Cache element indices for :nth-child and :nth-last-child selectors.

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index 0e83a6fc8b4c3f1f9d4761ba9695a15ff178e575..3ec67f73373a14d35c7d83f201945f89c0fb0cad 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -86,6 +86,7 @@
 #include "core/dom/NodeRenderingTraversal.h"
 #include "core/dom/NodeTraversal.h"
 #include "core/dom/NodeWithIndex.h"
+#include "core/dom/NthIndexCache.h"
 #include "core/dom/ProcessingInstruction.h"
 #include "core/dom/RequestAnimationFrameCallback.h"
 #include "core/dom/ScriptRunner.h"
@@ -474,6 +475,7 @@ Document::Document(const DocumentInit& initializer, DocumentClassFlags documentC
     , m_hasViewportUnits(false)
     , m_styleRecalcElementCounter(0)
     , m_parserSyncPolicy(AllowAsynchronousParsing)
+    , m_useNthIndexCache(false)
 {
     if (m_frame) {
         ASSERT(m_frame->page());
@@ -656,6 +658,22 @@ SelectorQueryCache& Document::selectorQueryCache()
     return *m_selectorQueryCache;
 }
 
+NthIndexCache* Document::nthIndexCache()
+{
+    if (!m_useNthIndexCache)
+        return nullptr;
+    if (!m_nthIndexCache)
+        m_nthIndexCache = NthIndexCache::create(*this);
+    return m_nthIndexCache.get();
+}
+
+void Document::setUseNthIndexCache(bool use)
+{
+    m_useNthIndexCache = use;
+    if (!use && m_nthIndexCache)
+        m_nthIndexCache->clear();
+}
+
 MediaQueryMatcher& Document::mediaQueryMatcher()
 {
     if (!m_mediaQueryMatcher)
@@ -1773,6 +1791,8 @@ void Document::updateStyle(StyleRecalcChange change)
     HTMLFrameOwnerElement::UpdateSuspendScope suspendWidgetHierarchyUpdates;
     m_lifecycle.advanceTo(DocumentLifecycle::InStyleRecalc);
 
+    UseNthIndexCacheScope nthIndexScope(*this);
+
     if (styleChangeType() >= SubtreeStyleChange)
         change = Force;
 
@@ -5714,6 +5734,7 @@ DEFINE_TRACE(Document)
     visitor->trace(m_visibilityObservers);
     visitor->trace(m_userActionElements);
     visitor->trace(m_svgExtensions);
+    visitor->trace(m_nthIndexCache);
     visitor->trace(m_timeline);
     visitor->trace(m_compositorPendingAnimations);
     visitor->trace(m_contextDocument);