Add mistagging-readbarrier to weak cell

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 5ee91913251c81f4833f9230afc419b59f7db384..f7cbb34cb1827baa96f5e72fef0d5f47075d41e0 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1949,22 +1949,66 @@ void PropertyCell::set_type_raw(Object* val, WriteBarrierMode ignored) {
 }
 
 
-Object* WeakCell::value() const { return READ_FIELD(this, kValueOffset); }
+static const intptr_t kReadBarrierTag = 2;
+static const intptr_t kReadBarrierMask = 2;
+
+
+inline bool HasReadBarrierTag(Object* value) {
+  DCHECK(!HAS_SMI_TAG(value));
+  return (reinterpret_cast<intptr_t>(value) & kReadBarrierMask) ==
+         kReadBarrierTag;
+}
+
+
+inline Object* ReadBarrierTag(Object* value) {
+  DCHECK(!HAS_SMI_TAG(value));
+  intptr_t tagged = reinterpret_cast<intptr_t>(value) | kReadBarrierTag;
+  return reinterpret_cast<Object*>(tagged);
+}
+
+
+inline Object* ReadBarrierUntag(Object* value) {
+  DCHECK(!HAS_SMI_TAG(value));
+  intptr_t untagged = reinterpret_cast<intptr_t>(value) & kReadBarrierTag;
+  return reinterpret_cast<Object*>(untagged);
+}
+
+
+Object* WeakCell::value() {
+  Object* pointer = READ_FIELD(this, kValueOffsetDontForgetTheReadBarrier);
+  if (pointer != Smi::FromInt(0) && HasReadBarrierTag(pointer)) {
+    pointer = ReadBarrierUntag(pointer);
+    WRITE_FIELD(this, kValueOffsetDontForgetTheReadBarrier, pointer);
+  }
+
+  return pointer;
+}
+
+
+Object* WeakCell::ValueNoReadBarrier() {
+  Object* pointer = READ_FIELD(this, kValueOffsetDontForgetTheReadBarrier);
+  if (pointer == Smi::FromInt(0)) return pointer;
+  DCHECK(!pointer->IsSmi());
+  return ReadBarrierUntag(pointer);
+}
 
 
 void WeakCell::clear() {
   DCHECK(GetHeap()->gc_state() == Heap::MARK_COMPACT);
-  WRITE_FIELD(this, kValueOffset, Smi::FromInt(0));
+  WRITE_FIELD(this, kValueOffsetDontForgetTheReadBarrier, Smi::FromInt(0));
 }
 
 
 void WeakCell::initialize(HeapObject* val) {
-  WRITE_FIELD(this, kValueOffset, val);
-  WRITE_BARRIER(GetHeap(), this, kValueOffset, val);
+  WRITE_FIELD(this, kValueOffsetDontForgetTheReadBarrier, val);
+  WRITE_BARRIER(GetHeap(), this, kValueOffsetDontForgetTheReadBarrier, val);
 }
 
 
-bool WeakCell::cleared() const { return value() == Smi::FromInt(0); }
+bool WeakCell::cleared() const {
+  Object* pointer = READ_FIELD(this, kValueOffsetDontForgetTheReadBarrier);
+  return pointer == Smi::FromInt(0);
+}
 
 
 Object* WeakCell::next() const { return READ_FIELD(this, kNextOffset); }