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

R=kmillikin@chromium.org
BUG=v8:1730

Review URL: http://codereview.chromium.org/8403037

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9835 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/isolate-inl.h

@@ -45,9 +45,7 @@ SaveContext::SaveContext(Isolate* isolate) : prev_(isolate->save_context()) {
   }
   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());
 }
 
 

src/isolate.h

@@ -1246,8 +1246,8 @@ class SaveContext BASE_EMBEDDED {
   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_);
+  bool IsBelowFrame(JavaScriptFrame* frame) {
+    return (c_entry_fp_ == 0) || (c_entry_fp_ > frame->sp());
   }
 
  private:
@@ -1256,7 +1256,7 @@ 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_;
 };
 
 

src/runtime.cc

@@ -10725,6 +10725,18 @@ static const int kFrameDetailsAtReturnIndex = 7;
 static const int kFrameDetailsFlagsIndex = 8;
 static const int kFrameDetailsFirstDynamicIndex = 9;
 
+
+static SaveContext* FindSavedContextForFrame(Isolate* isolate,
+                                             JavaScriptFrame* frame) {
+  SaveContext* save = isolate->save_context();
+  while (save != NULL && !save->IsBelowFrame(frame)) {
+    save = save->prev();
+  }
+  ASSERT(save != NULL);
+  return save;
+}
+
+
 // Return an array with frame details
 // args[0]: number: break id
 // args[1]: number: frame index
@@ -10780,11 +10792,7 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_GetFrameDetails) {
 
   // Traverse the saved contexts chain to find the active context for the
   // selected frame.
-  SaveContext* save = isolate->save_context();
-  while (save != NULL && !save->below(it.frame())) {
-    save = save->prev();
-  }
-  ASSERT(save != NULL);
+  SaveContext* save = FindSavedContextForFrame(isolate, it.frame());
 
   // Get the frame id.
   Handle<Object> frame_id(WrapFrameId(it.frame()->id()), isolate);
@@ -12062,11 +12070,8 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_DebugEvaluate) {
 
   // Traverse the saved contexts chain to find the active context for the
   // selected frame.
-  SaveContext* save = isolate->save_context();
-  while (save != NULL && !save->below(frame)) {
-    save = save->prev();
-  }
-  ASSERT(save != NULL);
+  SaveContext* save = FindSavedContextForFrame(isolate, frame);
+
   SaveContext savex(isolate);
   isolate->set_context(*(save->context()));