Remove unmarked entries from per context map caches.

src/mark-compact.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1406 matching lines @@
   heap()->isolate()->global_handles()->IterateWeakRoots(&root_visitor);
   while (marking_stack_.overflowed()) {
     RefillMarkingStack();
     EmptyMarkingStack();
   }
 
   // Repeat host application specific marking to mark unmarked objects
   // reachable from the weak roots.
   ProcessExternalMarking();
 
+  // Object literal map caches reference symbols (cache keys) and maps
+  // (cache values). At this point still useful maps have already been
+  // marked. Mark the keys for the alive values before we process the
+  // symbol table.
+  ProcessMapCaches();
+
   // Prune the symbol table removing all symbols only pointed to by the
   // symbol table.  Cannot use symbol_table() here because the symbol
   // table is marked.
   SymbolTable* symbol_table = heap()->raw_unchecked_symbol_table();
   SymbolTableCleaner v(heap());
   symbol_table->IterateElements(&v);
   symbol_table->ElementsRemoved(v.PointersRemoved());
   heap()->external_string_table_.Iterate(&v);
   heap()->external_string_table_.CleanUp();
 
   // Process the weak references.
   MarkCompactWeakObjectRetainer mark_compact_object_retainer;
   heap()->ProcessWeakReferences(&mark_compact_object_retainer);
 
   // Remove object groups after marking phase.
   heap()->isolate()->global_handles()->RemoveObjectGroups();
   heap()->isolate()->global_handles()->RemoveImplicitRefGroups();
 
   // Flush code from collected candidates.
   if (is_code_flushing_enabled()) {
     code_flusher_->ProcessCandidates();
   }
 
   // Clean up dead objects from the runtime profiler.
   heap()->isolate()->runtime_profiler()->RemoveDeadSamples();
 }
 
 
+void MarkCompactCollector::ProcessMapCaches() {
+  Object* raw_context = heap()->global_contexts_list_;
+  while (raw_context != heap()->undefined_value()) {
+    Context* context = reinterpret_cast<Context*>(raw_context);
+    if (context->IsMarked()) {
+      HeapObject* raw_map_cache =
+          HeapObject::cast(context->get(Context::MAP_CACHE_INDEX));
+      // A map cache may be reachable from the stack. In this case
+      // it's already transitively marked and it's too late clean up
+      // its parts.
+      if (!raw_map_cache->IsMarked() &&
+          raw_map_cache != heap()->undefined_value()) {
+        MapCache* map_cache = reinterpret_cast<MapCache*>(raw_map_cache);
+        int existing_elements = map_cache->NumberOfElements();
+        int used_elements = 0;
+        for (int i = MapCache::kElementsStartIndex;
+             i < map_cache->length();
+             i += MapCache::kEntrySize) {
+          Object* raw_key = map_cache->get(i);
+          if (raw_key == heap()->undefined_value() ||
+              raw_key == heap()->null_value()) continue;
+          STATIC_ASSERT(MapCache::kEntrySize == 2);
+          Object* raw_map = map_cache->get(i + 1);
+          if (raw_map->IsHeapObject() &&
+              HeapObject::cast(raw_map)->IsMarked()) {
+            ++used_elements;
+          } else {
+            // Delete useless entries with unmarked maps.
+            ASSERT(raw_map->IsMap());
+            map_cache->set_null_unchecked(heap(), i);
+            map_cache->set_null_unchecked(heap(), i + 1);
+          }
+        }
+        if (used_elements == 0) {
+          context->set(Context::MAP_CACHE_INDEX, heap()->undefined_value());
+        } else {
+          // Note: we don't actually shrink the cache here to avoid
+          // extra complexity during GC. We rely on subsequent cache
+          // usages (EnsureCapacity) to do this.
+          map_cache->ElementsRemoved(existing_elements - used_elements);
+          MarkObject(map_cache);
+        }
+      }
+    }
+    // Move to next element in the list.
+    raw_context = context->get(Context::NEXT_CONTEXT_LINK);
+  }
+  ProcessMarkingStack();
+}
+
+
 #ifdef DEBUG
 void MarkCompactCollector::UpdateLiveObjectCount(HeapObject* obj) {
   live_bytes_ += obj->Size();
   if (heap()->new_space()->Contains(obj)) {
     live_young_objects_size_ += obj->Size();
   } else if (heap()->map_space()->Contains(obj)) {
     ASSERT(obj->IsMap());
     live_map_objects_size_ += obj->Size();
   } else if (heap()->cell_space()->Contains(obj)) {
     ASSERT(obj->IsJSGlobalPropertyCell());
@@ skipping 1672 matching lines @@
 }
 
 
 void MarkCompactCollector::Initialize() {
   StaticPointersToNewGenUpdatingVisitor::Initialize();
   StaticMarkingVisitor::Initialize();
 }
 
 
 } }  // namespace v8::internal