LibJS: Add fast path for Array.prototype.shift

Makes `MicroBench/array-prototype-shift.js` 100x faster on my machine.

Progress on https://github.com/LadybirdBrowser/ladybird/issues/5725

Libraries/LibJS/Runtime/Array.cpp

@@ -299,35 +299,36 @@ ThrowCompletionOr<Optional<PropertyDescriptor>> Array::internal_get_own_property
     return Object::internal_get_own_property(property_key);
 }
 
+bool Array::default_prototype_chain_intact() const
+{
+    auto const& intrinsics = m_realm->intrinsics();
+    auto const* array_prototype = shape().prototype();
+    if (!array_prototype)
+        return false;
+    if (!array_prototype->indexed_properties().is_empty())
+        return false;
+    auto const& array_prototype_shape = shape().prototype()->shape();
+    if (intrinsics.default_array_prototype_shape().ptr() != &array_prototype_shape)
+        return false;
+
+    auto const* object_prototype = array_prototype_shape.prototype();
+    if (!object_prototype)
+        return false;
+    if (!object_prototype->indexed_properties().is_empty())
+        return false;
+    auto const& object_prototype_shape = array_prototype_shape.prototype()->shape();
+    if (intrinsics.default_object_prototype_shape().ptr() != &object_prototype_shape)
+        return false;
+    if (object_prototype_shape.prototype())
+        return false;
+
+    return true;
+}
+
 ThrowCompletionOr<bool> Array::internal_set(PropertyKey const& property_key, Value value, Value receiver, CacheablePropertyMetadata* cacheable_metadata, PropertyLookupPhase phase)
 {
     auto& vm = this->vm();
 
-    auto default_prototype_chain_intact = [&] {
-        auto const& intrinsics = m_realm->intrinsics();
-        auto* array_prototype = shape().prototype();
-        if (!array_prototype)
-            return false;
-        if (!array_prototype->indexed_properties().is_empty())
-            return false;
-        auto& array_prototype_shape = shape().prototype()->shape();
-        if (intrinsics.default_array_prototype_shape().ptr() != &array_prototype_shape)
-            return false;
-
-        auto* object_prototype = array_prototype_shape.prototype();
-        if (!object_prototype)
-            return false;
-        if (!object_prototype->indexed_properties().is_empty())
-            return false;
-        auto& object_prototype_shape = array_prototype_shape.prototype()->shape();
-        if (intrinsics.default_object_prototype_shape().ptr() != &object_prototype_shape)
-            return false;
-        if (object_prototype_shape.prototype())
-            return false;
-
-        return true;
-    };
-
     VERIFY(receiver.is_object());
     auto& receiver_object = receiver.as_object();
 

Libraries/LibJS/Runtime/Array.h

@@ -57,8 +57,11 @@ public:
 
     [[nodiscard]] bool length_is_writable() const { return m_length_writable; }
 
+    bool is_proxy_target() const { return m_is_proxy_target; }
     void set_is_proxy_target(bool is_proxy_target) { m_is_proxy_target = is_proxy_target; }
 
+    bool default_prototype_chain_intact() const;
+
     virtual void visit_edges(Cell::Visitor& visitor) override;
 
 protected:

Libraries/LibJS/Runtime/ArrayPrototype.cpp

@@ -1220,6 +1220,19 @@ JS_DEFINE_NATIVE_FUNCTION(ArrayPrototype::shift)
         TRY(this_object->set(vm.names.length, Value(0), Object::ShouldThrowExceptions::Yes));
         return js_undefined();
     }
+
+    // OPTIMIZATION: If this object is Array that:
+    // - is not a proxy target, which means get/set/has/delete will not trap.
+    // - has intact prototype chain, which means we don't have to worry about getters/setters potentially defined for holes.
+    // - has simple storage type, which means all values have default attributes (if some elements have configurable=false, we cannot use fast path, because delete operation will fail).
+    // then we could take a fast path by directly taking first element from indexed storage.
+    if (auto* array = as_if<Array>(*this_object); array && !array->is_proxy_target() && array->default_prototype_chain_intact() && array->indexed_properties().storage()->is_simple_storage()) {
+        auto first = array->indexed_properties().storage()->take_first().value;
+        if (first.is_special_empty_value())
+            return js_undefined();
+        return first;
+    }
+
     auto first = TRY(this_object->get(0));
     for (size_t k = 1; k < length; ++k) {
         size_t from = k;