the function that we are currently optimizing. However at the same
+        // time we assume that values of the final fields can be forwarded
+        // across side-effects. If we add 'null' as known values for these
+        // fields here we will incorrectly propagate this null across
+        // constructor invocation.
+        AllocateObjectInstr* alloc = instr->AsAllocateObject();
+        if ((alloc != NULL)) {
+          for (Value* use = alloc->input_use_list();
+               use != NULL;
+               use = use->next_use()) {
+            // Look for all immediate loads from this object.
+            if (use->use_index() != 0) {
+              continue;
+            }
+
+            LoadFieldInstr* load = use->instruction()->AsLoadField();
+            if (load != NULL) {
+              // Found a load. Initialize current value of the field to null for
+              // normal fields, or with type arguments.
+
+              // Forward for all fields for non-escaping objects and only
+              // non-final fields and type arguments for escaping ones.
+              if (aliased_set_->CanBeAliased(alloc) &&
+                  (load->field() != NULL) &&
+                  load->field()->is_final()) {
+                continue;
+              }
+
+              Definition* forward_def = graph_->constant_null();
+              if (alloc->ArgumentCount() > 0) {
+                ASSERT(alloc->ArgumentCount() == 1);
+                intptr_t type_args_offset =
+                    alloc->cls().type_arguments_field_offset();
+                if (load->offset_in_bytes() == type_args_offset) {
+                  forward_def = alloc->PushArgumentAt(0)->value()->definition();
+                }
+              }
+              gen->Add(load->place_id());
+              if (out_values == NULL) out_values = CreateBlockOutValues();
+              (*out_values)[load->place_id()] = forward_def;
+            }
+          }
+          continue;
+        }
+
+        if (!IsLoadEliminationCandidate(defn)) {
+          continue;
+        }
+
+        const intptr_t place_id = defn->place_id();
+        if (gen->Contains(place_id)) {
+          // This is a locally redundant load.
+          ASSERT((out_values != NULL) && ((*out_values)[place_id] != NULL));
+
+          Definition* replacement = (*out_values)[place_id];
+          graph_->EnsureSSATempIndex(defn, replacement);
+          if (FLAG_trace_optimization) {
+            THR_Print("Replacing load v%" Pd " with v%" Pd "\n",
+                      defn->ssa_temp_index(),
+                      replacement->ssa_temp_index());
+          }
+
+          defn->ReplaceUsesWith(replacement);
+          instr_it.RemoveCurrentFromGraph();
+          forwarded_ = true;
+          continue;
+        } else if (!kill->Contains(place_id)) {
+          // This is an exposed load: it is the first representative of a
+          // given expression id and it is not killed on the path from
+          // the block entry.
+          if (exposed_values == NULL) {
+            static const intptr_t kMaxExposedValuesInitialSize = 5;
+            exposed_values = new(Z) ZoneGrowableArray<Definition*>(
+                Utils::Minimum(kMaxExposedValuesInitialSize,
+                               aliased_set_->max_place_id()));
+          }
+
+          exposed_values->Add(defn);
+        }
+
+        gen->Add(place_id);
+
+        if (out_values == NULL) out_values = CreateBlockOutValues();
+        (*out_values)[place_id] = defn;
+      }
+
+      exposed_values_[preorder_number] = exposed_values;
+      out_values_[preorder_number] = out_values;
+    }
+  }
+
+  static void PerformPhiMoves(PhiPlaceMoves::MovesList phi_moves,
+                              BitVector* out,
+                              BitVector* forwarded_loads) {
+    forwarded_loads->Clear();
+
+    for (intptr_t i = 0; i < phi_moves->length(); i++) {
+      const intptr_t from = (*phi_moves)[i].from();
+      const intptr_t to = (*phi_moves)[i].to();
+      if (from == to) continue;
+
+      if (out->Contains(from)) {
+        forwarded_loads->Add(to);
+      }
+    }
+
+    for (intptr_t i = 0; i < phi_moves->length(); i++) {
+      const intptr_t from = (*phi_moves)[i].from();
+      const intptr_t to = (*phi_moves)[i].to();
+      if (from == to) continue;
+
+      out->Remove(to);
+    }
+
+    out->AddAll(forwarded_loads);
+  }
+
+  // Compute OUT sets by propagating them iteratively until fix point
+  // is reached.
+  void ComputeOutSets() {
+    BitVector* temp = new(Z) BitVector(Z, aliased_set_->max_place_id());
+    BitVector* forwarded_loads =
+        new(Z) BitVector(Z, aliased_set_->max_place_id());
+    BitVector* temp_out = new(Z) BitVector(Z, aliased_set_->max_place_id());
+
+    bool changed = true;
+    while (changed) {
+      changed = false;
+
+      for (BlockIterator block_it = graph_->reverse_postorder_iterator();
+           !block_it.Done();
+           block_it.Advance()) {
+        BlockEntryInstr* block = block_it.Current();
+
+        const intptr_t preorder_number = block->preorder_number();
+
+        BitVector* block_in = in_[preorder_number];
+        BitVector* block_out = out_[preorder_number];
+        BitVector* block_kill = kill_[preorder_number];
+        BitVector* block_gen = gen_[preorder_number];
+
+        // Compute block_in as the intersection of all out(p) where p
+        // is a predecessor of the current block.
+        if (block->IsGraphEntry()) {
+          temp->Clear();
+        } else {
+          temp->SetAll();
+          ASSERT(block->PredecessorCount() > 0);
+          for (intptr_t i = 0; i < block->PredecessorCount(); i++) {
+            BlockEntryInstr* pred = block->PredecessorAt(i);
+            BitVector* pred_out = out_[pred->preorder_number()];
+            if (pred_out == NULL) continue;
+            PhiPlaceMoves::MovesList phi_moves =
+                aliased_set_->phi_moves()->GetOutgoingMoves(pred);
+            if (phi_moves != NULL) {
+              // If there are phi moves, perform intersection with
+              // a copy of pred_out where the phi moves are applied.
+              temp_out->CopyFrom(pred_out);
+              PerformPhiMoves(phi_moves, temp_out, forwarded_loads);
+              pred_out = temp_out;
+            }
+            temp->Intersect(pred_out);
+          }
+        }
+
+        if (!temp->Equals(*block_in) || (block_out == NULL)) {
+          // If IN set has changed propagate the change to OUT set.
+          block_in->CopyFrom(temp);
+
+          temp->RemoveAll(block_kill);
+          temp->AddAll(block_gen);
+
+          if ((block_out == NULL) || !block_out->Equals(*temp)) {
+            if (block_out == NULL) {
+              block_out = out_[preorder_number] =
+                  new(Z) BitVector(Z, aliased_set_->max_place_id());
+            }
+            block_out->CopyFrom(temp);
+            changed = true;
+          }
+        }
+      }
+    }
+  }
+
+  // Compute out_values mappings by propagating them in reverse postorder once
+  // through the graph. Generate phis on back edges where eager merge is
+  // impossible.
+  // No replacement is done at this point and thus any out_value[place_id] is
+  // changed at most once: from NULL to an actual value.
+  // When merging incoming loads we might need to create a phi.
+  // These phis are not inserted at the graph immediately because some of them
+  // might become redundant after load forwarding is done.
+  void ComputeOutValues() {
+    GrowableArray<PhiInstr*> pending_phis(5);
+    ZoneGrowableArray<Definition*>* temp_forwarded_values = NULL;
+
+    for (BlockIterator block_it = graph_->reverse_postorder_iterator();
+         !block_it.Done();
+         block_it.Advance()) {
+      BlockEntryInstr* block = block_it.Current();
+
+      const bool can_merge_eagerly = CanMergeEagerly(block);
+
+      const intptr_t preorder_number = block->preorder_number();
+
+      ZoneGrowableArray<Definition*>* block_out_values =
+          out_values_[preorder_number];
+
+
+      // If OUT set has changed then we have new values available out of
+      // the block. Compute these values creating phi where necessary.
+      for (BitVector::Iterator it(out_[preorder_number]);
+           !it.Done();
+           it.Advance()) {
+        const intptr_t place_id = it.Current();
+
+        if (block_out_values == NULL) {
+          out_values_[preorder_number] = block_out_values =
+              CreateBlockOutValues();
+        }
+
+        if ((*block_out_values)[place_id] == NULL) {
+          ASSERT(block->PredecessorCount() > 0);
+          Definition* in_value = can_merge_eagerly ?
