Make C++ to JS transition faster by avoiding JavaScriptFrameIterator in SaveContext.

src/isolate-inl.h

 // Copyright 2011 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 27 matching lines @@
 
 SaveContext::SaveContext(Isolate* isolate) : prev_(isolate->save_context()) {
   if (isolate->context() != NULL) {
     context_ = Handle<Context>(isolate->context());
 #if __GNUC_VERSION__ >= 40100 && __GNUC_VERSION__ < 40300
     dummy_ = Handle<Context>(isolate->context());
 #endif
   }
   isolate->set_save_context(this);
 
-  // If there is no JS frame under the current C frame, use the value 0.
-  JavaScriptFrameIterator it(isolate);
-  js_sp_ = it.done() ? 0 : it.frame()->sp();
+  c_entry_fp_ = isolate->c_entry_fp(isolate->thread_local_top());
 }
 
 
 bool Isolate::DebuggerHasBreakPoints() {
 #ifdef ENABLE_DEBUGGER_SUPPORT
   return debug()->has_break_points();
 #else
   return false;
 #endif
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_ISOLATE_INL_H_