Swap bitfield3 and backpointer.

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 2494 matching lines @@
     ASSERT(HasEnumCache());
     return HeapObject::RawField(reinterpret_cast<HeapObject*>(this),
                                 kEnumerationIndexOffset);
   }
 
   Object** GetTransitionsSlot() {
     return HeapObject::RawField(reinterpret_cast<HeapObject*>(this),
                                 kTransitionsOffset);
   }
 
-  // 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).
-  inline int bit_field3_storage();
-  inline void set_bit_field3_storage(int value);
+  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);
@@ skipping 68 matching lines @@
   // empty descriptor array object if number_of_descriptors is 0.
   MUST_USE_RESULT static MaybeObject* Allocate(int number_of_descriptors,
                                                SharedMode shared_mode);
 
   // Casting.
   static inline DescriptorArray* cast(Object* obj);
 
   // Constant for denoting key was not found.
   static const int kNotFound = -1;
 
-  static const int kBitField3StorageIndex = 0;
+  static const int kBackPointerStorageIndex = 0;
   static const int kEnumerationIndexIndex = 1;
   static const int kTransitionsIndex = 2;
   static const int kFirstIndex = 3;
 
   // The length of the "bridge" to the enum cache.
   static const int kEnumCacheBridgeLength = 3;
   static const int kEnumCacheBridgeEnumIndex = 0;
   static const int kEnumCacheBridgeCacheIndex = 1;
   static const int kEnumCacheBridgeIndicesCacheIndex = 2;
 
   // Layout description.
-  static const int kBitField3StorageOffset = FixedArray::kHeaderSize;
-  static const int kEnumerationIndexOffset = kBitField3StorageOffset +
+  static const int kBackPointerStorageOffset = FixedArray::kHeaderSize;
+  static const int kEnumerationIndexOffset = kBackPointerStorageOffset +
                                              kPointerSize;
   static const int kTransitionsOffset = kEnumerationIndexOffset + kPointerSize;
   static const int kFirstOffset = kTransitionsOffset + kPointerSize;
 
   // Layout description for the bridge array.
   static const int kEnumCacheBridgeEnumOffset = FixedArray::kHeaderSize;
   static const int kEnumCacheBridgeCacheOffset =
     kEnumCacheBridgeEnumOffset + kPointerSize;
 
   // Layout of descriptor.
@@ skipping 1979 matching lines @@
 
   // Bit field.
   inline byte bit_field();
   inline void set_bit_field(byte value);
 
   // Bit field 2.
   inline byte bit_field2();
   inline void set_bit_field2(byte value);
 
   // Bit field 3.
   // TODO(1399): It should be possible to make room for bit_field3 in the map

[Jakob Kummerow, 2012/07/10 12:26:25]

outdated comment?

[Toon Verwaest, 2012/07/10 13:28:28]

Done.

   // without overloading the instance descriptors field (and storing it in the
   // DescriptorArray when the map has one).
   inline int bit_field3();
   inline void set_bit_field3(int value);
-  inline void SetOwnBitField3(int value);
 
   // Tells whether the object in the prototype property will be used
   // for instances created from this function.  If the prototype
   // property is set to a value that is not a JSObject, the prototype
   // property will not be used to create instances of the function.
   // See ECMA-262, 13.2.2.
   inline void set_non_instance_prototype(bool value);
   inline bool has_non_instance_prototype();
 
   // Tells whether function has special prototype property. If not, prototype
@@ skipping 110 matching lines @@
   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();
   MUST_USE_RESULT inline MaybeObject* AddTransition(String* key,
                                                     Object* value);
   MUST_USE_RESULT inline MaybeObject* set_transitions(
       TransitionArray* transitions);
-  inline void ClearTransitions();
+  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();
 
-  // Should only be called by the code that initializes map to set initial valid
-  // value of the instance descriptor member.
-  inline void init_instance_descriptors();
-
   // [instance descriptors]: describes the object.
   DECL_ACCESSORS(instance_descriptors, DescriptorArray)
 
   // 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();
-
-  // Sets the instance descriptor array for the map to be an empty descriptor
-  // array.
-  inline void clear_instance_descriptors();
+  inline void ClearDescriptorArray(Heap* heap, WriteBarrierMode mode);
 
   // [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 174 matching lines @@
 
   // 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.
   // TODO(1399): It should be possible to make room for bit_field3 in the map

[Jakob Kummerow, 2012/07/10 12:26:25]

outdated comment?

[Toon Verwaest, 2012/07/10 13:28:28]

Done.

   // without overloading the instance descriptors field, but the map is
   // currently perfectly aligned to 32 bytes and extending it at all would
   // double its size.  After the increment GC work lands, this size restriction
   // could be loosened and bit_field3 moved directly back in the map.
-  static const int kInstanceDescriptorsOrBitField3Offset =
+  static const int kInstanceDescriptorsOrBackPointerOffset =
       kConstructorOffset + kPointerSize;
   static const int kCodeCacheOffset =
-      kInstanceDescriptorsOrBitField3Offset + kPointerSize;
-  static const int kBackPointerOffset = kCodeCacheOffset + kPointerSize;
-  static const int kPadStart = kBackPointerOffset + kPointerSize;
+      kInstanceDescriptorsOrBackPointerOffset + 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 = kBackPointerOffset + kPointerSize;
+  static const int kPointerFieldsEndOffset = kBitField3Offset + kPointerSize;
 
   // Byte offsets within kInstanceSizesOffset.
   static const int kInstanceSizeOffset = kInstanceSizesOffset + 0;
   static const int kInObjectPropertiesByte = 1;
   static const int kInObjectPropertiesOffset =
       kInstanceSizesOffset + kInObjectPropertiesByte;
   static const int kPreAllocatedPropertyFieldsByte = 2;
   static const int kPreAllocatedPropertyFieldsOffset =
       kInstanceSizesOffset + kPreAllocatedPropertyFieldsByte;
   static const int kVisitorIdByte = 3;
@@ skipping 3805 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_