[snapshot] Factor out root serialization code

Factors out a new method Serializer::SerializeRoot which attempts to
serialize a given object as a Root if it is one and the Serializer's
policy allows that root to be serialized (implemented as a new virtual
method RootCanBeSerialized)..

This is in preparation for adding a ReadOnlySerializer which change the
way read-only roots are serialized.

Bug: v8:8191
Change-Id: I7fbb4e9520fba8b836a0b6bf95ca39abc3ded79e
Reviewed-on: https://chromium-review.googlesource.com/c/1264698Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56494}

src/address-map.h

@@ -57,7 +57,7 @@ class RootIndexMap {
   explicit RootIndexMap(Isolate* isolate);
 
   // Returns true on successful lookup and sets *|out_root_list|.
-  bool Lookup(HeapObject* obj, RootIndex* out_root_list) {
+  bool Lookup(HeapObject* obj, RootIndex* out_root_list) const {
     Maybe<uint32_t> maybe_index = map_->Get(obj);
     if (maybe_index.IsJust()) {
       *out_root_list = static_cast<RootIndex>(maybe_index.FromJust());

src/snapshot/builtin-serializer.cc

@@ -70,12 +70,7 @@ void BuiltinSerializer::SerializeObject(HeapObject* o, HowToCode how_to_code,
   DCHECK(!o->IsSmi());
 
   // Roots can simply be serialized as root references.
-  RootIndex root_index;
-  if (root_index_map()->Lookup(o, &root_index)) {
-    DCHECK(startup_serializer_->root_has_been_serialized(root_index));
-    PutRoot(root_index, o, how_to_code, where_to_point, skip);
-    return;
-  }
+  if (SerializeRoot(o, how_to_code, where_to_point, skip)) return;
 
   // Builtins are serialized using a dedicated bytecode. We only reach this
   // point if encountering a Builtin e.g. while iterating the body of another

src/snapshot/code-serializer.cc

@@ -124,11 +124,7 @@ void CodeSerializer::SerializeObject(HeapObject* obj, HowToCode how_to_code,
                                      WhereToPoint where_to_point, int skip) {
   if (SerializeHotObject(obj, how_to_code, where_to_point, skip)) return;
 
-  RootIndex root_index;
-  if (root_index_map()->Lookup(obj, &root_index)) {
-    PutRoot(root_index, obj, how_to_code, where_to_point, skip);
-    return;
-  }
+  if (SerializeRoot(obj, how_to_code, where_to_point, skip)) return;
 
   if (SerializeBackReference(obj, how_to_code, where_to_point, skip)) return;
 

src/snapshot/partial-serializer.cc

@@ -59,11 +59,7 @@ void PartialSerializer::SerializeObject(HeapObject* obj, HowToCode how_to_code,
   }
   if (SerializeHotObject(obj, how_to_code, where_to_point, skip)) return;
 
-  RootIndex root_index;
-  if (root_index_map()->Lookup(obj, &root_index)) {
-    PutRoot(root_index, obj, how_to_code, where_to_point, skip);
-    return;
-  }
+  if (SerializeRoot(obj, how_to_code, where_to_point, skip)) return;
 
   if (SerializeBackReference(obj, how_to_code, where_to_point, skip)) return;
 

src/snapshot/serializer.cc

@@ -138,6 +138,21 @@ void Serializer<AllocatorT>::PrintStack() {
 }
 #endif  // DEBUG
 
+template <class AllocatorT>
+bool Serializer<AllocatorT>::SerializeRoot(HeapObject* obj,
+                                           HowToCode how_to_code,
+                                           WhereToPoint where_to_point,
+                                           int skip) {
+  RootIndex root_index;
+  // Derived serializers are responsible for determining if the root has
+  // actually been serialized before calling this.
+  if (root_index_map()->Lookup(obj, &root_index)) {
+    PutRoot(root_index, obj, how_to_code, where_to_point, skip);
+    return true;
+  }
+  return false;
+}
+
 template <class AllocatorT>
 bool Serializer<AllocatorT>::SerializeHotObject(HeapObject* obj,
                                                 HowToCode how_to_code,

src/snapshot/serializer.h

@@ -182,6 +182,10 @@ class Serializer : public SerializerDeserializer {
   int PutAlignmentPrefix(HeapObject* object);
   void PutNextChunk(int space);
 
+  // Returns true if the object was successfully serialized as a root.
+  bool SerializeRoot(HeapObject* obj, HowToCode how_to_code,
+                     WhereToPoint where_to_point, int skip);
+
   // Returns true if the object was successfully serialized as hot object.
   bool SerializeHotObject(HeapObject* obj, HowToCode how_to_code,
                           WhereToPoint where_to_point, int skip);
@@ -237,7 +241,7 @@ class Serializer : public SerializerDeserializer {
 #endif  // DEBUG
 
   SerializerReferenceMap* reference_map() { return &reference_map_; }
-  RootIndexMap* root_index_map() { return &root_index_map_; }
+  const RootIndexMap* root_index_map() const { return &root_index_map_; }
   AllocatorT* allocator() { return &allocator_; }
 
   SnapshotByteSink sink_;  // Used directly by subclasses.

src/snapshot/startup-serializer.cc

@@ -33,17 +33,9 @@ void StartupSerializer::SerializeObject(HeapObject* obj, HowToCode how_to_code,
     return;
   }
   if (SerializeHotObject(obj, how_to_code, where_to_point, skip)) return;
-
-  RootIndex root_index;
-  // 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_map()->Lookup(obj, &root_index)) {
-    if (root_has_been_serialized(root_index)) {
-      PutRoot(root_index, obj, how_to_code, where_to_point, skip);
-      return;
-    }
-  }
-
+  if (IsRootAndHasBeenSerialized(obj) &&
+      SerializeRoot(obj, how_to_code, where_to_point, skip))
+    return;
   if (SerializeBackReference(obj, how_to_code, where_to_point, skip)) return;
 
   FlushSkip(skip);

src/snapshot/startup-serializer.h

@@ -32,6 +32,12 @@ class StartupSerializer : public Serializer<> {
     return root_has_been_serialized_.test(static_cast<size_t>(root_index));
   }
 
+  bool IsRootAndHasBeenSerialized(HeapObject* obj) const {
+    RootIndex root_index;
+    return root_index_map()->Lookup(obj, &root_index) &&
+           root_has_been_serialized(root_index);
+  }
+
  private:
   class PartialCacheIndexMap {
    public: