Simplify and compact transitions storage

src/transitions.h

 // Copyright 2012 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_TRANSITIONS_H_
 #define V8_TRANSITIONS_H_
 
 #include "src/checks.h"
 #include "src/elements-kind.h"
 #include "src/heap/heap.h"
 #include "src/isolate.h"
 #include "src/objects.h"
 
 namespace v8 {
 namespace internal {
 
 
 // TransitionArrays are fixed arrays used to hold map transitions for property,
-// constant, and element changes. They can either be simple transition arrays
-// that store a single property transition, or a full transition array that has
+// constant, and element changes. "Simple" transitions storing only a single
+// property transition are stored inline (i.e. the target map is stored
+// directly); otherwise a full transition array is used that has
 // prototype transitions and multiple property transitons. The details related
 // to property transitions are accessed in the descriptor array of the target
 // map. In the case of a simple transition, the key is also read from the
 // descriptor array of the target map.
 //
-// The simple format of the these objects is:
-// [0] Undefined or back pointer map
-// [1] Single transition
+// This class provides a static interface that operates directly on maps
+// and handles the distinction between simple and full transitions storage.
 //
 // The full format is:
-// [0] Undefined or back pointer map
-// [1] Smi(0) or fixed array of prototype transitions
-// [2] Number of transitions
-// [3] First transition
-// [3 + number of transitions * kTransitionSize]: start of slack
+// [0] Smi(0) or fixed array of prototype transitions
+// [1] Number of transitions
+// [2] First transition
+// [2 + number of transitions * kTransitionSize]: start of slack
 class TransitionArray: public FixedArray {
  public:
-  // Accessors for fetching instance transition at transition number.
-  inline Name* GetKey(int transition_number);
-  inline void SetKey(int transition_number, Name* value);
-  inline Object** GetKeySlot(int transition_number);
-  int GetSortedKeyIndex(int transition_number) { return transition_number; }
+  // Insert a new transition into |map|'s transition array, extending it
+  // as necessary.
+  static void Insert(Handle<Map> map, Handle<Name> name, Handle<Map> target,
+                     SimpleTransitionFlag flag);
 
-  Name* GetSortedKey(int transition_number) {
-    return GetKey(transition_number);
+  static Map* SearchTransition(Map* map, PropertyKind kind, Name* name,
+                               PropertyAttributes attributes);
+
+  static Map* SearchSpecial(Map* map, Symbol* name);
+
+  static Handle<Map> FindTransitionToField(Handle<Map> map, Handle<Name> name);
+
+  static Handle<String> ExpectedTransitionKey(Handle<Map> map);
+
+  static Handle<Map> ExpectedTransitionTarget(Handle<Map> map) {
+    DCHECK(!ExpectedTransitionKey(map).is_null());
+    return Handle<Map>(GetSimpleTransition(map->raw_transitions()));
+  }
+  static inline bool IsEmpty(Object* raw_transition) {
+    return raw_transition->IsSmi() ||
+           (raw_transition->IsWeakCell() &&
+            WeakCell::cast(raw_transition)->cleared());
+  }
+  static inline bool IsSimpleTransition(Object* raw_transition) {
+    DCHECK(!raw_transition->IsWeakCell() ||
+           WeakCell::cast(raw_transition)->cleared() ||
+           WeakCell::cast(raw_transition)->value()->IsMap());
+    return raw_transition->IsWeakCell() &&
+           !WeakCell::cast(raw_transition)->cleared();
+  }
+  static inline Map* GetSimpleTransition(Object* raw_transition) {
+    DCHECK(IsSimpleTransition(raw_transition));
+    return Map::cast(WeakCell::cast(raw_transition)->value());
+  }
+  static inline bool IsFullTransitionArray(Object* raw_transitions) {
+    return raw_transitions->IsTransitionArray();
   }
 
-  inline Map* GetTarget(int transition_number);
-  inline void SetTarget(int transition_number, Map* target);
+  // The size of transition arrays are limited so they do not end up in large
+  // object space. Otherwise ClearNonLiveReferences would leak memory while
+  // applying in-place right trimming.
+  static bool CanHaveMoreTransitions(Handle<Map> map);
 
