Swap bitfield3 and backpointer.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index e9cbb9cd51cf9fc55a501bb908a26d429d6b1c3f..31eb1b45e764f0e8afd6f01fc1a7834fe5590668 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -2512,11 +2512,7 @@ class DescriptorArray: public FixedArray {
                                 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".
@@ -2605,7 +2601,7 @@ class DescriptorArray: public FixedArray {
   // 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;
@@ -2617,8 +2613,8 @@ class DescriptorArray: public FixedArray {
   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;
@@ -4623,7 +4619,6 @@ class Map: public HeapObject {
   // 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
@@ -4754,7 +4749,8 @@ class Map: public HeapObject {
                                                     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).
@@ -4781,20 +4777,12 @@ class Map: public HeapObject {
 
   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)
@@ -4994,18 +4982,18 @@ class Map: public HeapObject {
   // 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;