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LibWeb: Make "assign slottables for a tree" fast when there are no slots

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We achieve this by keeping track of the number of HTMLSlotElements
inside each ShadowRoot (do via ad-hoc insertion and removal steps.)

This allows slottables assignment to skip over entire shadow roots when
we know they have no slots anyway.

Massive speedup on https://wpt.fyi/ which no longer takes minutes/hours
to load, but instead a "mere" 19 seconds. :^)
This commit is contained in:
Andreas Kling 2025-01-23 17:41:53 +01:00 • committed by Andreas Kling
commit adc25af8e2
Notes: github-actions[bot] 2025-01-23 20:40:06 +00:00
4 changed files with 49 additions and 11 deletions
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22
Libraries/LibWeb/DOM/Slottable.cpp
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@ -74,32 +74,32 @@ GC::Ptr<HTML::HTMLSlotElement> find_a_slot(Slottable const& slottable, OpenFlag
// 5. If shadow’s slot assignment is "manual", then return the slot in shadow’s descendants whose manually assigned
// nodes contains slottable, if any; otherwise null.
if (shadow->slot_assignment() == Bindings::SlotAssignmentMode::Manual) {
GC::Ptr<HTML::HTMLSlotElement> slot;
GC::Ptr<HTML::HTMLSlotElement> found_slot;
shadow->for_each_in_subtree_of_type<HTML::HTMLSlotElement>([&](auto& child) {
if (!child.manually_assigned_nodes().contains_slow(slottable))
shadow->for_each_slot([&](auto& slot) {
if (!slot.manually_assigned_nodes().contains_slow(slottable))
return TraversalDecision::Continue;
slot = child;
found_slot = slot;
return TraversalDecision::Break;
});
return slot;
return found_slot;
}
// 6. Return the first slot in tree order in shadow’s descendants whose name is slottable’s name, if any; otherwise null.
auto const& slottable_name = slottable.visit([](auto const& node) { return node->slottable_name(); });
GC::Ptr<HTML::HTMLSlotElement> slot;
GC::Ptr<HTML::HTMLSlotElement> found_slot;
shadow->for_each_in_subtree_of_type<HTML::HTMLSlotElement>([&](auto& child) {
if (child.slot_name() != slottable_name)
shadow->for_each_slot([&](auto& slot) {
if (slot.slot_name() != slottable_name)
return TraversalDecision::Continue;
slot = child;
found_slot = slot;
return TraversalDecision::Break;
});
return slot;
return found_slot;
}
// https://dom.spec.whatwg.org/#find-slotables
@ -186,7 +186,7 @@ void assign_slottables_for_a_tree(GC::Ref<Node> root)
// To assign slottables for a tree, given a node root, run assign slottables for each slot slot in root’s inclusive
// descendants, in tree order.
root->for_each_in_inclusive_subtree_of_type<HTML::HTMLSlotElement>([](auto& slot) {
static_cast<DOM::ShadowRoot&>(*root).for_each_slot([](auto& slot) {
assign_slottables(slot);
return TraversalDecision::Continue;
});