From 6bb4a2bfaa1dd919e3f5e6bc370e9ea5d449e8c6 Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Tue, 19 Mar 2024 15:41:02 +0100
Subject: [PATCH] LibWeb: Let HTMLCollection cache its element list

Use the new DOM tree version mechanism to allow HTMLCollection to
remember its internal list of elements instead of rebuilding it on
every access.

This avoids thousands of full DOM walks while loading our GitHub repo.

~15% speed-up on jQuery subtests in Speedometer 3.0 :^)
---
 .../Libraries/LibWeb/DOM/HTMLCollection.cpp   | 36 ++++++++++++-------
 .../Libraries/LibWeb/DOM/HTMLCollection.h     |  3 ++
 2 files changed, 26 insertions(+), 13 deletions(-)

diff --git a/Userland/Libraries/LibWeb/DOM/HTMLCollection.cpp b/Userland/Libraries/LibWeb/DOM/HTMLCollection.cpp
index b899f20c60c..da67dd22f97 100644
--- a/Userland/Libraries/LibWeb/DOM/HTMLCollection.cpp
+++ b/Userland/Libraries/LibWeb/DOM/HTMLCollection.cpp
@@ -6,6 +6,7 @@
  */
 
 #include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/DOM/Document.h>
 #include <LibWeb/DOM/Element.h>
 #include <LibWeb/DOM/HTMLCollection.h>
 #include <LibWeb/DOM/ParentNode.h>
@@ -45,24 +46,33 @@ void HTMLCollection::visit_edges(Cell::Visitor& visitor)
 {
     Base::visit_edges(visitor);
     visitor.visit(m_root);
+    for (auto& element : m_cached_elements)
+        visitor.visit(element);
 }
 
 JS::MarkedVector<Element*> HTMLCollection::collect_matching_elements() const
 {
-    JS::MarkedVector<Element*> elements(m_root->heap());
-    if (m_scope == Scope::Descendants) {
-        m_root->for_each_in_subtree_of_type<Element>([&](auto& element) {
-            if (m_filter(element))
-                elements.append(const_cast<Element*>(&element));
-            return IterationDecision::Continue;
-        });
-    } else {
-        m_root->for_each_child_of_type<Element>([&](auto& element) {
-            if (m_filter(element))
-                elements.append(const_cast<Element*>(&element));
-            return IterationDecision::Continue;
-        });
+    if (m_cached_dom_tree_version != root()->document().dom_tree_version()) {
+        m_cached_elements.clear();
+        if (m_scope == Scope::Descendants) {
+            m_root->for_each_in_subtree_of_type<Element>([&](auto& element) {
+                if (m_filter(element))
+                    m_cached_elements.append(element);
+                return IterationDecision::Continue;
+            });
+        } else {
+            m_root->for_each_child_of_type<Element>([&](auto& element) {
+                if (m_filter(element))
+                    m_cached_elements.append(element);
+                return IterationDecision::Continue;
+            });
+        }
+        m_cached_dom_tree_version = root()->document().dom_tree_version();
     }
+
+    JS::MarkedVector<Element*> elements(heap());
+    for (auto& element : m_cached_elements)
+        elements.append(element);
     return elements;
 }
 
diff --git a/Userland/Libraries/LibWeb/DOM/HTMLCollection.h b/Userland/Libraries/LibWeb/DOM/HTMLCollection.h
index 447f99ac0ac..563061c0820 100644
--- a/Userland/Libraries/LibWeb/DOM/HTMLCollection.h
+++ b/Userland/Libraries/LibWeb/DOM/HTMLCollection.h
@@ -60,6 +60,9 @@ protected:
 private:
     virtual void visit_edges(Cell::Visitor&) override;
 
+    mutable u64 m_cached_dom_tree_version { 0 };
+    mutable Vector<JS::NonnullGCPtr<Element>> m_cached_elements;
+
     JS::NonnullGCPtr<ParentNode> m_root;
     Function<bool(Element const&)> m_filter;