Avoid false-positives of under-invalidation checking

Source/platform/graphics/paint/DisplayItemList.cpp

 // 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.
 
 #include "config.h"
 #include "platform/graphics/paint/DisplayItemList.h"
 
 #include "platform/NotImplemented.h"
 #include "platform/RuntimeEnabledFeatures.h"
 #include "platform/TraceEvent.h"
@@ skipping 278 matching lines @@
 
 static void showUnderInvalidationError(const char* reason, const DisplayItem& displayItem)
 {
 #ifndef NDEBUG
     WTFLogAlways("%s: %s\nSee http://crbug.com/450725.", reason, displayItem.asDebugString().utf8().data());
 #else
     WTFLogAlways("%s. Run debug build to get more details\nSee http://crbug.com/450725.", reason);
 #endif // NDEBUG
 }
 
+static bool bitmapIsAllZero(const SkBitmap& bitmap)
+{
+    bitmap.lockPixels();
+    bool result = true;
+    for (int x = 0; result && x < bitmap.width(); ++x) {
+        for (int y = 0; result && y < bitmap.height(); ++y) {
+            if (SkColorSetA(bitmap.getColor(x, y), 0) != SK_ColorTRANSPARENT)
+                result = false;
+        }
+    }
+    bitmap.unlockPixels();
+    return result;
+}
+
 void DisplayItemList::checkCachedDisplayItemIsUnchanged(const DisplayItem& displayItem, DisplayItemIndicesByClientMap& displayItemIndicesByClient)
 {
     ASSERT(RuntimeEnabledFeatures::slimmingPaintUnderInvalidationCheckingEnabled());
 
     if (!displayItem.isDrawing() || !clientCacheIsValid(displayItem.client()))
         return;
 
     if (static_cast<const DrawingDisplayItem&>(displayItem).skipUnderInvalidationChecking())
         return;
 
@@ skipping 21 matching lines @@
         oldPicture->serialize(&oldPictureSerialized);
 
         if (newPictureSerialized.bytesWritten() == oldPictureSerialized.bytesWritten()) {
             RefPtr<SkData> oldData = adoptRef(oldPictureSerialized.copyToData());
             RefPtr<SkData> newData = adoptRef(newPictureSerialized.copyToData());
             if (oldData->equals(newData.get()))
                 return;
         }
     }
 
+    SkRect rect = newPicture->cullRect();

[chrishtr, 2015/06/01 19:04:28]

I think this would induce too many false negatives.

How about just putting a bit on a drawing recorder in ENABLE(ASSERT) mode that
can be set by canvas when it's doing a representational change?

[Xianzhu, 2015/06/01 19:10:15]

Could you give more details about the case of false negatives?

+    if (rect == oldPicture->cullRect()) {
+        // Some pictures may have same contents but different representations.
+        SkBitmap bitmap;
+        bitmap.allocPixels(SkImageInfo::MakeN32Premul(rect.width(), rect.height()));
+        SkCanvas canvas(bitmap);
+        canvas.translate(-rect.x(), -rect.y());
+        canvas.drawPicture(oldPicture.get());
+        SkPaint diffPaint;
+        diffPaint.setXfermodeMode(SkXfermode::kDifference_Mode);
+        canvas.drawPicture(newPicture.get(), nullptr, &diffPaint);
+        if (bitmapIsAllZero(bitmap)) // Contents are the same.
+            return;
+    }
+
     showUnderInvalidationError("ERROR: under-invalidation: display item changed", displayItem);
 #ifndef NDEBUG
     String oldPictureDebugString = oldPicture ? pictureAsDebugString(oldPicture.get()) : "None";
     String newPictureDebugString = newPicture ? pictureAsDebugString(newPicture.get()) : "None";
     WTFLogAlways("old picture:\n%s\n", oldPictureDebugString.utf8().data());
     WTFLogAlways("new picture:\n%s\n", newPictureDebugString.utf8().data());
 #endif // NDEBUG
 
     ASSERT_NOT_REACHED();
 }
@@ skipping 43 matching lines @@
 
 void DisplayItemList::replay(GraphicsContext& context) const
 {
     TRACE_EVENT0("blink,benchmark", "DisplayItemList::replay");
     ASSERT(m_newDisplayItems.isEmpty());
     for (auto& displayItem : m_currentDisplayItems)
         displayItem->replay(context);
 }
 
 } // namespace blink