[runtime] Add MapUpdater class that manages all kinds of map updates.

src/map-reconfigurer.cc

Index: src/map-reconfigurer.cc
diff --git a/src/map-reconfigurer.cc b/src/map-reconfigurer.cc
new file mode 100644
index 0000000000000000000000000000000000000000..b79255ad07ccfe7d61426319a1044908067784b5
--- /dev/null
+++ b/src/map-reconfigurer.cc
@@ -0,0 +1,617 @@
+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "src/map-reconfigurer.h"
+
+#include "src/field-type.h"
+#include "src/handles.h"
+#include "src/isolate.h"
+#include "src/objects-inl.h"
+#include "src/objects.h"
+#include "src/transitions.h"
+
+namespace v8 {
+namespace internal {
+
+namespace {
+
+inline bool EqualImmutableValues(Object* obj1, Object* obj2) {
+  if (obj1 == obj2) return true;  // Valid for both kData and kAccessor kinds.
+  // TODO(ishell): compare AccessorPairs.
+  return false;
+}
+
+}  // namespace
+
+Handle<Map> MapReconfigurer::Result() const {
+  DCHECK_EQ(kEnd, state_);
+  return result_map_;
+}
+
+Name* MapReconfigurer::GetKey(int descriptor) const {
+  return old_descriptors_->GetKey(descriptor);
+}
+
+PropertyDetails MapReconfigurer::GetDetails(int descriptor) const {
+  DCHECK_LE(0, descriptor);
+  if (descriptor == modified_descriptor_) {
+    return PropertyDetails(new_kind_, new_attributes_, new_location_,
+                           new_representation_);
+  }
+  return old_descriptors_->GetDetails(descriptor);
+}
+
+Object* MapReconfigurer::GetValue(int descriptor) const {
+  DCHECK_LE(0, descriptor);
+  if (descriptor == modified_descriptor_) {
+    DCHECK_EQ(kDescriptor, new_location_);
+    return *new_value_;
+  }
+  DCHECK_EQ(kDescriptor, GetDetails(descriptor).location());
+  return old_descriptors_->GetValue(descriptor);
+}
+
+FieldType* MapReconfigurer::GetFieldType(int descriptor) const {
+  DCHECK_LE(0, descriptor);
+  if (descriptor == modified_descriptor_) {
+    DCHECK_EQ(kField, new_location_);
+    return *new_field_type_;
+  }
+  DCHECK_EQ(kField, GetDetails(descriptor).location());
+  return old_descriptors_->GetFieldType(descriptor);
+}
+
+Handle<FieldType> MapReconfigurer::GetOrComputeFieldType(
+    int descriptor, PropertyLocation location,
+    Representation representation) const {
+  DCHECK_LE(0, descriptor);
+  // |location| is just a pre-fetched GetDetails(descriptor).location().
+  DCHECK_EQ(location, GetDetails(descriptor).location());
+  if (location == kField) {
+    return handle(GetFieldType(descriptor), isolate_);
+  } else {
+    return GetValue(descriptor)->OptimalType(isolate_, representation);
+  }
+}
+
+Handle<FieldType> MapReconfigurer::GetOrComputeFieldType(
+    Handle<DescriptorArray> descriptors, int descriptor,
+    PropertyLocation location, Representation representation) {
+  // |location| is just a pre-fetched GetDetails(descriptor).location().
+  DCHECK_EQ(descriptors->GetDetails(descriptor).location(), location);
+  if (location == kField) {
+    return handle(descriptors->GetFieldType(descriptor), isolate_);
+  } else {
+    return descriptors->GetValue(descriptor)
+        ->OptimalType(isolate_, representation);
+  }
+}
+
+Handle<Map> MapReconfigurer::ReconfigureToDataField(
+    int descriptor, PropertyAttributes attributes,
+    Representation representation, Handle<FieldType> field_type) {
+  DCHECK_EQ(kInitialized, state_);
+  DCHECK_LE(0, descriptor);
+  DCHECK(!old_map_->is_dictionary_map());
+  modified_descriptor_ = descriptor;
+  new_kind_ = kData;
+  new_attributes_ = attributes;
+  new_location_ = kField;
+  new_representation_ = representation;
+  new_field_type_ = field_type;
+
+  PropertyDetails old_details =
+      old_descriptors_->GetDetails(modified_descriptor_);
+
+  // If property kind is not reconfigured merge the result with
+  // representation/field type from the old descriptor.
+  if (old_details.kind() == new_kind_) {
+    Representation old_representation = old_details.representation();
+    new_representation_ = new_representation_.generalize(old_representation);
+
+    Handle<FieldType> old_field_type =
+        GetOrComputeFieldType(old_descriptors_, modified_descriptor_,
+                              old_details.location(), new_representation_);
+
+    new_field_type_ = Map::GeneralizeFieldTypes(
+        old_representation, old_field_type, new_representation_,
+        new_field_type_, isolate_);
+  }
+
+  if (TryRecofigureToDataFieldInplace()) return Result();
+  if (FindRootMap()) return Result();

