[cleanup] Remove dead code in StartupSerializer

Now that we don't iterate over the strong roots in the StartupSerializer
twice, remove code related to skipping non-immortal immovable roots.
Factor out code from Serializer::VisitRootPointers and use that method in
the StartSerializer override.

Also update comments that reflected the old way of serializing.

Change-Id: Ieb5e63389f455b963244717cada7e5ccde8e41cb
Reviewed-on: https://chromium-review.googlesource.com/1179669
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55218}

src/snapshot/serializer.cc

@@ -116,11 +116,16 @@ void Serializer<AllocatorT>::VisitRootPointers(Root root,
   if (root == Root::kBuiltins || root == Root::kDispatchTable) return;
 
   for (Object** current = start; current < end; current++) {
-    if ((*current)->IsSmi()) {
-      PutSmi(Smi::cast(*current));
-    } else {
-      SerializeObject(HeapObject::cast(*current), kPlain, kStartOfObject, 0);
-    }
+    SerializeRootObject(*current);
+  }
+}
+
+template <class AllocatorT>
+void Serializer<AllocatorT>::SerializeRootObject(Object* object) {
+  if (object->IsSmi()) {
+    PutSmi(Smi::cast(object));
+  } else {
+    SerializeObject(HeapObject::cast(object), kPlain, kStartOfObject, 0);
   }
 }
 

src/snapshot/serializer.h

@@ -170,6 +170,7 @@ class Serializer : public SerializerDeserializer {
 
   void VisitRootPointers(Root root, const char* description, Object** start,
                          Object** end) override;
+  void SerializeRootObject(Object* object);
 
   void PutRoot(int index, HeapObject* object, HowToCode how, WhereToPoint where,
                int skip);

src/snapshot/startup-serializer.cc

@@ -132,24 +132,13 @@ void StartupSerializer::VisitRootPointers(Root root, const char* description,
                                           Object** start, Object** end) {
   if (start == isolate()->heap()->roots_array_start()) {
     // Serializing the root list needs special handling:
-    // - The first pass over the root list only serializes immortal immovables.
-    // - The second pass over the root list serializes the rest.
     // - Only root list elements that have been fully serialized can be
-    //   referenced via as root by using kRootArray bytecodes.
-    int skip = 0;
+    //   referenced using kRootArray bytecodes.
     for (Object** current = start; current < end; current++) {
+      SerializeRootObject(*current);
       int root_index = static_cast<int>(current - start);
-      if ((*current)->IsSmi()) {
-        FlushSkip(skip);
-        PutSmi(Smi::cast(*current));
-      } else {
-        SerializeObject(HeapObject::cast(*current), kPlain, kStartOfObject,
-                        skip);
-      }
       root_has_been_serialized_.set(root_index);
-      skip = 0;
     }
-    FlushSkip(skip);
   } else {
     Serializer::VisitRootPointers(root, description, start, end);
   }

src/snapshot/startup-serializer.h

@@ -18,10 +18,10 @@ class StartupSerializer : public Serializer<> {
   ~StartupSerializer() override;
 
   // Serialize the current state of the heap.  The order is:
-  // 1) Immortal immovable roots
-  // 2) Remaining strong references.
-  // 3) Partial snapshot cache.
-  // 4) Weak references (e.g. the string table).
+  // 1) Strong roots
+  // 2) Builtins and bytecode handlers
+  // 3) Partial snapshot cache
+  // 4) Weak references (e.g. the string table)
   void SerializeStrongReferences();
   void SerializeWeakReferencesAndDeferred();