Enable HitTestCache by default.

Source/core/layout/LayoutView.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  *
  * 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,
@@ skipping 84 matching lines @@
 {
     TRACE_EVENT_BEGIN0("blink,devtools.timeline", "HitTest");
     m_hitTestCount++;
 
     ASSERT(!result.hitTestLocation().isRectBasedTest() || result.hitTestRequest().listBased());
 
     commitPendingSelection();
 
     uint64_t domTreeVersion = document().domTreeVersion();
     HitTestResult cacheResult = result;
-    bool cacheHit = m_hitTestCache->lookupCachedResult(cacheResult, domTreeVersion);
-    bool hitLayer = layer()->hitTest(result);
+    bool hitLayer = false;
+    if (m_hitTestCache->lookupCachedResult(cacheResult, domTreeVersion)) {
+        m_hitTestCacheHits++;
+        hitLayer = true;
+#if ENABLE(ASSERT)
+        m_hitTestCache->verifyCachedResult(result, cacheResult);
+#endif
+        result = cacheResult;
+    } else {
+        hitLayer = layer()->hitTest(result);
 
-    // FrameView scrollbars are not the same as Layer scrollbars tested by Layer::hitTestOverflowControls,
-    // so we need to test FrameView scrollbars separately here. Note that it's important we do this after
-    // the hit test above, because that may overwrite the entire HitTestResult when it finds a hit.
-    IntPoint framePoint = frameView()->contentsToFrame(result.hitTestLocation().roundedPoint());
-    if (Scrollbar* frameScrollbar = frameView()->scrollbarAtFramePoint(framePoint))
-        result.setScrollbar(frameScrollbar);
+        // FrameView scrollbars are not the same as Layer scrollbars tested by Layer::hitTestOverflowControls,
+        // so we need to test FrameView scrollbars separately here. Note that it's important we do this after
+        // the hit test above, because that may overwrite the entire HitTestResult when it finds a hit.
+        IntPoint framePoint = frameView()->contentsToFrame(result.hitTestLocation().roundedPoint());
+        if (Scrollbar* frameScrollbar = frameView()->scrollbarAtFramePoint(framePoint))
+            result.setScrollbar(frameScrollbar);
 
-    if (cacheHit) {
-        m_hitTestCacheHits++;
-        m_hitTestCache->verifyCachedResult(result, cacheResult);

[dtapuska, 2015/08/17 21:23:14]

verifyCacheResult logs some UMA metrics. rbyers@ indicated that this would lead
them to be orphaned. I do recognize this; although I want to keep them around as
I would like to try the rect based hit test cache next and I would use them in
that code path. (Although this code flow would need to be modified again; but
I'll cross that road later).

[esprehn, 2015/08/18 20:27:41]

Please delete the code for now and add it back later. Having UMA behind an
ENABLE(ASSERT) doesn't make sense.

[dtapuska, 2015/08/18 20:56:34]

Done.

+        if (hitLayer)
+            m_hitTestCache->addCachedResult(result, domTreeVersion);
     }
 
-    if (hitLayer)
-        m_hitTestCache->addCachedResult(result, domTreeVersion);
-
     TRACE_EVENT_END1("blink,devtools.timeline", "HitTest", "endData", InspectorHitTestEvent::endData(result.hitTestRequest(), result.hitTestLocation(), result));
     return hitLayer;
 }
 
 void LayoutView::clearHitTestCache()
 {
     m_hitTestCache->clear();
     if (LayoutPart* frameLayoutObject = frame()->ownerLayoutObject())
         frameLayoutObject->view()->clearHitTestCache();
 }
@@ skipping 872 matching lines @@
     return viewHeight(IncludeScrollbars) / scale;
 }
 
 void LayoutView::willBeDestroyed()
 {
     LayoutBlockFlow::willBeDestroyed();
     m_compositor.clear();
 }
 
 } // namespace blink