-  inline PropertyDetails GetTargetDetails(int transition_number);
-  inline Object* GetTargetValue(int transition_number);
+  // ===== PROTOTYPE TRANSITIONS =====
+  // When you set the prototype of an object using the __proto__ accessor you
+  // need a new map for the object (the prototype is stored in the map).  In
+  // order not to multiply maps unnecessarily we store these as transitions in
+  // the original map.  That way we can transition to the same map if the same
+  // prototype is set, rather than creating a new map every time.  The
+  // transitions are in the form of a map where the keys are prototype objects
+  // and the values are the maps they transition to.
+  // Cache format:
+  //    0: finger - index of the first free cell in the cache
+  //    1 + i: target map
+  static const int kMaxCachedPrototypeTransitions = 256;
+  static Handle<Map> PutPrototypeTransition(Handle<Map> map,
+                                            Handle<Object> prototype,
+                                            Handle<Map> target_map);
 
-  inline bool HasElementsTransition();
+  static Handle<Map> GetPrototypeTransition(Handle<Map> map,
+                                            Handle<Object> prototype);
 
-  inline Object* back_pointer_storage();
-  inline void set_back_pointer_storage(
-      Object* back_pointer,
-      WriteBarrierMode mode = UPDATE_WRITE_BARRIER);
+  static FixedArray* GetPrototypeTransitions(Map* map);
+
+  static int NumberOfPrototypeTransitions(FixedArray* proto_transitions) {
+    if (proto_transitions->length() == 0) return 0;
+    Object* raw = proto_transitions->get(kProtoTransitionNumberOfEntriesOffset);
+    return Smi::cast(raw)->value();
+  }
+
+  static void SetNumberOfPrototypeTransitions(FixedArray* proto_transitions,
+                                              int value);
 
   inline FixedArray* GetPrototypeTransitions();
   inline void SetPrototypeTransitions(
       FixedArray* prototype_transitions,
       WriteBarrierMode mode = UPDATE_WRITE_BARRIER);
   inline Object** GetPrototypeTransitionsSlot();
   inline bool HasPrototypeTransitions();
 
-  // Returns the number of transitions in the array.
-  int number_of_transitions() {
-    if (IsSimpleTransition()) return 1;
-    if (length() <= kFirstIndex) return 0;
-    return Smi::cast(get(kTransitionLengthIndex))->value();
+  // ===== ITERATION =====
+
+  typedef void (*TraverseCallback)(Map* map, void* data);
+
+  // Traverse the transition tree in postorder.
+  static void TraverseTransitionTree(Map* map, TraverseCallback callback,
+                                     void* data) {
+    // Make sure that we do not allocate in the callback.
+    DisallowHeapAllocation no_allocation;
+    TraverseTransitionTreeInternal(map, callback, data);
   }
 
-  int number_of_transitions_storage() {
-    if (IsSimpleTransition()) return 1;
-    if (length() <= kFirstIndex) return 0;
-    return (length() - kFirstIndex) / kTransitionSize;
+  // ===== FINE-GRAINED ACCESSORS =====
+
+  // Accessors for fetching instance transition at transition number.
+  static inline Name* GetKey(Object* raw_transitions, int transition_number);
+  inline Name* GetKey(int transition_number);
+  inline void SetKey(int transition_number, Name* value);
+  inline Object** GetKeySlot(int transition_number);
+  int GetSortedKeyIndex(int transition_number) { return transition_number; }
+
+  Name* GetSortedKey(int transition_number) {
+    return GetKey(transition_number);
   }
 
-  int NumberOfSlackTransitions() {
-    return number_of_transitions_storage() - number_of_transitions();
-  }
-
-  inline void SetNumberOfTransitions(int number_of_transitions);
-  inline int number_of_entries() { return number_of_transitions(); }
-
-  // Creates a FullTransitionArray from a SimpleTransitionArray in
-  // containing_map.
-  static Handle<TransitionArray> ExtendToFullTransitionArray(
-      Handle<Map> containing_map);
-
-  // Return a transition array, using the array from the owning map if it
-  // already has one (copying into a larger array if necessary), otherwise
-  // creating a new one according to flag.
-  // TODO(verwaest): This should not cause an existing transition to be
-  // overwritten.
-  static Handle<TransitionArray> Insert(Handle<Map> map, Handle<Name> name,
-                                        Handle<Map> target,
-                                        SimpleTransitionFlag flag);
-  // Search a  transition for a given kind, property name and attributes.
-  int Search(PropertyKind kind, Name* name, PropertyAttributes attributes,
-             int* out_insertion_index = NULL);
-
-  // Search a non-property transition (like elements kind, observe or frozen
-  // transitions).
-  inline int SearchSpecial(Symbol* symbol, int* out_insertion_index = NULL) {
-    return SearchName(symbol, out_insertion_index);
-  }
+  static inline Map* GetTarget(Object* raw_transitions, int transition_number);
+  inline Map* GetTarget(int transition_number);
+  inline void SetTarget(int transition_number, Map* target);
 
   static inline PropertyDetails GetTargetDetails(Name* name, Map* target);
 
