Don't cache matched properties for elements without a flat-tree parent.

third_party/WebKit/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 1515 matching lines @@
 {
     const Element* element = state.element();
     ASSERT(element);
 
     INCREMENT_STYLE_STATS_COUNTER(document().styleEngine(), matchedPropertyApply, 1);
 
     unsigned cacheHash = RuntimeEnabledFeatures::styleMatchedPropertiesCacheEnabled() && matchResult.isCacheable() ? computeMatchedPropertiesHash(matchResult.matchedProperties().data(), matchResult.matchedProperties().size()) : 0;
     bool applyInheritedOnly = false;
     const CachedMatchedProperties* cachedMatchedProperties = cacheHash ? m_matchedPropertiesCache.find(cacheHash, state, matchResult.matchedProperties()) : nullptr;
 
-    if (cachedMatchedProperties && MatchedPropertiesCache::isCacheable(*state.style(), *state.parentStyle())) {
+    if (cachedMatchedProperties && MatchedPropertiesCache::isCacheable(*state.style(), *state.parentStyle(), state.parentNode())) {

[hayato, 2016/08/30 02:01:05]

Could you explain briefly what parentStyle means if flatTreeParent is null? It
is never used?

[esprehn, 2016/08/30 21:37:10]

Can we just pass State into isCacheable? Passing all these members is kind of
silly

         INCREMENT_STYLE_STATS_COUNTER(document().styleEngine(), matchedPropertyCacheHit, 1);
         // 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()->copyNonInheritedFromCached(*cachedMatchedProperties->computedStyle);
         if (state.parentStyle()->inheritedDataShared(*cachedMatchedProperties->parentComputedStyle) && !isAtShadowBoundary(element)
             && (!state.distributedToInsertionPoint() || state.style()->userModify() == READ_ONLY)) {
             INCREMENT_STYLE_STATS_COUNTER(document().styleEngine(), matchedPropertyCacheInheritedHit, 1);
 
             EInsideLink linkStatus = state.style()->insideLink();
@@ skipping 73 matching lines @@
         // Check whether the final border and background differs from the cached UA ones.
         // When there is a partial match in the MatchedPropertiesCache, these flags will already be set correctly
         // and the value stored in cacheUserAgentBorderAndBackground is incorrect, so doing this check again
         // would give the wrong answer.
         state.style()->setHasAuthorBackground(hasAuthorBackground(state));
         state.style()->setHasAuthorBorder(hasAuthorBorder(state));
     }
 
     loadPendingResources(state);
 
-    if (!cachedMatchedProperties && cacheHash && MatchedPropertiesCache::isCacheable(*state.style(), *state.parentStyle())) {
+    if (!cachedMatchedProperties && cacheHash && MatchedPropertiesCache::isCacheable(*state.style(), *state.parentStyle(), state.parentNode())) {
         ASSERT(RuntimeEnabledFeatures::styleMatchedPropertiesCacheEnabled());
         INCREMENT_STYLE_STATS_COUNTER(document().styleEngine(), matchedPropertyCacheAdded, 1);
         m_matchedPropertiesCache.add(*state.style(), *state.parentStyle(), cacheHash, matchResult.matchedProperties());
     }
 
     ASSERT(!state.fontBuilder().fontDirty());
 }
 
 bool StyleResolver::hasAuthorBackground(const StyleResolverState& state)
 {
@@ skipping 103 matching lines @@
     visitor->trace(m_siblingRuleSet);
     visitor->trace(m_uncommonAttributeRuleSet);
     visitor->trace(m_watchedSelectorsRules);
     visitor->trace(m_treeBoundaryCrossingScopes);
     visitor->trace(m_styleSharingLists);
     visitor->trace(m_pendingStyleSheets);
     visitor->trace(m_document);
 }
 
 } // namespace blink