[heap] Process live weak cells directly in the marking visitor.

src/heap/objects-visiting-inl.h

Index: src/heap/objects-visiting-inl.h
diff --git a/src/heap/objects-visiting-inl.h b/src/heap/objects-visiting-inl.h
index 29a5afc320a370d4e9ee2802865353eb8054fe80..24eab712fe70a3403a4e74ff30249fd1532798c4 100644
--- a/src/heap/objects-visiting-inl.h
+++ b/src/heap/objects-visiting-inl.h
@@ -354,9 +354,16 @@ void StaticMarkingVisitor<StaticVisitor>::VisitWeakCell(Map* map,
   // We can ignore weak cells with cleared values because they will always
   // contain smi zero.
   if (weak_cell->next_cleared() && !weak_cell->cleared()) {
-    weak_cell->set_next(heap->encountered_weak_cells(),
-                        UPDATE_WEAK_WRITE_BARRIER);
-    heap->set_encountered_weak_cells(weak_cell);
+    HeapObject* value = HeapObject::cast(weak_cell->value());
+    if (MarkCompactCollector::IsMarked(value)) {

[ulan, 2015/09/22 11:30:46]

Please add a comment explaining that this is done for performance and the
general case is processed in the "else" branch.

[Hannes Payer (out of office), 2015/09/22 13:22:57]

Done.

+      Object** slot = HeapObject::RawField(weak_cell, WeakCell::kValueOffset);
+      map->GetHeap()->mark_compact_collector()->RecordSlot(weak_cell, slot,
+                                                           *slot);
+    } else {
+      weak_cell->set_next(heap->encountered_weak_cells(),
+                          UPDATE_WEAK_WRITE_BARRIER);
+      heap->set_encountered_weak_cells(weak_cell);
+    }
   }
 }