+              MergeIncomingValues(block, place_id) : NULL;
+          if ((in_value == NULL) &&
+              (in_[preorder_number]->Contains(place_id))) {
+            PhiInstr* phi = new(Z) PhiInstr(block->AsJoinEntry(),
+                                            block->PredecessorCount());
+            phi->set_place_id(place_id);
+            pending_phis.Add(phi);
+            in_value = phi;
+          }
+          (*block_out_values)[place_id] = in_value;
+        }
+      }
+
+      // If the block has outgoing phi moves perform them. Use temporary list
+      // of values to ensure that cyclic moves are performed correctly.
+      PhiPlaceMoves::MovesList phi_moves =
+          aliased_set_->phi_moves()->GetOutgoingMoves(block);
+      if ((phi_moves != NULL) && (block_out_values != NULL)) {
+        if (temp_forwarded_values == NULL) {
+          temp_forwarded_values = CreateBlockOutValues();
+        }
+
+        for (intptr_t i = 0; i < phi_moves->length(); i++) {
+          const intptr_t from = (*phi_moves)[i].from();
+          const intptr_t to = (*phi_moves)[i].to();
+          if (from == to) continue;
+
+          (*temp_forwarded_values)[to] = (*block_out_values)[from];
+        }
+
+        for (intptr_t i = 0; i < phi_moves->length(); i++) {
+          const intptr_t from = (*phi_moves)[i].from();
+          const intptr_t to = (*phi_moves)[i].to();
+          if (from == to) continue;
+
+          (*block_out_values)[to] = (*temp_forwarded_values)[to];
+        }
+      }
+
+      if (FLAG_trace_load_optimization) {
+        THR_Print("B%" Pd "\n", block->block_id());
+        THR_Print("  IN: ");
+        aliased_set_->PrintSet(in_[preorder_number]);
+        THR_Print("\n");
+
+        THR_Print("  KILL: ");
+        aliased_set_->PrintSet(kill_[preorder_number]);
+        THR_Print("\n");
+
+        THR_Print("  OUT: ");
+        aliased_set_->PrintSet(out_[preorder_number]);
+        THR_Print("\n");
+      }
+    }
+
+    // All blocks were visited. Fill pending phis with inputs
+    // that flow on back edges.
+    for (intptr_t i = 0; i < pending_phis.length(); i++) {
+      FillPhiInputs(pending_phis[i]);
+    }
+  }
+
+  bool CanMergeEagerly(BlockEntryInstr* block) {
+    for (intptr_t i = 0; i < block->PredecessorCount(); i++) {
+      BlockEntryInstr* pred = block->PredecessorAt(i);
+      if (pred->postorder_number() < block->postorder_number()) {
+        return false;
+      }
+    }
+    return true;
+  }
+
+  void MarkLoopInvariantLoads() {
+    const ZoneGrowableArray<BlockEntryInstr*>& loop_headers =
+        graph_->LoopHeaders();
+
+    ZoneGrowableArray<BitVector*>* invariant_loads =
+        new(Z) ZoneGrowableArray<BitVector*>(loop_headers.length());
+
+    for (intptr_t i = 0; i < loop_headers.length(); i++) {
+      BlockEntryInstr* header = loop_headers[i];
+      BlockEntryInstr* pre_header = header->ImmediateDominator();
+      if (pre_header == NULL) {
+        invariant_loads->Add(NULL);
+        continue;
+      }
+
+      BitVector* loop_gen = new(Z) BitVector(Z, aliased_set_->max_place_id());
+      for (BitVector::Iterator loop_it(header->loop_info());
+           !loop_it.Done();
+           loop_it.Advance()) {
+        const intptr_t preorder_number = loop_it.Current();
+        loop_gen->AddAll(gen_[preorder_number]);
+      }
+
+      for (BitVector::Iterator loop_it(header->loop_info());
+           !loop_it.Done();
+           loop_it.Advance()) {
+        const intptr_t preorder_number = loop_it.Current();
+        loop_gen->RemoveAll(kill_[preorder_number]);
+      }
+
+      if (FLAG_trace_optimization) {
+        for (BitVector::Iterator it(loop_gen); !it.Done(); it.Advance()) {
+          THR_Print("place %s is loop invariant for B%" Pd "\n",
+                    aliased_set_->places()[it.Current()]->ToCString(),
+                    header->block_id());
+        }
+      }
+
+      invariant_loads->Add(loop_gen);
+    }
+
+    graph_->set_loop_invariant_loads(invariant_loads);
+  }
+
+  // Compute incoming value for the given expression id.
+  // Will create a phi if different values are incoming from multiple
+  // predecessors.
+  Definition* MergeIncomingValues(BlockEntryInstr* block, intptr_t place_id) {
+    // First check if the same value is coming in from all predecessors.
+    static Definition* const kDifferentValuesMarker =
+        reinterpret_cast<Definition*>(-1);
+    Definition* incoming = NULL;
+    for (intptr_t i = 0; i < block->PredecessorCount(); i++) {
+      BlockEntryInstr* pred = block->PredecessorAt(i);
+      ZoneGrowableArray<Definition*>* pred_out_values =
+          out_values_[pred->preorder_number()];
+      if ((pred_out_values == NULL) || ((*pred_out_values)[place_id] == NULL)) {
+        return NULL;
+      } else if (incoming == NULL) {
+        incoming = (*pred_out_values)[place_id];
+      } else if (incoming != (*pred_out_values)[place_id]) {
+        incoming = kDifferentValuesMarker;
+      }
+    }
+
+    if (incoming != kDifferentValuesMarker) {
+      ASSERT(incoming != NULL);
+      return incoming;
+    }
+
+    // Incoming values are different. Phi is required to merge.
+    PhiInstr* phi = new(Z) PhiInstr(
+        block->AsJoinEntry(), block->PredecessorCount());
+    phi->set_place_id(place_id);
+    FillPhiInputs(phi);
+    return phi;
+  }
+
+  void FillPhiInputs(PhiInstr* phi) {
+    BlockEntryInstr* block = phi->GetBlock();
+    const intptr_t place_id = phi->place_id();
+
+    for (intptr_t i = 0; i < block->PredecessorCount(); i++) {
+      BlockEntryInstr* pred = block->PredecessorAt(i);
+      ZoneGrowableArray<Definition*>* pred_out_values =
+          out_values_[pred->preorder_number()];
+      ASSERT((*pred_out_values)[place_id] != NULL);
+
+      // Sets of outgoing values are not linked into use lists so
+      // they might contain values that were replaced and removed
+      // from the graph by this iteration.
+      // To prevent using them we additionally mark definitions themselves
+      // as replaced and store a pointer to the replacement.
+      Definition* replacement = (*pred_out_values)[place_id]->Replacement();
+      Value* input = new(Z) Value(replacement);
+      phi->SetInputAt(i, input);
+      replacement->AddInputUse(input);
+    }
+
+    graph_->AllocateSSAIndexes(phi);
+    phis_.Add(phi);  // Postpone phi insertion until after load forwarding.
+
+    if (FLAG_trace_load_optimization) {
+      THR_Print("created pending phi %s for %s at B%" Pd "\n",
+                phi->ToCString(),
+                aliased_set_->places()[place_id]->ToCString(),
+                block->block_id());
+    }
+  }
+
+  // Iterate over basic blocks and replace exposed loads with incoming
+  // values.
+  void ForwardLoads() {
+    for (BlockIterator block_it = graph_->reverse_postorder_iterator();
+         !block_it.Done();
+         block_it.Advance()) {
+      BlockEntryInstr* block = block_it.Current();
+
+      ZoneGrowableArray<Definition*>* loads =
+          exposed_values_[block->preorder_number()];
+      if (loads == NULL) continue;  // No exposed loads.
+
+      BitVector* in = in_[block->preorder_number()];
+
+      for (intptr_t i = 0; i < loads->length(); i++) {
+        Definition* load = (*loads)[i];
+        if (!in->Contains(load->place_id())) continue;  // No incoming value.
+
+        Definition* replacement = MergeIncomingValues(block, load->place_id());
+        ASSERT(replacement != NULL);
+
+        // Sets of outgoing values are not linked into use lists so
+        // they might contain values that were replace and removed
+        // from the graph by this iteration.
