Avoid style sharing with mis-matched descendant selectors

Source/core/css/resolver/SharedStyleFinder.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (c) 2011, Code Aurora Forum. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
@@ skipping 252 matching lines @@
         if (parent->hasScopedHTMLStyleChild())
             return false;
         if (parent->inlineStyle())
             return false;
         if (parent->isSVGElement() && toSVGElement(parent)->animatedSMILStyleProperties())
             return false;
         if (parent->hasID() && m_features.idsInRules.contains(parent->idForStyleResolution().impl()))
             return false;
         if (parentElementPreventsSharing(parent))
             return false;
+        if (!candidateMatchesDescendantRules(context, element))

[esprehn, 2013/10/21 19:19:08]

The hotest part of style sharing is matching the rulesets at the end. Matching
against almost the entire sheet (since nearly every rule is a descendant rule)
is going to be very very slow.

+            return false;
     }
 
     return true;
 }
 
 #ifdef STYLE_STATS
 Element* SharedStyleFinder::searchDocumentForSharedStyle(const ElementResolveContext& context) const
 {
     for (Element* element = context.element()->document().documentElement(); element; element = ElementTraversal::next(element)) {
         if (canShareStyleWithElement(context, element))
@@ skipping 22 matching lines @@
 }
 
 bool SharedStyleFinder::matchesRuleSet(const ElementResolveContext& context, RuleSet* ruleSet)
 {
     if (!ruleSet)
         return false;
     ElementRuleCollector collector(context, m_styleResolver->selectorFilter());
     return collector.hasAnyMatchingRules(ruleSet);
 }
 
+bool SharedStyleFinder::candidateMatchesDescendantRules(const ElementResolveContext& context, Element* candidate) const
+{
+    if (!m_descendantRuleSet)
+        return false;
+
+    ElementResolveContext candidateContext(*candidate);
+
+    ElementRuleCollector collector(context, m_styleResolver->selectorFilter());
+    ElementRuleCollector candidateCollector(candidateContext, m_styleResolver->selectorFilter());
+    int firstRuleIndex = -1, lastRuleIndex = -1;
+    RuleRange ruleRange(firstRuleIndex, lastRuleIndex);
+    collector.collectMatchingRules(MatchRequest(m_descendantRuleSet), ruleRange, SelectorChecker::StaysWithinTreeScope);
+    candidateCollector.collectMatchingRules(MatchRequest(m_descendantRuleSet), ruleRange, SelectorChecker::StaysWithinTreeScope);
+    return collector.matchedResult().ranges == candidateCollector.matchedResult().ranges;

[esprehn, 2013/10/21 19:19:08]

Matching rules like this is really expensive.

+}
+
 RenderStyle* SharedStyleFinder::locateSharedStyle(const ElementResolveContext& context)
 {
     STYLE_STATS_ADD_SEARCH();
 
     if (!context.element()->supportsStyleSharing())
         return 0;
 
     STYLE_STATS_ADD_ELEMENT_ELIGIBLE_FOR_SHARING();
 
     // Cache whether context.element() is affected by any known class selectors.
@@ skipping 23 matching lines @@
     if (matchesRuleSet(context, m_siblingRuleSet) || matchesRuleSet(context, m_uncommonAttributeRuleSet))
         return 0;
     // Tracking child index requires unique style for each node. This may get set by the sibling rule match above.
     if (parentElementPreventsSharing(context.element()->parentElement()))
         return 0;
     STYLE_STATS_ADD_STYLE_SHARED();
     return shareElement->renderStyle();
 }
 
 }