Improve generalization / migration tracing.

src/objects-inl.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1549 matching lines @@
   }
   set_map(map);
   return this;
 }
 
 
 MaybeObject* JSObject::MigrateInstance() {
   // Converting any field to the most specific type will cause the
   // GeneralizeFieldRepresentation algorithm to create the most general existing
   // transition that matches the object. This achieves what is needed.
-  return GeneralizeFieldRepresentation(0, Representation::None());
+  Map* original_map = map();
+  MaybeObject* maybe_result = GeneralizeFieldRepresentation(
+      0, Representation::None());
+  JSObject* result;
+  if (FLAG_trace_migration && maybe_result->To(&result)) {
+    PrintInstanceMigration(stdout, original_map, result->map());
+  }
+  return maybe_result;
 }
 
 
 MaybeObject* JSObject::TryMigrateInstance() {
   Map* new_map = map()->CurrentMapForDeprecated();
   if (new_map == NULL) return Smi::FromInt(0);
-  return MigrateToMap(new_map);
+  Map* original_map = map();
+  MaybeObject* maybe_result = MigrateToMap(new_map);
+  JSObject* result;
+  if (FLAG_trace_migration && maybe_result->To(&result)) {
+    PrintInstanceMigration(stdout, original_map, result->map());
+  }
+  return maybe_result;
 }
 
 
 Handle<String> JSObject::ExpectedTransitionKey(Handle<Map> map) {
   DisallowHeapAllocation no_gc;
   if (!map->HasTransitionArray()) return Handle<String>::null();
   TransitionArray* transitions = map->transitions();
   if (!transitions->IsSimpleTransition()) return Handle<String>::null();
   int transition = TransitionArray::kSimpleTransitionIndex;
   PropertyDetails details = transitions->GetTargetDetails(transition);
@@ skipping 4656 matching lines @@
 #undef WRITE_UINT32_FIELD
 #undef READ_SHORT_FIELD
 #undef WRITE_SHORT_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_INL_H_