+        // To prevent using them we additionally mark definitions themselves
+        // as replaced and store a pointer to the replacement.
+        replacement = replacement->Replacement();
+
+        if (load != replacement) {
+          graph_->EnsureSSATempIndex(load, replacement);
+
+          if (FLAG_trace_optimization) {
+            THR_Print("Replacing load v%" Pd " with v%" Pd "\n",
+                      load->ssa_temp_index(),
+                      replacement->ssa_temp_index());
+          }
+
+          load->ReplaceUsesWith(replacement);
+          load->RemoveFromGraph();
+          load->SetReplacement(replacement);
+          forwarded_ = true;
+        }
+      }
+    }
+  }
+
+  // Check if the given phi take the same value on all code paths.
+  // Eliminate it as redundant if this is the case.
+  // When analyzing phi operands assumes that only generated during
+  // this load phase can be redundant. They can be distinguished because
+  // they are not marked alive.
+  // TODO(vegorov): move this into a separate phase over all phis.
+  bool EliminateRedundantPhi(PhiInstr* phi) {
+    Definition* value = NULL;  // Possible value of this phi.
+
+    worklist_.Clear();
+    if (in_worklist_ == NULL) {
+      in_worklist_ = new(Z) BitVector(Z, graph_->current_ssa_temp_index());
+    } else {
+      in_worklist_->Clear();
+    }
+
+    worklist_.Add(phi);
+    in_worklist_->Add(phi->ssa_temp_index());
+
+    for (intptr_t i = 0; i < worklist_.length(); i++) {
+      PhiInstr* phi = worklist_[i];
+
+      for (intptr_t i = 0; i < phi->InputCount(); i++) {
+        Definition* input = phi->InputAt(i)->definition();
+        if (input == phi) continue;
+
+        PhiInstr* phi_input = input->AsPhi();
+        if ((phi_input != NULL) && !phi_input->is_alive()) {
+          if (!in_worklist_->Contains(phi_input->ssa_temp_index())) {
+            worklist_.Add(phi_input);
+            in_worklist_->Add(phi_input->ssa_temp_index());
+          }
+          continue;
+        }
+
+        if (value == NULL) {
+          value = input;
+        } else if (value != input) {
+          return false;  // This phi is not redundant.
+        }
+      }
+    }
+
+    // All phis in the worklist are redundant and have the same computed
+    // value on all code paths.
+    ASSERT(value != NULL);
+    for (intptr_t i = 0; i < worklist_.length(); i++) {
+      worklist_[i]->ReplaceUsesWith(value);
+    }
+
+    return true;
+  }
+
+  // Returns true if definitions are congruent assuming their inputs
+  // are congruent.
+  bool CanBeCongruent(Definition* a, Definition* b) {
+    return (a->tag() == b->tag()) &&
+       ((a->IsPhi() && (a->GetBlock() == b->GetBlock())) ||
+        (a->AllowsCSE() && a->Dependencies().IsNone() &&
+         a->AttributesEqual(b)));
+  }
+
+  // Given two definitions check if they are congruent under assumption that
+  // their inputs will be proven congruent. If they are - add them to the
+  // worklist to check their inputs' congruency.
+  // Returns true if pair was added to the worklist or is already in the
+  // worklist and false if a and b are not congruent.
+  bool AddPairToCongruencyWorklist(Definition* a, Definition* b) {
+    if (!CanBeCongruent(a, b)) {
+      return false;
+    }
+
+    // If a is already in the worklist check if it is being compared to b.
+    // Give up if it is not.
+    if (in_worklist_->Contains(a->ssa_temp_index())) {
+      for (intptr_t i = 0; i < congruency_worklist_.length(); i += 2) {
+        if (a == congruency_worklist_[i]) {
+          return (b == congruency_worklist_[i + 1]);
+        }
+      }
+      UNREACHABLE();
+    } else if (in_worklist_->Contains(b->ssa_temp_index())) {
+      return AddPairToCongruencyWorklist(b, a);
+    }
+
+    congruency_worklist_.Add(a);
+    congruency_worklist_.Add(b);
+    in_worklist_->Add(a->ssa_temp_index());
+    return true;
+  }
+
+  bool AreInputsCongruent(Definition* a, Definition* b) {
+    ASSERT(a->tag() == b->tag());
+    ASSERT(a->InputCount() == b->InputCount());
+    for (intptr_t j = 0; j < a->InputCount(); j++) {
+      Definition* inputA = a->InputAt(j)->definition();
+      Definition* inputB = b->InputAt(j)->definition();
+
+      if (inputA != inputB) {
+        if (!AddPairToCongruencyWorklist(inputA, inputB)) {
+          return false;
+        }
+      }
+    }
+    return true;
+  }
+
+  // Returns true if instruction dom dominates instruction other.
+  static bool Dominates(Instruction* dom, Instruction* other) {
+    BlockEntryInstr* dom_block = dom->GetBlock();
+    BlockEntryInstr* other_block = other->GetBlock();
+
+    if (dom_block == other_block) {
+      for (Instruction* current = dom->next();
+           current != NULL;
+           current = current->next()) {
+        if (current == other) {
+          return true;
+        }
+      }
+      return false;
+    }
+
+    return dom_block->Dominates(other_block);
+  }
+
+  // Replace the given phi with another if they are congruent.
+  // Returns true if succeeds.
+  bool ReplacePhiWith(PhiInstr* phi, PhiInstr* replacement) {
+    ASSERT(phi->InputCount() == replacement->InputCount());
+    ASSERT(phi->block() == replacement->block());
+
+    congruency_worklist_.Clear();
+    if (in_worklist_ == NULL) {
+      in_worklist_ = new(Z) BitVector(Z, graph_->current_ssa_temp_index());
+    } else {
+      in_worklist_->Clear();
+    }
+
+    // During the comparison worklist contains pairs of definitions to be
+    // compared.
+    if (!AddPairToCongruencyWorklist(phi, replacement)) {
+      return false;
+    }
+
+    // Process the worklist. It might grow during each comparison step.
+    for (intptr_t i = 0; i < congruency_worklist_.length(); i += 2) {
+      if (!AreInputsCongruent(congruency_worklist_[i],
+                              congruency_worklist_[i + 1])) {
+        return false;
+      }
+    }
+
+    // At this point worklist contains pairs of congruent definitions.
+    // Replace the one member of the pair with another maintaining proper
+    // domination relation between definitions and uses.
+    for (intptr_t i = 0; i < congruency_worklist_.length(); i += 2) {
+      Definition* a = congruency_worklist_[i];
+      Definition* b = congruency_worklist_[i + 1];
+
+      // If these definitions are not phis then we need to pick up one
+      // that dominates another as the replacement: if a dominates b swap them.
+      // Note: both a and b are used as a phi input at the same block B which
+      // means a dominates B and b dominates B, which guarantees that either
+      // a dominates b or b dominates a.
+      if (!a->IsPhi()) {
+        if (Dominates(a, b)) {
+          Definition* t = a;
+          a = b;
+          b = t;
+        }
+        ASSERT(Dominates(b, a));
+      }
+
+      if (FLAG_trace_load_optimization) {
+        THR_Print("Replacing %s with congruent %s\n",
+                  a->ToCString(),
+                  b->ToCString());
+      }
+
+      a->ReplaceUsesWith(b);
+      if (a->IsPhi()) {
+        // We might be replacing a phi introduced by the load forwarding
+        // that is not inserted in the graph yet.
+        ASSERT(b->IsPhi());
+        PhiInstr* phi_a = a->AsPhi();
+        if (phi_a->is_alive()) {
+          phi_a->mark_dead();
+          phi_a->block()->RemovePhi(phi_a);
+          phi_a->UnuseAllInputs();
+        }
+      } else {
+        a->RemoveFromGraph();
+      }
+    }
+
+    return true;
+  }
+
+  // Insert the given phi into the graph. Attempt to find an equal one in the
+  // target block first.
+  // Returns true if the phi was inserted and false if it was replaced.
+  bool EmitPhi(PhiInstr* phi) {
+    for (PhiIterator it(phi->block()); !it.Done(); it.Advance()) {
+      if (ReplacePhiWith(phi, it.Current())) {
+        return false;
+      }
+    }
+
+    phi->mark_alive();
+    phi->block()->InsertPhi(phi);
+    return true;
+  }
+
+  // Phis have not yet been inserted into the graph but they have uses of
+  // their inputs.  Insert the non-redundant ones and clear the input uses
+  // of the redundant ones.
