LibJS: Cache the Intl.Collator in String.prototype.localeCompare()

In the very common case that no special constructor options are provided
for the Intl.Collator when calling localeCompare() on a string, we can
cache and reuse a default-constructed Intl.Collator, saving lots of time
and space.

This shaves a fair bit of load time off of https://wpt.fyi/ where they
use Array.prototype.sort() and localeCompare() to sort a big JSON thing.

Time spent in sort():
- Before: 1656 ms
- After: 135 ms

Libraries/LibJS/Runtime/Intrinsics.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2022-2023, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2020-2025, Andreas Kling <andreas@ladybird.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -41,6 +42,7 @@
 #include <LibJS/Runtime/GeneratorFunctionConstructor.h>
 #include <LibJS/Runtime/GeneratorFunctionPrototype.h>
 #include <LibJS/Runtime/GeneratorPrototype.h>
+#include <LibJS/Runtime/Intl/Collator.h>
 #include <LibJS/Runtime/Intl/CollatorConstructor.h>
 #include <LibJS/Runtime/Intl/CollatorPrototype.h>
 #include <LibJS/Runtime/Intl/DateTimeFormatConstructor.h>
@@ -418,6 +420,16 @@ void Intrinsics::visit_edges(Visitor& visitor)
     visitor.visit(m_##snake_name##_prototype);
     JS_ENUMERATE_ITERATOR_PROTOTYPES
 #undef __JS_ENUMERATE
+
+    visitor.visit(m_default_collator);
+}
+
+GC::Ref<Intl::Collator> Intrinsics::default_collator()
+{
+    if (!m_default_collator) {
+        m_default_collator = as<Intl::Collator>(*MUST(construct(this->vm(), intl_collator_constructor(), js_undefined(), js_undefined())));
+    }
+    return *m_default_collator;
 }
 
 // 10.2.4 AddRestrictedFunctionProperties ( F, realm ), https://tc39.es/ecma262/#sec-addrestrictedfunctionproperties

Libraries/LibJS/Runtime/Intrinsics.h

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2022-2023, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2020-2025, Andreas Kling <andreas@ladybird.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -95,6 +96,8 @@ public:
     JS_ENUMERATE_ITERATOR_PROTOTYPES
 #undef __JS_ENUMERATE
 
+    [[nodiscard]] GC::Ref<Intl::Collator> default_collator();
+
 private:
     Intrinsics(Realm& realm)
         : m_realm(realm)
@@ -189,6 +192,8 @@ private:
     GC::Ptr<Object> m_##snake_name##_prototype;
     JS_ENUMERATE_ITERATOR_PROTOTYPES
 #undef __JS_ENUMERATE
+
+    GC::Ptr<Intl::Collator> m_default_collator;
 };
 
 void add_restricted_function_properties(FunctionObject&, Realm&);

Libraries/LibJS/Runtime/StringPrototype.cpp

@@ -544,7 +544,15 @@ JS_DEFINE_NATIVE_FUNCTION(StringPrototype::locale_compare)
     auto that_value = TRY(vm.argument(0).to_string(vm));
 
     // 4. Let collator be ? Construct(%Collator%, « locales, options »).
-    auto collator = TRY(construct(vm, realm.intrinsics().intl_collator_constructor(), vm.argument(1), vm.argument(2)));
+    auto locales = vm.argument(1);
+    auto options = vm.argument(2);
+
+    // OPTIMIZATION: If both locales and options are undefined, we can use a cached default-constructed Collator.
+    GC::Ptr<Object> collator;
+    if (locales.is_undefined() && options.is_undefined())
+        collator = realm.intrinsics().default_collator();
+    else
+        collator = TRY(construct(vm, realm.intrinsics().intl_collator_constructor(), locales, options));
 
     // 5. Return CompareStrings(collator, S, thatValue).
     return Intl::compare_strings(static_cast<Intl::Collator const&>(*collator), string, that_value);