Client side display item cache flag

third_party/WebKit/Source/platform/graphics/paint/DisplayItemClient.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 DisplayItemClient_h
 #define DisplayItemClient_h
 
 #include "platform/PlatformExport.h"
 #include "platform/geometry/LayoutRect.h"
 #include "wtf/text/WTFString.h"
 
 namespace blink {
 
+// Holds a unique cache generation id of display items and paint controllers.
+//
+// A paint controller sets its cache generation to DisplayItemCacheGeneration::next()
+// at the end of each commitNewDisplayItems, and updates the cache generation of each
+// client with cached drawings by calling DisplayItemClient::setDisplayItemsCached().
+// A display item is treated as validly cached in a paint controller if its cache generation
+// matches the paint controller's cache generation.
+//
+// SPv1 only: If a display item is painted on multiple paint controllers, because cache
+// generations are unique, the client's cache generation matches the last paint controller
+// only. The client will be treated as invalid on other paint controllers regardless if
+// it's validly cached by these paint controllers. The situation is very rare (about 0.07%
+// clients were painted on multiple paint controllers) so the performance penalty is trivial.
+class PLATFORM_EXPORT DisplayItemCacheGeneration {
+    DISALLOW_NEW();
+public:
+    DisplayItemCacheGeneration() : m_value(kInvalidGeneration) { }
+
+    void invalidate() { m_value = kInvalidGeneration; }
+    static DisplayItemCacheGeneration next() { return DisplayItemCacheGeneration(s_nextGeneration++); }
+    bool matches(const DisplayItemCacheGeneration& other)
+    {
+        return m_value != kInvalidGeneration && other.m_value != kInvalidGeneration && m_value == other.m_value;
+    }
+
+private:
+    typedef uint32_t Generation;
+    DisplayItemCacheGeneration(Generation value) : m_value(value) { }
+
+    static const Generation kInvalidGeneration = 0;
+    static Generation s_nextGeneration;
+    Generation m_value;
+};
+
 // The interface for objects that can be associated with display items.
 // A DisplayItemClient object should live at least longer than the document cycle
 // in which its display items are created during painting.
 // After the document cycle, a pointer/reference to DisplayItemClient should be
 // no longer dereferenced unless we can make sure the client is still valid.
 class PLATFORM_EXPORT DisplayItemClient {
 public:
 #if ENABLE(ASSERT)
     DisplayItemClient();
     virtual ~DisplayItemClient();
 #else
     virtual ~DisplayItemClient() { }
 #endif
 
     virtual String debugName() const = 0;
 
     // The visual rect of this DisplayItemClient, in object space of the object that owns the GraphicsLayer, i.e.
     // offset by offsetFromLayoutObjectWithSubpixelAccumulation().
     virtual LayoutRect visualRect() const = 0;
 
+    // The default implementation is uncacheable.
+    // Use DISPLAY_ITEM_CACHE_STATUS_IMPLEMENTATION for cacheable implementation.

[chrishtr, 2016/04/29 15:55:17]

I think it would be better to do the opposite, so we don't accidentally omit
caching where
not intended.

[Xianzhu, 2016/04/29 16:37:11]

Agreed. Done.

+    virtual bool displayItemsAreCached(DisplayItemCacheGeneration) const { return false; }
+    virtual void setDisplayItemsCached(DisplayItemCacheGeneration) const { }
+    virtual void setDisplayItemsUncached() const { }
+
 #if ENABLE(ASSERT)
     // Tests if a DisplayItemClient object has been created and has not been deleted yet.
     static bool isAlive(const DisplayItemClient&);
 #endif
+
+protected:
 };
 
+#define DISPLAY_ITEM_CACHE_STATUS_IMPLEMENTATION \
+    bool displayItemsAreCached(DisplayItemCacheGeneration cacheGeneration) const final { return m_cacheGeneration.matches(cacheGeneration); } \
+    void setDisplayItemsCached(DisplayItemCacheGeneration cacheGeneration) const final { m_cacheGeneration = cacheGeneration; } \
+    void setDisplayItemsUncached() const final { m_cacheGeneration.invalidate(); } \
+    mutable DisplayItemCacheGeneration m_cacheGeneration;
+
 inline bool operator==(const DisplayItemClient& client1, const DisplayItemClient& client2) { return &client1 == &client2; }
 inline bool operator!=(const DisplayItemClient& client1, const DisplayItemClient& client2) { return &client1 != &client2; }
 
 } // namespace blink
 
 #endif // DisplayItemClient_h