+  void EmitPhis() {
+    // First eliminate all redundant phis.
+    for (intptr_t i = 0; i < phis_.length(); i++) {
+      PhiInstr* phi = phis_[i];
+      if (!phi->HasUses() || EliminateRedundantPhi(phi)) {
+        phi->UnuseAllInputs();
+        phis_[i] = NULL;
+      }
+    }
+
+    // Now emit phis or replace them with equal phis already present in the
+    // graph.
+    for (intptr_t i = 0; i < phis_.length(); i++) {
+      PhiInstr* phi = phis_[i];
+      if ((phi != NULL) && (!phi->HasUses() || !EmitPhi(phi))) {
+        phi->UnuseAllInputs();
+      }
+    }
+  }
+
+  ZoneGrowableArray<Definition*>* CreateBlockOutValues() {
+    ZoneGrowableArray<Definition*>* out =
+        new(Z) ZoneGrowableArray<Definition*>(aliased_set_->max_place_id());
+    for (intptr_t i = 0; i < aliased_set_->max_place_id(); i++) {
+      out->Add(NULL);
+    }
+    return out;
+  }
+
+  FlowGraph* graph_;
+  DirectChainedHashMap<PointerKeyValueTrait<Place> >* map_;
+
+  // Mapping between field offsets in words and expression ids of loads from
+  // that offset.
+  AliasedSet* aliased_set_;
+
+  // Per block sets of expression ids for loads that are: incoming (available
+  // on the entry), outgoing (available on the exit), generated and killed.
+  GrowableArray<BitVector*> in_;
+  GrowableArray<BitVector*> out_;
+  GrowableArray<BitVector*> gen_;
+  GrowableArray<BitVector*> kill_;
+
+  // Per block list of upwards exposed loads.
+  GrowableArray<ZoneGrowableArray<Definition*>*> exposed_values_;
+
+  // Per block mappings between expression ids and outgoing definitions that
+  // represent those ids.
+  GrowableArray<ZoneGrowableArray<Definition*>*> out_values_;
+
+  // List of phis generated during ComputeOutValues and ForwardLoads.
+  // Some of these phis might be redundant and thus a separate pass is
+  // needed to emit only non-redundant ones.
+  GrowableArray<PhiInstr*> phis_;
+
+  // Auxiliary worklist used by redundant phi elimination.
+  GrowableArray<PhiInstr*> worklist_;
+  GrowableArray<Definition*> congruency_worklist_;
+  BitVector* in_worklist_;
+
+
+  // True if any load was eliminated.
+  bool forwarded_;
+
+  DISALLOW_COPY_AND_ASSIGN(LoadOptimizer);
+};
+
+
+bool DominatorBasedCSE::Optimize(FlowGraph* graph) {
+  bool changed = false;
+  if (FLAG_load_cse) {
+    changed = LoadOptimizer::OptimizeGraph(graph) || changed;
+  }
+
+  CSEInstructionMap map;
+  changed = OptimizeRecursive(graph, graph->graph_entry(), &map) || changed;
+
+  return changed;
+}
+
+
+bool DominatorBasedCSE::OptimizeRecursive(
+    FlowGraph* graph,
+    BlockEntryInstr* block,
+    CSEInstructionMap* map) {
+  bool changed = false;
+  for (ForwardInstructionIterator it(block); !it.Done(); it.Advance()) {
+    Instruction* current = it.Current();
+    if (current->AllowsCSE()) {
+      Instruction* replacement = map->Lookup(current);
+      if ((replacement != NULL) &&
+          graph->block_effects()->IsAvailableAt(replacement, block)) {
+        // Replace current with lookup result.
+        graph->ReplaceCurrentInstruction(&it, current, replacement);
+        changed = true;
+        continue;
+      }
+
+      // For simplicity we assume that instruction either does not depend on
+      // anything or does not affect anything. If this is not the case then
+      // we should first remove affected instructions from the map and
+      // then add instruction to the map so that it does not kill itself.
+      ASSERT(current->Effects().IsNone() || current->Dependencies().IsNone());
+      map->Insert(current);
+    }
+
+    map->RemoveAffected(current->Effects());
+  }
+
+  // Process children in the dominator tree recursively.
+  intptr_t num_children = block->dominated_blocks().length();
+  for (intptr_t i = 0; i < num_children; ++i) {
+    BlockEntryInstr* child = block->dominated_blocks()[i];
+    if (i  < num_children - 1) {
+      // Copy map.
+      CSEInstructionMap child_map(*map);
+      changed = OptimizeRecursive(graph, child, &child_map) || changed;
+    } else {
+      // Reuse map for the last child.
+      changed = OptimizeRecursive(graph, child, map) || changed;
+    }
+  }
+  return changed;
+}
+
+
+class StoreOptimizer : public LivenessAnalysis {
+ public:
+  StoreOptimizer(FlowGraph* graph,
+                 AliasedSet* aliased_set,
+                 DirectChainedHashMap<PointerKeyValueTrait<Place> >* map)
+      : LivenessAnalysis(aliased_set->max_place_id(), graph->postorder()),
+        graph_(graph),
+        map_(map),
+        aliased_set_(aliased_set),
+        exposed_stores_(graph_->postorder().length()) {
+    const intptr_t num_blocks = graph_->postorder().length();
+    for (intptr_t i = 0; i < num_blocks; i++) {
+      exposed_stores_.Add(NULL);
+    }
+  }
+
+  static void OptimizeGraph(FlowGraph* graph) {
+    ASSERT(FLAG_load_cse);
+    if (FLAG_trace_load_optimization) {
+      FlowGraphPrinter::PrintGraph("Before StoreOptimizer", graph);
+    }
+
+    DirectChainedHashMap<PointerKeyValueTrait<Place> > map;
+    AliasedSet* aliased_set = NumberPlaces(graph, &map, kOptimizeStores);
+    if ((aliased_set != NULL) && !aliased_set->IsEmpty()) {
+      StoreOptimizer store_optimizer(graph, aliased_set, &map);
+      store_optimizer.Optimize();
+    }
+  }
+
+ private:
+  void Optimize() {
+    Analyze();
+    if (FLAG_trace_load_optimization) {
+      Dump();
+    }
+    EliminateDeadStores();
+    if (FLAG_trace_load_optimization) {
+      FlowGraphPrinter::PrintGraph("After StoreOptimizer", graph_);
+    }
+  }
+
+  bool CanEliminateStore(Instruction* instr) {
+    switch (instr->tag()) {
+      case Instruction::kStoreInstanceField: {
+        StoreInstanceFieldInstr* store_instance = instr->AsStoreInstanceField();
+        // Can't eliminate stores that initialize fields.
+        return !(store_instance->is_potential_unboxed_initialization() ||
+                 store_instance->is_object_reference_initialization());
+      }
+      case Instruction::kStoreIndexed:
+      case Instruction::kStoreStaticField:
+        return true;
+      default:
+        UNREACHABLE();
+        return false;
+    }
+  }
+
+  virtual void ComputeInitialSets() {
+    Zone* zone = graph_->zone();
+    BitVector* all_places = new(zone) BitVector(zone,
+        aliased_set_->max_place_id());
+    all_places->SetAll();
+    for (BlockIterator block_it = graph_->postorder_iterator();
+         !block_it.Done();
+         block_it.Advance()) {
+      BlockEntryInstr* block = block_it.Current();
+      const intptr_t postorder_number = block->postorder_number();
+
+      BitVector* kill = kill_[postorder_number];
+      BitVector* live_in = live_in_[postorder_number];
+      BitVector* live_out = live_out_[postorder_number];
+
+      ZoneGrowableArray<Instruction*>* exposed_stores = NULL;
+
+      // Iterate backwards starting at the last instruction.
+      for (BackwardInstructionIterator instr_it(block);
+           !instr_it.Done();
+           instr_it.Advance()) {
+        Instruction* instr = instr_it.Current();
+
+        bool is_load = false;
+        bool is_store = false;
+        Place place(instr, &is_load, &is_store);
+        if (place.IsImmutableField()) {
+          // Loads/stores of final fields do not participate.
+          continue;
+        }
+
+        // Handle stores.
