Moving prototype transitions into the header of the transition array.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index c479a14aa07eec86c620b40fdfef1f4229aa35f4..3db5c739133583c18b0add8ed5ad4f7828f4c24d 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -4807,6 +4807,8 @@ class Map: public HeapObject {
   inline void SetBackPointer(Object* value,
                              WriteBarrierMode mode = UPDATE_WRITE_BARRIER);
 
+  inline void init_back_pointer(Object* undefined);

[Michael Starzinger, 2012/07/06 13:53:18]

Lets make that a camel-case method (i.e. InitializeBackPointer), since it will
soon become more complex. Also drop the empty newline above.

[Toon Verwaest, 2012/07/09 12:45:27]

As discussed offline, will leave as is. Not complex since only used while
bootstrapping.

On 2012/07/06 13:53:18, Michael Starzinger wrote:

+
   // [prototype transitions]: cache of prototype transitions.
   // Prototype transition is a transition that happens
   // when we change object's prototype to a new one.
@@ -4815,14 +4817,16 @@ class Map: public HeapObject {
   //    1: back pointer that overlaps with prototype transitions field.
   //    2 + 2 * i: prototype
   //    3 + 2 * i: target map
-  DECL_ACCESSORS(prototype_transitions, FixedArray)
+  inline FixedArray* prototype_transitions();
+  MUST_USE_RESULT inline MaybeObject* set_prototype_transitions(
+      FixedArray* prototype_transitions);
+  inline bool HasPrototypeTransitions();
 
-  inline void init_prototype_transitions(Object* undefined);
   inline HeapObject* unchecked_prototype_transitions();
+  inline TransitionArray* unchecked_transition_array();
 
-  static const int kProtoTransitionHeaderSize = 2;
+  static const int kProtoTransitionHeaderSize = 1;
   static const int kProtoTransitionNumberOfEntriesOffset = 0;
-  static const int kProtoTransitionBackPointerOffset = 1;
   static const int kProtoTransitionElementsPerEntry = 2;
   static const int kProtoTransitionPrototypeOffset = 0;
   static const int kProtoTransitionMapOffset = 1;
@@ -4995,17 +4999,16 @@ class Map: public HeapObject {
       kConstructorOffset + kPointerSize;
   static const int kCodeCacheOffset =
       kInstanceDescriptorsOrBitField3Offset + kPointerSize;
-  static const int kPrototypeTransitionsOrBackPointerOffset =
+  static const int kBackPointerOffset =

[Michael Starzinger, 2012/07/06 13:53:18]

This should fit in one line now?

[Toon Verwaest, 2012/07/09 12:45:27]

Done.

       kCodeCacheOffset + kPointerSize;
   static const int kPadStart =

[Michael Starzinger, 2012/07/06 13:53:18]

Likewise.

[Toon Verwaest, 2012/07/09 12:45:27]

Done.

-      kPrototypeTransitionsOrBackPointerOffset + kPointerSize;
+      kBackPointerOffset + 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 =
-      kPrototypeTransitionsOrBackPointerOffset + kPointerSize;
+  static const int kPointerFieldsEndOffset = kBackPointerOffset + kPointerSize;
 
   // Byte offsets within kInstanceSizesOffset.
   static const int kInstanceSizeOffset = kInstanceSizesOffset + 0;