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

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 5453bf249ad3f756cbd97944d94dd018f79aa389..75d6c7f0a9f016266e2869c7f60a510250817ac9 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1245,9 +1245,10 @@ class SaveContext BASE_EMBEDDED {
   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:
@@ -1256,7 +1257,7 @@ class SaveContext BASE_EMBEDDED {
   Handle<Context> dummy_;
 #endif
   SaveContext* prev_;
-  Address js_sp_;  // The top JS frame's sp when saving context.
+  Address c_entry_fp_;
 };