+        if (is_store) {
+          if (kill->Contains(instr->place_id())) {
+            if (!live_in->Contains(instr->place_id()) &&
+                CanEliminateStore(instr)) {
+              if (FLAG_trace_optimization) {
+                THR_Print(
+                    "Removing dead store to place %" Pd " in block B%" Pd "\n",
+                    instr->place_id(), block->block_id());
+              }
+              instr_it.RemoveCurrentFromGraph();
+            }
+          } else if (!live_in->Contains(instr->place_id())) {
+            // Mark this store as down-ward exposed: They are the only
+            // candidates for the global store elimination.
+            if (exposed_stores == NULL) {
+              const intptr_t kMaxExposedStoresInitialSize = 5;
+              exposed_stores = new(zone) ZoneGrowableArray<Instruction*>(
+                  Utils::Minimum(kMaxExposedStoresInitialSize,
+                                 aliased_set_->max_place_id()));
+            }
+            exposed_stores->Add(instr);
+          }
+          // Interfering stores kill only loads from the same place.
+          kill->Add(instr->place_id());
+          live_in->Remove(instr->place_id());
+          continue;
+        }
+
+        // Handle side effects, deoptimization and function return.
+        if (!instr->Effects().IsNone() ||
+            instr->CanDeoptimize() ||
+            instr->IsThrow() ||
+            instr->IsReThrow() ||
+            instr->IsReturn()) {
+          // Instructions that return from the function, instructions with side
+          // effects and instructions that can deoptimize are considered as
+          // loads from all places.
+          live_in->CopyFrom(all_places);
+          if (instr->IsThrow() || instr->IsReThrow() || instr->IsReturn()) {
+            // Initialize live-out for exit blocks since it won't be computed
+            // otherwise during the fixed point iteration.
+            live_out->CopyFrom(all_places);
+          }
+          continue;
+        }
+
+        // Handle loads.
+        Definition* defn = instr->AsDefinition();
+        if ((defn != NULL) && IsLoadEliminationCandidate(defn)) {
+          const intptr_t alias = aliased_set_->LookupAliasId(place.ToAlias());
+          live_in->AddAll(aliased_set_->GetKilledSet(alias));
+          continue;
+        }
+      }
+      exposed_stores_[postorder_number] = exposed_stores;
+    }
+    if (FLAG_trace_load_optimization) {
+      Dump();
+      THR_Print("---\n");
+    }
+  }
+
+  void EliminateDeadStores() {
+    // Iteration order does not matter here.
+    for (BlockIterator block_it = graph_->postorder_iterator();
+         !block_it.Done();
+         block_it.Advance()) {
+      BlockEntryInstr* block = block_it.Current();
+      const intptr_t postorder_number = block->postorder_number();
+
+      BitVector* live_out = live_out_[postorder_number];
+
+      ZoneGrowableArray<Instruction*>* exposed_stores =
+        exposed_stores_[postorder_number];
+      if (exposed_stores == NULL) continue;  // No exposed stores.
+
+      // Iterate over candidate stores.
+      for (intptr_t i = 0; i < exposed_stores->length(); ++i) {
+        Instruction* instr = (*exposed_stores)[i];
+        bool is_load = false;
+        bool is_store = false;
+        Place place(instr, &is_load, &is_store);
+        ASSERT(!is_load && is_store);
+        if (place.IsImmutableField()) {
+          // Final field do not participate in dead store elimination.
+          continue;
+        }
+        // Eliminate a downward exposed store if the corresponding place is not
+        // in live-out.
+        if (!live_out->Contains(instr->place_id()) &&
+            CanEliminateStore(instr)) {
+          if (FLAG_trace_optimization) {
+            THR_Print("Removing dead store to place %" Pd " block B%" Pd "\n",
+                      instr->place_id(), block->block_id());
+          }
+          instr->RemoveFromGraph(/* ignored */ false);
+        }
+      }
+    }
+  }
+
+  FlowGraph* graph_;
+  DirectChainedHashMap<PointerKeyValueTrait<Place> >* map_;
+
+  // Mapping between field offsets in words and expression ids of loads from
+  // that offset.
+  AliasedSet* aliased_set_;
+
+  // Per block list of downward exposed stores.
+  GrowableArray<ZoneGrowableArray<Instruction*>*> exposed_stores_;
+
+  DISALLOW_COPY_AND_ASSIGN(StoreOptimizer);
+};
+
+
+void DeadStoreElimination::Optimize(FlowGraph* graph) {
+  if (FLAG_dead_store_elimination) {
+    StoreOptimizer::OptimizeGraph(graph);
+  }
+}
+
+
+enum SafeUseCheck { kOptimisticCheck, kStrictCheck };
+
+// Check if the use is safe for allocation sinking. Allocation sinking
+// candidates can only be used at store instructions:
+//
+//     - any store into the allocation candidate itself is unconditionally safe
+//       as it just changes the rematerialization state of this candidate;
+//     - store into another object is only safe if another object is allocation
+//       candidate.
+//
+// We use a simple fix-point algorithm to discover the set of valid candidates
+// (see CollectCandidates method), that's why this IsSafeUse can operate in two
+// modes:
+//
+//     - optimistic, when every allocation is assumed to be an allocation
+//       sinking candidate;
+//     - strict, when only marked allocations are assumed to be allocation
+//       sinking candidates.
+//
+// Fix-point algorithm in CollectCandiates first collects a set of allocations
+// optimistically and then checks each collected candidate strictly and unmarks
+// invalid candidates transitively until only strictly valid ones remain.
+static bool IsSafeUse(Value* use, SafeUseCheck check_type) {
+  if (use->instruction()->IsMaterializeObject()) {
+    return true;
+  }
+
+  StoreInstanceFieldInstr* store = use->instruction()->AsStoreInstanceField();
+  if (store != NULL) {
+    if (use == store->value()) {
+      Definition* instance = store->instance()->definition();
+      return instance->IsAllocateObject() &&
+          ((check_type == kOptimisticCheck) ||
+           instance->Identity().IsAllocationSinkingCandidate());
+    }
+    return true;
+  }
+
+  return false;
+}
+
+
+// Right now we are attempting to sink allocation only into
+// deoptimization exit. So candidate should only be used in StoreInstanceField
+// instructions that write into fields of the allocated object.
+// We do not support materialization of the object that has type arguments.
+static bool IsAllocationSinkingCandidate(Definition* alloc,
+                                         SafeUseCheck check_type) {
+  for (Value* use = alloc->input_use_list();
+       use != NULL;
+       use = use->next_use()) {
+    if (!IsSafeUse(use, check_type)) {
+      if (FLAG_trace_optimization) {
+        THR_Print("use of %s at %s is unsafe for allocation sinking\n",
+                  alloc->ToCString(),
+                  use->instruction()->ToCString());
+      }
+      return false;
+    }
+  }
+
+  return true;
+}
+
+
+// If the given use is a store into an object then return an object we are
+// storing into.
+static Definition* StoreInto(Value* use) {
+  StoreInstanceFieldInstr* store = use->instruction()->AsStoreInstanceField();
+  if (store != NULL) {
+    return store->instance()->definition();
+  }
+
+  return NULL;
+}
+
+
+// Remove the given allocation from the graph. It is not observable.
+// If deoptimization occurs the object will be materialized.
+void AllocationSinking::EliminateAllocation(Definition* alloc) {
+  ASSERT(IsAllocationSinkingCandidate(alloc, kStrictCheck));
+
+  if (FLAG_trace_optimization) {
+    THR_Print("removing allocation from the graph: v%" Pd "\n",
+              alloc->ssa_temp_index());
+  }
+
+  // As an allocation sinking candidate it is only used in stores to its own
+  // fields. Remove these stores.
+  for (Value* use = alloc->input_use_list();
+       use != NULL;
+       use = alloc->input_use_list()) {
+    use->instruction()->RemoveFromGraph();
+  }
+
+  // There should be no environment uses. The pass replaced them with
+  // MaterializeObject instructions.
+#ifdef DEBUG
+  for (Value* use = alloc->env_use_list();
+       use != NULL;
+       use = use->next_use()) {
+    ASSERT(use->instruction()->IsMaterializeObject());
+  }
+#endif
+  ASSERT(alloc->input_use_list() == NULL);
+  alloc->RemoveFromGraph();
+  if (alloc->ArgumentCount() > 0) {
+    ASSERT(alloc->ArgumentCount() == 1);
+    for (intptr_t i = 0; i < alloc->ArgumentCount(); ++i) {
+      alloc->PushArgumentAt(i)->RemoveFromGraph();
+    }
+  }
+}
+
+
+// Find allocation instructions that can be potentially eliminated and
+// rematerialized at deoptimization exits if needed. See IsSafeUse
+// for the description of algorithm used below.
