Add map-based read barrier to WeakCell

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index 1992c23925a061d7d334d12dc69b43dbc25b162e..9a462dcc98bbb96d5b67ffef2ed969d2aae87abe 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -2679,14 +2679,21 @@ void MarkCompactCollector::ProcessAndClearWeakCells() {
   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);