From 7ee47d81ca52017178c44b4c605eb03b6d3fa090 Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Sun, 19 Nov 2023 21:53:58 +0100
Subject: [PATCH] LibWeb: Allocate custom element reactions queue on demand

This shrinks most DOM elements by 16 bytes.
---
 Userland/Libraries/LibWeb/Bindings/MainThreadVM.cpp |  8 +++++---
 Userland/Libraries/LibWeb/DOM/Element.cpp           | 13 ++++++++++---
 Userland/Libraries/LibWeb/DOM/Element.h             |  8 +++++---
 3 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/Userland/Libraries/LibWeb/Bindings/MainThreadVM.cpp b/Userland/Libraries/LibWeb/Bindings/MainThreadVM.cpp
index 82d9ab92574..8eb01466946 100644
--- a/Userland/Libraries/LibWeb/Bindings/MainThreadVM.cpp
+++ b/Userland/Libraries/LibWeb/Bindings/MainThreadVM.cpp
@@ -675,12 +675,14 @@ void invoke_custom_element_reactions(Vector<JS::Handle<DOM::Element>>& element_q
         auto element = element_queue.take_first();
 
         // 2. Let reactions be element's custom element reaction queue.
-        auto& reactions = element->custom_element_reaction_queue();
+        auto* reactions = element->custom_element_reaction_queue();
 
         // 3. Repeat until reactions is empty:
-        while (!reactions.is_empty()) {
+        if (!reactions)
+            continue;
+        while (!reactions->is_empty()) {
             // 1. Remove the first element of reactions, and let reaction be that element. Switch on reaction's type:
-            auto reaction = reactions.take_first();
+            auto reaction = reactions->take_first();
 
             auto maybe_exception = reaction.visit(
                 [&](DOM::CustomElementUpgradeReaction const& custom_element_upgrade_reaction) -> JS::ThrowCompletionOr<void> {
diff --git a/Userland/Libraries/LibWeb/DOM/Element.cpp b/Userland/Libraries/LibWeb/DOM/Element.cpp
index 39c193cb04d..8fde3fd27f3 100644
--- a/Userland/Libraries/LibWeb/DOM/Element.cpp
+++ b/Userland/Libraries/LibWeb/DOM/Element.cpp
@@ -1811,7 +1811,7 @@ void Element::enqueue_an_element_on_the_appropriate_element_queue()
 void Element::enqueue_a_custom_element_upgrade_reaction(HTML::CustomElementDefinition& custom_element_definition)
 {
     // 1. Add a new upgrade reaction to element's custom element reaction queue, with custom element definition definition.
-    m_custom_element_reaction_queue.append(CustomElementUpgradeReaction { .custom_element_definition = custom_element_definition });
+    ensure_custom_element_reaction_queue().append(CustomElementUpgradeReaction { .custom_element_definition = custom_element_definition });
 
     // 2. Enqueue an element on the appropriate element queue given element.
     enqueue_an_element_on_the_appropriate_element_queue();
@@ -1846,7 +1846,7 @@ void Element::enqueue_a_custom_element_callback_reaction(FlyString const& callba
     }
 
     // 5. Add a new callback reaction to element's custom element reaction queue, with callback function callback and arguments args.
-    m_custom_element_reaction_queue.append(CustomElementCallbackReaction { .callback = callback_iterator->value, .arguments = move(arguments) });
+    ensure_custom_element_reaction_queue().append(CustomElementCallbackReaction { .callback = callback_iterator->value, .arguments = move(arguments) });
 
     // 6. Enqueue an element on the appropriate element queue given element.
     enqueue_an_element_on_the_appropriate_element_queue();
@@ -1929,7 +1929,7 @@ JS::ThrowCompletionOr<void> Element::upgrade_element(JS::NonnullGCPtr<HTML::Cust
         m_custom_element_definition = nullptr;
 
         // 2. Empty element's custom element reaction queue.
-        m_custom_element_reaction_queue.clear();
+        m_custom_element_reaction_queue = nullptr;
 
         // 3. Rethrow the exception (thus terminating this algorithm).
         return maybe_exception.release_error();
@@ -2271,4 +2271,11 @@ void Element::attribute_change_steps(FlyString const& local_name, Optional<Strin
     }
 }
 
+auto Element::ensure_custom_element_reaction_queue() -> CustomElementReactionQueue&
+{
+    if (!m_custom_element_reaction_queue)
+        m_custom_element_reaction_queue = make<CustomElementReactionQueue>();
+    return *m_custom_element_reaction_queue;
+}
+
 }
diff --git a/Userland/Libraries/LibWeb/DOM/Element.h b/Userland/Libraries/LibWeb/DOM/Element.h
index 75db7d60e29..b6bc89a4d65 100644
--- a/Userland/Libraries/LibWeb/DOM/Element.h
+++ b/Userland/Libraries/LibWeb/DOM/Element.h
@@ -325,8 +325,10 @@ public:
     void enqueue_a_custom_element_upgrade_reaction(HTML::CustomElementDefinition& custom_element_definition);
     void enqueue_a_custom_element_callback_reaction(FlyString const& callback_name, JS::MarkedVector<JS::Value> arguments);
 
-    Vector<Variant<CustomElementUpgradeReaction, CustomElementCallbackReaction>>& custom_element_reaction_queue() { return m_custom_element_reaction_queue; }
-    Vector<Variant<CustomElementUpgradeReaction, CustomElementCallbackReaction>> const& custom_element_reaction_queue() const { return m_custom_element_reaction_queue; }
+    using CustomElementReactionQueue = Vector<Variant<CustomElementUpgradeReaction, CustomElementCallbackReaction>>;
+    CustomElementReactionQueue* custom_element_reaction_queue() { return m_custom_element_reaction_queue; }
+    CustomElementReactionQueue const* custom_element_reaction_queue() const { return m_custom_element_reaction_queue; }
+    CustomElementReactionQueue& ensure_custom_element_reaction_queue();
 
     JS::ThrowCompletionOr<void> upgrade_element(JS::NonnullGCPtr<HTML::CustomElementDefinition> custom_element_definition);
     void try_to_upgrade();
@@ -416,7 +418,7 @@ private:
     // https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-reaction-queue
     // All elements have an associated custom element reaction queue, initially empty. Each item in the custom element reaction queue is of one of two types:
     // NOTE: See the structs at the top of this header.
-    Vector<Variant<CustomElementUpgradeReaction, CustomElementCallbackReaction>> m_custom_element_reaction_queue;
+    OwnPtr<CustomElementReactionQueue> m_custom_element_reaction_queue;
 
     // https://dom.spec.whatwg.org/#concept-element-custom-element-state
     CustomElementState m_custom_element_state { CustomElementState::Undefined };