[serializer][ptr-compr] Add byte code for cleared weak reference

in order to be able to properly deserialize the latter in case of enabled
pointer compression where the cleared weak reference value is Isolate-dependent.

Bug: v8:7703
Change-Id: Ied995bc15317c83fcc03cf27de3fbf135659a9d5
Reviewed-on: https://chromium-review.googlesource.com/c/1333676
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57510}

src/snapshot/deserializer.cc

@@ -649,6 +649,11 @@ bool Deserializer::ReadData(UnalignedSlot current, UnalignedSlot limit,
         break;
       }
 
+      case kClearedWeakReference:
+        UnalignedCopy(current, HeapObjectReference::ClearedValue(isolate_));
+        current.Advance();
+        break;
+
       case kWeakPrefix:
         DCHECK(!allocator()->next_reference_is_weak());
         allocator()->set_next_reference_is_weak(true);

src/snapshot/serializer-common.h

@@ -142,8 +142,7 @@ class SerializerDeserializer : public RootVisitor {
   V(0x79)                               \
   V(0x7a)                               \
   V(0x7b)                               \
-  V(0x7c)                               \
-  V(0x7d)
+  V(0x7c)
 
   // ---------- byte code range 0x00..0x7f ----------
   // Byte codes in this range represent Where, HowToCode and WhereToPoint.
@@ -236,6 +235,7 @@ class SerializerDeserializer : public RootVisitor {
   static const int kApiReference = 0x3d;
 
   // In-place weak references
+  static const int kClearedWeakReference = 0x7d;
   static const int kWeakPrefix = 0x7e;
 
   // Encodes an off-heap instruction stream target.

src/snapshot/serializer.cc

@@ -669,12 +669,19 @@ void Serializer::ObjectSerializer::VisitPointers(HeapObject* host,
                                                  MaybeObjectSlot end) {
   MaybeObjectSlot current = start;
   while (current < end) {
-    while (current < end && ((*current)->IsSmi() || (*current)->IsCleared())) {
+    while (current < end && (*current)->IsSmi()) {
       ++current;
     }
     if (current < end) {
       OutputRawData(current.address());
     }
+    // TODO(ishell): Revisit this change once we stick to 32-bit compressed
+    // tagged values.
+    while (current < end && (*current)->IsCleared()) {
+      sink_->Put(kClearedWeakReference, "ClearedWeakReference");
+      bytes_processed_so_far_ += kPointerSize;
+      ++current;
+    }
     HeapObject* current_contents;
     HeapObjectReferenceType reference_type;
     while (current < end &&