[Toon Verwaest, 2017/01/09 13:01:19]

This is odd to read... I'd expect if (!FindRootMap()) return Result()
(Failure()?)

[Igor Sheludko, 2017/01/09 14:54:25]

true means "done", therefore it returns. And all these functions never fail,
they just sometimes return generalized copy of a map.

[Igor Sheludko, 2017/01/10 10:28:08]

As discussed offline, these methods now return an updated state and we can check
it is equal to kEnd explicitly which should improve code readability.

+  if (FindTargetMap()) return Result();
+  ConstructNewMap();
+  return Result();
+}
+
+Handle<Map> MapReconfigurer::ReconfigureElementsKind(
+    ElementsKind elements_kind) {
+  DCHECK_EQ(kInitialized, state_);
+  new_elements_kind_ = elements_kind;
+
+  if (FindRootMap()) return Result();
+  if (FindTargetMap()) return Result();
+  ConstructNewMap();
+  return Result();
+}
+
+Handle<Map> MapReconfigurer::Update() {
+  DCHECK_EQ(kInitialized, state_);
+  DCHECK(old_map_->is_deprecated());
+
+  if (FindRootMap()) return Result();
+  if (FindTargetMap()) return Result();
+  ConstructNewMap();
+  return Result();
+}
+
+bool MapReconfigurer::CopyGeneralizeAllRepresentations(const char* reason) {
+  StoreMode store_mode =
+      modified_descriptor_ >= 0 ? FORCE_FIELD : ALLOW_IN_DESCRIPTOR;
+  result_map_ = Map::CopyGeneralizeAllRepresentations(
+      old_map_, new_elements_kind_, modified_descriptor_, store_mode, new_kind_,
+      new_attributes_, reason);
+  state_ = kEnd;

[Toon Verwaest, 2017/01/09 13:01:19]

these states are a little odd. That's just for debugging right?

[Igor Sheludko, 2017/01/09 14:54:25]

Right. And the updater instance should not be used twice. 

I can remove the state field if you don't like it.

