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LibJS: Make mapped arguments objects way less allocation-happy
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Browse source 
By following the spec to the letter, our mapped arguments objects ended
up with many extra GC allocations:

- 1 extra Object for the internal [[ParameterMap]].
- 2 extra NativeFunctions for each mapped parameter accessor.
- 1 extra Accessor to hold the aforementioned NativeFunctions.

This patch removes all those allocations and lets ArgumentsObject model
the desired behavior in custom C++ instead of using script primitives.

1.06x speedup on Speedometer's TodoMVC-jQuery.
This commit is contained in:
Andreas Kling 2025-05-11 12:32:24 +02:00 • committed by Andreas Kling
commit a0864dbb26
Notes: github-actions[bot] 2025-05-11 12:01:34 +00:00
3 changed files with 63 additions and 44 deletions
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62
Libraries/LibJS/Runtime/ArgumentsObject.cpp
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@ -1,5 +1,5 @@
/*
* Copyright (c) 2021, Andreas Kling <andreas@ladybird.org>
* Copyright (c) 2021-2025, Andreas Kling <andreas@ladybird.org>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
@ -22,24 +22,27 @@ void ArgumentsObject::initialize(Realm& realm)
{
Base::initialize(realm);
set_has_parameter_map();
m_parameter_map = Object::create(realm, nullptr);
}
void ArgumentsObject::visit_edges(Cell::Visitor& visitor)
{
Base::visit_edges(visitor);
visitor.visit(m_environment);
visitor.visit(m_parameter_map);
}
bool ArgumentsObject::parameter_map_has(PropertyKey const& key) const
{
return key.is_number()
&& key.as_number() < m_mapped_names.size()
&& !m_mapped_names[key.as_number()].is_empty();
}
// 10.4.4.3 [[Get]] ( P, Receiver ), https://tc39.es/ecma262/#sec-arguments-exotic-objects-get-p-receiver
ThrowCompletionOr<Value> ArgumentsObject::internal_get(PropertyKey const& property_key, Value receiver, CacheablePropertyMetadata* cacheable_metadata, PropertyLookupPhase phase) const
{
// 1. Let map be args.[[ParameterMap]].
auto& map = *m_parameter_map;
// 2. Let isMapped be ! HasOwnProperty(map, P).
bool is_mapped = MUST(m_parameter_map->has_own_property(property_key));
bool is_mapped = parameter_map_has(property_key);
// 3. If isMapped is false, then
if (!is_mapped) {
@ -47,10 +50,9 @@ ThrowCompletionOr<Value> ArgumentsObject::internal_get(PropertyKey const& proper
return Object::internal_get(property_key, receiver, cacheable_metadata, phase);
}
// FIXME: a. Assert: map contains a formal parameter mapping for P.
// a. Assert: map contains a formal parameter mapping for P.
// b. Return ! Get(map, P).
return MUST(map.get(property_key));
return get_from_parameter_map(property_key);
}
// 10.4.4.4 [[Set]] ( P, V, Receiver ), https://tc39.es/ecma262/#sec-arguments-exotic-objects-set-p-v-receiver
@ -65,15 +67,14 @@ ThrowCompletionOr<bool> ArgumentsObject::internal_set(PropertyKey const& propert
} else {
// a. Let map be args.[[ParameterMap]].
// b. Let isMapped be ! HasOwnProperty(map, P).
is_mapped = MUST(parameter_map().has_own_property(property_key));
is_mapped = parameter_map_has(property_key);
}
// 3. If isMapped is true, then
if (is_mapped) {
// a. Assert: The following Set will succeed, since formal parameters mapped by arguments objects are always writable.
// b. Perform ! Set(map, P, V, false).
MUST(m_parameter_map->set(property_key, value, Object::ShouldThrowExceptions::No));
set_in_parameter_map(property_key, value);
}
// 4. Return ? OrdinarySet(args, P, V, Receiver).
@ -84,10 +85,8 @@ ThrowCompletionOr<bool> ArgumentsObject::internal_set(PropertyKey const& propert
ThrowCompletionOr<bool> ArgumentsObject::internal_delete(PropertyKey const& property_key)
