Add comment on StyleInvalidator trace events

Source/core/css/invalidation/StyleInvalidator.cpp

 
 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 
 #include "core/css/invalidation/StyleInvalidator.h"
 
 #include "core/css/invalidation/DescendantInvalidationSet.h"
 #include "core/dom/Document.h"
 #include "core/dom/Element.h"
 #include "core/dom/ElementTraversal.h"
 #include "core/dom/shadow/ElementShadow.h"
 #include "core/dom/shadow/ShadowRoot.h"
 #include "core/inspector/InspectorTraceEvents.h"
 #include "core/rendering/RenderObject.h"
 
 namespace blink {
 
+// StyleInvalidator methods are super sensitive to performance benchmarks.
+// We easily get 1% regression per additional if statement on recursive
+// invalidate methods.
+// To minimize performance impact, we wrap trace events with a lookup of
+// cached flag. The cached flag is made "static const" and is not shared
+// with DescendantInvalidationSet to avoid additional GOT lookup cost.
 static const unsigned char* s_tracingEnabled = nullptr;
 
 #define TRACE_STYLE_INVALIDATOR_INVALIDATION_IF_ENABLED(element, reason) \
     if (UNLIKELY(*s_tracingEnabled)) \
         TRACE_STYLE_INVALIDATOR_INVALIDATION(element, reason);
 
 void StyleInvalidator::invalidate(Document& document)
 {
     RecursionData recursionData;
     if (Element* documentElement = document.documentElement())
@@ skipping 157 matching lines @@
 }
 
 void StyleInvalidator::trace(Visitor* visitor)
 {
 #if ENABLE(OILPAN)
     visitor->trace(m_pendingInvalidationMap);
 #endif
 }
 
 } // namespace blink