From c933e28935a5bd7bb9f44c74450ce7220996b581 Mon Sep 17 00:00:00 2001
From: "vegorov@chromium.org"
 <vegorov@chromium.org@ce2b1a6d-e550-0410-aec6-3dcde31c8c00>
Date: Fri, 28 Oct 2011 12:49:09 +0000
Subject: [PATCH] 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 |  4 +---
 src/isolate.h     |  6 +++---
 src/runtime.cc    | 25 +++++++++++++++----------
 3 files changed, 19 insertions(+), 16 deletions(-)

diff --git a/src/isolate-inl.h b/src/isolate-inl.h
index d6e613176d..0a2c17404e 100644
--- a/src/isolate-inl.h
+++ b/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());
 }
 
 
diff --git a/src/isolate.h b/src/isolate.h
index 5453bf249a..116b802670 100644
--- a/src/isolate.h
+++ b/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_;
 };
 
 
diff --git a/src/runtime.cc b/src/runtime.cc
index 51ed26d889..3161614e7c 100644
--- a/src/runtime.cc
+++ b/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()));