Optimize testing for an index's existence in packed Arrays

src/runtime/runtime-object.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/arguments.h"
 #include "src/bootstrapper.h"
 #include "src/debug.h"
 #include "src/runtime/runtime.h"
@@ skipping 1573 matching lines @@
 }
 
 
 RUNTIME_FUNCTION(RuntimeReference_ClassOf) {
   SealHandleScope shs(isolate);
   DCHECK(args.length() == 1);
   CONVERT_ARG_CHECKED(Object, obj, 0);
   if (!obj->IsJSReceiver()) return isolate->heap()->null_value();
   return JSReceiver::cast(obj)->class_name();
 }
+
+
+RUNTIME_FUNCTION(RuntimeReference_HasFastPackedElements) {
+  SealHandleScope shs(isolate);
+  DCHECK(args.length() == 1);
+  CONVERT_ARG_CHECKED(HeapObject, obj, 0);
+  return isolate->heap()->ToBoolean(
+      IsFastPackedElementsKind(obj->map()->elements_kind()));
+}
 }
 }  // namespace v8::internal