Don't assume so easily that RenderStyle data can be shared.

Source/core/css/resolver/StyleResolver.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 587 matching lines @@
 
     {
         ElementRuleCollector collector(state.elementContext(), m_selectorFilter, state.style(), m_inspectorCSSOMWrappers);
         collector.setRegionForStyling(regionForStyling);
 
         if (matchingBehavior == MatchOnlyUserAgentRules)
             matchUARules(collector);
         else
             matchAllRules(state, collector, m_matchAuthorAndUserStyles, matchingBehavior != MatchAllRulesExcludingSMIL);
 
-        applyMatchedProperties(state, collector.matchedResult(), element);
+        applyMatchedProperties(state, collector.matchedResult());
     }
     {
         StyleAdjuster adjuster(state.cachedUAStyle(), m_document->inQuirksMode());
         adjuster.adjustRenderStyle(state.style(), state.parentStyle(), element);
     }
     document()->didAccessStyleResolver();
 
     // FIXME: Shouldn't this be on RenderBody::styleDidChange?
     if (element->hasTagName(bodyTag))
         document()->textLinkColors().setTextColor(state.style()->visitedDependentColor(CSSPropertyColor));
@@ skipping 174 matching lines @@
         if (m_matchAuthorAndUserStyles) {
             matchUserRules(collector, false);
             matchAuthorRules(state.element(), collector, false);
         }
 
         if (collector.matchedResult().matchedProperties.isEmpty())
             return 0;
 
         state.style()->setStyleType(pseudoStyleRequest.pseudoId);
 
-        applyMatchedProperties(state, collector.matchedResult(), e);
+        applyMatchedProperties(state, collector.matchedResult());
     }
     {
         StyleAdjuster adjuster(state.cachedUAStyle(), m_document->inQuirksMode());
         // FIXME: Passing 0 as the Element* introduces a lot of complexity
         // in the adjustRenderStyle code.
         adjuster.adjustRenderStyle(state.style(), state.parentStyle(), 0);
     }
 
     // Start loading resources referenced by this style.
     m_styleResourceLoader.loadPendingResources(state.style(), state.elementStyleResources());
@@ skipping 367 matching lines @@
 static unsigned computeMatchedPropertiesHash(const MatchedProperties* properties, unsigned size)
 {
     return StringHasher::hashMemory(properties, sizeof(MatchedProperties) * size);
 }
 
 void StyleResolver::invalidateMatchedPropertiesCache()
 {
     m_matchedPropertiesCache.clear();
 }
 
-void StyleResolver::applyMatchedProperties(StyleResolverState& state, const MatchResult& matchResult, const Element* element)
+void StyleResolver::applyMatchedProperties(StyleResolverState& state, const MatchResult& matchResult)
 {
+    const Element* element = state.element();
     ASSERT(element);
     STYLE_STATS_ADD_MATCHED_PROPERTIES_SEARCH();
 
     unsigned cacheHash = matchResult.isCacheable ? computeMatchedPropertiesHash(matchResult.matchedProperties.data(), matchResult.matchedProperties.size()) : 0;
     bool applyInheritedOnly = false;
     const CachedMatchedProperties* cachedMatchedProperties = 0;
 
-    if (cacheHash && (cachedMatchedProperties = m_matchedPropertiesCache.find(cacheHash, state, matchResult))) {
+    if (cacheHash && (cachedMatchedProperties = m_matchedPropertiesCache.find(cacheHash, state, matchResult))
+        && MatchedPropertiesCache::isCacheable(element, state.style(), state.parentStyle())) {

[ojan, 2013/07/28 17:42:31]

Not a big deal, but it'd be nice if we saved the isCacheable bool so we don't
have to call this method twice. It would also simplify the if statements below a
bit.

bool isCacheable = cacheHash && MatchedPropertiesCache::isCacheable(element,
state.style(), state.parentStyle());

Then, down below, the early return just becomes:
if (cachedMatchedProperties || !isCacheable)

Given how much we call applyMatchedProperties, I could imagine doing all the
branching in MatchedPropertiesCache::isCacheable could end up mattering, but
even if that's not the case, I think the code is nicer this way.

         STYLE_STATS_ADD_MATCHED_PROPERTIES_HIT();
         // We can build up the style by copying non-inherited properties from an earlier style object built using the same exact
         // style declarations. We then only need to apply the inherited properties, if any, as their values can depend on the
         // element context. This is fast and saves memory by reusing the style data structures.
         state.style()->copyNonInheritedFrom(cachedMatchedProperties->renderStyle.get());
         if (state.parentStyle()->inheritedDataShared(cachedMatchedProperties->parentRenderStyle.get()) && !isAtShadowBoundary(element)) {
             STYLE_STATS_ADD_MATCHED_PROPERTIES_HIT_SHARED_INHERITED();
 
             EInsideLink linkStatus = state.style()->insideLink();
             // If the cache item parent style has identical inherited properties to the current parent style then the
@@ skipping 68 matching lines @@
 
     ASSERT(!state.fontBuilder().fontDirty());
 
 #ifdef STYLE_STATS
     if (!cachedMatchedProperties)
         STYLE_STATS_ADD_MATCHED_PROPERTIES_TO_CACHE();
 #endif
 
     if (cachedMatchedProperties || !cacheHash)
         return;
-    if (!MatchedPropertiesCache::isCacheable(state.element(), state.style(), state.parentStyle()))
+    if (!MatchedPropertiesCache::isCacheable(element, state.style(), state.parentStyle()))
         return;
     STYLE_STATS_ADD_MATCHED_PROPERTIES_ENTERED_INTO_CACHE();
     m_matchedPropertiesCache.add(state.style(), state.parentStyle(), cacheHash, matchResult);
 }
 
 CSSPropertyValue::CSSPropertyValue(CSSPropertyID id, const StylePropertySet& propertySet)
     : property(id), value(propertySet.getPropertyCSSValue(id).get())
 { }
 
 void StyleResolver::applyPropertiesToStyle(const CSSPropertyValue* properties, size_t count, RenderStyle* style)
@@ skipping 158 matching lines @@
         m_matchedPropertiesSearches, m_matchedPropertiesHit, m_matchedPropertiesSharedInheritedHit, m_matchedPropertiesToCache, m_matchedPropertiesEnteredIntoCache);
 
     fprintf(stderr, "Total:\n");
     printStyleStats(m_totalSearches, m_totalElementsEligibleForSharing, m_totalStylesShared, m_totalSearchFoundSiblingForSharing, m_totalSearchesMissedSharing,
         m_totalMatchedPropertiesSearches, m_totalMatchedPropertiesHit, m_totalMatchedPropertiesSharedInheritedHit, m_totalMatchedPropertiesToCache, m_totalMatchedPropertiesEnteredIntoCache);
     fprintf(stderr, "--------------------------------------------------------------------------------\n");
 }
 #endif
 
 } // namespace WebCore