PrototypeInfo.validity_cell: adding comment

src/objects.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_OBJECTS_H_
 #define V8_OBJECTS_H_
 
 #include <iosfwd>
 
 #include "src/allocation.h"
@@ skipping 5972 matching lines @@
 
   // [prototype_users]: WeakFixedArray containing maps using this prototype,
   // or Smi(0) if uninitialized.
   DECL_ACCESSORS(prototype_users, Object)
   // [registry_slot]: Slot in prototype's user registry where this user
   // is stored. Returns UNREGISTERED if this prototype has not been registered.
   inline int registry_slot() const;
   inline void set_registry_slot(int slot);
   // [validity_cell]: Cell containing the validity bit for prototype chains
   // going through this object, or Smi(0) if uninitialized.
+  // When a prototype object changes its map, then both its own validity cell
+  // and those of all "downstream" prototypes are invalidated; handlers for a
+  // given receiver embed the currently valid cell for that receiver's prototype
+  // during their compilation and check it on execution.
   DECL_ACCESSORS(validity_cell, Object)
   // [constructor_name]: User-friendly name of the original constructor.
   DECL_ACCESSORS(constructor_name, Object)
 
   DECLARE_CAST(PrototypeInfo)
 
   // Dispatched behavior.
   DECLARE_PRINTER(PrototypeInfo)
   DECLARE_VERIFIER(PrototypeInfo)
 
@@ skipping 4570 matching lines @@
 
   Isolate* isolate_;
   Handle<FixedArray> keys_;
   Handle<OrderedHashSet> set_;
   int length_;
   DISALLOW_COPY_AND_ASSIGN(KeyAccumulator);
 };
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_