[serializer] track objects being serialized in debug mode.

This is to make debugging serialization easier by having a way
to inspect the object graph. Serializer::PrintStack() can be
invoked from the debugger to print the current traversal path.

R=jgruber@chromium.org

Change-Id: Ie67408ade7989dc559904821b3f009fdfe3e459f
Reviewed-on: https://chromium-review.googlesource.com/571219Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46662}

src/snapshot/serializer.cc

@@ -147,6 +147,13 @@ bool Serializer::BackReferenceIsAlreadyAllocated(
     }
   }
 }
+
+void Serializer::PrintStack() {
+  for (const auto& o : stack_) {
+    o->Print();
+    PrintF("\n");
+  }
+}
 #endif  // DEBUG
 
 bool Serializer::SerializeHotObject(HeapObject* obj, HowToCode how_to_code,

src/snapshot/serializer.h

@@ -186,8 +186,6 @@ class Serializer : public SerializerDeserializer {
     }
   }
 
-  bool BackReferenceIsAlreadyAllocated(SerializerReference back_reference);
-
   // This will return the space for an object.
   SerializerReference AllocateLargeObject(int size);
   SerializerReference AllocateMap();
@@ -222,6 +220,14 @@ class Serializer : public SerializerDeserializer {
 
   void OutputStatistics(const char* name);
 
+#ifdef DEBUG
+  void PushStack(HeapObject* o) { stack_.Add(o); }
+  void PopStack() { stack_.RemoveLast(); }
+  void PrintStack();
+
+  bool BackReferenceIsAlreadyAllocated(SerializerReference back_reference);
+#endif  // DEBUG
+
   Isolate* isolate_;
 
   SnapshotByteSink sink_;
@@ -264,6 +270,10 @@ class Serializer : public SerializerDeserializer {
   size_t* instance_type_size_;
 #endif  // OBJECT_PRINT
 
+#ifdef DEBUG
+  List<HeapObject*> stack_;
+#endif  // DEBUG
+
   DISALLOW_COPY_AND_ASSIGN(Serializer);
 };
 
@@ -277,8 +287,16 @@ class Serializer::ObjectSerializer : public ObjectVisitor {
         sink_(sink),
         reference_representation_(how_to_code + where_to_point),
         bytes_processed_so_far_(0),
-        code_has_been_output_(false) {}
-  ~ObjectSerializer() override {}
+        code_has_been_output_(false) {
+#ifdef DEBUG
+    serializer_->PushStack(obj);
+#endif  // DEBUG
+  }
+  ~ObjectSerializer() override {
+#ifdef DEBUG
+    serializer_->PopStack();
+#endif  // DEBUG
+  }
   void Serialize();
   void SerializeContent();
   void SerializeDeferred();