[in-place weak refs] Use in-place weak refs in Context::map_cache

BUG=v8:7308

Change-Id: I940ed715949307abb831d47c197b7e75ed5b0fca
Reviewed-on: https://chromium-review.googlesource.com/1169047
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55019}

src/heap/factory.cc

@@ -3742,27 +3742,24 @@ Handle<Map> Factory::ObjectLiteralMapFromCache(Handle<Context> native_context,
   Handle<Object> maybe_cache(native_context->map_cache(), isolate());
   if (maybe_cache->IsUndefined(isolate())) {
     // Allocate the new map cache for the native context.
-    maybe_cache = NewFixedArray(kMapCacheSize, TENURED);
+    maybe_cache = NewWeakFixedArray(kMapCacheSize, TENURED);
     native_context->set_map_cache(*maybe_cache);
   } else {
     // Check to see whether there is a matching element in the cache.
-    Handle<FixedArray> cache = Handle<FixedArray>::cast(maybe_cache);
-    Object* result = cache->get(cache_index);
-    if (result->IsWeakCell()) {
-      WeakCell* cell = WeakCell::cast(result);
-      if (!cell->cleared()) {
-        Map* map = Map::cast(cell->value());
-        DCHECK(!map->is_dictionary_map());
-        return handle(map, isolate());
-      }
+    Handle<WeakFixedArray> cache = Handle<WeakFixedArray>::cast(maybe_cache);
+    MaybeObject* result = cache->Get(cache_index);
+    HeapObject* heap_object;
+    if (result->ToWeakHeapObject(&heap_object)) {
+      Map* map = Map::cast(heap_object);
+      DCHECK(!map->is_dictionary_map());
+      return handle(map, isolate());
     }
   }
   // Create a new map and add it to the cache.
-  Handle<FixedArray> cache = Handle<FixedArray>::cast(maybe_cache);
+  Handle<WeakFixedArray> cache = Handle<WeakFixedArray>::cast(maybe_cache);
   Handle<Map> map = Map::Create(isolate(), number_of_properties);
   DCHECK(!map->is_dictionary_map());
-  Handle<WeakCell> cell = NewWeakCell(map);
-  cache->set(cache_index, *cell);
+  cache->Set(cache_index, HeapObjectReference::Weak(*map));
   return map;
 }
 

test/cctest/test-api.cc

@@ -21986,12 +21986,11 @@ THREADED_TEST(ReadOnlyIndexedProperties) {
 
 
 static int CountLiveMapsInMapCache(i::Context* context) {
-  i::FixedArray* map_cache = i::FixedArray::cast(context->map_cache());
+  i::WeakFixedArray* map_cache = i::WeakFixedArray::cast(context->map_cache());
   int length = map_cache->length();
   int count = 0;
   for (int i = 0; i < length; i++) {
-    i::Object* value = map_cache->get(i);
-    if (value->IsWeakCell() && !i::WeakCell::cast(value)->cleared()) count++;
+    if (map_cache->Get(i)->IsWeakHeapObject()) count++;
   }
   return count;
 }