RUNTIME_FUNCTION(MaybeObject*, Runtime_RegExpInitializeObject) {
  HandleScope scope(isolate);
  DisallowHeapAllocation no_allocation;
  ASSERT(args.length() == 5);
  CONVERT_ARG_HANDLE_CHECKED(JSRegExp, regexp, 0);
  CONVERT_ARG_HANDLE_CHECKED(String, source, 1);
  // If source is the empty string we set it to "(?:)" instead as
  // suggested by ECMA-262, 5th, section 15.10.4.1.
  if (source->length() == 0) source = isolate->factory()->query_colon_string();

  CONVERT_ARG_HANDLE_CHECKED(Object, global, 2);
  if (!global->IsTrue()) global = isolate->factory()->false_value();

  CONVERT_ARG_HANDLE_CHECKED(Object, ignoreCase, 3);
  if (!ignoreCase->IsTrue()) ignoreCase = isolate->factory()->false_value();

  CONVERT_ARG_HANDLE_CHECKED(Object, multiline, 4);
  if (!multiline->IsTrue()) multiline = isolate->factory()->false_value();

  Map* map = regexp->map();
  Object* constructor = map->constructor();
  if (constructor->IsJSFunction() &&
      JSFunction::cast(constructor)->initial_map() == map) {
    // If we still have the original map, set in-object properties directly.
    regexp->InObjectPropertyAtPut(JSRegExp::kSourceFieldIndex, *source);
    // Both true and false are immovable immortal objects so no need for write
    // barrier.
    regexp->InObjectPropertyAtPut(
        JSRegExp::kGlobalFieldIndex, *global, SKIP_WRITE_BARRIER);
    regexp->InObjectPropertyAtPut(
        JSRegExp::kIgnoreCaseFieldIndex, *ignoreCase, SKIP_WRITE_BARRIER);
    regexp->InObjectPropertyAtPut(
        JSRegExp::kMultilineFieldIndex, *multiline, SKIP_WRITE_BARRIER);
    regexp->InObjectPropertyAtPut(
        JSRegExp::kLastIndexFieldIndex, Smi::FromInt(0), SKIP_WRITE_BARRIER);
    return *regexp;
  }