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

Source/core/dom/Document.h

Index: Source/core/dom/Document.h
diff --git a/Source/core/dom/Document.h b/Source/core/dom/Document.h
index 51a3c60befaffd8c8722eedee272a4a7ce04c092..bf14e6dbb835fbc03e33beceabca0fa03b59cbdb 100644
--- a/Source/core/dom/Document.h
+++ b/Source/core/dom/Document.h
@@ -128,6 +128,7 @@ class MediaQueryListListener;
 class MediaQueryMatcher;
 class NodeFilter;
 class NodeIterator;
+class NthIndexCache;
 class Page;
 class PlatformMouseEvent;
 class ProcessingInstruction;
@@ -247,6 +248,7 @@ public:
     virtual bool canContainRangeEndPoint() const override { return true; }
 
     SelectorQueryCache& selectorQueryCache();
+    NthIndexCache* nthIndexCache();
 
     // Focus Management.
     Element* activeElement() const;
@@ -1061,6 +1063,25 @@ public:
 
     OriginsUsingFeatures::Value& originsUsingFeaturesValue() { return m_originsUsingFeaturesValue; }
 
+    class UseNthIndexCacheScope {
+        STACK_ALLOCATED();
+        WTF_MAKE_NONCOPYABLE(UseNthIndexCacheScope);
+    public:
+        UseNthIndexCacheScope(Document& document)
+            : m_document(&document)
+        {
+            ASSERT(!document.useNthIndexCache());

[sof, 2015/03/30 13:27:53]

Just to verify: there's no possibility that you may ever enter nested scopes?

[rune, 2015/03/30 20:14:40]

Shouldn't with the current uses of this scope. querySelector(All) should not
nest, and the DocumentLifecycle should ensure we don't nest updateStyle. The
assert here is to introduce future nesting problems. I'd rather avoid nested
scopes than mending them by checking/storing the current useNthIndexCache().

+            document.setUseNthIndexCache(true);
+        }
+        ~UseNthIndexCacheScope()
+        {
+            ASSERT(m_document->useNthIndexCache());
+            m_document->setUseNthIndexCache(false);
+        }
+    private:
+        RawPtrWillBeMember<Document> m_document;
+    };
+
 protected:
     Document(const DocumentInit&, DocumentClassFlags = DefaultDocumentClass);
 
@@ -1317,6 +1338,7 @@ private:
     WillBeHeapHashMap<String, RefPtrWillBeMember<HTMLCanvasElement>> m_cssCanvasElements;
 
     OwnPtr<SelectorQueryCache> m_selectorQueryCache;
+    OwnPtrWillBeMember<NthIndexCache> m_nthIndexCache;
 
     bool m_useSecureKeyboardEntryWhenActive;
 
@@ -1397,6 +1419,11 @@ private:
     ParserSynchronizationPolicy m_parserSyncPolicy;
 
     OriginsUsingFeatures::Value m_originsUsingFeaturesValue;
+
+    bool useNthIndexCache() const { return m_useNthIndexCache; }
+    void setUseNthIndexCache(bool);
+
+    bool m_useNthIndexCache;
 };
 
 inline bool Document::shouldOverrideLegacyDescription(ViewportDescription::Type origin)