Avoid style sharing with mis-matched descendant selectors

Source/core/css/resolver/SharedStyleFinder.cpp

Index: Source/core/css/resolver/SharedStyleFinder.cpp
diff --git a/Source/core/css/resolver/SharedStyleFinder.cpp b/Source/core/css/resolver/SharedStyleFinder.cpp
index a2969bf327f2d5412942178edc39876e520ab4b2..578f20570a17575981a6478a5b7306e223a75587 100644
--- a/Source/core/css/resolver/SharedStyleFinder.cpp
+++ b/Source/core/css/resolver/SharedStyleFinder.cpp
@@ -270,6 +270,8 @@ bool SharedStyleFinder::canShareStyleWithElement(const ElementResolveContext& co
             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;
@@ -312,6 +314,22 @@ bool SharedStyleFinder::matchesRuleSet(const ElementResolveContext& context, Rul
     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();