Don't move the object start of objects on non-swept pages.

src/heap.cc

 // 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 3433 matching lines @@
 }
 
 
 bool Heap::CanMoveObjectStart(HeapObject* object) {
   Address address = object->address();
   bool is_in_old_pointer_space = InOldPointerSpace(address);
   bool is_in_old_data_space = InOldDataSpace(address);
 
   if (lo_space()->Contains(object)) return false;
 
-  // We cannot move the object start if the given old space page is
-  // concurrently swept.
+  Page* page = Page::FromAddress(address);
+  // We can move the object start if:
+  // (1) the object is not in old pointer or old data space,
+  // (2) the page of the object was already swept,
+  // (3) the page was already concurrently swept. This case is an optimization
+  // for concurrent sweeping. The WasSwept predicate for concurrently swept
+  // pages is set after sweeping all pages.
   return (!is_in_old_pointer_space && !is_in_old_data_space) ||
-      Page::FromAddress(address)->parallel_sweeping() <=
-          MemoryChunk::PARALLEL_SWEEPING_FINALIZE;
+         page->WasSwept() ||
+         (mark_compact_collector()->AreSweeperThreadsActivated() &&
+              page->parallel_sweeping() <=
+                  MemoryChunk::PARALLEL_SWEEPING_FINALIZE);
 }
 
 
 void Heap::AdjustLiveBytes(Address address, int by, InvocationMode mode) {
   if (incremental_marking()->IsMarking() &&
       Marking::IsBlack(Marking::MarkBitFrom(address))) {
     if (mode == FROM_GC) {
       MemoryChunk::IncrementLiveBytesFromGC(address, by);
     } else {
       MemoryChunk::IncrementLiveBytesFromMutator(address, by);
@@ skipping 3268 matching lines @@
       static_cast<int>(object_sizes_last_time_[index]));
   CODE_AGE_LIST_COMPLETE(ADJUST_LAST_TIME_OBJECT_COUNT)
 #undef ADJUST_LAST_TIME_OBJECT_COUNT
 
   OS::MemCopy(object_counts_last_time_, object_counts_, sizeof(object_counts_));
   OS::MemCopy(object_sizes_last_time_, object_sizes_, sizeof(object_sizes_));
   ClearObjectStats();
 }
 
 } }  // namespace v8::internal