Skip PaintLayer empty paint phases if it previously painted nothing

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

Index: third_party/WebKit/Source/core/paint/README.md
diff --git a/third_party/WebKit/Source/core/paint/README.md b/third_party/WebKit/Source/core/paint/README.md
index 290425340c63f49fbb1706b05f33605e5fcfc4bf..049c92e457f758de557398e3fa9ed3476700f577 100644
--- a/third_party/WebKit/Source/core/paint/README.md
+++ b/third_party/WebKit/Source/core/paint/README.md
@@ -90,3 +90,28 @@ There are many conditions affecting
 *   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.