+  return true;  // Done.
+}
+
+bool MapReconfigurer::TryRecofigureToDataFieldInplace() {
+  // If it's just a representation generalization case (i.e. property kind and
+  // attributes stays unchanged) it's fine to transition from None to anything
+  // but double without any modification to the object, because the default
+  // uninitialized value for representation None can be overwritten by both
+  // smi and tagged values. Doubles, however, would require a box allocation.
+  if (new_representation_.IsNone() || new_representation_.IsDouble()) {
+    return false;  // Not done yet.
+  }
+
+  PropertyDetails old_details =
+      old_descriptors_->GetDetails(modified_descriptor_);
+  Representation old_representation = old_details.representation();
+  if (!old_representation.IsNone()) {
+    return false;  // Not done yet.
+  }
+
+  DCHECK_EQ(new_kind_, old_details.kind());
+  DCHECK_EQ(new_attributes_, old_details.attributes());
+  DCHECK_EQ(kField, old_details.location());
+  if (FLAG_trace_generalization) {
+    old_map_->PrintGeneralization(
+        stdout, "uninitialized field", modified_descriptor_, old_nof_, old_nof_,
+        false, old_representation, new_representation_,
+        handle(old_descriptors_->GetFieldType(modified_descriptor_), isolate_),
+        MaybeHandle<Object>(), new_field_type_, MaybeHandle<Object>());
+  }
+  Handle<Map> field_owner(old_map_->FindFieldOwner(modified_descriptor_),
+                          isolate_);
+
+  Map::GeneralizeFieldType(field_owner, modified_descriptor_,
+                           new_representation_, new_field_type_);
+  // Check that the descriptor array was updated.
+  DCHECK(old_descriptors_->GetDetails(modified_descriptor_)
+             .representation()
+             .Equals(new_representation_));
+  DCHECK(old_descriptors_->GetFieldType(modified_descriptor_)
+             ->NowIs(new_field_type_));
+
+  result_map_ = old_map_;
+  state_ = kEnd;
+  return true;  // Done.
+}
+
+bool MapReconfigurer::FindRootMap() {
+  DCHECK_EQ(kInitialized, state_);
+  // Check the state of the root map.
+  root_map_ = handle(old_map_->FindRootMap(), isolate_);
+  int root_nof = root_map_->NumberOfOwnDescriptors();
+  if (!old_map_->EquivalentToForTransition(*root_map_)) {
+    return CopyGeneralizeAllRepresentations("GenAll_NotEquivalent");
+  }
+
+  ElementsKind from_kind = root_map_->elements_kind();
+  ElementsKind to_kind = new_elements_kind_;
+  // TODO(ishell): Add a test for SLOW_SLOPPY_ARGUMENTS_ELEMENTS.
+  if (from_kind != to_kind && to_kind != DICTIONARY_ELEMENTS &&
+      to_kind != SLOW_STRING_WRAPPER_ELEMENTS &&
+      to_kind != SLOW_SLOPPY_ARGUMENTS_ELEMENTS &&
+      !(IsTransitionableFastElementsKind(from_kind) &&
+        IsMoreGeneralElementsKindTransition(from_kind, to_kind))) {
+    return CopyGeneralizeAllRepresentations("GenAll_InvalidElementsTransition");
+  }
+
+  if (modified_descriptor_ >= 0 && modified_descriptor_ < root_nof) {
+    PropertyDetails old_details =
+        old_descriptors_->GetDetails(modified_descriptor_);
+    if (old_details.kind() != new_kind_ ||
+        old_details.attributes() != new_attributes_) {
+      return CopyGeneralizeAllRepresentations("GenAll_RootModification1");
+    }
+    if (!new_representation_.fits_into(old_details.representation())) {
+      return CopyGeneralizeAllRepresentations("GenAll_RootModification2");
+    }
+    if (old_details.location() != kField) {
+      return CopyGeneralizeAllRepresentations("GenAll_RootModification3");
+    }
+    DCHECK_EQ(kData, old_details.kind());
+    DCHECK_EQ(kData, new_kind_);
+    DCHECK_EQ(kField, new_location_);
+    FieldType* old_field_type =
+        old_descriptors_->GetFieldType(modified_descriptor_);
