AX: Reduce parent lookups in unignoredChildren pre-order traversal

[twilco]

058338e

AX: Reduce parent lookups in unignoredChildren pre-order traversal
https://bugs.webkit.org/show_bug.cgi?id=314691
rdar://176932652

Reviewed by Dominic Mazzoni.

Samples taken during AX queries showed nextSiblingIncludingIgnored
as ~95% of time in stitchedUnignoredChildren -> unignoredChildren ->
nextInPreOrder. Each call went through parentObject(), a virtual call
whose cost varies by subclass (HashMap lookup on AXIsolatedObject; DOM
traversal + axObjectCache lookup on AccessibilityRenderObject /
AccessibilityNodeObject) but is always non-trivial. The traversal was
paying for that lookup twice per ascended level. Two adjustments cut
the per-hop work:

1. nextInPreOrder: replace the pre-loop nextSiblingIncludingIgnored()
   call + in-loop parentObject() ascent with a single loop that fetches
   parent once per level. In the non-cross-frame case, the sibling-search
   parent and ascent parent are the same value, so the ascent reuses it
   via WTF::move, eliminating the duplicate parentObject() call and the
   accompanying RefPtr ref()/deref() per ascended level.

2. unignoredChildren: for ignored or invalid descendants, inline the
   descent step (matching nextInPreOrder's Column / TableHeaderContainer
   role guard via shouldSetChildIndexInParent()) instead of falling back
   to nextInPreOrder(). The function already had its own sibling-or-
   ascend loop that caches parent + siblings across sibling hops, but the
   nextInPreOrder() fallback was invalidating that cache every time the
   traversal crossed an ignored descendant. With the fallback gone,
   cached parent/siblings now survive through ignored-descendant
   traversal, and the per-hop cost drops to descendant->indexInParent() + a vector index.

* Source/WebCore/accessibility/AXCoreObject.cpp:
(WebCore::AXCoreObject::unignoredChildren):
(WebCore::AXCoreObject::nextInPreOrder):

Canonical link: https://commits.webkit.org/313164@main

05fa6d2

Misc	iOS, visionOS, tvOS & watchOS	macOS	Linux	Windows	Apple Internal
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 win	✅ 🛠 ios-apple
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🧪 wpe-wk2	🧪 win-tests	✅ 🛠 mac-apple
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	❌ 🧪 api-mac	✅ 🧪 api-wpe		✅ 🛠 vision-apple
	✅ 🧪 ios-wk2-wpt	✅ 🧪 api-mac-debug	✅ 🛠 gtk3-libwebrtc
⏳ 🛠 🧪 jsc	✅ 🧪 api-ios	✅ 🧪 mac-wk1	✅ 🛠 gtk
	✅ 🛠 ios-safer-cpp	✅ 🧪 mac-wk2	✅ 🧪 gtk-wk2
	✅ 🛠 vision	✅ 🧪 mac-AS-debug-wk2	✅ 🧪 api-gtk
✅ 🛠 🧪 merge	✅ 🛠 vision-sim	✅ 🧪 mac-wk2-stress	✅ 🛠 playstation
	✅ 🧪 vision-wk2	✅ 🧪 mac-intel-wk2
	✅ 🛠 tv	✅ 🛠 mac-safer-cpp
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on previous version of this PR (hash 7358b48)

Details

Misc	iOS, visionOS, tvOS & watchOS	macOS	Linux	Windows	Apple Internal
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	🛠 wpe	🛠 win	⏳ 🛠 ios-apple
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	🧪 wpe-wk2	🧪 win-tests	⏳ 🛠 mac-apple
✅ 🧪 webkitperl	🧪 ios-wk2	🧪 api-mac	🧪 api-wpe		✅ 🛠 vision-apple
	🧪 ios-wk2-wpt	🧪 api-mac-debug	🛠 gtk3-libwebrtc
⏳ 🛠 🧪 jsc	🧪 api-ios	✅ 🧪 mac-wk1	🛠 gtk
	🛠 ios-safer-cpp	✅ 🧪 mac-wk2	🧪 gtk-wk2
	✅ 🛠 vision	🧪 mac-AS-debug-wk2	🧪 api-gtk
	✅ 🛠 vision-sim	✅ 🧪 mac-wk2-stress	✅ 🛠 playstation
	✅ 🧪 vision-wk2	🧪 mac-intel-wk2
	✅ 🛠 tv	✅ 🛠 mac-safer-cpp
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on current version of this PR (hash 05fa6d2)

Details

Misc	iOS, visionOS, tvOS & watchOS	macOS	Linux	Windows	Apple Internal
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 win	✅ 🛠 ios-apple
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🧪 wpe-wk2	🧪 win-tests	✅ 🛠 mac-apple
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	❌ 🧪 api-mac	✅ 🧪 api-wpe		✅ 🛠 vision-apple
	✅ 🧪 ios-wk2-wpt	✅ 🧪 api-mac-debug	✅ 🛠 gtk3-libwebrtc
⏳ 🛠 🧪 jsc	✅ 🧪 api-ios	✅ 🧪 mac-wk1	✅ 🛠 gtk
	✅ 🛠 ios-safer-cpp	✅ 🧪 mac-wk2	✅ 🧪 gtk-wk2
	✅ 🛠 vision	✅ 🧪 mac-AS-debug-wk2	✅ 🧪 api-gtk
✅ 🛠 🧪 merge	✅ 🛠 vision-sim	✅ 🧪 mac-wk2-stress	✅ 🛠 playstation
	✅ 🧪 vision-wk2	✅ 🧪 mac-intel-wk2
	✅ 🛠 tv	✅ 🛠 mac-safer-cpp
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-commit-queue]

Committed 313164@main (058338e): https://commits.webkit.org/313164@main

Reviewed commits have been landed. Closing PR #64803 and removing active labels.