1356519 - Retain clips and clip chain on frame tree

Status: NEW

(Core :: Web Painting, enhancement, P5)

Description

[Thinker Li [:sinker]]

Clips and clip chains are recomputed and recreated every time building display list, but
they are rarely/or usually minor changed.  So, for most of time they are redundant.
Retaining clip chains and clips would save time.

For now, clip chains are maintained by |DisplayListClipState| owned by
|DisplayListBuilder|.  There are 4 use cases of clip chains.
 - Apply and get clips during the process of |BuildDisplayList()| &
   |BuildDisplayForChild()|.
 - Mark out-of-flow/or absolute frames during the process of |BuildDisplayList()|.
   - see Mark*ForDisplayList().
 - Display items apply special clips
   - PresShell::ClipListToRange()
   - ClipItemsExceptCaret() in nsGfxScrollFrame
   - *Item::ApplyOpacity()
 - FuseClipChainUpTo() called by ProcessDisplayItems()

|DisplayListClipState| is changed by following calls,
 - ClipContainingBlockDescendants(),
 - ClipContentDescendants(),
 - ClipContainingBlockDescendantsToContentBox(), and
 - ClearUpToASR() of DisplayListClipState().
They are called by |BuildDisplayListForChild()|,
|BuildDisplayListForStackingContext()|, and |BuildDisplayList()| of frame types.

The cliip chains are retrieved from |DisplayListClipState| through following calls,
 - nsDisplayItem::IntersectClip(),
 - DisplayListClipState::GetCurrentCombinedClipChain(), and
 - DisplayListClipState::GetClipChainForContainingBlockDescendants()
   with nsDisplayListBuilder::CopyWholeChain().
 - nsDisplayItem::FuseClipChainUpTo()
They are majorly called by nsFrame.cpp and nsDisplayList.cpp.
IntersectClip() would be called by nsGfxScrollFrame and PresShell too.
|FueClipChainUpTo()| is called by FrameLayerBuilder.cpp.

All clip chains are allocated by
|nsDisplayListBuilder::AllocateDisplayItemClipChain()|.

With these information, we can retain clip chains and reuse them by frame keys and/or frame itself.

Retained clip chains are invalidated every time the frame or one of its ancestor is restyled. (nsFrame::DidSetStyleContext()).

Comment 1

[Thinker Li [:sinker]]

This bug would depend on ASR. We need a way to make them can be done separated.
Or, assign two bugs to the same developer.