{
// 1. Let map be args.[[ParameterMap]].
auto& map = parameter_map();
// 2. Let isMapped be ! HasOwnProperty(map, P).
bool is_mapped = MUST(map.has_own_property(property_key));
bool is_mapped = parameter_map_has(property_key);
// 3. Let result be ? OrdinaryDelete(args, P).
bool result = TRY(Object::internal_delete(property_key));
@ -95,7 +94,7 @@ ThrowCompletionOr<bool> ArgumentsObject::internal_delete(PropertyKey const& prop
// 4. If result is true and isMapped is true, then
if (result && is_mapped) {
// a. Perform ! map.[[Delete]](P).
MUST(map.internal_delete(property_key));
delete_from_parameter_map(property_key);
}
// 5. Return result.
@ -114,12 +113,12 @@ ThrowCompletionOr<Optional<PropertyDescriptor>> ArgumentsObject::internal_get_ow
// 3. Let map be args.[[ParameterMap]].
// 4. Let isMapped be ! HasOwnProperty(map, P).
bool is_mapped = MUST(m_parameter_map->has_own_property(property_key));
bool is_mapped = parameter_map_has(property_key);
// 5. If isMapped is true, then
if (is_mapped) {
// a. Set desc.[[Value]] to ! Get(map, P).
desc->value = MUST(m_parameter_map->get(property_key));
desc->value = get_from_parameter_map(property_key);
}
// 6. Return desc.
@ -130,10 +129,8 @@ ThrowCompletionOr<Optional<PropertyDescriptor>> ArgumentsObject::internal_get_ow
ThrowCompletionOr<bool> ArgumentsObject::internal_define_own_property(PropertyKey const& property_key, PropertyDescriptor const& descriptor, Optional<PropertyDescriptor>* precomputed_get_own_property)
{
// 1. Let map be args.[[ParameterMap]].
auto& map = parameter_map();
// 2. Let isMapped be ! HasOwnProperty(map, P).
bool is_mapped = MUST(map.has_own_property(property_key));
bool is_mapped = parameter_map_has(property_key);
// 3. Let newArgDesc be Desc.
auto new_arg_desc = descriptor;
@ -145,7 +142,7 @@ ThrowCompletionOr<bool> ArgumentsObject::internal_define_own_property(PropertyKe
// i. Set newArgDesc to a copy of Desc.
new_arg_desc = descriptor;
// ii. Set newArgDesc.[[Value]] to ! Get(map, P).
new_arg_desc.value = MUST(map.get(property_key));
new_arg_desc.value = get_from_parameter_map(property_key);
}
}
@ -161,19 +158,19 @@ ThrowCompletionOr<bool> ArgumentsObject::internal_define_own_property(PropertyKe
// a. If IsAccessorDescriptor(Desc) is true, then
if (descriptor.is_accessor_descriptor()) {
// i. Perform ! map.[[Delete]](P).
MUST(map.internal_delete(property_key));
delete_from_parameter_map(property_key);
} else {
// i. If Desc has a [[Value]] field, then
if (descriptor.value.has_value()) {
// 1. Assert: The following Set will succeed, since formal parameters mapped by arguments objects are always writable.
// 2. Perform ! Set(map, P, Desc.[[Value]], false).
MUST(map.set(property_key, descriptor.value.value(), Object::ShouldThrowExceptions::No));
set_in_parameter_map(property_key, descriptor.value.value());
}
// ii. If Desc has a [[Writable]] field and Desc.[[Writable]] is false, then
if (descriptor.writable == false) {
// 1. Perform ! map.[[Delete]](P).
MUST(map.internal_delete(property_key));
delete_from_parameter_map(property_key);
}
}
}
@ -182,4 +179,19 @@ ThrowCompletionOr<bool> ArgumentsObject::internal_define_own_property(PropertyKe
return true;
}
void ArgumentsObject::delete_from_parameter_map(PropertyKey const& property_key)
{
m_mapped_names[property_key.as_number()] = FlyString {};
}
Value ArgumentsObject::get_from_parameter_map(PropertyKey const& property_key) const
{
return MUST(m_environment->get_binding_value(vm(), m_mapped_names[property_key.as_number()], false));
}
void ArgumentsObject::set_in_parameter_map(PropertyKey const& property_key, Value value)
{
MUST(m_environment->set_mutable_binding(vm(), m_mapped_names[property_key.as_number()], value, false));
}
}