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diff --git a/Source/WebCore/style/StyleSharingResolver.cpp b/Source/WebCore/style/StyleSharingResolver.cpp
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+++ b/Source/WebCore/style/StyleSharingResolver.cpp
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+/*
+ * Copyright (C) 2016 Apple Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
+ * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "config.h"
+#include "StyleSharingResolver.h"
+
+#include "DocumentRuleSets.h"
+#include "ElementRuleCollector.h"
+#include "HTMLInputElement.h"
+#include "HTMLNames.h"
+#include "NodeRenderStyle.h"
+#include "RenderStyle.h"
+#include "SVGElement.h"
+#include "ShadowRoot.h"
+#include "StyleScope.h"
+#include "StyleUpdate.h"
+#include "StyledElement.h"
+#include "VisitedLinkState.h"
+#include "WebVTTElement.h"
+#include "XMLNames.h"
+
+namespace WebCore {
+namespace Style {
+
+static const unsigned cStyleSearchThreshold = 10;
+
+struct SharingResolver::Context {
+    const Update& update;
+    const StyledElement& element;
+    bool elementAffectedByClassRules;
+    EInsideLink elementLinkState;
+};
+
+SharingResolver::SharingResolver(const Document& document, const DocumentRuleSets& ruleSets, const SelectorFilter& selectorFilter)
+    : m_document(document)
+    , m_ruleSets(ruleSets)
+    , m_selectorFilter(selectorFilter)
+{
+}
+
+static inline bool parentElementPreventsSharing(const Element& parentElement)
+{
+    return parentElement.hasFlagsSetDuringStylingOfChildren();
+}
+
+static inline bool elementHasDirectionAuto(const Element& element)
+{
+    // FIXME: This line is surprisingly hot, we may wish to inline hasDirectionAuto into StyleResolver.
+    return is<HTMLElement>(element) && downcast<HTMLElement>(element).hasDirectionAuto();
+}
+
+std::unique_ptr<RenderStyle> SharingResolver::resolve(const Element& searchElement, const Update& update)
+{
+    if (!is<StyledElement>(searchElement))
+        return nullptr;
+    auto& element = downcast<StyledElement>(searchElement);
+    if (!element.parentElement())
+        return nullptr;
+    auto& parentElement = *element.parentElement();
+    if (parentElement.shadowRoot())
+        return nullptr;
+    if (!update.elementStyle(parentElement))
+        return nullptr;
+    // If the element has inline style it is probably unique.
+    if (element.inlineStyle())
+        return nullptr;
+    if (element.isSVGElement() && downcast<SVGElement>(element).animatedSMILStyleProperties())
+        return nullptr;
+    // Ids stop style sharing if they show up in the stylesheets.
+    auto& id = element.idForStyleResolution();
+    if (!id.isNull() && m_ruleSets.features().idsInRules.contains(id))
+        return nullptr;
+    if (parentElementPreventsSharing(parentElement))
+        return nullptr;
+    if (&element == m_document.cssTarget())
+        return nullptr;
+    if (elementHasDirectionAuto(element))
+        return nullptr;
+    if (element.shadowRoot() && !element.shadowRoot()->styleScope().resolver().ruleSets().authorStyle().hostPseudoClassRules().isEmpty())
+        return nullptr;
+
+    Context context {
+        update,
+        element,
+        element.hasClass() && classNamesAffectedByRules(element.classNames()),
+        m_document.visitedLinkState().determineLinkState(element)
+    };
+
+    // Check previous siblings and their cousins.
+    unsigned count = 0;
+    StyledElement* shareElement = nullptr;
+    Node* cousinList = element.previousSibling();
+    while (cousinList) {
+        shareElement = findSibling(context, cousinList, count);
+        if (shareElement)
+            break;
+        if (count >= cStyleSearchThreshold)
+            break;
+        cousinList = locateCousinList(cousinList->parentElement());
+    }
+
+    // If we have exhausted all our budget or our cousins.
+    if (!shareElement)
+        return nullptr;
+
+    // Can't share if sibling rules apply. This is checked at the end as it should rarely fail.
+    if (styleSharingCandidateMatchesRuleSet(element, m_ruleSets.sibling()))
+        return nullptr;
+    // Can't share if attribute rules apply.
+    if (styleSharingCandidateMatchesRuleSet(element, m_ruleSets.uncommonAttribute()))
+        return nullptr;
+    // Tracking child index requires unique style for each node. This may get set by the sibling rule match above.
