Implement a Hit Test Cache.

Source/core/layout/HitTestResult.h

Index: Source/core/layout/HitTestResult.h
diff --git a/Source/core/layout/HitTestResult.h b/Source/core/layout/HitTestResult.h
index 01127a7ecd36ce969ce251bf51e202cc8a8e05e0..a65add09507a6928294b1f9b836c7c95c1538583 100644
--- a/Source/core/layout/HitTestResult.h
+++ b/Source/core/layout/HitTestResult.h
@@ -34,6 +34,7 @@
 #include "wtf/ListHashSet.h"
 #include "wtf/OwnPtr.h"
 #include "wtf/RefPtr.h"
+#include "wtf/VectorTraits.h"
 
 namespace blink {
 
@@ -49,7 +50,8 @@ class PositionWithAffinity;
 class Scrollbar;
 
 class CORE_EXPORT HitTestResult {
-    DISALLOW_ALLOCATION();
+    ALLOW_ONLY_INLINE_ALLOCATION();
+
 public:
     typedef WillBeHeapListHashSet<RefPtrWillBeMember<Node>> NodeSet;
 
@@ -63,6 +65,14 @@ public:
     HitTestResult& operator=(const HitTestResult&);
     DECLARE_TRACE();
 
+    bool equalForCacheability(const HitTestResult&) const;
+    void cacheValues(const HitTestResult&);
+
+    // Populate this object based on another HitTestResult; similar to assignment operator
+    // but don't assign any of the request parameters. ie. Thie method avoids setting
+    // |m_hitTestLocation|, |m_hitTestRequest|.
+    void populateFromCachedResult(const HitTestResult&);
+
     // For point-based hit tests, these accessors provide information about the node
     // under the point. For rect-based hit tests they are meaningless (reflect the
     // last candidate node observed in the rect).
@@ -124,6 +134,9 @@ public:
 
     bool isOverLink() const;
 
+    bool isCacheable() const { return m_cacheable; }
+    void setCacheable(bool cacheable) { m_cacheable = cacheable; }
+
     // Return true if the test is a list-based test and we should continue testing.
     bool addNodeToListBasedTestResult(Node*, const HitTestLocation& pointInContainer, const LayoutRect& = LayoutRect());
     bool addNodeToListBasedTestResult(Node*, const HitTestLocation& pointInContainer, const FloatRect&);
@@ -143,6 +156,7 @@ private:
 
     HitTestLocation m_hitTestLocation;
     HitTestRequest m_hitTestRequest;
+    bool m_cacheable;
 
     RefPtrWillBeMember<Node> m_innerNode;
     RefPtrWillBeMember<Node> m_innerPossiblyPseudoNode;
@@ -159,4 +173,16 @@ private:
 
 } // namespace blink
 
+#if ENABLE(OILPAN)
+// TODO(sof): the trait override/specialization is needed by HitTestCache's
+// HeapVector<> to handle unused slots. It is not correct for HitTestResult
+// in the wider sense of what canInitializeWithMemset provides, so fix this
+// by introducing a trait that encompasses "unused slot" handling and use it
+// here instead.
+//
+// Until that time, make this trait specialization conditional on OILPAN to
+// limit exposure.
+WTF_ALLOW_INIT_WITH_MEM_FUNCTIONS(blink::HitTestResult);
+#endif
+
 #endif // HitTestResult_h