Add filler at the new space top when forcing scavenge.

src/heap.h

 // Copyright 2012 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 1241 matching lines @@
   // Check whether the heap is currently iterable.
   bool IsHeapIterable();
 
   // Ensure that we have swept all spaces in such a way that we can iterate
   // over all objects.  May cause a GC.
   void EnsureHeapIsIterable();
 
   // Notify the heap that a context has been disposed.
   int NotifyContextDisposed();
 
-  // Utility to invoke the scavenger. This is needed in test code to
-  // ensure correct callback for weak global handles.
-  void PerformScavenge();
-
   inline void increment_scan_on_scavenge_pages() {
     scan_on_scavenge_pages_++;
     if (FLAG_gc_verbose) {
       PrintF("Scan-on-scavenge pages: %d\n", scan_on_scavenge_pages_);
     }
   }
 
   inline void decrement_scan_on_scavenge_pages() {
     scan_on_scavenge_pages_--;
     if (FLAG_gc_verbose) {
@@ skipping 852 matching lines @@
   // Pretenuring decisions are made based on feedback collected during new
   // space evacuation. Note that between feedback collection and calling this
   // method object in old space must not move.
   // Right now we only process pretenuring feedback in high promotion mode.
   void ProcessPretenuringFeedback();
 
   // Checks whether a global GC is necessary
   GarbageCollector SelectGarbageCollector(AllocationSpace space,
                                           const char** reason);
 
+  // Make sure there is a filler value behind the top of the new space
+  // so that the GC does not confuse some unintialized/stale memory
+  // with the allocation memento of the object at the top
+  void EnsureFillerObjectAtTop();
+
   // Performs garbage collection operation.
   // Returns whether there is a chance that another major GC could
   // collect more garbage.
   bool CollectGarbage(
       GarbageCollector collector,
       const char* gc_reason,
       const char* collector_reason,
       const GCCallbackFlags gc_callback_flags = kNoGCCallbackFlags);
 
   // Performs garbage collection
@@ skipping 965 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_H_