+    if (!new_field_type_->NowIs(old_field_type)) {
+      return CopyGeneralizeAllRepresentations("GenAll_RootModification4");
+    }
+  }
+
+  // From here on, use the map with correct elements kind as root map.
+  if (from_kind != to_kind) {
+    root_map_ = Map::AsElementsKind(root_map_, to_kind);
+  }
+  state_ = kAtRootMap;
+  return false;  // Not done yet.
+}
+
+bool MapReconfigurer::FindTargetMap() {
+  DCHECK_EQ(kAtRootMap, state_);
+  target_map_ = root_map_;
+
+  int root_nof = root_map_->NumberOfOwnDescriptors();
+  for (int i = root_nof; i < old_nof_; ++i) {
+    PropertyDetails old_details = GetDetails(i);
+    Map* transition = TransitionArray::SearchTransition(
+        *target_map_, old_details.kind(), GetKey(i), old_details.attributes());
+    if (transition == NULL) break;
+    Handle<Map> tmp_map(transition, isolate_);
+
+    Handle<DescriptorArray> tmp_descriptors(tmp_map->instance_descriptors(),
+                                            isolate_);
+
+    // Check if target map is incompatible.
+    PropertyDetails tmp_details = tmp_descriptors->GetDetails(i);
+    DCHECK_EQ(old_details.kind(), tmp_details.kind());
+    DCHECK_EQ(old_details.attributes(), tmp_details.attributes());
+    if (old_details.kind() == kAccessor &&
+        !EqualImmutableValues(GetValue(i), tmp_descriptors->GetValue(i))) {
+      // TODO(ishell): mutable accessors are not implemented yet.
+      return CopyGeneralizeAllRepresentations("GenAll_Incompatible");
+    }
+    // Check if old location fits into tmp location.
+    if (old_details.location() == kField &&
+        tmp_details.location() == kDescriptor) {
+      break;
+    }
+
+    // Check if old representation fits into tmp representation.
+    Representation tmp_representation = tmp_details.representation();
+    if (!old_details.representation().fits_into(tmp_representation)) {
+      break;
+    }
+
+    if (tmp_details.location() == kField) {
+      Handle<FieldType> old_field_type =
+          GetOrComputeFieldType(i, old_details.location(), tmp_representation);
+      Map::GeneralizeFieldType(tmp_map, i, tmp_representation, old_field_type);
+    } else {
+      // kDescriptor: Check that the value matches.
+      if (!EqualImmutableValues(GetValue(i), tmp_descriptors->GetValue(i))) {
+        break;
+      }
+    }
+    DCHECK(!tmp_map->is_deprecated());
+    target_map_ = tmp_map;
+  }
+
+  // Directly change the map if the target map is more general.
+  int target_nof = target_map_->NumberOfOwnDescriptors();
+  if (target_nof == old_nof_) {
+#ifdef DEBUG
+    if (modified_descriptor_ >= 0) {
+      DescriptorArray* target_descriptors = target_map_->instance_descriptors();
+      PropertyDetails details =
+          target_descriptors->GetDetails(modified_descriptor_);
+      DCHECK_EQ(new_kind_, details.kind());
+      DCHECK_EQ(new_attributes_, details.attributes());
+      DCHECK_EQ(new_location_, details.location());
+      DCHECK(new_representation_.fits_into(details.representation()));
+      if (new_location_ == kField) {
+        DCHECK_EQ(kField, details.location());
+        DCHECK(new_field_type_->NowIs(
+            target_descriptors->GetFieldType(modified_descriptor_)));
+      } else {
+        DCHECK(details.location() == kField ||
+               EqualImmutableValues(*new_value_, target_descriptors->GetValue(
+                                                     modified_descriptor_)));
+      }
+    }
+#endif
+    if (*target_map_ != *old_map_) {
+      old_map_->NotifyLeafMapLayoutChange();
+    }
+    result_map_ = target_map_;
+    state_ = kEnd;
+    return true;  // Done.
+  }
+
+  // Find the last compatible target map in the transition tree.
