[SPInvalidation] Micro-optimize PaintLayerClipper::calculateRects

third_party/WebKit/Source/core/paint/PaintLayerClipper.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights
  * reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 248 matching lines @@
   } while (false);
 #endif
 
 void PaintLayerClipper::CalculateRectsWithGeometryMapper(
     const ClipRectsContext& context,
     const LayoutRect& paint_dirty_rect,
     LayoutRect& layer_bounds,
     ClipRect& background_rect,
     ClipRect& foreground_rect,
     const LayoutPoint* offset_from_root) const {
+  LayoutPoint offset;
+  if (offset_from_root)
+    offset = *offset_from_root;
+  else
+    layer_.ConvertToLayerCoords(context.root_layer, offset);
+  layer_bounds = LayoutRect(offset, LayoutSize(layer_.size()));
+
   // TODO(chrishtr): fix the underlying bug that causes this situation.
   if (!layer_.GetLayoutObject().PaintProperties() &&
       !layer_.GetLayoutObject().LocalBorderBoxProperties()) {
     background_rect = ClipRect(LayoutRect(LayoutRect::InfiniteIntRect()));
     foreground_rect = ClipRect(LayoutRect(LayoutRect::InfiniteIntRect()));
   } else {
     CalculateBackgroundClipRectWithGeometryMapper(context, background_rect);
     background_rect.Move(context.sub_pixel_accumulation);
     background_rect.Intersect(paint_dirty_rect);
 
-    CalculateForegroundClipRectWithGeometryMapper(context, foreground_rect);
-    foreground_rect.Move(context.sub_pixel_accumulation);
-    foreground_rect.Intersect(paint_dirty_rect);
+    foreground_rect = background_rect;
+    LayoutBoxModelObject& layout_object = layer_.GetLayoutObject();
+    if (ShouldClipOverflow(context)) {
+      LayoutRect overflow_and_clip_rect =
+        ToLayoutBox(layout_object)
+            .OverflowClipRect(offset, context.overlay_scrollbar_clip_behavior);

[trchen, 2017/04/14 23:35:35]

Should it be (offset + context.sub_pixel_accumulation)?

[chrishtr, 2017/04/14 23:51:56]

Do you mean I should adjust OverflowClipRect() by an additional
sub_pixel_accumulation? It seems not if one is trying to copy 
non-GM behavior in CalculateRects(), which does not do that I
believe. Maybe that behavior is incorrect?

[trchen, 2017/04/15 00:09:51]

Hmmmmm. That looks very weird indeed.

The non-GM version seemed to me that every time we inherit the parent layer's
foreground rect, we shift it by context.sub_pixel_accumulation. (Feels wrong,
and doesn't match this CL's version either.)

If I understand it correctly, context.sub_pixel_accumulation is the difference
between root layer origin and backing origin (i.e. paint offset for root layer).
I feel it is cleaner to just change #269 to:
LayoutPoint offset = context.sub_pixel_accumulation;

Then the question becomes whether *offset_from_root already included
context.sub_pixel_accumulation. :/

+      foreground_rect.Intersect(overflow_and_clip_rect);
+      if (layout_object.StyleRef().HasBorderRadius())
+        foreground_rect.SetHasRadius(true);
+    }
+    
+    if (layout_object.HasClip()) {

[trchen, 2017/04/14 23:35:35]

nit: When constructed from bottom-up, apply CSS clip before overflow clip.

[chrishtr, 2017/04/14 23:51:56]

Done, except now it technically disagrees with non-GM. :)

+      LayoutRect new_pos_clip = ToLayoutBox(layout_object).ClipRect(offset);

[trchen, 2017/04/14 23:35:35]

Ditto. (offset + context.sub_pixel_accumulation)?

[chrishtr, 2017/04/14 23:51:56]

Same answer as above.

+      foreground_rect.Intersect(new_pos_clip);
+    }
   }
 
-  LayoutPoint offset;
-  if (offset_from_root)
-    offset = *offset_from_root;
-  else
-    layer_.ConvertToLayerCoords(context.root_layer, offset);
-  layer_bounds = LayoutRect(offset, LayoutSize(layer_.size()));
-
 #ifdef CHECK_CLIP_RECTS
   ClipRect testBackgroundRect, testForegroundRect;
   LayoutRect testLayerBounds;
   PaintLayerClipper(m_layer, nullptr)
       .calculateRects(context, paintDirtyRect, testLayerBounds,
                       testBackgroundRect, testForegroundRect, offsetFromRoot);
   CHECK_RECTS_EQ(testBackgroundRect, backgroundRect);
   CHECK_RECTS_EQ(testForegroundRect, foregroundRect);
   CHECK_RECTS_EQ(testLayerBounds, layerBounds);
 #endif
@@ skipping 331 matching lines @@
   DCHECK(!use_geometry_mapper_);
   ClipRectsContext context(root_layer, kPaintingClipRects,
                            kIgnorePlatformOverlayScrollbarSize,
                            subpixel_accumulation);
   if (respect_overflow_clip == kIgnoreOverflowClip)
     context.SetIgnoreOverflowClip();
   return GetClipRects(context);
 }
 
 }  // namespace blink