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

src/runtime.cc

 // 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 10681 matching lines @@
 static const int kFrameDetailsReceiverIndex = 1;
 static const int kFrameDetailsFunctionIndex = 2;
 static const int kFrameDetailsArgumentCountIndex = 3;
 static const int kFrameDetailsLocalCountIndex = 4;
 static const int kFrameDetailsSourcePositionIndex = 5;
 static const int kFrameDetailsConstructCallIndex = 6;
 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->IsAboveFrame(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
 //
 // The array returned contains the following information:
 // 0: Frame id
 // 1: Receiver
 // 2: Function
 // 3: Argument count
 // 4: Local count
@@ skipping 35 matching lines @@
   if (it.done()) return heap->undefined_value();
 
   if (it.frame()->is_optimized()) {
     inlined_frame_index =
         it.frame()->GetInlineCount() - (index - count) - 1;
   }
   FrameInspector frame_inspector(it.frame(), inlined_frame_index, isolate);
 
   // 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);
 
   // Find source position.
   int position =
       it.frame()->LookupCode()->SourcePosition(it.frame()->pc());
 
   // Check for constructor frame. Inlined frames cannot be construct calls.
   bool inlined_frame =
@@ skipping 1257 matching lines @@
   // Get the frame where the debugging is performed.
   StackFrame::Id id = UnwrapFrameId(wrapped_id);
   JavaScriptFrameIterator it(isolate, id);
   JavaScriptFrame* frame = it.frame();
   Handle<JSFunction> function(JSFunction::cast(frame->function()));
   Handle<SerializedScopeInfo> scope_info(function->shared()->scope_info());
   ScopeInfo<> sinfo(*scope_info);
 
   // 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()));
 
   // Create the (empty) function replacing the function on the stack frame for
   // the purpose of evaluating in the context created below. It is important
   // that this function does not describe any parameters and local variables
   // in the context. If it does then this will cause problems with the lookup
   // in Context::Lookup, where context slots for parameters and local variables
   // are looked at before the extension object.
   Handle<JSFunction> go_between =
@@ skipping 1472 matching lines @@
   } else {
     // Handle last resort GC and make sure to allow future allocations
     // to grow the heap without causing GCs (if possible).
     isolate->counters()->gc_last_resort_from_js()->Increment();
     isolate->heap()->CollectAllGarbage(Heap::kNoGCFlags);
   }
 }
 
 
 } }  // namespace v8::internal