+  for (int i = target_nof; i < old_nof_; ++i) {
+    PropertyDetails old_details = GetDetails(i);
+    Map* transition = TransitionArray::SearchTransition(
+        *target_map_, old_details.kind(), GetKey(i), old_details.attributes());
+    if (transition == NULL) break;
+    Handle<Map> tmp_map(transition, isolate_);
+    Handle<DescriptorArray> tmp_descriptors(tmp_map->instance_descriptors(),
+                                            isolate_);
+
+#ifdef DEBUG
+    // Check that target map is compatible.
+    PropertyDetails tmp_details = tmp_descriptors->GetDetails(i);
+    DCHECK_EQ(old_details.kind(), tmp_details.kind());
+    DCHECK_EQ(old_details.attributes(), tmp_details.attributes());
+#endif
+    if (old_details.kind() == kAccessor &&
+        !EqualImmutableValues(GetValue(i), tmp_descriptors->GetValue(i))) {
+      return CopyGeneralizeAllRepresentations("GenAll_Incompatible");
+    }
+    DCHECK(!tmp_map->is_deprecated());
+    target_map_ = tmp_map;
+  }
+
+  state_ = kAtTargetMap;
+  return false;  // Not done yet.
+}
+
+Handle<DescriptorArray> MapReconfigurer::BuildDescriptorArray() {
+  int target_nof = target_map_->NumberOfOwnDescriptors();
+  Handle<DescriptorArray> target_descriptors(
+      target_map_->instance_descriptors(), isolate_);
+
+  // Allocate a new descriptor array large enough to hold the required
+  // descriptors, with minimally the exact same size as the old descriptor
+  // array.
+  int new_slack =
+      Max(old_nof_, old_descriptors_->number_of_descriptors()) - old_nof_;
+  Handle<DescriptorArray> new_descriptors =
+      DescriptorArray::Allocate(isolate_, old_nof_, new_slack);
+  DCHECK(new_descriptors->length() > target_descriptors->length() ||
+         new_descriptors->NumberOfSlackDescriptors() > 0 ||
+         new_descriptors->number_of_descriptors() ==
+             old_descriptors_->number_of_descriptors());
+  DCHECK(new_descriptors->number_of_descriptors() == old_nof_);
+
+  int root_nof = root_map_->NumberOfOwnDescriptors();
+
+  // Given that we passed root modification check in FindRootMap() so
+  // the root descriptors are either not modified at all or already more
+  // general than we requested. Take |root_nof| entries as is.
+  // 0 -> |root_nof|
+  int current_offset = 0;
+  for (int i = 0; i < root_nof; ++i) {
+    PropertyDetails old_details = old_descriptors_->GetDetails(i);
+    if (old_details.location() == kField) {
+      current_offset += old_details.field_width_in_words();
+    }
+    Descriptor d(handle(GetKey(i), isolate_),
+                 handle(old_descriptors_->GetValue(i), isolate_), old_details);
+    new_descriptors->Set(i, &d);
+  }
+
+  // Merge virtually modified old_descriptor entries with target_descriptor
+  // entries.
+  // |root_nof| -> |target_nof|
+  for (int i = root_nof; i < target_nof; ++i) {
+    Handle<Name> key(GetKey(i), isolate_);
+    PropertyDetails old_details = GetDetails(i);
+    PropertyDetails target_details = target_descriptors->GetDetails(i);
+
+    PropertyKind next_kind = old_details.kind();
+    PropertyAttributes next_attributes = old_details.attributes();
+    PropertyLocation next_location =
+        old_details.location() == kField ||
+                target_details.location() == kField ||
+                !EqualImmutableValues(target_descriptors->GetValue(i),
+                                      GetValue(i))
+            ? kField
+            : kDescriptor;
+
+    Representation next_representation =
+        old_details.representation().generalize(
+            target_details.representation());
+
+    DCHECK_EQ(next_kind, target_details.kind());
+    DCHECK_EQ(next_attributes, target_details.attributes());
+
+    if (next_location == kField) {
