Optimize MarkCompactCollector::ClearNonLiveReferences.

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index ffda9f159d263d2c40e22c3c013881ef1be7f106..2f56b0c6ad5a8cd0b8025ddaf1efc9c786f6c0fa 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -2198,14 +2198,13 @@ void MarkCompactCollector::ClearNonLiveReferences() {
   for (HeapObject* obj = map_iterator.Next(); obj != NULL;
        obj = map_iterator.Next()) {
     Map* map = Map::cast(obj);
-
     if (!map->CanTransition()) continue;
-
     MarkBit map_mark = Marking::MarkBitFrom(map);
-    ClearNonLivePrototypeTransitions(map);
-    ClearNonLiveMapTransitions(map, map_mark);
-
-    if (Marking::IsWhite(map_mark)) {
+    bool alive = !Marking::IsWhite(map_mark);

[Hannes Payer (out of office), 2015/11/12 19:35:59]

Let's ask Marking::IsBlackOrGrey here for liveness, it is more direct.

[ulan, 2015/11/17 11:29:46]

Done.

+    if (alive) {
+      ClearNonLivePrototypeTransitions(map);

[ulan, 2015/11/12 12:00:37]

We don't need to compact prototype transitions for dead maps because the array
will not be used anymore and weak cells make sure that there are no dangling
pointers.

[Hannes Payer (out of office), 2015/11/12 19:35:59]

Awesome!

+    } else {
+      ClearNonLiveMapTransitions(map);

[ulan, 2015/11/12 12:00:37]

We don't need to call this function for live maps because it does meaningful
work only for (live parent) -> (dead map) transitions.

[Hannes Payer (out of office), 2015/11/12 19:35:59]

Awesome^2!

       have_code_to_deoptimize_ |=
           map->dependent_code()->MarkCodeForDeoptimization(
               isolate(), DependentCode::kWeakCodeGroup);
@@ -2267,17 +2266,16 @@ void MarkCompactCollector::ClearNonLivePrototypeTransitions(Map* map) {
 }
 
 
-void MarkCompactCollector::ClearNonLiveMapTransitions(Map* map,
-                                                      MarkBit map_mark) {
+void MarkCompactCollector::ClearNonLiveMapTransitions(Map* map) {
   Object* potential_parent = map->GetBackPointer();
   if (!potential_parent->IsMap()) return;
   Map* parent = Map::cast(potential_parent);
 
   // Follow back pointer, check whether we are dealing with a map transition
   // from a live map to a dead path and in case clear transitions of parent.
-  bool current_is_alive = Marking::IsBlackOrGrey(map_mark);
+  DCHECK(Marking::IsBlackOrGrey(Marking::MarkBitFrom(map)));
   bool parent_is_alive = Marking::IsBlackOrGrey(Marking::MarkBitFrom(parent));
-  if (!current_is_alive && parent_is_alive) {
+  if (parent_is_alive) {
     ClearMapTransitions(parent, map);
   }
 }