-  // Allocates a TransitionArray.
-  static Handle<TransitionArray> Allocate(Isolate* isolate,
-                                          int number_of_transitions,
-                                          int slack = 0);
+  // Returns the number of transitions in the array.
+  static int NumberOfTransitions(Object* raw_transitions);
+  // Required for templatized Search interface.
+  inline int number_of_entries() { return number_of_transitions(); }
 
-  bool IsSimpleTransition() {
-    return length() == kSimpleTransitionSize &&
-        get(kSimpleTransitionTarget)->IsHeapObject() &&
-        // The IntrusivePrototypeTransitionIterator may have set the map of the
-        // prototype transitions array to a smi. In that case, there are
-        // prototype transitions, hence this transition array is a full
-        // transition array.
-        HeapObject::cast(get(kSimpleTransitionTarget))->map()->IsMap() &&
-        get(kSimpleTransitionTarget)->IsMap();
-  }
+  inline void SetNumberOfTransitions(int number_of_transitions);
 
-  bool IsFullTransitionArray() {
-    return length() > kFirstIndex ||
-        (length() == kFirstIndex && !IsSimpleTransition());
-  }
+  static int Capacity(Object* raw_transitions);
 
   // Casting.
   static inline TransitionArray* cast(Object* obj);
 
-  // Constant for denoting key was not found.
-  static const int kNotFound = -1;
-
-  static const int kBackPointerStorageIndex = 0;
-
-  // Layout for full transition arrays.
-  static const int kPrototypeTransitionsIndex = 1;
-  static const int kTransitionLengthIndex = 2;
-  static const int kFirstIndex = 3;
-
-  // Layout for simple transition arrays.
-  static const int kSimpleTransitionTarget = 1;
-  static const int kSimpleTransitionSize = 2;
-  static const int kSimpleTransitionIndex = 0;
-  STATIC_ASSERT(kSimpleTransitionIndex != kNotFound);
-
-  static const int kBackPointerStorageOffset = FixedArray::kHeaderSize;
-
-  // Layout for the full transition array header.
-  static const int kPrototypeTransitionsOffset = kBackPointerStorageOffset +
-                                                 kPointerSize;
-  static const int kTransitionLengthOffset =
-      kPrototypeTransitionsOffset + kPointerSize;
-
-  // Layout of map transition entries in full transition arrays.
-  static const int kTransitionKey = 0;
-  static const int kTransitionTarget = 1;
   static const int kTransitionSize = 2;
+  static const int kProtoTransitionHeaderSize = 1;
 
 #if defined(DEBUG) || defined(OBJECT_PRINT)
   // For our gdb macros, we should perhaps change these in the future.
   void Print();
 
   // Print all the transitions.
-  void PrintTransitions(std::ostream& os, bool print_header = true);  // NOLINT
+  static void PrintTransitions(std::ostream& os, Object* transitions,
+                               bool print_header = true);  // NOLINT
 #endif
 
 #ifdef DEBUG
   bool IsSortedNoDuplicates(int valid_entries = -1);
-  bool IsConsistentWithBackPointers(Map* current_map);
-  bool IsEqualTo(TransitionArray* other);
+  static bool IsSortedNoDuplicates(Map* map);
+  static bool IsConsistentWithBackPointers(Map* map);
 
   // Returns true for a non-property transitions like elements kind, observed
   // or frozen transitions.
   static inline bool IsSpecialTransition(Name* name);
 #endif
 
+  // Constant for denoting key was not found.
+  static const int kNotFound = -1;
+
   // The maximum number of transitions we want in a transition array (should
   // fit in a page).
   static const int kMaxNumberOfTransitions = 1024 + 512;
 
-  // Returns the fixed array length required to hold number_of_transitions
-  // transitions.
-  static int LengthFor(int number_of_transitions) {
-    return ToKeyIndex(number_of_transitions);
-  }
+ private:
+  // Layout for full transition arrays.
+  static const int kPrototypeTransitionsIndex = 0;
+  static const int kTransitionLengthIndex = 1;
+  static const int kFirstIndex = 2;
 
