LibWeb+AK: Use AK::Queue for the microtask queue

The microtask queue is a pure FIFO (enqueue at back, dequeue from
front) but was using a Vector, making every dequeue O(n) due to
element shifting.

Replace it with AK::Queue which has O(1) dequeue. This makes a huge
difference when processing large numbers of microtasks, e.g. during
async-heavy JavaScript workloads where each `await` generates a
microtask.

Also add a for_each() method to AK::Queue so the GC can visit the
queued tasks.

AK/Queue.h

@@ -76,6 +76,18 @@ public:
         return m_segments.last()->data.last();
     }
 
+    template<typename F>
+    void for_each(F&& callback) const
+    {
+        bool first = true;
+        for (auto const& segment : m_segments) {
+            size_t start = first ? m_index_into_first : 0;
+            first = false;
+            for (size_t i = start; i < segment.data.size(); ++i)
+                callback(segment.data[i]);
+        }
+    }
+
     void clear()
     {
         while (auto* segment = m_segments.take_first())

Libraries/LibWeb/HTML/EventLoop/EventLoop.cpp

@@ -38,7 +38,6 @@ EventLoop::EventLoop(Type type)
     : m_type(type)
 {
     m_task_queue = heap().allocate<TaskQueue>(*this);
-    m_microtask_queue = heap().allocate<TaskQueue>(*this);
 
     m_rendering_task_function = GC::create_function(heap(), [this] {
         update_the_rendering();
@@ -52,7 +51,7 @@ void EventLoop::visit_edges(Visitor& visitor)
     Base::visit_edges(visitor);
     visitor.visit(m_reached_step_1_tasks);
     visitor.visit(m_task_queue);
-    visitor.visit(m_microtask_queue);
+    m_microtask_queue.for_each([&](auto& task) { visitor.visit(task); });
     visitor.visit(m_currently_running_task);
     visitor.visit(m_backup_incumbent_realm_stack);
     visitor.visit(m_rendering_task_function);
@@ -227,7 +226,7 @@ void EventLoop::process()
     }
 
     // If there are eligible tasks in the queue, schedule a new round of processing. :^)
-    if (m_task_queue->has_runnable_tasks() || (!m_microtask_queue->is_empty() && !m_performing_a_microtask_checkpoint)) {
+    if (m_task_queue->has_runnable_tasks() || (!m_microtask_queue.is_empty() && !m_performing_a_microtask_checkpoint)) {
         schedule();
     }
 }
@@ -574,10 +573,11 @@ TaskID queue_a_task(HTML::Task::Source source, GC::Ptr<EventLoop> event_loop, GC
     auto task = HTML::Task::create(event_loop->vm(), source, document, steps);
 
     // 8. Let queue be the task queue to which source is associated on event loop.
-    auto& queue = source == HTML::Task::Source::Microtask ? event_loop->microtask_queue() : event_loop->task_queue();
-
     // 9. Append task to queue.
-    queue.add(task);
+    if (source == HTML::Task::Source::Microtask)
+        event_loop->enqueue_microtask(task);
+    else
+        event_loop->task_queue().add(task);
 
     return task->id();
 }
@@ -615,7 +615,7 @@ void queue_a_microtask(DOM::Document const* document, GC::Ref<GC::Function<void(
     // FIXME: 7. Set microtask's script evaluation environment settings object set to an empty set.
 
     // 8. Enqueue microtask on event loop's microtask queue.
-    (void)event_loop.microtask_queue().enqueue(microtask);
+    event_loop.enqueue_microtask(microtask);
 }
 
 void perform_a_microtask_checkpoint()
@@ -641,9 +641,9 @@ void EventLoop::perform_a_microtask_checkpoint()
     m_performing_a_microtask_checkpoint = true;
 
     // 3. While the event loop's microtask queue is not empty:
-    while (!m_microtask_queue->is_empty()) {
+    while (!m_microtask_queue.is_empty()) {
         // 1. Let oldestMicrotask be the result of dequeuing from the event loop's microtask queue.
-        auto oldest_microtask = m_microtask_queue->dequeue();
+        auto oldest_microtask = m_microtask_queue.dequeue();
 
         // 2. Set the event loop's currently running task to oldestMicrotask.
         m_currently_running_task = oldest_microtask;

Libraries/LibWeb/HTML/EventLoop/EventLoop.h

@@ -8,6 +8,7 @@
 
 #include <AK/Function.h>
 #include <AK/Noncopyable.h>
+#include <AK/Queue.h>
 #include <LibCore/Forward.h>
 #include <LibGC/Ptr.h>
 #include <LibGC/Weak.h>
@@ -53,8 +54,9 @@ public:
     TaskQueue& task_queue() { return *m_task_queue; }
     TaskQueue const& task_queue() const { return *m_task_queue; }
 
-    TaskQueue& microtask_queue() { return *m_microtask_queue; }
-    TaskQueue const& microtask_queue() const { return *m_microtask_queue; }
+    bool microtask_queue_empty() const { return m_microtask_queue.is_empty(); }
+    void enqueue_microtask(GC::Ref<HTML::Task> task) { m_microtask_queue.enqueue(task); }
+    GC::Ref<HTML::Task> dequeue_microtask() { return m_microtask_queue.dequeue(); }
 
     void spin_until(GC::Ref<GC::Function<bool()>> goal_condition);
     void spin_processing_tasks_with_source_until(Task::Source, GC::Ref<GC::Function<bool()>> goal_condition);
@@ -108,7 +110,7 @@ private:
     Vector<GC::Ref<GC::Function<void()>>> m_reached_step_1_tasks;
 
     GC::Ptr<TaskQueue> m_task_queue;
-    GC::Ptr<TaskQueue> m_microtask_queue;
+    Queue<GC::Ref<HTML::Task>> m_microtask_queue;
 
     // https://html.spec.whatwg.org/multipage/webappapis.html#currently-running-task
     GC::Ptr<Task> m_currently_running_task { nullptr };