[serializer] encode recent long-encoded root list items as hot objects.

src/snapshot/startup-serializer.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/snapshot/startup-serializer.h"
 
 #include "src/objects-inl.h"
 #include "src/v8threads.h"
 
 namespace v8 {
@@ skipping 32 matching lines @@
       obj = isolate()->heap()->undefined_value();
     }
   } else if (obj->IsCode()) {
     Code* code = Code::cast(obj);
     if (code->kind() == Code::FUNCTION) {
       code->ClearInlineCaches();
       code->set_profiler_ticks(0);
     }
   }
 
+  if (SerializeHotObject(obj, how_to_code, where_to_point, skip)) return;
+
   int root_index = root_index_map_.Lookup(obj);
   // We can only encode roots as such if it has already been serialized.
   // That applies to root indices below the wave front.
   if (root_index != RootIndexMap::kInvalidRootIndex) {
     if (root_has_been_serialized_.test(root_index)) {
       PutRoot(root_index, obj, how_to_code, where_to_point, skip);
       return;
     }
   }
 
-  if (SerializeKnownObject(obj, how_to_code, where_to_point, skip)) return;
+  if (SerializeBackReference(obj, how_to_code, where_to_point, skip)) return;
 
   FlushSkip(skip);
 
   // Object has not yet been serialized.  Serialize it here.
   ObjectSerializer object_serializer(this, obj, &sink_, how_to_code,
                                      where_to_point);
   object_serializer.Serialize();
 
   if (serializing_immortal_immovables_roots_ &&
       root_index != RootIndexMap::kInvalidRootIndex) {
@@ skipping 96 matching lines @@
   if (root_index == Heap::kStackLimitRootIndex ||
       root_index == Heap::kRealStackLimitRootIndex) {
     return true;
   }
   return Heap::RootIsImmortalImmovable(root_index) !=
          serializing_immortal_immovables_roots_;
 }
 
 }  // namespace internal
 }  // namespace v8