Avoid false-positives of under-invalidation checking

Source/platform/graphics/paint/DisplayItemList.cpp

Index: Source/platform/graphics/paint/DisplayItemList.cpp
diff --git a/Source/platform/graphics/paint/DisplayItemList.cpp b/Source/platform/graphics/paint/DisplayItemList.cpp
index 24227418e7fa6ce1df0b008ca6f5c38a18e2876e..2338bdde40d99165eebc440065cabd8c48499a01 100644
--- a/Source/platform/graphics/paint/DisplayItemList.cpp
+++ b/Source/platform/graphics/paint/DisplayItemList.cpp
@@ -296,6 +296,20 @@ static void showUnderInvalidationError(const char* reason, const DisplayItem& di
 #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());
@@ -337,6 +351,21 @@ void DisplayItemList::checkCachedDisplayItemIsUnchanged(const DisplayItem& displ
         }
     }
 
+    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";