[heap] Simplify clearing of normalized map caches.

Currently the incremental marking visitor treats elements of normalized
map caches weakly by coloring the caches grey without pusing to marking
deque.

The mark-compact prologue then clears all normalized map caches.

We can achieve similar effect by just clearing the caches in the marking
visitor.

BUG=chromium:694255

Review-Url: https://codereview.chromium.org/2745183002
Cr-Commit-Position: refs/heads/master@{#43941}

src/heap/heap.cc

@@ -1254,26 +1254,6 @@ void Heap::EnsureFromSpaceIsCommitted() {
 }
 
 
-void Heap::ClearNormalizedMapCaches() {
-  if (isolate_->bootstrapper()->IsActive() &&
-      !incremental_marking()->IsMarking()) {
-    return;
-  }
-
-  Object* context = native_contexts_list();
-  while (!context->IsUndefined(isolate())) {
-    // GC can happen when the context is not fully initialized,
-    // so the cache can be undefined.
-    Object* cache =
-        Context::cast(context)->get(Context::NORMALIZED_MAP_CACHE_INDEX);
-    if (!cache->IsUndefined(isolate())) {
-      NormalizedMapCache::cast(cache)->Clear();
-    }
-    context = Context::cast(context)->next_context_link();
-  }
-}
-
-
 void Heap::UpdateSurvivalStatistics(int start_new_space_size) {
   if (start_new_space_size == 0) return;
 
@@ -1515,7 +1495,6 @@ void Heap::MarkCompactPrologue() {
   CompletelyClearInstanceofCache();
 
   FlushNumberStringCache();
-  ClearNormalizedMapCaches();
 }
 
 

src/heap/heap.h

@@ -861,8 +861,6 @@ class Heap {
   // scavenge operation.
   inline bool ShouldBePromoted(Address old_address, int object_size);
 
-  void ClearNormalizedMapCaches();
-
   void IncrementDeferredCount(v8::Isolate::UseCounterFeature feature);
 
   // Completely clear the Instanceof cache (to stop it keeping objects alive

src/heap/incremental-marking.cc

@@ -153,7 +153,6 @@ class IncrementalMarkingMarkingVisitor
   static void Initialize() {
     StaticMarkingVisitor<IncrementalMarkingMarkingVisitor>::Initialize();
     table_.Register(kVisitFixedArray, &VisitFixedArrayIncremental);
-    table_.Register(kVisitNativeContext, &VisitNativeContextIncremental);
   }
 
   static const int kProgressBarScanningChunk = 32 * 1024;
@@ -199,26 +198,6 @@ class IncrementalMarkingMarkingVisitor
     }
   }
 
-  static void VisitNativeContextIncremental(Map* map, HeapObject* object) {
-    Context* context = Context::cast(object);
-
-    // We will mark cache black with a separate pass when we finish marking.
-    // Note that GC can happen when the context is not fully initialized,
-    // so the cache can be undefined.
-    Object* cache = context->get(Context::NORMALIZED_MAP_CACHE_INDEX);
-    if (!cache->IsUndefined(map->GetIsolate())) {
-      if (cache->IsHeapObject()) {
-        HeapObject* heap_obj = HeapObject::cast(cache);
-        // Mark the object grey if it is white, do not enque it into the marking
-        // deque.
-        if (ObjectMarking::IsWhite(heap_obj)) {
-          ObjectMarking::WhiteToGrey(heap_obj);
-        }
-      }
-    }
-    VisitNativeContext(map, context);
-  }
-
   INLINE(static void VisitPointer(Heap* heap, HeapObject* object, Object** p)) {
     Object* target = *p;
     if (target->IsHeapObject()) {

src/heap/objects-visiting-inl.h

@@ -295,6 +295,13 @@ void StaticMarkingVisitor<StaticVisitor>::VisitBytecodeArray(
 template <typename StaticVisitor>
 void StaticMarkingVisitor<StaticVisitor>::VisitNativeContext(
     Map* map, HeapObject* object) {
+  // GC can happen when the context is not fully initialized,
+  // so the cache can be undefined.
+  Object* cache =
+      Context::cast(object)->get(Context::NORMALIZED_MAP_CACHE_INDEX);
+  if (cache->IsNormalizedMapCache()) {
+    NormalizedMapCache::cast(cache)->Clear();
+  }
   FixedBodyVisitor<StaticVisitor, Context::MarkCompactBodyDescriptor,
                    void>::Visit(map, object);
 }