+    if (parentElementPreventsSharing(parentElement))
+        return nullptr;
+
+    m_elementsSharingStyle.add(&element, shareElement);
+
+    return RenderStyle::clonePtr(*update.elementStyle(*shareElement));
+}
+
+StyledElement* SharingResolver::findSibling(const Context& context, Node* node, unsigned& count) const
+{
+    for (; node; node = node->previousSibling()) {
+        if (!is<StyledElement>(*node))
+            continue;
+        if (canShareStyleWithElement(context, downcast<StyledElement>(*node)))
+            break;
+        if (count++ >= cStyleSearchThreshold)
+            return nullptr;
+    }
+    return downcast<StyledElement>(node);
+}
+
+Node* SharingResolver::locateCousinList(const Element* parent) const
+{
+    for (unsigned count = 0; count < cStyleSearchThreshold; ++count) {
+        auto* elementSharingParentStyle = m_elementsSharingStyle.get(parent);
+        if (!elementSharingParentStyle)
+            return nullptr;
+        if (!parentElementPreventsSharing(*elementSharingParentStyle)) {
+            if (auto* cousin = elementSharingParentStyle->lastChild())
+                return cousin;
+        }
+        parent = elementSharingParentStyle;
+    }
+
+    return nullptr;
+}
+
+static bool canShareStyleWithControl(const HTMLFormControlElement& element, const HTMLFormControlElement& formElement)
+{
+    if (!is<HTMLInputElement>(formElement) || !is<HTMLInputElement>(element))
+        return false;
+
+    auto& thisInputElement = downcast<HTMLInputElement>(formElement);
+    auto& otherInputElement = downcast<HTMLInputElement>(element);
+
+    if (thisInputElement.isAutoFilled() != otherInputElement.isAutoFilled())
+        return false;
+    if (thisInputElement.shouldAppearChecked() != otherInputElement.shouldAppearChecked())
+        return false;
+    if (thisInputElement.isRequired() != otherInputElement.isRequired())
+        return false;
+
+    if (formElement.isDisabledFormControl() != element.isDisabledFormControl())
+        return false;
+
+    if (formElement.isInRange() != element.isInRange())
+        return false;
+
+    if (formElement.isOutOfRange() != element.isOutOfRange())
+        return false;
+
+    return true;
+}
+
+bool SharingResolver::canShareStyleWithElement(const Context& context, const StyledElement& candidateElement) const
+{
+    auto& element = context.element;
+    auto* style = context.update.elementStyle(candidateElement);
+    if (!style)
+        return false;
+    if (style->unique())
+        return false;
+    if (style->hasUniquePseudoStyle())
+        return false;
+    if (candidateElement.tagQName() != element.tagQName())
+        return false;
+    if (candidateElement.inlineStyle())
+        return false;
+    if (candidateElement.needsStyleRecalc())
+        return false;
+    if (candidateElement.isSVGElement() && downcast<SVGElement>(candidateElement).animatedSMILStyleProperties())
+        return false;
+    if (candidateElement.isLink() != element.isLink())
+        return false;
+    if (candidateElement.hovered() != element.hovered())
+        return false;
+    if (candidateElement.active() != element.active())
+        return false;
+    if (candidateElement.focused() != element.focused())
+        return false;
+    if (candidateElement.shadowPseudoId() != element.shadowPseudoId())
+        return false;
+    if (&candidateElement == m_document.cssTarget())
+        return false;
+    if (!sharingCandidateHasIdenticalStyleAffectingAttributes(context, candidateElement))
+        return false;
+    if (const_cast<StyledElement&>(candidateElement).additionalPresentationAttributeStyle() != const_cast<StyledElement&>(element).additionalPresentationAttributeStyle())
+        return false;
+    if (candidateElement.affectsNextSiblingElementStyle() || candidateElement.styleIsAffectedByPreviousSibling())
+        return false;
+    if (candidateElement.styleAffectedByFocusWithin() || element.styleAffectedByFocusWithin())
+        return false;
+
+    auto& candidateElementId = candidateElement.idForStyleResolution();
+    if (!candidateElementId.isNull() && m_ruleSets.features().idsInRules.contains(candidateElementId))
+        return false;
+
+    bool isControl = is<HTMLFormControlElement>(candidateElement);
+
+    if (isControl != is<HTMLFormControlElement>(element))
+        return false;
+
+    if (isControl && !canShareStyleWithControl(downcast<HTMLFormControlElement>(element), downcast<HTMLFormControlElement>(candidateElement)))
+        return false;
+
+    if (style->transitions() || style->animations())
+        return false;
+
+    // Turn off style sharing for elements that can gain layers for reasons outside of the style system.
