Simplify subtree (now subsequence) caching

Source/platform/graphics/paint/SubtreeRecorder.cpp

 // 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.
 
 #include "config.h"
 #include "platform/graphics/paint/SubtreeRecorder.h"
 
 #include "platform/RuntimeEnabledFeatures.h"
 #include "platform/graphics/GraphicsContext.h"
 #include "platform/graphics/paint/CachedDisplayItem.h"
 #include "platform/graphics/paint/DisplayItemList.h"
 #include "platform/graphics/paint/SubtreeDisplayItem.h"
 
 namespace blink {
 
-SubtreeRecorder::SubtreeRecorder(GraphicsContext& context, const DisplayItemClientWrapper& client, int paintPhase)
+bool SubtreeRecorder::useCachedSubtreeIfPossible(GraphicsContext& context, const DisplayItemClientWrapper& client)
+{
+    // The conditions of returning false is not complete for a specific client.
+    // The client must check additional conditions before calling this function.
+
+    ASSERT(RuntimeEnabledFeatures::slimmingPaintV2Enabled());
+    ASSERT(context.displayItemList());
+
+    if (context.displayItemList()->displayItemConstructionIsDisabled() || RuntimeEnabledFeatures::slimmingPaintUnderInvalidationCheckingEnabled())
+        return false;
+
+    if (!context.displayItemList()->clientCacheIsValid(client.displayItemClient()))
+        return false;
+
+    context.displayItemList()->createAndAppend<CachedDisplayItem>(client, DisplayItem::CachedSubtree);
+    return true;
+}
+
+SubtreeRecorder::SubtreeRecorder(GraphicsContext& context, const DisplayItemClientWrapper& client)
     : m_displayItemList(context.displayItemList())
     , m_client(client)
-    , m_paintPhase(paintPhase)
-    , m_canUseCache(false)
-#if ENABLE(ASSERT)
-    , m_checkedCanUseCache(false)
-#endif
 {
-    if (!RuntimeEnabledFeatures::slimmingPaintV2Enabled())
-        return;
-
+    ASSERT(RuntimeEnabledFeatures::slimmingPaintV2Enabled());
     ASSERT(m_displayItemList);
-
-    // TODO(wangxianzhu): Implement subtree caching.
-
-    if (!m_canUseCache)
-        m_displayItemList->createAndAppend<BeginSubtreeDisplayItem>(m_client, DisplayItem::paintPhaseToBeginSubtreeType(paintPhase));
+    m_displayItemList->createAndAppend<BeginSubtreeDisplayItem>(m_client);
 }
 
 SubtreeRecorder::~SubtreeRecorder()
 {
-    if (!RuntimeEnabledFeatures::slimmingPaintV2Enabled())
-        return;
-
-    ASSERT(m_checkedCanUseCache);
-    if (m_canUseCache)
-        m_displayItemList->createAndAppend<CachedDisplayItem>(m_client, DisplayItem::paintPhaseToCachedSubtreeType(m_paintPhase));
-    else if (m_displayItemList->lastDisplayItemIsNoopBegin())

[chrishtr, 2015/08/25 23:48:19]

The no-op optimization is no longer useful?

[Xianzhu, 2015/08/26 23:21:06]

Right. The situation is similar to that of empty drawing display items:
- Empty subtrees will seldom happen when we let DPL issue them;
- CachedSubtrees without previously cached empty subtrees will break
synchronized matching of the merge algorithm.

-        m_displayItemList->removeLastDisplayItem();
-    else
-        m_displayItemList->createAndAppend<EndSubtreeDisplayItem>(m_client, DisplayItem::paintPhaseToEndSubtreeType(m_paintPhase));
-}
-
-bool SubtreeRecorder::canUseCache() const
-{
-#if ENABLE(ASSERT)
-    m_checkedCanUseCache = true;
-#endif
-    return m_canUseCache;
+    ASSERT(RuntimeEnabledFeatures::slimmingPaintV2Enabled());
+    m_displayItemList->createAndAppend<EndSubtreeDisplayItem>(m_client);
 }
 
 } // namespace blink