+void AllocationSinking::CollectCandidates() {
+  // Optimistically collect all potential candidates.
+  for (BlockIterator block_it = flow_graph_->reverse_postorder_iterator();
+       !block_it.Done();
+       block_it.Advance()) {
+    BlockEntryInstr* block = block_it.Current();
+    for (ForwardInstructionIterator it(block); !it.Done(); it.Advance()) {
+      { AllocateObjectInstr* alloc = it.Current()->AsAllocateObject();
+        if ((alloc != NULL) &&
+            IsAllocationSinkingCandidate(alloc, kOptimisticCheck)) {
+          alloc->SetIdentity(AliasIdentity::AllocationSinkingCandidate());
+          candidates_.Add(alloc);
+        }
+      }
+      { AllocateUninitializedContextInstr* alloc =
+            it.Current()->AsAllocateUninitializedContext();
+        if ((alloc != NULL) &&
+            IsAllocationSinkingCandidate(alloc, kOptimisticCheck)) {
+          alloc->SetIdentity(AliasIdentity::AllocationSinkingCandidate());
+          candidates_.Add(alloc);
+        }
+      }
+    }
+  }
+
+  // Transitively unmark all candidates that are not strictly valid.
+  bool changed;
+  do {
+    changed = false;
+    for (intptr_t i = 0; i < candidates_.length(); i++) {
+      Definition* alloc = candidates_[i];
+      if (alloc->Identity().IsAllocationSinkingCandidate()) {
+        if (!IsAllocationSinkingCandidate(alloc, kStrictCheck)) {
+          alloc->SetIdentity(AliasIdentity::Unknown());
+          changed = true;
+        }
+      }
+    }
+  } while (changed);
+
+  // Shrink the list of candidates removing all unmarked ones.
+  intptr_t j = 0;
+  for (intptr_t i = 0; i < candidates_.length(); i++) {
+    Definition* alloc = candidates_[i];
+    if (alloc->Identity().IsAllocationSinkingCandidate()) {
+      if (FLAG_trace_optimization) {
+        THR_Print("discovered allocation sinking candidate: v%" Pd "\n",
+                  alloc->ssa_temp_index());
+      }
+
+      if (j != i) {
+        candidates_[j] = alloc;
+      }
+      j++;
+    }
+  }
+  candidates_.TruncateTo(j);
+}
+
+
+// If materialization references an allocation sinking candidate then replace
+// this reference with a materialization which should have been computed for
+// this side-exit. CollectAllExits should have collected this exit.
+void AllocationSinking::NormalizeMaterializations() {
+  for (intptr_t i = 0; i < candidates_.length(); i++) {
+    Definition* alloc = candidates_[i];
+
+    Value* next_use;
+    for (Value* use = alloc->input_use_list();
+         use != NULL;
+         use = next_use) {
+      next_use = use->next_use();
+      if (use->instruction()->IsMaterializeObject()) {
+        use->BindTo(MaterializationFor(alloc, use->instruction()));
+      }
+    }
+  }
+}
+
+
+// We transitively insert materializations at each deoptimization exit that
+// might see the given allocation (see ExitsCollector). Some of this
+// materializations are not actually used and some fail to compute because
+// they are inserted in the block that is not dominated by the allocation.
+// Remove them unused materializations from the graph.
+void AllocationSinking::RemoveUnusedMaterializations() {
+  intptr_t j = 0;
+  for (intptr_t i = 0; i < materializations_.length(); i++) {
+    MaterializeObjectInstr* mat = materializations_[i];
+    if ((mat->input_use_list() == NULL) && (mat->env_use_list() == NULL)) {
+      // Check if this materialization failed to compute and remove any
+      // unforwarded loads. There were no loads from any allocation sinking
+      // candidate in the beggining so it is safe to assume that any encountered
+      // load was inserted by CreateMaterializationAt.
+      for (intptr_t i = 0; i < mat->InputCount(); i++) {
+        LoadFieldInstr* load = mat->InputAt(i)->definition()->AsLoadField();
+        if ((load != NULL) &&
+            (load->instance()->definition() == mat->allocation())) {
+          load->ReplaceUsesWith(flow_graph_->constant_null());
+          load->RemoveFromGraph();
+        }
+      }
+      mat->RemoveFromGraph();
+    } else {
+      if (j != i) {
+        materializations_[j] = mat;
+      }
+      j++;
+    }
+  }
+  materializations_.TruncateTo(j);
+}
+
+
+// Some candidates might stop being eligible for allocation sinking after
+// the load forwarding because they flow into phis that load forwarding
+// inserts. Discover such allocations and remove them from the list
+// of allocation sinking candidates undoing all changes that we did
+// in preparation for sinking these allocations.
+void AllocationSinking::DiscoverFailedCandidates() {
+  // Transitively unmark all candidates that are not strictly valid.
+  bool changed;
+  do {
+    changed = false;
+    for (intptr_t i = 0; i < candidates_.length(); i++) {
+      Definition* alloc = candidates_[i];
+      if (alloc->Identity().IsAllocationSinkingCandidate()) {
+        if (!IsAllocationSinkingCandidate(alloc, kStrictCheck)) {
+          alloc->SetIdentity(AliasIdentity::Unknown());
+          changed = true;
+        }
+      }
+    }
+  } while (changed);
+
+  // Remove all failed candidates from the candidates list.
+  intptr_t j = 0;
+  for (intptr_t i = 0; i < candidates_.length(); i++) {
+    Definition* alloc = candidates_[i];
+    if (!alloc->Identity().IsAllocationSinkingCandidate()) {
+      if (FLAG_trace_optimization) {
+        THR_Print("allocation v%" Pd " can't be eliminated\n",
+                  alloc->ssa_temp_index());
+      }
+
+#ifdef DEBUG
+      for (Value* use = alloc->env_use_list();
+           use != NULL;
+           use = use->next_use()) {
+        ASSERT(use->instruction()->IsMaterializeObject());
+      }
+#endif
+
+      // All materializations will be removed from the graph. Remove inserted
+      // loads first and detach materializations from allocation's environment
+      // use list: we will reconstruct it when we start removing
+      // materializations.
+      alloc->set_env_use_list(NULL);
+      for (Value* use = alloc->input_use_list();
+           use != NULL;
+           use = use->next_use()) {
+        if (use->instruction()->IsLoadField()) {
+          LoadFieldInstr* load = use->instruction()->AsLoadField();
+          load->ReplaceUsesWith(flow_graph_->constant_null());
+          load->RemoveFromGraph();
+        } else {
+          ASSERT(use->instruction()->IsMaterializeObject() ||
+                 use->instruction()->IsPhi() ||
+                 use->instruction()->IsStoreInstanceField());
+        }
+      }
+    } else {
+      if (j != i) {
+        candidates_[j] = alloc;
+      }
+      j++;
+    }
+  }
+
+  if (j != candidates_.length()) {  // Something was removed from candidates.
+    intptr_t k = 0;
+    for (intptr_t i = 0; i < materializations_.length(); i++) {
+      MaterializeObjectInstr* mat = materializations_[i];
+      if (!mat->allocation()->Identity().IsAllocationSinkingCandidate()) {
+        // Restore environment uses of the allocation that were replaced
+        // by this materialization and drop materialization.
+        mat->ReplaceUsesWith(mat->allocation());
+        mat->RemoveFromGraph();
+      } else {
+        if (k != i) {
+          materializations_[k] = mat;
+        }
+        k++;
+      }
+    }
+    materializations_.TruncateTo(k);
+  }
+
+  candidates_.TruncateTo(j);
+}
+
+
+void AllocationSinking::Optimize() {
+  CollectCandidates();
+
+  // Insert MaterializeObject instructions that will describe the state of the
+  // object at all deoptimization points. Each inserted materialization looks
+  // like this (where v_0 is allocation that we are going to eliminate):
+  //   v_1     <- LoadField(v_0, field_1)
+  //           ...