+    // See comments in RenderObject::setStyle().
+    if (candidateElement.hasTagName(HTMLNames::iframeTag) || candidateElement.hasTagName(HTMLNames::frameTag))
+        return false;
+
+    if (candidateElement.hasTagName(HTMLNames::embedTag) || candidateElement.hasTagName(HTMLNames::objectTag) || candidateElement.hasTagName(HTMLNames::appletTag) || candidateElement.hasTagName(HTMLNames::canvasTag))
+        return false;
+
+    if (elementHasDirectionAuto(candidateElement))
+        return false;
+
+    if (candidateElement.isLink() && context.elementLinkState != style->insideLink())
+        return false;
+
+    if (candidateElement.elementData() != element.elementData()) {
+        if (candidateElement.attributeWithoutSynchronization(HTMLNames::readonlyAttr) != element.attributeWithoutSynchronization(HTMLNames::readonlyAttr))
+            return false;
+        if (candidateElement.isSVGElement()) {
+            if (candidateElement.getAttribute(HTMLNames::typeAttr) != element.getAttribute(HTMLNames::typeAttr))
+                return false;
+        } else {
+            if (candidateElement.attributeWithoutSynchronization(HTMLNames::typeAttr) != element.attributeWithoutSynchronization(HTMLNames::typeAttr))
+                return false;
+        }
+    }
+
+    if (candidateElement.matchesValidPseudoClass() != element.matchesValidPseudoClass())
+        return false;
+
+    if (element.matchesInvalidPseudoClass() != element.matchesValidPseudoClass())
+        return false;
+
+    if (candidateElement.matchesIndeterminatePseudoClass() != element.matchesIndeterminatePseudoClass())
+        return false;
+
+    if (candidateElement.matchesDefaultPseudoClass() != element.matchesDefaultPseudoClass())
+        return false;
+
+    if (candidateElement.shadowRoot() && !candidateElement.shadowRoot()->styleScope().resolver().ruleSets().authorStyle().hostPseudoClassRules().isEmpty())
+        return false;
+
+#if ENABLE(FULLSCREEN_API)
+    if (&element == m_document.webkitCurrentFullScreenElement() || &element == m_document.webkitCurrentFullScreenElement())
+        return false;
+#endif
+    return true;
+}
+
+bool SharingResolver::styleSharingCandidateMatchesRuleSet(const StyledElement& element, const RuleSet* ruleSet) const
+{
+    if (!ruleSet)
+        return false;
+
+    ElementRuleCollector collector(const_cast<StyledElement&>(element), m_ruleSets, &m_selectorFilter);
+    return collector.hasAnyMatchingRules(ruleSet);
+}
+
+bool SharingResolver::sharingCandidateHasIdenticalStyleAffectingAttributes(const Context& context, const StyledElement& sharingCandidate) const
+{
+    auto& element = context.element;
+    if (element.elementData() == sharingCandidate.elementData())
+        return true;
+    if (element.attributeWithoutSynchronization(XMLNames::langAttr) != sharingCandidate.attributeWithoutSynchronization(XMLNames::langAttr))
+        return false;
+    if (element.attributeWithoutSynchronization(HTMLNames::langAttr) != sharingCandidate.attributeWithoutSynchronization(HTMLNames::langAttr))
+        return false;
+
+    if (context.elementAffectedByClassRules) {
+        if (!sharingCandidate.hasClass())
+            return false;
+        // SVG elements require a (slow!) getAttribute comparision because "class" is an animatable attribute for SVG.
+        if (element.isSVGElement()) {
+            if (element.getAttribute(HTMLNames::classAttr) != sharingCandidate.getAttribute(HTMLNames::classAttr))
+                return false;
+        } else {
+            if (element.classNames() != sharingCandidate.classNames())
+                return false;
+        }
+    } else if (sharingCandidate.hasClass() && classNamesAffectedByRules(sharingCandidate.classNames()))
+        return false;
+
+    if (const_cast<StyledElement&>(element).presentationAttributeStyle() != const_cast<StyledElement&>(sharingCandidate).presentationAttributeStyle())
+        return false;
+
+    return true;
+}
+
+bool SharingResolver::classNamesAffectedByRules(const SpaceSplitString& classNames) const
+{
+    for (unsigned i = 0; i < classNames.size(); ++i) {
+        if (m_ruleSets.features().classesInRules.contains(classNames[i]))
+            return true;
+    }
+    return false;
+}
+
+
+}
+}