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

src/map-updater.h

+// 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.
+
+#ifndef V8_MAP_RECONFIGURER_H_
+#define V8_MAP_RECONFIGURER_H_
+
+#include "src/elements-kind.h"
+#include "src/globals.h"
+#include "src/handles.h"
+#include "src/objects.h"
+#include "src/property-details.h"
+
+namespace v8 {
+namespace internal {
+
+// The |MapUpdater| class implements all sorts of map reconfigurations
+// including changes of elements kind, property attributes, property kind,
+// property location and field representations/type changes. It ensures that
+// the reconfigured map and all the intermediate maps are properly integrated
+// into the exising transition tree.
+//
+// To avoid high degrees over polymorphism, and to stabilize quickly, on every
+// rewrite the new type is deduced by merging the current type with any
+// potential new (partial) version of the type in the transition tree.
+// To do this, on each rewrite:
+// - Search the root of the transition tree using FindRootMap.
+// - Find/create a |root_map| with requested |new_elements_kind|.
+// - Find |target_map|, the newest matching version of this map using the
+//   "updated" |old_map|'s descriptor array (i.e. whose entry at |modify_index|
+//   is considered to be of |new_kind| and having |new_attributes|) to walk
+//   the transition tree.
+// - Merge/generalize the "updated" descriptor array of the |old_map| and
+//   descriptor array of the |target_map|.
+// - Generalize the |modify_index| descriptor using |new_representation| and
+//   |new_field_type|.
+// - Walk the tree again starting from the root towards |target_map|. Stop at
+//   |split_map|, the first map who's descriptor array does not match the merged
+//   descriptor array.
+// - If |target_map| == |split_map|, |target_map| is in the expected state.
+//   Return it.
+// - Otherwise, invalidate the outdated transition target from |target_map|, and
+//   replace its transition tree with a new branch for the updated descriptors.
+class MapUpdater {
+ public:
+  MapUpdater(Isolate* isolate, Handle<Map> old_map)
+      : isolate_(isolate),
+        old_map_(old_map),
+        old_descriptors_(old_map->instance_descriptors(), isolate_),
+        old_nof_(old_map_->NumberOfOwnDescriptors()),
+        new_elements_kind_(old_map_->elements_kind()) {}
+
+  // Prepares for reconfiguring of a property at |descriptor| to data field
+  // with given |attributes| and |representation|/|field_type| and
+  // performs the steps 1-5.
+  Handle<Map> ReconfigureToDataField(int descriptor,
+                                     PropertyAttributes attributes,
+                                     Representation representation,
+                                     Handle<FieldType> field_type);
+
+  // Prepares for reconfiguring elements kind and performs the steps 1-5.
+  Handle<Map> ReconfigureElementsKind(ElementsKind elements_kind);
+
+  // Prepares for updating deprecated map to most up-to-date non-deprecated
+  // version and performs the steps 1-5.
+  Handle<Map> Update();
+
+ private:
+  enum State { kInitialized, kAtRootMap, kAtTargetMap, kEnd };
+
+  // Try to reconfigure property in-place without rebuilding transition tree
+  // and creating new maps. See implementation for details.
+  State TryRecofigureToDataFieldInplace();
+
+  // Step 1.
+  // - Search the root of the transition tree using FindRootMap.
+  // - Find/create a |root_map_| with requested |new_elements_kind_|.
+  State FindRootMap();
+
+  // Step 2.
+  // - Find |target_map_|, the newest matching version of this map using the
+  //   "updated" |old_map|'s descriptor array (i.e. whose entry at
+  //   |modified_descriptor_| is considered to be of |new_kind| and having
+  //   |new_attributes|) to walk the transition tree.
+  State FindTargetMap();
+
+  // Step 3.
+  // - Merge/generalize the "updated" descriptor array of the |old_map_| and
+  //   descriptor array of the |target_map_|.
+  // - Generalize the |modified_descriptor_| using |new_representation| and
+  //   |new_field_type_|.
+  Handle<DescriptorArray> BuildDescriptorArray();
+
+  // Step 4.
+  // - Walk the tree again starting from the root towards |target_map|. Stop at
+  //   |split_map|, the first map who's descriptor array does not match the
+  //   merged descriptor array.
+  Handle<Map> FindSplitMap(Handle<DescriptorArray> descriptors);
+
+  // Step 5.
+  // - If |target_map| == |split_map|, |target_map| is in the expected state.
+  //   Return it.
+  // - Otherwise, invalidate the outdated transition target from |target_map|,
+  //   and replace its transition tree with a new branch for the updated
+  //   descriptors.
+  State ConstructNewMap();
+
+  // When a requested reconfiguration can not be done the result is a copy
+  // of |old_map_| where every field has |Tagged| representation and |Any|
+  // field type. This map is disconnected from the transition tree.
+  State CopyGeneralizeAllRepresentations(const char* reason);
+
+  // Returns name of a |descriptor| property.
+  inline Name* GetKey(int descriptor) const;
+
+  // Returns property details of a |descriptor| in "updated" |old_descrtiptors_|
+  // array.
+  inline PropertyDetails GetDetails(int descriptor) const;
+
+  // Returns value of a |descriptor| with kDescriptor location in "updated"
+  // |old_descrtiptors_| array.
+  inline Object* GetValue(int descriptor) const;
+
+  // Returns field type for a |descriptor| with kField location in "updated"
+  // |old_descrtiptors_| array.
+  inline FieldType* GetFieldType(int descriptor) const;
+
+  // If a |descriptor| property in "updated" |old_descriptors_| has kField
+  // location then returns it's field type otherwise computes optimal field
+  // type for the descriptor's value and |representation|. The |location|
+  // value must be a pre-fetched location for |descriptor|.
+  inline Handle<FieldType> GetOrComputeFieldType(
+      int descriptor, PropertyLocation location,
+      Representation representation) const;
+
+  // If a |descriptor| property in given |descriptors| array has kField
+  // location then returns it's field type otherwise computes optimal field
+  // type for the descriptor's value and |representation|.
+  // The |location| value must be a pre-fetched location for |descriptor|.
+  inline Handle<FieldType> GetOrComputeFieldType(
+      Handle<DescriptorArray> descriptors, int descriptor,
+      PropertyLocation location, Representation representation);
+
+  Isolate* isolate_;
+  Handle<Map> old_map_;
+  Handle<DescriptorArray> old_descriptors_;
+  Handle<Map> root_map_;
+  Handle<Map> target_map_;
+  Handle<Map> result_map_;
+  int old_nof_;
+
+  State state_ = kInitialized;
+  ElementsKind new_elements_kind_;
+
+  // If |modified_descriptor_| is not equal to -1 them the fields below form
+  // an "update" of the |old_map_|'s descriptors.
+  int modified_descriptor_ = -1;
+  PropertyKind new_kind_ = kData;
+  PropertyAttributes new_attributes_ = NONE;
+  PropertyLocation new_location_ = kField;
+  Representation new_representation_ = Representation::None();
+
+  // Data specific to kField location.
+  Handle<FieldType> new_field_type_;
+
+  // Data specific to kDescriptor location.
+  Handle<Object> new_value_;
+};
+
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_MAP_RECONFIGURER_H_