Add map-based read barrier to WeakCell

src/heap/mark-compact.cc

 // Copyright 2012 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/base/atomicops.h"
 #include "src/base/bits.h"
 #include "src/code-stubs.h"
 #include "src/compilation-cache.h"
@@ skipping 2661 matching lines @@
   }
   heap()->set_encountered_weak_collections(Smi::FromInt(0));
 }
 
 
 void MarkCompactCollector::ProcessAndClearWeakCells() {
   HeapObject* undefined = heap()->undefined_value();
   Object* weak_cell_obj = heap()->encountered_weak_cells();
   while (weak_cell_obj != Smi::FromInt(0)) {
     WeakCell* weak_cell = reinterpret_cast<WeakCell*>(weak_cell_obj);
-    // We do not insert cleared weak cells into the list, so the value
-    // cannot be a Smi here.
-    HeapObject* value = HeapObject::cast(weak_cell->value());
-    if (!MarkCompactCollector::IsMarked(value)) {
-      weak_cell->clear();
-    } else {
-      Object** slot = HeapObject::RawField(weak_cell, WeakCell::kValueOffset);
-      heap()->mark_compact_collector()->RecordSlot(slot, slot, value);
+    // Raw read avoids read barrier.
+    Object* value = weak_cell->ValueNoReadBarrier();
+    if (value != Smi::FromInt(0)) {
+      HeapObject* heap_value = HeapObject::cast(value);
+      // TODO(erikcorry): Also check whether the cell has been used.
+      if (!MarkCompactCollector::IsMarked(heap_value)) {
+        weak_cell->clear();
+      } else {
+        Object** slot = HeapObject::RawField(
+            weak_cell, WeakCell::kValueOffsetDontForgetTheReadBarrier);
+        heap()->mark_compact_collector()->RecordSlot(slot, slot, value);
+      }
+      // Tag for the next round of GC.
+      *HeapObject::RawField(weak_cell, HeapObject::kMapOffset) =
+          heap()->unused_weak_cell_map();
     }
     weak_cell_obj = weak_cell->next();
     weak_cell->set_next(undefined, SKIP_WRITE_BARRIER);
   }
   heap()->set_encountered_weak_cells(Smi::FromInt(0));
 }
 
 
 void MarkCompactCollector::AbortWeakCells() {
   Object* undefined = heap()->undefined_value();
@@ skipping 1707 matching lines @@
   SlotsBuffer* buffer = *buffer_address;
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 }
 }  // namespace v8::internal