Subtree caching implementation in blink-core

Source/core/paint/LayoutObjectSubtreeRecorder.h

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef LayoutObjectSubtreeRecorder_h
+#define LayoutObjectSubtreeRecorder_h
+
+#include "core/layout/LayoutObject.h"
+#include "core/paint/PaintPhase.h"
+#include "platform/graphics/paint/SubtreeRecorder.h"
+
+namespace blink {
+
+class GraphicsContext;
+
+// Convenience wrapper of SubtreeRecorder for LayoutObject painters.

[pdr., 2015/08/20 22:45:06]

Do we need subtree recorders for anything but layout objects? It would be nice
if we could combine these.

[Xianzhu, 2015/08/21 00:31:38]

Done.

Previously I moved SubtreeRecorder from core/paint to platform/graphics/paint
for testing subtree merging algorithm in DisplayItemListTest. Added
SimpleSubtreeRecorder in the test just to issue BeginSubtree/EndSubtree display
items so that the test no longer requires SubtreeRedorder.

+class LayoutObjectSubtreeRecorder final {
+public:
+    LayoutObjectSubtreeRecorder(GraphicsContext& context, const LayoutObject& layoutObject, PaintPhase phase)
+        : m_canUseCache(false)
+#if ENABLE(ASSERT)
+#endif
+    {
+        if (!RuntimeEnabledFeatures::slimmingPaintV2Enabled())
+            return;
+        if (!needsSubtreeRecorder(layoutObject))
+            return;
+
+        m_canUseCache = !layoutObject.shouldCheckForRepaint() && SubtreeRecorder::useCachedSubtreeIfPossible(context, layoutObject, phase);
+        if (!m_canUseCache)
+            m_subtreeRecorder.emplace(context, layoutObject, phase);
+    }
+
+    bool canUseCache() const { return m_canUseCache; }
+
+private:
+    // We need to balance the benefit of subtree optimization and the cost of subtree display items.
+    // Only output subtree information if the block has multiple children or multiple line boxes.
+    static bool needsSubtreeRecorder(const LayoutObject& layoutObject)
+    {
+        return (layoutObject.slowFirstChild() && layoutObject.slowFirstChild()->nextSibling())
+            || (layoutObject.isLayoutBlockFlow() && toLayoutBlockFlow(layoutObject).firstLineBox() && toLayoutBlockFlow(layoutObject).firstLineBox()->nextLineBox());
+    }
+
+    Optional<SubtreeRecorder> m_subtreeRecorder;
+    bool m_canUseCache;

[pdr., 2015/08/20 22:45:06]

I think we can reuse the pointer in m_subtreeRecorder instead of needing this
extra boolean.

[Xianzhu, 2015/08/21 00:31:38]

Done.

+#if ENABLE(ASSERT)

[pdr., 2015/08/20 22:45:06]

Oops

[Xianzhu, 2015/08/21 00:31:38]

Done.

+
+#endif
+};
+
+} // namespace blink
+
+#endif // LayoutObjectSubtreeRecorder_h