Parallelize marking phase of mark-sweep/compact collection cycle.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index f63d6725ec2981afdf13b294c8a7de8a76d51f31..68664885349b21822a3fc0ab9049899952929ad1 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1110,11 +1110,35 @@ bool HeapObject::IsMarked() {
 }
 
 
-void HeapObject::SetMark() {
-  ASSERT(!IsMarked());
+bool HeapObject::SetMark() {
   MapWord first_word = map_word();
-  first_word.SetMark();
-  set_map_word(first_word);
+
+  if (!first_word.IsMarked()) {
+    first_word.SetMark();
+    set_map_word(first_word);
+    return true;
+  }
+
+  return false;
+}
+
+
+bool HeapObject::SetMarkExclusively() {
+  MapWord first_word = map_word();
+
+  if (!first_word.IsMarked()) {
+    MapWord new_first_word = first_word;
+    new_first_word.SetMark();
+
+    AtomicWord* map_word_slot =
+        reinterpret_cast<AtomicWord*>(this->address() + kMapOffset);
+
+    return AtomicCompareAndSwap(map_word_slot,
+                                static_cast<AtomicWord>(first_word.value_),
+                                static_cast<AtomicWord>(new_first_word.value_));
+  }
+
+  return false;
 }