Calculate and track display item opaqueness

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

 // Copyright 2014 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 DrawingDisplayItem_h
 #define DrawingDisplayItem_h
 
 #include "platform/PlatformExport.h"
+#include "platform/RuntimeEnabledFeatures.h"
 #include "platform/geometry/FloatPoint.h"
 #include "platform/graphics/paint/DisplayItem.h"
 #include "third_party/skia/include/core/SkPicture.h"
 #include "wtf/PassOwnPtr.h"
 
 namespace blink {
 
 class PLATFORM_EXPORT DrawingDisplayItem final : public DisplayItem {
 public:
 #if ENABLE(ASSERT)
     enum UnderInvalidationCheckingMode {
         CheckPicture, // Check if the new picture and the old picture are the same
         CheckBitmap, // Check if the new picture and the old picture produce the same bitmap
     };
 #endif
 
     DrawingDisplayItem(const DisplayItemClient& client
         , Type type
         , PassRefPtr<const SkPicture> picture
+        , bool knownToBeOpaque = false
 #if ENABLE(ASSERT)
         , UnderInvalidationCheckingMode underInvalidationCheckingMode = CheckPicture
 #endif
         )
         : DisplayItem(client, type, sizeof(*this))
         , m_picture(picture && picture->approximateOpCount() ? picture : nullptr)
+        , m_knownToBeOpaque(knownToBeOpaque)
 #if ENABLE(ASSERT)
         , m_underInvalidationCheckingMode(underInvalidationCheckingMode)
 #endif
     {
         ASSERT(isDrawingType(type));
     }
 
     void replay(GraphicsContext&) const override;
     void appendToWebDisplayItemList(const IntRect&, WebDisplayItemList*) const override;
     bool drawsContent() const override;
 
     const SkPicture* picture() const { return m_picture.get(); }
 
+    bool knownToBeOpaque() const { ASSERT(RuntimeEnabledFeatures::slimmingPaintV2Enabled()); return m_knownToBeOpaque; }
+
 #if ENABLE(ASSERT)
     UnderInvalidationCheckingMode underInvalidationCheckingMode() const { return m_underInvalidationCheckingMode; }
     bool equals(const DisplayItem& other) const final;
 #endif
 
 private:
 #ifndef NDEBUG
     void dumpPropertiesAsDebugString(WTF::StringBuilder&) const override;
 #endif
 
     RefPtr<const SkPicture> m_picture;
 
+    // True if there are no transparent areas. Only used for SlimmingPaintV2.

[pdr., 2016/01/27 05:07:46]

Jbroman, what do you think about this?

It's very gross but we have several free bits in DisplayItem and bloating
DrawingDisplayItem may hurt memory without spv2. Unless there's a better idea, I
think we should move it.

[jbroman, 2016/01/27 16:04:27]

Ugh, it probably costs us 4-8 bytes per due to pointer alignment, doesn't it?
You could quickly check to see how much memory in total that is in practice
(it's still not massive compared to the usual size of a DrawingDisplayItem +
associated SkPicture + SkRecord). I'm worried it may be premature optimization.

But if you'd really rather have DisplayItem::m_derivedClassDefinedBit1 or
something, we _could_.

+    // TODO(pdr): It's messy, but we may want to move this to DisplayItem which has free bits.
+    const bool m_knownToBeOpaque;
+
 #if ENABLE(ASSERT)
     UnderInvalidationCheckingMode m_underInvalidationCheckingMode;
 #endif
 };
 
 } // namespace blink
 
 #endif // DrawingDisplayItem_h