Add bit_field3 to Map objects

src/objects.h

 // Copyright 2011 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 2021 matching lines @@
   // only be used for storing old space objects or smis.
   static inline void fast_set(FixedArray* array, int index, Object* value);
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(FixedArray);
 };
 
 
 // DescriptorArrays are fixed arrays used to hold instance descriptors.
 // The format of the these objects is:
-//   [0]: point to a fixed array with (value, detail) pairs.
-//   [1]: next enumeration index (Smi), or pointer to small fixed array:
+//   [0]: storage for bit_field3 for Map owning this object (Smi)

[Mads Ager (chromium), 2011/05/23 10:14:51]

Add a TODO(bugnumber) comment about moving this to the map where it belongs.

[danno, 2011/06/01 13:15:11]

Done.

+//   [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
-//   [2]: first key
+//   [3]: first key
 //   [length() - 1]: last key
 //
 class DescriptorArray: public FixedArray {
  public:
-  // Is this the singleton empty_descriptor_array?
+  // 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();
 
   // Returns the number of descriptors in the array.
   int number_of_descriptors() {
     ASSERT(length() > kFirstIndex || IsEmpty());
     int len = length();
     return len <= kFirstIndex ? 0 : len - kFirstIndex;
   }
 
   int NextEnumerationIndex() {
@@ skipping 16 matching lines @@
   bool HasEnumCache() {
     return !IsEmpty() && !get(kEnumerationIndexIndex)->IsSmi();
   }
 
   Object* GetEnumCache() {
     ASSERT(HasEnumCache());
     FixedArray* bridge = FixedArray::cast(get(kEnumerationIndexIndex));
     return bridge->get(kEnumCacheBridgeCacheIndex);
   }
 
+  inline int bit_field3_storage();

[Mads Ager (chromium), 2011/05/23 10:14:51]

I would probably repeat the TODO(bugnumber) comment here.

[danno, 2011/06/01 13:15:11]

Done.

+  inline void set_bit_field3_storage(int value);
+
   // Initialize or change the enum cache,
   // using the supplied storage for the small "bridge".
   void SetEnumCache(FixedArray* bridge_storage, FixedArray* new_cache);
 
   // Accessors for fetching instance descriptor at descriptor number.
   inline String* GetKey(int descriptor_number);
   inline Object* GetValue(int descriptor_number);
   inline Smi* GetDetails(int descriptor_number);
   inline PropertyType GetType(int descriptor_number);
   inline int GetFieldIndex(int descriptor_number);
@@ skipping 58 matching lines @@
   // 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);
 
   // Casting.
   static inline DescriptorArray* cast(Object* obj);
 
   // Constant for denoting key was not found.
   static const int kNotFound = -1;
 
-  static const int kContentArrayIndex = 0;
-  static const int kEnumerationIndexIndex = 1;
-  static const int kFirstIndex = 2;
+  static const int kBitField3StorageIndex = 0;
+  static const int kContentArrayIndex = 1;
+  static const int kEnumerationIndexIndex = 2;
+  static const int kFirstIndex = 3;
 
   // The length of the "bridge" to the enum cache.
   static const int kEnumCacheBridgeLength = 2;
   static const int kEnumCacheBridgeEnumIndex = 0;
   static const int kEnumCacheBridgeCacheIndex = 1;
 
   // Layout description.
-  static const int kContentArrayOffset = FixedArray::kHeaderSize;
+  static const int kBitField3StorageOffset = FixedArray::kHeaderSize;
+  static const int kContentArrayOffset = kBitField3StorageOffset + kPointerSize;
   static const int kEnumerationIndexOffset = kContentArrayOffset + kPointerSize;
   static const int kFirstOffset = kEnumerationIndexOffset + kPointerSize;
 
   // Layout description for the bridge array.
   static const int kEnumCacheBridgeEnumOffset = FixedArray::kHeaderSize;
   static const int kEnumCacheBridgeCacheOffset =
     kEnumCacheBridgeEnumOffset + kPointerSize;
 
 #ifdef OBJECT_PRINT
   // Print all the descriptors.
@@ skipping 1451 matching lines @@
   inline void set_unused_property_fields(int value);
 
   // 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.

[Mads Ager (chromium), 2011/05/23 10:14:51]

TODO(bugnumber) comment explaining that this is really in the descriptor array
at this point and that we need to fix that when map pointers are no longer
encoded for GC.

[danno, 2011/06/01 13:15:11]

Done.

+  inline int bit_field3();
+  inline void set_bit_field3(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
   // property will not be created when accessed (will return undefined),
@@ skipping 101 matching lines @@
   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 the instance descriptor member.

[Mads Ager (chromium), 2011/05/23 10:14:51]

value the -> value of the?

[danno, 2011/06/01 13:15:11]

Done.

+  inline void init_instance_descriptors();
+
   // [instance descriptors]: describes the object.
   DECL_ACCESSORS(instance_descriptors, DescriptorArray)
 
+  // Sets the instance descriptor array for the map to be an empty descriptor
+  // list.

[Mads Ager (chromium), 2011/05/23 10:14:51]

list -> array

[danno, 2011/06/01 13:15:11]

Done.

+  inline void clear_instance_descriptors();
+
   // [stub cache]: contains stubs compiled for this map.
   DECL_ACCESSORS(code_cache, Object)
 
   // [prototype transitions]: cache of prototype transitions.
   // Prototype transition is a transition that happens
   // when we change object's prototype to a new one.
   // Cache format:
   //    0: finger - index of the first free cell in the cache
   //    1 + 2 * i: prototype
   //    2 + 2 * i: target map
@@ skipping 105 matching lines @@
 
   MaybeObject* PutPrototypeTransition(Object* prototype, Map* map);
 
   static const int kMaxPreAllocatedPropertyFields = 255;
 
   // 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;
-  static const int kInstanceDescriptorsOffset =
+  // 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
+  // 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 =
       kConstructorOffset + kPointerSize;
-  static const int kCodeCacheOffset = kInstanceDescriptorsOffset + kPointerSize;
+  static const int kCodeCacheOffset =
+      kInstanceDescriptorsOrBitField3Offset + kPointerSize;
   static const int kPrototypeTransitionsOffset =
       kCodeCacheOffset + kPointerSize;
   static const int kPadStart = kPrototypeTransitionsOffset + kPointerSize;
   static const int kSize = MAP_POINTER_ALIGN(kPadStart);
 
   // Layout of pointer fields. Heap iteration code relies on them
   // being continiously allocated.
   static const int kPointerFieldsBeginOffset = Map::kPrototypeOffset;
   static const int kPointerFieldsEndOffset =
       Map::kPrototypeTransitionsOffset + kPointerSize;
@@ skipping 2866 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_