[runtime] Optimize MigrateFastToFast for the transition case

This avoids quite expensive computation further downstream.

Review URL: https://codereview.chromium.org/1697173002

Cr-Commit-Position: refs/heads/master@{#34007}

src/objects.cc

@@ -2883,45 +2883,43 @@ void JSObject::MigrateToMap(Handle<JSObject> object, Handle<Map> new_map,
 void JSObject::MigrateFastToFast(Handle<JSObject> object, Handle<Map> new_map) {
   Isolate* isolate = object->GetIsolate();
   Handle<Map> old_map(object->map());
-  int old_number_of_fields;
-  int number_of_fields = new_map->NumberOfFields();
-  int inobject = new_map->GetInObjectProperties();
-  int unused = new_map->unused_property_fields();
-
-  // Nothing to do if no functions were converted to fields and no smis were
-  // converted to doubles.
-  if (!old_map->InstancesNeedRewriting(*new_map, number_of_fields, inobject,
-                                       unused, &old_number_of_fields)) {
+  // In case of a regular transition.
+  if (new_map->GetBackPointer() == *old_map) {
+    // If the map does not add named properties, simply set the map.
+    if (old_map->NumberOfOwnDescriptors() ==
+        new_map->NumberOfOwnDescriptors()) {
       object->synchronized_set_map(*new_map);
       return;
     }
 
-  int total_size = number_of_fields + unused;
-  int external = total_size - inobject;
-
-  if (number_of_fields != old_number_of_fields &&
-      new_map->GetBackPointer() == *old_map) {
     PropertyDetails details = new_map->GetLastDescriptorDetails();
+    // Either new_map adds an kDescriptor property, or a kField property for
+    // which there is still space, and which does not require a mutable double
+    // box (an out-of-object double).
+    if (details.location() == kDescriptor ||
+        (old_map->unused_property_fields() > 0 &&
+         ((FLAG_unbox_double_fields && object->properties()->length() == 0) ||
+          !details.representation().IsDouble()))) {
+      object->synchronized_set_map(*new_map);
+      return;
+    }
 
+    // If there is still space in the object, we need to allocate a mutable
+    // double box.
     if (old_map->unused_property_fields() > 0) {
-      if (details.representation().IsDouble()) {
       FieldIndex index =
           FieldIndex::ForDescriptor(*new_map, new_map->LastAdded());
-        if (new_map->IsUnboxedDoubleField(index)) {
-          object->RawFastDoublePropertyAtPut(index, 0);
-        } else {
+      DCHECK(details.representation().IsDouble());
+      DCHECK(!new_map->IsUnboxedDoubleField(index));
       Handle<Object> value = isolate->factory()->NewHeapNumber(0, MUTABLE);
       object->RawFastPropertyAtPut(index, *value);
-        }
-      }
       object->synchronized_set_map(*new_map);
       return;
     }
 
-    DCHECK(number_of_fields == old_number_of_fields + 1);
     // This migration is a transition from a map that has run out of property
-    // space. Therefore it could be done by extending the backing store.
-    int grow_by = external - object->properties()->length();
+    // space. Extend the backing store.
+    int grow_by = new_map->unused_property_fields() + 1;
     Handle<FixedArray> old_storage = handle(object->properties(), isolate);
     Handle<FixedArray> new_storage =
         isolate->factory()->CopyFixedArrayAndGrow(old_storage, grow_by);
@@ -2933,8 +2931,8 @@ void JSObject::MigrateFastToFast(Handle<JSObject> object, Handle<Map> new_map) {
     } else {
       value = isolate->factory()->uninitialized_value();
     }
-    DCHECK(details.type() == DATA);
-    int target_index = details.field_index() - inobject;
+    DCHECK_EQ(DATA, details.type());
+    int target_index = details.field_index() - new_map->GetInObjectProperties();
     DCHECK(target_index >= 0);  // Must be a backing store index.
     new_storage->set(target_index, *value);
 
@@ -2946,6 +2944,23 @@ void JSObject::MigrateFastToFast(Handle<JSObject> object, Handle<Map> new_map) {
     object->synchronized_set_map(*new_map);
     return;
   }
+
+  int old_number_of_fields;
+  int number_of_fields = new_map->NumberOfFields();
+  int inobject = new_map->GetInObjectProperties();
+  int unused = new_map->unused_property_fields();
+
+  // Nothing to do if no functions were converted to fields and no smis were
+  // converted to doubles.
+  if (!old_map->InstancesNeedRewriting(*new_map, number_of_fields, inobject,
+                                       unused, &old_number_of_fields)) {
+    object->synchronized_set_map(*new_map);
+    return;
+  }
+
+  int total_size = number_of_fields + unused;
+  int external = total_size - inobject;
+
   Handle<FixedArray> array = isolate->factory()->NewFixedArray(total_size);
 
   Handle<DescriptorArray> old_descriptors(old_map->instance_descriptors());