+      Handle<FieldType> old_field_type =
+          GetOrComputeFieldType(i, old_details.location(), next_representation);
+
+      Handle<FieldType> target_field_type =
+          GetOrComputeFieldType(target_descriptors, i,
+                                target_details.location(), next_representation);
+
+      Handle<FieldType> next_field_type = Map::GeneralizeFieldTypes(
+          old_details.representation(), old_field_type, next_representation,
+          target_field_type, isolate_);
+
+      Handle<Object> wrapped_type(Map::WrapType(next_field_type));
+      Descriptor d;
+      if (next_kind == kData) {
+        d = Descriptor::DataField(key, current_offset, wrapped_type,
+                                  next_attributes, next_representation);
+      } else {
+        // TODO(ishell): mutable accessors are not implemented yet.
+        UNIMPLEMENTED();
+      }
+      current_offset += d.GetDetails().field_width_in_words();
+      new_descriptors->Set(i, &d);
+    } else {
+      DCHECK_EQ(kDescriptor, next_location);
+
+      Handle<Object> value(GetValue(i), isolate_);
+      Descriptor d;
+      if (next_kind == kData) {
+        d = Descriptor::DataConstant(key, value, next_attributes);
+      } else {
+        DCHECK_EQ(kAccessor, next_kind);
+        d = Descriptor::AccessorConstant(key, value, next_attributes);
+      }
+      new_descriptors->Set(i, &d);
+    }
+  }
+
+  // Take virtually modified old_descriptor entries.
+  // |target_nof| -> |old_nof|
+  for (int i = target_nof; i < old_nof_; ++i) {
+    PropertyDetails old_details = GetDetails(i);
+    Handle<Name> key(GetKey(i), isolate_);
+
+    PropertyKind next_kind = old_details.kind();
+    PropertyAttributes next_attributes = old_details.attributes();
+    PropertyLocation next_location = old_details.location();
+    Representation next_representation = old_details.representation();
+
+    Descriptor d;
+    if (next_location == kField) {
+      Handle<FieldType> old_field_type =
+          GetOrComputeFieldType(i, old_details.location(), next_representation);
+
+      Handle<Object> wrapped_type(Map::WrapType(old_field_type));
+      Descriptor d;
+      if (next_kind == kData) {
+        d = Descriptor::DataField(key, current_offset, wrapped_type,
+                                  next_attributes, next_representation);
+      } else {
+        // TODO(ishell): mutable accessors are not implemented yet.
+        UNIMPLEMENTED();
+      }
+      current_offset += d.GetDetails().field_width_in_words();
+      new_descriptors->Set(i, &d);
+    } else {
+      DCHECK_EQ(kDescriptor, next_location);
+
+      Handle<Object> value(GetValue(i), isolate_);
+      if (next_kind == kData) {
+        d = Descriptor::DataConstant(key, value, next_attributes);
+      } else {
+        DCHECK_EQ(kAccessor, next_kind);
+        d = Descriptor::AccessorConstant(key, value, next_attributes);
+      }
+      new_descriptors->Set(i, &d);
+    }
+  }
+
+  new_descriptors->Sort();
+  return new_descriptors;
+}
+
+Handle<Map> MapReconfigurer::FindSplitMap(Handle<DescriptorArray> descriptors) {
+  DisallowHeapAllocation no_allocation;
+
+  int root_nof = root_map_->NumberOfOwnDescriptors();
+  Map* current = *root_map_;
+  for (int i = root_nof; i < old_nof_; i++) {
+    Name* name = descriptors->GetKey(i);
+    PropertyDetails details = descriptors->GetDetails(i);
+    Map* next = TransitionArray::SearchTransition(current, details.kind(), name,
+                                                  details.attributes());
+    if (next == NULL) break;
+    DescriptorArray* next_descriptors = next->instance_descriptors();
+
+    PropertyDetails next_details = next_descriptors->GetDetails(i);
+    DCHECK_EQ(details.kind(), next_details.kind());