- private:
+  // Layout of map transition entries in full transition arrays.
+  static const int kTransitionKey = 0;
+  static const int kTransitionTarget = 1;
+  STATIC_ASSERT(kTransitionSize == 2);
+
+  static const int kProtoTransitionNumberOfEntriesOffset = 0;
+  STATIC_ASSERT(kProtoTransitionHeaderSize == 1);
+
   // Conversion from transition number to array indices.
   static int ToKeyIndex(int transition_number) {
     return kFirstIndex +
            (transition_number * kTransitionSize) +
            kTransitionKey;
   }
 
   static int ToTargetIndex(int transition_number) {
     return kFirstIndex +
            (transition_number * kTransitionSize) +
            kTransitionTarget;
   }
 
-  static Handle<TransitionArray> AllocateSimple(
-      Isolate* isolate, Handle<Map> target);
+  // Returns the fixed array length required to hold number_of_transitions
+  // transitions.
+  static int LengthFor(int number_of_transitions) {
+    return ToKeyIndex(number_of_transitions);
+  }
 
-  // Allocate a new transition array with a single entry.
-  static Handle<TransitionArray> NewWith(Handle<Map> map,
-                                         Handle<Name> name,
-                                         Handle<Map> target,
-                                         SimpleTransitionFlag flag);
+  // Allocates a TransitionArray.
+  static Handle<TransitionArray> Allocate(Isolate* isolate,
+                                          int number_of_transitions,
+                                          int slack = 0);
 
+  static void EnsureHasFullTransitionArray(Handle<Map> map);
+  static void ReplaceTransitions(Handle<Map> map, Object* new_transitions);
+
+  // Search a  transition for a given kind, property name and attributes.
+  int Search(PropertyKind kind, Name* name, PropertyAttributes attributes,
+             int* out_insertion_index = NULL);
+
+  // Search a non-property transition (like elements kind, observe or frozen
+  // transitions).
+  inline int SearchSpecial(Symbol* symbol, int* out_insertion_index = NULL) {
+    return SearchName(symbol, out_insertion_index);
+  }
   // Search a first transition for a given property name.
   inline int SearchName(Name* name, int* out_insertion_index = NULL);
   int SearchDetails(int transition, PropertyKind kind,
                     PropertyAttributes attributes, int* out_insertion_index);
 
+  int number_of_transitions() {
+    if (length() < kFirstIndex) return 0;
+    return Smi::cast(get(kTransitionLengthIndex))->value();
+  }
+
+  static inline PropertyDetails GetSimpleTargetDetails(Map* transition) {
+    return transition->GetLastDescriptorDetails();
+  }
+
+  static inline Name* GetSimpleTransitionKey(Map* transition) {
+    int descriptor = transition->LastAdded();
+    return transition->instance_descriptors()->GetKey(descriptor);
+  }
+
+  static void TraverseTransitionTreeInternal(Map* map,
+                                             TraverseCallback callback,
+                                             void* data);
+
+  static void SetPrototypeTransitions(Handle<Map> map,
+                                      Handle<FixedArray> proto_transitions);
+
   // Compares two tuples <key, kind, attributes>, returns -1 if
   // tuple1 is "less" than tuple2, 0 if tuple1 equal to tuple2 and 1 otherwise.
   static inline int CompareKeys(Name* key1, uint32_t hash1, PropertyKind kind1,
                                 PropertyAttributes attributes1, Name* key2,
                                 uint32_t hash2, PropertyKind kind2,
                                 PropertyAttributes attributes2);
 
   // Compares keys, returns -1 if key1 is "less" than key2,
   // 0 if key1 equal to key2 and 1 otherwise.
   static inline int CompareNames(Name* key1, uint32_t hash1, Name* key2,
                                  uint32_t hash2);
 
   // Compares two details, returns -1 if details1 is "less" than details2,
   // 0 if details1 equal to details2 and 1 otherwise.
   static inline int CompareDetails(PropertyKind kind1,
                                    PropertyAttributes attributes1,
                                    PropertyKind kind2,
                                    PropertyAttributes attributes2);
 
   inline void NoIncrementalWriteBarrierSet(int transition_number,
                                            Name* key,
                                            Map* target);
 
   // Copy a single transition from the origin array.
   inline void NoIncrementalWriteBarrierCopyFrom(TransitionArray* origin,
                                                 int origin_transition,
                                                 int target_transition);
 
+#ifdef DEBUG
+  static void CheckNewTransitionsAreConsistent(Handle<Map> map,
+                                               TransitionArray* old_transitions,
+                                               Object* transitions);
+#endif
+
   DISALLOW_IMPLICIT_CONSTRUCTORS(TransitionArray);
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TRANSITIONS_H_