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

src/isolate.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 1227 matching lines @@
     } else {
       Isolate* isolate = context_->GetIsolate();
       isolate->set_context(*context_);
       isolate->set_save_context(prev_);
     }
   }
 
   Handle<Context> context() { return context_; }
   SaveContext* prev() { return prev_; }
 
-  // Returns true if this save context is below a given JavaScript frame.
-  bool below(JavaScriptFrame* frame) {
-    return (js_sp_ == 0) || (frame->sp() < js_sp_);
+  // Returns true if this save context is above a given JavaScript frame.
+  bool IsAboveFrame(JavaScriptFrame* frame) {

[Kevin Millikin (Chromium), 2011/10/28 11:09:08]

Let's rename this to use "Below", and you can change the comment to say:

Returns true if this save context is below a given JavaScript frame (i.e., at a
higher address).

+    // Every frame is below topmost SaveContext.
+    return (c_entry_fp_ == 0) || (c_entry_fp_ > frame->sp());
   }
 
  private:
   Handle<Context> context_;
 #if __GNUC_VERSION__ >= 40100 && __GNUC_VERSION__ < 40300
   Handle<Context> dummy_;
 #endif
   SaveContext* prev_;
-  Address js_sp_;  // The top JS frame's sp when saving context.
+  Address c_entry_fp_;
 };
 
 
 class AssertNoContextChange BASE_EMBEDDED {
 #ifdef DEBUG
  public:
   AssertNoContextChange() :
       scope_(Isolate::Current()),
       context_(Isolate::Current()->context(), Isolate::Current()) {
   }
@@ skipping 89 matching lines @@
 
 // Mark the global context with out of memory.
 inline void Context::mark_out_of_memory() {
   global_context()->set_out_of_memory(HEAP->true_value());
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_ISOLATE_H_