Link all global contexts into a weak list.

src/contexts.h

 // Copyright 2006-2008 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 207 matching lines @@
     NORMALIZED_MAP_CACHE_INDEX,
     RUNTIME_CONTEXT_INDEX,
     CALL_AS_FUNCTION_DELEGATE_INDEX,
     CALL_AS_CONSTRUCTOR_DELEGATE_INDEX,
     SCRIPT_FUNCTION_INDEX,
     OPAQUE_REFERENCE_FUNCTION_INDEX,
     CONTEXT_EXTENSION_FUNCTION_INDEX,
     OUT_OF_MEMORY_INDEX,
     MAP_CACHE_INDEX,
     CONTEXT_DATA_INDEX,
-    GLOBAL_CONTEXT_SLOTS
+
+    // Properties from here get weak reference treatment by the GC.
+    NEXT_CONTEXT_LINK,  // Weak.
+
+    // Total number of slots.
+    GLOBAL_CONTEXT_SLOTS,
+
+    FIRST_WEAK_SLOT = NEXT_CONTEXT_LINK
   };
 
   // Direct slot access.
   JSFunction* closure() { return JSFunction::cast(get(CLOSURE_INDEX)); }
   void set_closure(JSFunction* closure) { set(CLOSURE_INDEX, closure); }
 
   Context* fcontext() { return Context::cast(get(FCONTEXT_INDEX)); }
   void set_fcontext(Context* context) { set(FCONTEXT_INDEX, context); }
 
   Context* previous() {
@@ skipping 87 matching lines @@
   // does not contain a with statement the property is global unless
   // it is shadowed by a property in an extension object introduced by
   // eval.
   bool GlobalIfNotShadowedByEval(Handle<String> name);
 
   // Code generation support.
   static int SlotOffset(int index) {
     return kHeaderSize + index * kPointerSize - kHeapObjectTag;
   }
 
+  static const int kSize = kHeaderSize + GLOBAL_CONTEXT_SLOTS * kPointerSize;
+
+  // GC support.

[Søren Thygesen Gjesse, 2010/10/18 14:31:10]

Maybe extend the comment to say that during scavenge all slots are strong and
only during mark sweep/mark compact does the weak slots receive weak treatment.

+  typedef FixedBodyDescriptor<
+      kHeaderSize, kSize, kSize> ScavengeBodyDescriptor;
+
+  typedef FixedBodyDescriptor<
+      kHeaderSize,
+      kHeaderSize + FIRST_WEAK_SLOT * kPointerSize,
+      kSize> MarkCompactBodyDescriptor;
+
  private:
   // Unchecked access to the slots.
   Object* unchecked_previous() { return get(PREVIOUS_INDEX); }
   Object* unchecked_extension() { return get(EXTENSION_INDEX); }
 
 #ifdef DEBUG
   // Bootstrapping-aware type checks.
   static bool IsBootstrappingOrContext(Object* object);
   static bool IsBootstrappingOrGlobalObject(Object* object);
 #endif
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_CONTEXTS_H_