Implement a Hit Test Cache.

Source/core/layout/LayoutView.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2006 Apple Computer, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  *
  */
 
 #ifndef LayoutView_h
 #define LayoutView_h
 
 #include "core/CoreExport.h"
 #include "core/dom/Position.h"
 #include "core/frame/FrameView.h"
+#include "core/layout/HitTestCache.h"
+#include "core/layout/HitTestResult.h"
 #include "core/layout/LayoutBlockFlow.h"
 #include "core/layout/LayoutState.h"
 #include "core/layout/PaintInvalidationState.h"
 #include "core/layout/PendingSelection.h"
 #include "platform/PODFreeListArena.h"
 #include "platform/RuntimeEnabledFeatures.h"
 #include "platform/heap/Handle.h"
 #include "platform/scroll/ScrollableArea.h"
 #include "wtf/OwnPtr.h"
 
 namespace blink {
 
 class DeprecatedPaintLayerCompositor;
 class LayoutQuote;
 
 // The root of the layout tree, corresponding to the CSS initial containing block.
 // It's dimensions match that of the logical viewport (which may be different from
 // the visible viewport in fixed-layout mode), and it is always at position (0,0)
 // relative to the document (and so isn't necessarily in view).
 class CORE_EXPORT LayoutView final : public LayoutBlockFlow {
 public:
     explicit LayoutView(Document*);
     virtual ~LayoutView();
     void willBeDestroyed() override;
 
     bool hitTest(HitTestResult&);
-    bool hitTest(const HitTestRequest&, const HitTestLocation&, HitTestResult&);
 
     // Returns the total count of calls to HitTest, for testing.
     unsigned hitTestCount() const { return m_hitTestCount; }
+    unsigned hitTestCacheHits() const { return m_hitTestCacheHits; }
+
+    void clearHitTestCache() { m_hitTestCache.clear(); }
 
     virtual const char* name() const override { return "LayoutView"; }
 
     virtual bool isOfType(LayoutObjectType type) const override { return type == LayoutObjectLayoutView || LayoutBlockFlow::isOfType(type); }
 
     virtual DeprecatedPaintLayerType layerTypeRequired() const override { return NormalDeprecatedPaintLayer; }
 
     virtual bool isChildAllowed(LayoutObject*, const ComputedStyle&) const override;
 
     virtual void layout() override;
@@ skipping 136 matching lines @@
     LayoutUnit m_pageLogicalHeight;
     bool m_pageLogicalHeightChanged;
     LayoutState* m_layoutState;
     OwnPtr<DeprecatedPaintLayerCompositor> m_compositor;
     RefPtr<IntervalArena> m_intervalArena;
 
     LayoutQuote* m_layoutQuoteHead;
     unsigned m_layoutCounterCount;
 
     unsigned m_hitTestCount;
+    unsigned m_hitTestCacheHits;
+    HitTestCache m_hitTestCache;

[sof, 2015/06/09 18:59:18]

Oilpan-wise, to make this work on LayoutView, it needs to be handled&represented
like PendingSelection below. i.e., HitTestCache needs to be on the heap. (If
LayoutView had been on the heap itself, keeping it as a part object would have
been fine.)

[dtapuska, 2015/06/09 19:39:15]

In the posted version:
The pointers inside the HitTestResult are in fact raw ptrs which are really weak
because and are checked based on the dom tree version; if it doesn't match they
aren't used. So stale pointers won't get used.

Does turning HitTestCache into a GarbageCollected object imply that I need to
turn HitTestResult into a GarbageCollected object as well; that way I could
trace them and have the WeakMembers inside that class? 

If HitTestResult is created on the stack in a few places.. does it imply that I
need to turn all those stack allocations into heap allocations too?

 
     OwnPtrWillBePersistent<PendingSelection> m_pendingSelection;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutView, isLayoutView());
 
 // Suspends the LayoutState cached offset and clipRect optimization. Used under transforms
 // that cannot be represented by LayoutState (common in SVG) and when manipulating the layout
 // tree during layout in ways that can trigger paint invalidation of a non-child (e.g. when a list item
 // moves its list marker around). Note that even when disabled, LayoutState is still used to
@@ skipping 15 matching lines @@
             m_paintInvalidationState->m_cachedOffsetsEnabled = true;
     }
 private:
     const PaintInvalidationState* m_paintInvalidationState;
     bool m_didDisable;
 };
 
 } // namespace blink
 
 #endif // LayoutView_h