[serializer] Move WeakFixedArray compaction to separate heap walk phase

This avoids discovering and compacting the same array repeatedly. No change in resulting snapshots intended.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1290393002

Cr-Commit-Position: refs/heads/master@{#30177}

src/api.cc

@@ -394,6 +394,28 @@ StartupData V8::CreateSnapshotDataBlob(const char* custom_source) {
       // If we don't do this then we end up with a stray root pointing at the
       // context even after we have disposed of the context.
       internal_isolate->heap()->CollectAllAvailableGarbage("mksnapshot");
+
+      // GC may have cleared weak cells, so compact any WeakFixedArrays
+      // found on the heap.
+      i::HeapIterator iterator(internal_isolate->heap(),
+                               i::HeapIterator::kFilterUnreachable);
+      for (i::HeapObject* o = iterator.next(); o != NULL; o = iterator.next()) {
+        if (o->IsPrototypeInfo()) {
+          i::Object* prototype_users =
+              i::PrototypeInfo::cast(o)->prototype_users();
+          if (prototype_users->IsWeakFixedArray()) {
+            i::WeakFixedArray* array = i::WeakFixedArray::cast(prototype_users);
+            array->Compact<i::JSObject::PrototypeRegistryCompactionCallback>();
+          }
+        } else if (o->IsScript()) {
+          i::Object* shared_list = i::Script::cast(o)->shared_function_infos();
+          if (shared_list->IsWeakFixedArray()) {
+            i::WeakFixedArray* array = i::WeakFixedArray::cast(shared_list);
+            array->Compact<i::WeakFixedArray::NullCallback>();
+          }
+        }
+      }
+
       i::Object* raw_context = *v8::Utils::OpenPersistent(context);
       context.Reset();
 

src/snapshot/serialize.cc

@@ -1923,38 +1923,10 @@ void Serializer::ObjectSerializer::Serialize() {
   // We don't expect fillers.
   DCHECK(!object_->IsFiller());
 
-  if (object_->IsPrototypeInfo()) {
-    Object* prototype_users = PrototypeInfo::cast(object_)->prototype_users();
-    if (prototype_users->IsWeakFixedArray()) {
-      WeakFixedArray* array = WeakFixedArray::cast(prototype_users);
-      array->Compact<JSObject::PrototypeRegistryCompactionCallback>();
-    }
-  }
-  // Compaction of a prototype users list can require the registered users
-  // to update their remembered slots. That doesn't work if those users
-  // have already been serialized themselves. So if this object is a
-  // registered user, compact its prototype's user list now.
-  if (object_->IsMap()) {
-    Map* map = Map::cast(object_);
-    if (map->is_prototype_map() && map->prototype_info()->IsPrototypeInfo() &&
-        PrototypeInfo::cast(map->prototype_info())->registry_slot() !=
-            PrototypeInfo::UNREGISTERED) {
-      JSObject* proto = JSObject::cast(map->prototype());
-      PrototypeInfo* info = PrototypeInfo::cast(proto->map()->prototype_info());
-      WeakFixedArray* array = WeakFixedArray::cast(info->prototype_users());
-      array->Compact<JSObject::PrototypeRegistryCompactionCallback>();
-    }
-  }
-
   if (object_->IsScript()) {
     // Clear cached line ends.
     Object* undefined = serializer_->isolate()->heap()->undefined_value();
     Script::cast(object_)->set_line_ends(undefined);
-    Object* shared_list = Script::cast(object_)->shared_function_infos();
-    if (shared_list->IsWeakFixedArray()) {
-      WeakFixedArray::cast(shared_list)
-          ->Compact<WeakFixedArray::NullCallback>();
-    }
   }
 
   if (object_->IsExternalString()) {