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..967a7400af57c4e5a3f0e59a2975990fc3e3be19 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 +1063,23 @@ public:
 
     OriginsUsingFeatures::Value& originsUsingFeaturesValue() { return m_originsUsingFeaturesValue; }
 
+    class UseNthIndexCacheScope {
+    public:

[sof, 2015/03/28 08:33:59]

If you add STACK_ALLOCATED(); here and turn the Document& reference into a
RawPtrWillBeMember<Document>, the scope object will be handled as expected
during conservative stack scanning.

(It's a somewhat specialized scope object to expose via Document, so perhaps
consider lifting it out into a separate file?)

[rune, 2015/03/30 09:00:03]

Done.
It only makes sense in the context of Document as it's about temporarily
changing the state of Document, and I wanted to make sure setUseNthIndexCache is
not used by other classes, at least not outside of Document, thus making it a
member class. The alternative would be to make it a friend of Document.

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

...
I won't insist, but having the scope object be a friend would look tidier. It
wouldn't be out of place to expose UseNthIndexCacheScope via NthIndexCache.h

+        UseNthIndexCacheScope(Document& document)
+            : m_document(document)
+        {
+            ASSERT(!m_document.useNthIndexCache());
+            m_document.setUseNthIndexCache(true);
+        }
+        ~UseNthIndexCacheScope()
+        {
+            ASSERT(m_document.useNthIndexCache());
+            m_document.setUseNthIndexCache(false);
+        }
+    private:
+        Document& m_document;
+    };
+
 protected:
     Document(const DocumentInit&, DocumentClassFlags = DefaultDocumentClass);
 
@@ -1317,6 +1336,7 @@ private:
     WillBeHeapHashMap<String, RefPtrWillBeMember<HTMLCanvasElement>> m_cssCanvasElements;
 
     OwnPtr<SelectorQueryCache> m_selectorQueryCache;
+    OwnPtrWillBeMember<NthIndexCache> m_nthIndexCache;

[sof, 2015/03/28 08:33:59]

Add a trace() call for this to Document's trace() implementation.

[rune, 2015/03/30 09:00:03]

Done.

 
     bool m_useSecureKeyboardEntryWhenActive;
 
@@ -1397,6 +1417,11 @@ 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)