Optimize ClearNonLiveReferences: do not compact prototype transitions in GC.

src/transitions.h

Index: src/transitions.h
diff --git a/src/transitions.h b/src/transitions.h
index 1fcd3860d0bdc552490d42d00a84e97d41dbf26b..72356beb2ee733232a6a88d710ff73699bd1f580 100644
--- a/src/transitions.h
+++ b/src/transitions.h
@@ -102,6 +102,9 @@ class TransitionArray: public FixedArray {
   static const int kMaxCachedPrototypeTransitions = 256;
   static void PutPrototypeTransition(Handle<Map> map, Handle<Object> prototype,
                                      Handle<Map> target_map);
+  static bool CompactPrototypeTransitionArray(FixedArray* array);

[Jakob Kummerow, 2015/11/25 12:38:05]

AFAICS both of these new functions are internal except for testing. I'd like to
keep the TransitionArray's public interface as lean as possible, so here's my
suggestion how to structure the code:

transitions.h:

static int NumberOfPrototypeTransitionsForTest(Map* map);

transitions.cc:

namespace {

bool CompactPrototypeTransitionArray(...) { ... }
Handle<FixedArray> GrowPrototypeTransitionArray(...) { ... }

}  // namespace

static bool NumberOfPrototypeTransitionsForTest(Map* map) {
  FixedArray* transitions = TransitionArray::GetPrototypeTransitions(map);
  CompactPrototypeTransitionArray(transitions);
  return TransitionArray::NumberOfPrototypeTransitions(transitions);
}

// Two more call sites from PutPrototypeTransition.

test-heap.cc: 

Drop "static int NumberOfProtoTransitions(Map* map)", call
NumberOfPrototypeTransitionsForTest directly.

[ulan, 2015/11/25 13:12:48]

Done.

+  static Handle<FixedArray> GrowPrototypeTransitionArray(
+      Handle<FixedArray> array, int new_capacity, Isolate* isolate);
 
   static Handle<Map> GetPrototypeTransition(Handle<Map> map,
                                             Handle<Object> prototype);