Client side display item cache flag

third_party/WebKit/Source/platform/graphics/paint/DisplayItemClient.h

Index: third_party/WebKit/Source/platform/graphics/paint/DisplayItemClient.h
diff --git a/third_party/WebKit/Source/platform/graphics/paint/DisplayItemClient.h b/third_party/WebKit/Source/platform/graphics/paint/DisplayItemClient.h
index 784aa638560e0a48fbaf3b4cdf80129dc29ac672..9aeabff11990e723286fc9e40c99ddf6b060976d 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/DisplayItemClient.h
+++ b/third_party/WebKit/Source/platform/graphics/paint/DisplayItemClient.h
@@ -11,6 +11,40 @@
 
 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.
@@ -31,12 +65,29 @@ public:
     // offset by offsetFromLayoutObjectWithSubpixelAccumulation().
     virtual LayoutRect visualRect() const = 0;
 
+    virtual bool displayItemsAreCached(DisplayItemCacheGeneration) const = 0;

[chrishtr, 2016/04/29 16:51:15]

Just move the definitions from DISPLAY_ITEM_CACHE_STATUS_IMPLEMENTATION here?

[Xianzhu, 2016/04/29 17:17:30]

That would make DisplayItemClient no longer a pure interface which may violate
style guide about multiple inheritance (though I'm not sure the real problems,
perhaps pointer offset when cast?).

Another problem is that the 32-bit data field may cause a 32-bit hole in the
derived class because of alignment. With the macro, the subclass can put the
macro at a proper place to avoid the hole. In this Cl I put the macro at the end
of the classes.

+    virtual void setDisplayItemsCached(DisplayItemCacheGeneration) const = 0;
+    virtual void setDisplayItemsUncached() const = 0;
+
 #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;
+
+#define DISPLAY_ITEM_CACHE_STATUS_UNCACHEABLE_IMPLEMENTATION \
+    bool displayItemsAreCached(DisplayItemCacheGeneration) const final { return false; } \
+    void setDisplayItemsCached(DisplayItemCacheGeneration) const final { } \
+    void setDisplayItemsUncached() const final { }
+
 inline bool operator==(const DisplayItemClient& client1, const DisplayItemClient& client2) { return &client1 == &client2; }
 inline bool operator!=(const DisplayItemClient& client1, const DisplayItemClient& client2) { return &client1 != &client2; }