+  //   v_N     <- LoadField(v_0, field_N)
+  //   v_{N+1} <- MaterializeObject(field_1 = v_1, ..., field_N = v_{N})
+  for (intptr_t i = 0; i < candidates_.length(); i++) {
+    InsertMaterializations(candidates_[i]);
+  }
+
+  // Run load forwarding to eliminate LoadField instructions inserted above.
+  // All loads will be successfully eliminated because:
+  //   a) they use fields (not offsets) and thus provide precise aliasing
+  //      information
+  //   b) candidate does not escape and thus its fields is not affected by
+  //      external effects from calls.
+  LoadOptimizer::OptimizeGraph(flow_graph_);
+
+  NormalizeMaterializations();
+
+  RemoveUnusedMaterializations();
+
+  // If any candidates are no longer eligible for allocation sinking abort
+  // the optimization for them and undo any changes we did in preparation.
+  DiscoverFailedCandidates();
+
+  // At this point we have computed the state of object at each deoptimization
+  // point and we can eliminate it. Loads inserted above were forwarded so there
+  // are no uses of the allocation just as in the begging of the pass.
+  for (intptr_t i = 0; i < candidates_.length(); i++) {
+    EliminateAllocation(candidates_[i]);
+  }
+
+  // Process materializations and unbox their arguments: materializations
+  // are part of the environment and can materialize boxes for double/mint/simd
+  // values when needed.
+  // TODO(vegorov): handle all box types here.
+  for (intptr_t i = 0; i < materializations_.length(); i++) {
+    MaterializeObjectInstr* mat = materializations_[i];
+    for (intptr_t j = 0; j < mat->InputCount(); j++) {
+      Definition* defn = mat->InputAt(j)->definition();
+      if (defn->IsBox()) {
+        mat->InputAt(j)->BindTo(defn->InputAt(0)->definition());
+      }
+    }
+  }
+}
+
+
+// Remove materializations from the graph. Register allocator will treat them
+// as part of the environment not as a real instruction.
+void AllocationSinking::DetachMaterializations() {
+  for (intptr_t i = 0; i < materializations_.length(); i++) {
+    materializations_[i]->previous()->LinkTo(materializations_[i]->next());
+  }
+}
+
+
+// Add a field/offset to the list of fields if it is not yet present there.
+static bool AddSlot(ZoneGrowableArray<const Object*>* slots,
+                    const Object& slot) {
+  for (intptr_t i = 0; i < slots->length(); i++) {
+    if ((*slots)[i]->raw() == slot.raw()) {
+      return false;
+    }
+  }
+  slots->Add(&slot);
+  return true;
+}
+
+
+// Find deoptimization exit for the given materialization assuming that all
+// materializations are emitted right before the instruction which is a
+// deoptimization exit.
+static Instruction* ExitForMaterialization(MaterializeObjectInstr* mat) {
+  while (mat->next()->IsMaterializeObject()) {
+    mat = mat->next()->AsMaterializeObject();
+  }
+  return mat->next();
+}
+
+
+// Given the deoptimization exit find first materialization that was inserted
+// before it.
+static Instruction* FirstMaterializationAt(Instruction* exit) {
+  while (exit->previous()->IsMaterializeObject()) {
+    exit = exit->previous();
+  }
+  return exit;
+}
+
+
+// Given the allocation and deoptimization exit try to find MaterializeObject
+// instruction corresponding to this allocation at this exit.
+MaterializeObjectInstr* AllocationSinking::MaterializationFor(
+    Definition* alloc, Instruction* exit) {
+  if (exit->IsMaterializeObject()) {
+    exit = ExitForMaterialization(exit->AsMaterializeObject());
+  }
+
+  for (MaterializeObjectInstr* mat = exit->previous()->AsMaterializeObject();
+       mat != NULL;
+       mat = mat->previous()->AsMaterializeObject()) {
+    if (mat->allocation() == alloc) {
+      return mat;
+    }
+  }
+
+  return NULL;
+}
+
+
+// Insert MaterializeObject instruction for the given allocation before
+// the given instruction that can deoptimize.
+void AllocationSinking::CreateMaterializationAt(
+    Instruction* exit,
+    Definition* alloc,
+    const ZoneGrowableArray<const Object*>& slots) {
+  ZoneGrowableArray<Value*>* values =
+      new(Z) ZoneGrowableArray<Value*>(slots.length());
+
+  // All loads should be inserted before the first materialization so that
+  // IR follows the following pattern: loads, materializations, deoptimizing
+  // instruction.
+  Instruction* load_point = FirstMaterializationAt(exit);
+
+  // Insert load instruction for every field.
+  for (intptr_t i = 0; i < slots.length(); i++) {
+    LoadFieldInstr* load = slots[i]->IsField()
+        ? new(Z) LoadFieldInstr(
+            new(Z) Value(alloc),
+            &Field::Cast(*slots[i]),
+            AbstractType::ZoneHandle(Z),
+            alloc->token_pos())
+        : new(Z) LoadFieldInstr(
+            new(Z) Value(alloc),
+            Smi::Cast(*slots[i]).Value(),
+            AbstractType::ZoneHandle(Z),
+            alloc->token_pos());
+    flow_graph_->InsertBefore(
+        load_point, load, NULL, FlowGraph::kValue);
+    values->Add(new(Z) Value(load));
+  }
+
+  MaterializeObjectInstr* mat = NULL;
+  if (alloc->IsAllocateObject()) {
+    mat = new(Z) MaterializeObjectInstr(
+        alloc->AsAllocateObject(), slots, values);
+  } else {
+    ASSERT(alloc->IsAllocateUninitializedContext());
+    mat = new(Z) MaterializeObjectInstr(
+        alloc->AsAllocateUninitializedContext(), slots, values);
+  }
+
+  flow_graph_->InsertBefore(exit, mat, NULL, FlowGraph::kValue);
+
+  // Replace all mentions of this allocation with a newly inserted
+  // MaterializeObject instruction.
+  // We must preserve the identity: all mentions are replaced by the same
+  // materialization.
+  for (Environment::DeepIterator env_it(exit->env());
+       !env_it.Done();
+       env_it.Advance()) {
+    Value* use = env_it.CurrentValue();
+    if (use->definition() == alloc) {
+      use->RemoveFromUseList();
+      use->set_definition(mat);
+      mat->AddEnvUse(use);
+    }
+  }
+
+  // Mark MaterializeObject as an environment use of this allocation.
+  // This will allow us to discover it when we are looking for deoptimization
+  // exits for another allocation that potentially flows into this one.
+  Value* val = new(Z) Value(alloc);
+  val->set_instruction(mat);
+  alloc->AddEnvUse(val);
+
+  // Record inserted materialization.
+  materializations_.Add(mat);
+}
+
+
+// Add given instruction to the list of the instructions if it is not yet
+// present there.
+template<typename T>
+void AddInstruction(GrowableArray<T*>* list, T* value) {
+  ASSERT(!value->IsGraphEntry());
+  for (intptr_t i = 0; i < list->length(); i++) {
+    if ((*list)[i] == value) {
+      return;
+    }
+  }
+  list->Add(value);
+}
+
+
+// Transitively collect all deoptimization exits that might need this allocation
+// rematerialized. It is not enough to collect only environment uses of this
+// allocation because it can flow into other objects that will be
+// dematerialized and that are referenced by deopt environments that
+// don't contain this allocation explicitly.
+void AllocationSinking::ExitsCollector::Collect(Definition* alloc) {
+  for (Value* use = alloc->env_use_list();
+       use != NULL;
+       use = use->next_use()) {
+    if (use->instruction()->IsMaterializeObject()) {
+      AddInstruction(&exits_, ExitForMaterialization(
+          use->instruction()->AsMaterializeObject()));
+    } else {
+      AddInstruction(&exits_, use->instruction());
+    }
+  }
+
+  // Check if this allocation is stored into any other allocation sinking
+  // candidate and put it on worklist so that we conservatively collect all
+  // exits for that candidate as well because they potentially might see
+  // this object.
+  for (Value* use = alloc->input_use_list();
+       use != NULL;
+       use = use->next_use()) {
+    Definition* obj = StoreInto(use);
+    if ((obj != NULL) && (obj != alloc)) {
+      AddInstruction(&worklist_, obj);
+    }
+  }
+}
+
+
+void AllocationSinking::ExitsCollector::CollectTransitively(Definition* alloc) {
+  exits_.TruncateTo(0);
+  worklist_.TruncateTo(0);
+
+  worklist_.Add(alloc);
+
+  // Note: worklist potentially will grow while we are iterating over it.
