Swapped transition array and descriptor array.

src/objects.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 66 matching lines @@
 //           - DescriptorArray
 //           - HashTable
 //             - Dictionary
 //             - SymbolTable
 //             - CompilationCacheTable
 //             - CodeCacheHashTable
 //             - MapCache
 //           - Context
 //           - JSFunctionResultCache
 //           - ScopeInfo
+//           - TransitionArray
 //         - FixedDoubleArray
 //         - ExternalArray
 //           - ExternalPixelArray
 //           - ExternalByteArray
 //           - ExternalUnsignedByteArray
 //           - ExternalShortArray
 //           - ExternalUnsignedShortArray
 //           - ExternalIntArray
 //           - ExternalUnsignedIntArray
 //           - ExternalFloatArray
@@ skipping 2354 matching lines @@
   void FixedDoubleArrayVerify();
 #endif
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(FixedDoubleArray);
 };
 
 
 // DescriptorArrays are fixed arrays used to hold instance descriptors.
 // The format of the these objects is:
-// TODO(1399): It should be possible to make room for bit_field3 in the map
-//             without overloading the instance descriptors field in the map
-//             (and storing it in the DescriptorArray when the map has one).
-//   [0]: storage for bit_field3 for Map owning this object (Smi)
-//   [1]: point to a fixed array with (value, detail) pairs.
-//   [2]: next enumeration index (Smi), or pointer to small fixed array:
-//          [0]: next enumeration index (Smi)
-//          [1]: pointer to fixed array with enum cache
-//   [3]: first key
+//   [0]: Either Smi(0) if uninitialized, or a pointer to small fixed array:
+//          [0]: pointer to fixed array with enum cache
+//          [1]: either Smi(0) or pointer to fixed array with indices
+//   [1]: first key
 //   [length() - kDescriptorSize]: last key
-//
 class DescriptorArray: public FixedArray {
  public:
   // Returns true for both shared empty_descriptor_array and for smis, which the
   // map uses to encode additional bit fields when the descriptor array is not
   // yet used.
   inline bool IsEmpty();
-  inline bool MayContainTransitions();
-  inline bool HasTransitionArray();
-
-  DECL_ACCESSORS(transitions, TransitionArray)
-  inline void ClearTransitions();
 
   // Returns the number of descriptors in the array.
   int number_of_descriptors() {
-    ASSERT(MayContainTransitions() || IsEmpty());
+    ASSERT(length() >= kFirstIndex || IsEmpty());
     int len = length();
     return len <= kFirstIndex ? 0 : (len - kFirstIndex) / kDescriptorSize;
   }
 
   inline int number_of_entries() { return number_of_descriptors(); }
   inline int NextEnumerationIndex() { return number_of_descriptors() + 1; }
 
   bool HasEnumCache() {
     return !IsEmpty() && !get(kEnumCacheIndex)->IsSmi();
   }
 
   Object* GetEnumCache() {
     ASSERT(HasEnumCache());
     FixedArray* bridge = FixedArray::cast(get(kEnumCacheIndex));
     return bridge->get(kEnumCacheBridgeCacheIndex);
   }
 
   Object** GetEnumCacheSlot() {
     ASSERT(HasEnumCache());
     return HeapObject::RawField(reinterpret_cast<HeapObject*>(this),
                                 kEnumCacheOffset);
   }
 
-  Object** GetTransitionsSlot() {
-    return HeapObject::RawField(reinterpret_cast<HeapObject*>(this),
-                                kTransitionsOffset);
-  }
-
-  DECL_ACCESSORS(back_pointer_storage, Object)
-
   // Initialize or change the enum cache,
   // using the supplied storage for the small "bridge".
   void SetEnumCache(FixedArray* bridge_storage,
                     FixedArray* new_cache,
                     Object* new_index_cache);
 
   // Accessors for fetching instance descriptor at descriptor number.
   inline String* GetKey(int descriptor_number);
   inline Object** GetKeySlot(int descriptor_number);
   inline Object* GetValue(int descriptor_number);
@@ skipping 17 matching lines @@
                     const WhitenessWitness&,
                     int number_of_set_descriptors);
 
   // Transfer a complete descriptor from the src descriptor array to this
   // descriptor array.
   void CopyFrom(int dst_index,
                 DescriptorArray* src,
                 int src_index,
                 const WhitenessWitness&);
 
