Precisely sweep scan on scavenge pages and use heap iterator to iterate over them.

src/store-buffer.cc

Index: src/store-buffer.cc
diff --git a/src/store-buffer.cc b/src/store-buffer.cc
index 4ab5c33c8cc439b59033c85c4be1ae5b92c67d49..e4cef27b4a5433d71e934425247767216aebbb02 100644
--- a/src/store-buffer.cc
+++ b/src/store-buffer.cc
@@ -552,8 +552,21 @@ void StoreBuffer::IteratePointersToNewSpace(ObjectSlotCallback slot_callback,
             FindPointersToNewSpaceInMapsRegion(
                 start, end, slot_callback, clear_maps);
           } else {
-            FindPointersToNewSpaceInRegion(
-                start, end, slot_callback, clear_maps);
+            ASSERT(page->WasSweptPrecisely());
+            HeapObjectIterator iterator(page, NULL);
+            for (HeapObject* heap_object = iterator.Next();
+                 heap_object != NULL;
+                 heap_object = iterator.Next()) {
+              // We skip filler, free space, and constant pool objects.

[rmcilroy, 2014/07/02 12:39:07]

Could you add back the old comment here please:

// Skip fillers, free space, and constant pool arrays (which never 
// contain new-space pointers but can contain pointers which can 
// be confused for fillers).

+              if (!heap_object->IsFiller() &&
+                  !heap_object->IsConstantPoolArray()) {
+                FindPointersToNewSpaceInRegion(
+                    heap_object->address() + HeapObject::kHeaderSize,
+                    heap_object->address() + heap_object->Size(),
+                    slot_callback,
+                    clear_maps);
+              }
+            }
           }
         }
       }