+  // We are not removing allocations from the worklist not to waste space on
+  // the side maintaining BitVector of already processed allocations: worklist
+  // is expected to be very small thus linear search in it is just as effecient
+  // as a bitvector.
+  for (intptr_t i = 0; i < worklist_.length(); i++) {
+    Collect(worklist_[i]);
+  }
+}
+
+
+void AllocationSinking::InsertMaterializations(Definition* alloc) {
+  // Collect all fields that are written for this instance.
+  ZoneGrowableArray<const Object*>* slots =
+      new(Z) ZoneGrowableArray<const Object*>(5);
+
+  for (Value* use = alloc->input_use_list();
+       use != NULL;
+       use = use->next_use()) {
+    StoreInstanceFieldInstr* store = use->instruction()->AsStoreInstanceField();
+    if ((store != NULL) && (store->instance()->definition() == alloc)) {
+      if (!store->field().IsNull()) {
+        AddSlot(slots, store->field());
+      } else {
+        AddSlot(slots, Smi::ZoneHandle(Z, Smi::New(store->offset_in_bytes())));
+      }
+    }
+  }
+
+  if (alloc->ArgumentCount() > 0) {
+    AllocateObjectInstr* alloc_object = alloc->AsAllocateObject();
+    ASSERT(alloc_object->ArgumentCount() == 1);
+    intptr_t type_args_offset =
+        alloc_object->cls().type_arguments_field_offset();
+    AddSlot(slots, Smi::ZoneHandle(Z, Smi::New(type_args_offset)));
+  }
+
+  // Collect all instructions that mention this object in the environment.
+  exits_collector_.CollectTransitively(alloc);
+
+  // Insert materializations at environment uses.
+  for (intptr_t i = 0; i < exits_collector_.exits().length(); i++) {
+    CreateMaterializationAt(
+        exits_collector_.exits()[i], alloc, *slots);
+  }
+}
+
+
+void TryCatchAnalyzer::Optimize(FlowGraph* flow_graph) {
+  // For every catch-block: Iterate over all call instructions inside the
+  // corresponding try-block and figure out for each environment value if it
+  // is the same constant at all calls. If yes, replace the initial definition
+  // at the catch-entry with this constant.
+  const GrowableArray<CatchBlockEntryInstr*>& catch_entries =
+      flow_graph->graph_entry()->catch_entries();
+  intptr_t base = kFirstLocalSlotFromFp + flow_graph->num_non_copied_params();
+  for (intptr_t catch_idx = 0;
+       catch_idx < catch_entries.length();
+       ++catch_idx) {
+    CatchBlockEntryInstr* catch_entry = catch_entries[catch_idx];
+
+    // Initialize cdefs with the original initial definitions (ParameterInstr).
+    // The following representation is used:
+    // ParameterInstr => unknown
+    // ConstantInstr => known constant
+    // NULL => non-constant
+    GrowableArray<Definition*>* idefs = catch_entry->initial_definitions();
+    GrowableArray<Definition*> cdefs(idefs->length());
+    cdefs.AddArray(*idefs);
+
+    // exception_var and stacktrace_var are never constant.
+    intptr_t ex_idx = base - catch_entry->exception_var().index();
+    intptr_t st_idx = base - catch_entry->stacktrace_var().index();
+    cdefs[ex_idx] = cdefs[st_idx] = NULL;
+
+    for (BlockIterator block_it = flow_graph->reverse_postorder_iterator();
+         !block_it.Done();
+         block_it.Advance()) {
+      BlockEntryInstr* block = block_it.Current();
+      if (block->try_index() == catch_entry->catch_try_index()) {
+        for (ForwardInstructionIterator instr_it(block);
+             !instr_it.Done();
+             instr_it.Advance()) {
+          Instruction* current = instr_it.Current();
+          if (current->MayThrow()) {
+            Environment* env = current->env()->Outermost();
+            ASSERT(env != NULL);
+            for (intptr_t env_idx = 0; env_idx < cdefs.length(); ++env_idx) {
+              if (cdefs[env_idx] != NULL &&
+                  env->ValueAt(env_idx)->BindsToConstant()) {
+                cdefs[env_idx] = env->ValueAt(env_idx)->definition();
+              }
+              if (cdefs[env_idx] != env->ValueAt(env_idx)->definition()) {
+                cdefs[env_idx] = NULL;
+              }
+            }
+          }
+        }
+      }
+    }
+    for (intptr_t j = 0; j < idefs->length(); ++j) {
+      if (cdefs[j] != NULL && cdefs[j]->IsConstant()) {
+        // TODO(fschneider): Use constants from the constant pool.
+        Definition* old = (*idefs)[j];
+        ConstantInstr* orig = cdefs[j]->AsConstant();
+        ConstantInstr* copy =
+            new(flow_graph->zone()) ConstantInstr(orig->value());
+        copy->set_ssa_temp_index(flow_graph->alloc_ssa_temp_index());
+        old->ReplaceUsesWith(copy);
+        (*idefs)[j] = copy;
+      }
+    }
+  }
+}
+
+
+// Returns true iff this definition is used in a non-phi instruction.
+static bool HasRealUse(Definition* def) {
+  // Environment uses are real (non-phi) uses.
+  if (def->env_use_list() != NULL) return true;
+
+  for (Value::Iterator it(def->input_use_list());
+       !it.Done();
+       it.Advance()) {
+    if (!it.Current()->instruction()->IsPhi()) return true;
+  }
+  return false;
+}
+
+
+void DeadCodeElimination::EliminateDeadPhis(FlowGraph* flow_graph) {
+  GrowableArray<PhiInstr*> live_phis;
+  for (BlockIterator b = flow_graph->postorder_iterator();
+       !b.Done();
+       b.Advance()) {
+    JoinEntryInstr* join = b.Current()->AsJoinEntry();
+    if (join != NULL) {
+      for (PhiIterator it(join); !it.Done(); it.Advance()) {
+        PhiInstr* phi = it.Current();
+        // Phis that have uses and phis inside try blocks are
+        // marked as live.
+        if (HasRealUse(phi) || join->InsideTryBlock()) {
+          live_phis.Add(phi);
+          phi->mark_alive();
+        } else {
+          phi->mark_dead();
+        }
+      }
+    }
+  }
+
+  while (!live_phis.is_empty()) {
+    PhiInstr* phi = live_phis.RemoveLast();
+    for (intptr_t i = 0; i < phi->InputCount(); i++) {
+      Value* val = phi->InputAt(i);
+      PhiInstr* used_phi = val->definition()->AsPhi();
+      if ((used_phi != NULL) && !used_phi->is_alive()) {
+        used_phi->mark_alive();
+        live_phis.Add(used_phi);
+      }
+    }
+  }
+
+  for (BlockIterator it(flow_graph->postorder_iterator());
+       !it.Done();
+       it.Advance()) {
+    JoinEntryInstr* join = it.Current()->AsJoinEntry();
+    if (join != NULL) {
+      if (join->phis_ == NULL) continue;
+
+      // Eliminate dead phis and compact the phis_ array of the block.
+      intptr_t to_index = 0;
+      for (intptr_t i = 0; i < join->phis_->length(); ++i) {
+        PhiInstr* phi = (*join->phis_)[i];
+        if (phi != NULL) {
+          if (!phi->is_alive()) {
+            phi->ReplaceUsesWith(flow_graph->constant_null());
+            phi->UnuseAllInputs();
+            (*join->phis_)[i] = NULL;
+            if (FLAG_trace_optimization) {
+              THR_Print("Removing dead phi v%" Pd "\n", phi->ssa_temp_index());
+            }
+          } else if (phi->IsRedundant()) {
+            phi->ReplaceUsesWith(phi->InputAt(0)->definition());
+            phi->UnuseAllInputs();
+            (*join->phis_)[i] = NULL;
+            if (FLAG_trace_optimization) {
+              THR_Print("Removing redundant phi v%" Pd "\n",
+                         phi->ssa_temp_index());
+            }
+          } else {
+            (*join->phis_)[to_index++] = phi;
+          }
+        }
+      }
+      if (to_index == 0) {
+        join->phis_ = NULL;
+      } else {
+        join->phis_->TruncateTo(to_index);
+      }
+    }
+  }
+}
+
+
+}  // namespace dart