-  // Indicates whether the search function should expect a sorted or an unsorted
-  // descriptor array as input.
-  enum SharedMode {
-    MAY_BE_SHARED,
-    CANNOT_BE_SHARED
-  };
-
-  // Return a copy of the array with all transitions and null descriptors
-  // removed. Return a Failure object in case of an allocation failure.
-  MUST_USE_RESULT MaybeObject* Copy(SharedMode shared_mode);
-
   // Sort the instance descriptors by the hash codes of their keys.
   void Sort(const WhitenessWitness&);
 
   // Search the instance descriptors for given name.
   INLINE(int Search(String* name));
 
   // As the above, but uses DescriptorLookupCache and updates it when
   // necessary.
   INLINE(int SearchWithCache(String* name));
 
   // Tells whether the name is present int the array.
   bool Contains(String* name) { return kNotFound != Search(name); }
 
   // Allocates a DescriptorArray, but returns the singleton
   // empty descriptor array object if number_of_descriptors is 0.
-  MUST_USE_RESULT static MaybeObject* Allocate(int number_of_descriptors,
-                                               SharedMode shared_mode);
+  MUST_USE_RESULT static MaybeObject* Allocate(int number_of_descriptors);
 
   // Casting.
   static inline DescriptorArray* cast(Object* obj);
 
   // Constant for denoting key was not found.
   static const int kNotFound = -1;
 
-  static const int kBackPointerStorageIndex = 0;
-  static const int kEnumCacheIndex = 1;
-  static const int kTransitionsIndex = 2;
-  static const int kFirstIndex = 3;
+  static const int kEnumCacheIndex = 0;
+  static const int kFirstIndex = 1;
 
   // The length of the "bridge" to the enum cache.
   static const int kEnumCacheBridgeLength = 2;
   static const int kEnumCacheBridgeCacheIndex = 0;
   static const int kEnumCacheBridgeIndicesCacheIndex = 1;
 
   // Layout description.
-  static const int kBackPointerStorageOffset = FixedArray::kHeaderSize;
-  static const int kEnumCacheOffset = kBackPointerStorageOffset +
-                                      kPointerSize;
-  static const int kTransitionsOffset = kEnumCacheOffset + kPointerSize;
-  static const int kFirstOffset = kTransitionsOffset + kPointerSize;
+  static const int kEnumCacheOffset = FixedArray::kHeaderSize;
+  static const int kFirstOffset = kEnumCacheOffset + kPointerSize;
 
   // Layout description for the bridge array.
   static const int kEnumCacheBridgeCacheOffset = FixedArray::kHeaderSize;
 
   // Layout of descriptor.
   static const int kDescriptorKey = 0;
   static const int kDescriptorDetails = 1;
   static const int kDescriptorValue = 2;
   static const int kDescriptorSize = 3;
 
@@ skipping 2176 matching lines @@
   }
 
   static bool IsValidElementsTransition(ElementsKind from_kind,
                                         ElementsKind to_kind);
 
   inline bool HasTransitionArray();
   inline bool HasElementsTransition();
   inline Map* elements_transition_map();
   MUST_USE_RESULT inline MaybeObject* set_elements_transition_map(
       Map* transitioned_map);
-  inline TransitionArray* transitions();
   inline void SetTransition(int index, Map* target);
   MUST_USE_RESULT inline MaybeObject* AddTransition(String* key, Map* target);
-  MUST_USE_RESULT inline MaybeObject* set_transitions(
-      TransitionArray* transitions);
+  DECL_ACCESSORS(transitions, TransitionArray)
   inline void ClearTransitions(Heap* heap,
                                WriteBarrierMode mode = UPDATE_WRITE_BARRIER);
 
   // Tells whether the map is attached to SharedFunctionInfo
   // (for inobject slack tracking).
   inline void set_attached_to_shared_function_info(bool value);
 
   inline bool attached_to_shared_function_info();
 
   // Tells whether the map is shared between objects that may have different
   // behavior. If true, the map should never be modified, instead a clone
   // should be created and modified.
   inline void set_is_shared(bool value);
   inline bool is_shared();
 
   // Tells whether the instance needs security checks when accessing its
   // properties.
   inline void set_is_access_check_needed(bool access_check_needed);
   inline bool is_access_check_needed();
 
   // [prototype]: implicit prototype object.
   DECL_ACCESSORS(prototype, Object)
 
