Add mistagging-readbarrier to weak cell

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 fa127db6129c0f1598eb2140f3d1fdb294ff4e90..d20377753b47505c014d7d8b2f8caaf0285ecc7c 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -2682,12 +2682,21 @@ void MarkCompactCollector::ProcessAndClearWeakCells() {
     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)) {
+    Object* value = ReadBarrierUntag(
+        *HeapObject::RawField(weak_cell,
+                              WeakCell::kValueOffsetDontForgetTheReadBarrier));
+    if (!MarkCompactCollector::IsMarked(HeapObject::cast(weak_cell->value()))) {
       weak_cell->clear();
     } else {
-      Object** slot = HeapObject::RawField(weak_cell, WeakCell::kValueOffset);
-      heap()->mark_compact_collector()->RecordSlot(slot, slot, value);
+      Object** slot = HeapObject::RawField(
+          weak_cell, WeakCell::kValueOffsetDontForgetTheReadBarrier);
+      if (value != Smi::FromInt(0)) {
+        heap()->mark_compact_collector()->RecordSlot(slot, slot, value);
+        // Tag for the next round of GC.
+        *HeapObject::RawField(weak_cell,
+                              WeakCell::kValueOffsetDontForgetTheReadBarrier) =
+            ReadBarrierTag(value);
+      }
     }
     weak_cell_obj = weak_cell->next();
     weak_cell->set_next(undefined, SKIP_WRITE_BARRIER);