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LibWeb: Reduce allocations in IntersectionObserver hot path

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Pre-compute per-observer values (root node, root paintable, root
bounds, is_implicit_root, root_is_element) once before the per-target
loop instead of recomputing them for each target.

Add intersection_root_node() which returns GC::Ref<DOM::Node> without
creating GC::Root wrappers. Previously, intersection_root() was called
multiple times per target (in the skip condition check, in
compute_intersection, and again in root_intersection_rectangle inside
compute_intersection), each call creating GC::Root objects that require
heap allocation via new RootImpl.

Pass the pre-computed root bounds and root paintable into
compute_intersection instead of having it call
root_intersection_rectangle() and intersection_root() internally.

For an observer watching N targets, this eliminates roughly 4N heap
allocations per frame.
This commit is contained in:
Andreas Kling
2026-03-22 09:11:37 -05:00
committed by Andreas Kling
parent 9abb7e4517
commit e2d5d72f55
Notes: github-actions[bot] 2026-03-22 19:10:45 +00:00 
Author: https://github.com/awesomekling
Commit: https://github.com/LadybirdBrowser/ladybird/commit/e2d5d72f55a
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/8574
3 changed files with 26 additions and 30 deletions
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23
Libraries/LibWeb/IntersectionObserver/IntersectionObserver.cpp
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@@ -243,18 +243,19 @@ String IntersectionObserver::scroll_margin() const
// https://www.w3.org/TR/intersection-observer/#intersectionobserver-intersection-root
Variant<GC::Root<DOM::Element>, GC::Root<DOM::Document>> IntersectionObserver::intersection_root() const
{
// The intersection root for an IntersectionObserver is the value of its root attribute
// if the attribute is non-null;
if (m_root) {
if (m_root->is_element())
return GC::make_root(static_cast<DOM::Element&>(*m_root));
if (m_root->is_document())
return GC::make_root(static_cast<DOM::Document&>(*m_root));
VERIFY_NOT_REACHED();
}
auto root_node = intersection_root_node();
if (root_node->is_element())
return GC::make_root(static_cast<DOM::Element&>(*root_node));
return GC::make_root(static_cast<DOM::Document&>(*root_node));
}
// otherwise, it is the top-level browsing context’s document node, referred to as the implicit root.
return GC::make_root(as<HTML::Window>(HTML::relevant_global_object(*this)).page().top_level_browsing_context().active_document());
GC::Ref<DOM::Node> IntersectionObserver::intersection_root_node() const
{
if (m_root)
return *m_root;
// The implicit root is the top-level browsing context’s document node.
return *as<HTML::Window>(HTML::relevant_global_object(*this)).page().top_level_browsing_context().active_document();
}
// https://www.w3.org/TR/intersection-observer/#intersectionobserver-root-intersection-rectangle