[FeedbackVector] Clarify the way the feedback vector is installed.

src/objects-inl.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // Review notes:
 //
 // - The use of macros in these inline functions may seem superfluous
 // but it is absolutely needed to make sure gcc generates optimal
 // code. gcc is not happy when attempting to inline too deep.
 //
@@ skipping 6578 matching lines @@
   if (was_optimized && !is_optimized) {
     // TODO(titzer): linear in the number of optimized functions; fix!
     context()->native_context()->RemoveOptimizedFunction(this);
   }
 }
 
 bool JSFunction::has_feedback_vector() const {
   return !feedback_vector_cell()->value()->IsUndefined(GetIsolate());
 }
 
+JSFunction::FeedbackVectorState JSFunction::GetFeedbackVectorState(
+    Isolate* isolate) const {
+  Cell* cell = feedback_vector_cell();
+  if (cell == isolate->heap()->undefined_cell()) {
+    return TOP_LEVEL_SCRIPT_NEEDS_VECTOR;
+  } else if (cell->value() == isolate->heap()->undefined_value() ||
+             !has_feedback_vector()) {
+    return NEEDS_VECTOR;
+  }
+  return HAS_VECTOR;
+}
+
 Context* JSFunction::context() {
   return Context::cast(READ_FIELD(this, kContextOffset));
 }
 
 
 JSObject* JSFunction::global_proxy() {
   return context()->global_proxy();
 }
 
 
@@ skipping 1774 matching lines @@
 #undef WRITE_INT64_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 #undef NOBARRIER_READ_BYTE_FIELD
 #undef NOBARRIER_WRITE_BYTE_FIELD
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_OBJECTS_INL_H_