   // [constructor]: points back to the function responsible for this map.
   DECL_ACCESSORS(constructor, Object)
 
   inline JSFunction* unchecked_constructor();
 
   // [instance descriptors]: describes the object.
-  DECL_ACCESSORS(instance_descriptors, DescriptorArray)
-  inline void InitializeDescriptors(DescriptorArray* descriptors);
-
-  // Should only be called to clear a descriptor array that was only used to
-  // store transitions and does not contain any live transitions anymore.
-  inline void ClearDescriptorArray(Heap* heap, WriteBarrierMode mode);
+  inline DescriptorArray* instance_descriptors();
+  MUST_USE_RESULT inline MaybeObject* SetDescriptors(
+      DescriptorArray* descriptors,
+      WriteBarrierMode mode = UPDATE_WRITE_BARRIER);
+  static void SetDescriptors(Handle<Map> map,
+                             Handle<DescriptorArray> descriptors);
+  MUST_USE_RESULT inline MaybeObject* InitializeDescriptors(
+      DescriptorArray* descriptors);
 
   // [stub cache]: contains stubs compiled for this map.
   DECL_ACCESSORS(code_cache, Object)
 
   // [back pointer]: points back to the parent map from which a transition
   // leads to this map. The field overlaps with prototype transitions and the
   // back pointer will be moved into the prototype transitions array if
   // required.
   inline Object* GetBackPointer();
   inline void SetBackPointer(Object* value,
@@ skipping 85 matching lines @@
                                                   TransitionFlag flag);
 
   MUST_USE_RESULT MaybeObject* CopyNormalized(PropertyNormalizationMode mode,
                                               NormalizedMapSharingMode sharing);
 
   inline void AppendDescriptor(Descriptor* desc,
                                const DescriptorArray::WhitenessWitness&);
 
   // Returns a copy of the map, with all transitions dropped from the
   // instance descriptors.
-  MUST_USE_RESULT MaybeObject* Copy(DescriptorArray::SharedMode shared_mode);
+  MUST_USE_RESULT MaybeObject* Copy();
 
   // Returns the property index for name (only valid for FAST MODE).
   int PropertyIndexFor(String* name);
 
   // Returns the next free property index (only valid for FAST MODE).
   int NextFreePropertyIndex();
 
   // Returns the number of properties described in instance_descriptors
   // filtering out properties with the specified attributes.
   int NumberOfDescribedProperties(PropertyAttributes filter = NONE);
@@ skipping 101 matching lines @@
   static const int kMaxPreAllocatedPropertyFields = 255;
 
   // Constant for denoting that the LastAdded field was not yet set.
   static const int kNoneAdded = LastAddedBits::kMax;
 
   // Layout description.
   static const int kInstanceSizesOffset = HeapObject::kHeaderSize;
   static const int kInstanceAttributesOffset = kInstanceSizesOffset + kIntSize;
   static const int kPrototypeOffset = kInstanceAttributesOffset + kIntSize;
   static const int kConstructorOffset = kPrototypeOffset + kPointerSize;
-  // Storage for instance descriptors is overloaded to also contain additional
-  // map flags when unused (bit_field3). When the map has instance descriptors,
-  // the flags are transferred to the instance descriptor array and accessed
-  // through an extra indirection.
-  static const int kInstanceDescriptorsOrBackPointerOffset =
+  // Storage for the transition array is overloaded to directly contain a back
+  // pointer if unused. When the map has transitions, the back pointer is
+  // transferred to the transition array and accessed through an extra
+  // indirection.
+  static const int kTransitionsOrBackPointerOffset =
       kConstructorOffset + kPointerSize;
   static const int kCodeCacheOffset =
-      kInstanceDescriptorsOrBackPointerOffset + kPointerSize;
+      kTransitionsOrBackPointerOffset + kPointerSize;
   static const int kBitField3Offset = kCodeCacheOffset + kPointerSize;
   static const int kPadStart = kBitField3Offset + kPointerSize;
   static const int kSize = MAP_POINTER_ALIGN(kPadStart);
 
   // Layout of pointer fields. Heap iteration code relies on them
   // being continuously allocated.
   static const int kPointerFieldsBeginOffset = Map::kPrototypeOffset;
   static const int kPointerFieldsEndOffset = kBitField3Offset + kPointerSize;
 
   // Byte offsets within kInstanceSizesOffset.
@@ skipping 3824 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_