+    DCHECK_EQ(details.attributes(), next_details.attributes());
+    if (details.location() != next_details.location()) break;
+    if (!details.representation().Equals(next_details.representation())) break;
+
+    if (next_details.location() == kField) {
+      FieldType* next_field_type = next_descriptors->GetFieldType(i);
+      if (!descriptors->GetFieldType(i)->NowIs(next_field_type)) {
+        break;
+      }
+    } else {
+      if (!EqualImmutableValues(descriptors->GetValue(i),
+                                next_descriptors->GetValue(i))) {
+        break;
+      }
+    }
+    current = next;
+  }
+  return handle(current, isolate_);
+}
+
+bool MapReconfigurer::ConstructNewMap() {
+  Handle<DescriptorArray> new_descriptors = BuildDescriptorArray();
+
+  Handle<Map> split_map = FindSplitMap(new_descriptors);
+  int split_nof = split_map->NumberOfOwnDescriptors();
+  DCHECK_NE(old_nof_, split_nof);
+
+  PropertyDetails split_details = GetDetails(split_nof);
+
+  // Invalidate a transition target at |key|.
+  Map* maybe_transition = TransitionArray::SearchTransition(
+      *split_map, split_details.kind(), GetKey(split_nof),
+      split_details.attributes());
+  if (maybe_transition != NULL) {
+    maybe_transition->DeprecateTransitionTree();
+  }
+
+  // If |maybe_transition| is not NULL then the transition array already
+  // contains entry for given descriptor. This means that the transition
+  // could be inserted regardless of whether transitions array is full or not.
+  if (maybe_transition == NULL &&
+      !TransitionArray::CanHaveMoreTransitions(split_map)) {
+    return CopyGeneralizeAllRepresentations("GenAll_CantHaveMoreTransitions");
+  }
+
+  old_map_->NotifyLeafMapLayoutChange();
+
+  if (FLAG_trace_generalization && modified_descriptor_ >= 0) {
+    PropertyDetails old_details =
+        old_descriptors_->GetDetails(modified_descriptor_);
+    PropertyDetails new_details =
+        new_descriptors->GetDetails(modified_descriptor_);
+    MaybeHandle<FieldType> old_field_type;
+    MaybeHandle<FieldType> new_field_type;
+    MaybeHandle<Object> old_value;
+    MaybeHandle<Object> new_value;
+    if (old_details.type() == DATA) {
+      old_field_type = handle(
+          old_descriptors_->GetFieldType(modified_descriptor_), isolate_);
+    } else {
+      old_value =
+          handle(old_descriptors_->GetValue(modified_descriptor_), isolate_);
+    }
+    if (new_details.type() == DATA) {
+      new_field_type =
+          handle(new_descriptors->GetFieldType(modified_descriptor_), isolate_);
+    } else {
+      new_value =
+          handle(new_descriptors->GetValue(modified_descriptor_), isolate_);
+    }
+
+    old_map_->PrintGeneralization(
+        stdout, "", modified_descriptor_, split_nof, old_nof_,
+        old_details.location() == kDescriptor && new_location_ == kField,
+        old_details.representation(), new_details.representation(),
+        old_field_type, old_value, new_field_type, new_value);
+  }
+
+  Handle<LayoutDescriptor> new_layout_descriptor =
+      LayoutDescriptor::New(split_map, new_descriptors, old_nof_);
+
+  Handle<Map> new_map = Map::AddMissingTransitions(split_map, new_descriptors,
+                                                   new_layout_descriptor);
+
+  // Deprecated part of the transition tree is no longer reachable, so replace
+  // current instance descriptors in the "survived" part of the tree with
+  // the new descriptors to maintain descriptors sharing invariant.
+  split_map->ReplaceDescriptors(*new_descriptors, *new_layout_descriptor);
+
+  result_map_ = new_map;
+  state_ = kEnd;
+  return true;  // Done.
+}
+
+}  // namespace internal
+}  // namespace v8