Skip PaintLayer empty paint phases if it previously painted nothing

third_party/WebKit/Source/core/paint/README.md

 # `Source/core/paint`
 
 This directory contains implementation of painters of layout objects.
 
 ## Painters
 
 ## Paint invalidation
 
 Paint invalidation marks anything that need to be painted differently from the original
 cached painting.
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 In a subsequence paint, if the layer would generate exactly the same display items, we'll simply
 output a `CachedSubsequence` display item in place of the display items, and skip all paintings
 of the layer and its descendants in painting order. After painting, `PaintController` will
 replace `CacheSubsequence` with cached display items created in previous paintings.
 
 There are many conditions affecting
 *   whether we need to generate subsequence for a PaintLayer;
 *   whether we can use cached subsequence for a PaintLayer.
 See `shouldCreateSubsequence()` and `shouldRepaintSubsequence()` in `PaintLayerPainter.cpp` for
 the conditions.
+
+## Optimization for empty paint phases
+
+During painting, we walk the layout tree multiple times for multiple paint phases. Sometimes
+a layer contain nothing needing a certain paint phase and we can skip tree walk for such
+empty phases. Now we have optimized `PaintPhaseDescendantBlockBackgroundsOnly`,
+`PaintPhaseDescendantOutlinesOnly` and `PaintPhaseFloat` for empty paint phases.
+
+During paint invaliidation, we set the containing self-painting layer's `needsPaintPhaseXXX`
+flag if the object has something needing to be painted in the paint phase.
+
+During painting, we check the flag before painting a paint phase and skip the tree walk if
+the flag is not set.
+
+It's hard to clear a `needsPaintPhaseXXX` flag when a layer no longer needs the paint phase,
+so we never clear the flags. Instead, we use another set of flags (`previousPaintPhaseXXXWasEmpty`)
+to record if a painting of a phase actually produced nothing. We'll skip the next
+painting of the phase if the flag is set, regardless of the corresponding
+`needsPaintPhaseXXX` flag. We will clear the `previousPaintPhaseXXXWasEmpty` flags when
+we paint with different clipping, scroll offset or interest rect from the previous paint.
+
+We don't clear the `previousPaintPhaseXXXWasEmpty` flags when the layer is marked `needsRepaint`.
+Instead we clear the flag when the corresponding `needsPaintPhaseXXX` is set. This ensures that
+we won't clear `previousPaintPhaseXXXWasEmpty` flags when unrelated things changed which won't
+cause the paint phases to become non-empty.