Add map-based read barrier to WeakCell

src/type-feedback-vector.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/ic/ic.h"
 #include "src/ic/ic-state.h"
 #include "src/objects.h"
 #include "src/type-feedback-vector-inl.h"
@@ skipping 375 matching lines @@
   Object* feedback = GetFeedback();
   if (feedback->IsFixedArray()) {
     int found = 0;
     FixedArray* array = FixedArray::cast(feedback);
     // The array should be of the form [<optional name>], then
     // [map, handler, map, handler, ... ]
     DCHECK(array->length() >= (2 + start_index));
     for (int i = start_index; i < array->length(); i += 2) {
       WeakCell* cell = WeakCell::cast(array->get(i));
       if (!cell->cleared()) {
-        Map* map = Map::cast(cell->value());
+        Map* map = Map::cast(cell->value(isolate->heap()));
         maps->Add(handle(map, isolate));
         found++;
       }
     }
     return found;
   }
 
   return 0;
 }
 
 
 MaybeHandle<Code> FeedbackNexus::FindHandlerForMap(int start_index,
                                                    Handle<Map> map) const {
   Object* feedback = GetFeedback();
   if (feedback->IsFixedArray()) {
     FixedArray* array = FixedArray::cast(feedback);
     for (int i = start_index; i < array->length(); i += 2) {
       WeakCell* cell = WeakCell::cast(array->get(i));
       if (!cell->cleared()) {
-        Map* array_map = Map::cast(cell->value());
+        // Don't need a read barrier because we immediately
+        // discard the value unless we already had it from
+        // somewhere else.
+        Map* array_map = Map::cast(cell->ValueNoReadBarrier());
         if (array_map == *map) {
           Code* code = Code::cast(array->get(i + 1));
           DCHECK(code->kind() == Code::HANDLER);
           return handle(code);
         }
       }
     }
   }
 
   return MaybeHandle<Code>();
@@ skipping 68 matching lines @@
   if (feedback->IsFixedArray()) {
     FixedArray* array = FixedArray::cast(feedback);
     DCHECK(array->length() >= 3);
     Object* name = array->get(0);
     if (name->IsName()) return Name::cast(name);
   }
   return NULL;
 }
 }
 }  // namespace v8::internal