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LibJS: Cache stable for-in iteration at bytecode sites

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Cache the flattened enumerable key snapshot for each `for..in` site and
reuse a `PropertyNameIterator` when the receiver shape, dictionary
generation, indexed storage kind and length, prototype chain
validity, and magical-length state still match.

Handle packed indexed receivers as well as plain named-property
objects. Teach `ObjectPropertyIteratorNext` in `asmint.asm` to return
cached property values directly and to fall back to the slow iterator
logic when any guard fails.

Treat arrays' hidden non-enumerable `length` property as a visited
name for for-in shadowing, and include the receiver's magical-length
state in the cache key so arrays and plain objects do not share
snapshots.

Add `test-js` and `test-js-bytecode` coverage for mixed numeric and
named keys, packed receiver transitions, re-entry, iterator reuse, GC
retention, array length shadowing, and same-site cache reuse.
This commit is contained in:
Andreas Kling 2026-04-10 00:56:49 +02:00 • committed by Andreas Kling
parent 114eeddea1
commit 879ac36e45
Notes: github-actions[bot] 2026-04-10 13:14:02 +00:00
27 changed files with 1453 additions and 157 deletions
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41
Libraries/LibJS/Bytecode/Executable.cpp
View file

@ -19,6 +19,38 @@
namespace JS::Bytecode {
GC_DEFINE_ALLOCATOR(Executable);
GC_DEFINE_ALLOCATOR(ObjectPropertyIteratorCacheData);
ObjectPropertyIteratorCacheData::ObjectPropertyIteratorCacheData(VM& vm, Vector<PropertyKey> properties, ObjectPropertyIteratorFastPath fast_path, u32 indexed_property_count, bool receiver_has_magical_length_property, GC::Ref<Shape> shape, GC::Ptr<PrototypeChainValidity> prototype_chain_validity)
: m_properties(move(properties))
, m_shape(shape)
, m_prototype_chain_validity(prototype_chain_validity)
, m_indexed_property_count(indexed_property_count)
, m_receiver_has_magical_length_property(receiver_has_magical_length_property)
, m_fast_path(fast_path)
{
// The iterator fast path returns JS Values directly, so materialize the
// cached key list once up front instead of converting PropertyKeys during
// every ObjectPropertyIteratorNext.
m_property_values.ensure_capacity(indexed_property_count + m_properties.size());
for (u32 i = 0; i < indexed_property_count; ++i)
m_property_values.append(PropertyKey { i }.to_value(vm));
for (auto const& key : m_properties)
m_property_values.append(key.to_value(vm));
if (m_shape->is_dictionary())
m_shape_dictionary_generation = m_shape->dictionary_generation();
}
void ObjectPropertyIteratorCacheData::visit_edges(Visitor& visitor)
{
Base::visit_edges(visitor);
visitor.visit(m_shape);
visitor.visit(m_prototype_chain_validity);
visitor.visit(m_property_values.span());
for (auto& key : m_properties)
key.visit_edges(visitor);
}
Executable::Executable(
Vector<u8> bytecode,
@ -32,6 +64,7 @@ Executable::Executable(
size_t number_of_global_variable_caches,
size_t number_of_template_object_caches,
size_t number_of_object_shape_caches,
size_t number_of_object_property_iterator_caches,
size_t number_of_registers,
Strict strict)
: GC::WeakContainer(heap())
@ -49,6 +82,7 @@ Executable::Executable(
global_variable_caches.resize(number_of_global_variable_caches);
template_object_caches.resize(number_of_template_object_caches);
object_shape_caches.resize(number_of_object_shape_caches);
object_property_iterator_caches.resize(number_of_object_property_iterator_caches);
}
Executable::~Executable() = default;
@ -61,7 +95,8 @@ void Executable::fixup_cache_pointers()
property_lookup_caches.span(),
global_variable_caches.span(),
template_object_caches.span(),
object_shape_caches.span());
object_shape_caches.span(),
object_property_iterator_caches.span());
}
}
@ -239,6 +274,10 @@ void Executable::visit_edges(Visitor& visitor)
visitor.visit(constants);
for (auto& cache : template_object_caches)
visitor.visit(cache.cached_template_object);
for (auto& cache : object_property_iterator_caches)
visitor.visit(cache.data);
for (auto& cache : object_property_iterator_caches)
visitor.visit(cache.reusable_property_name_iterator);
for (auto& data : shared_function_data)
visitor.visit(data);
for (auto& blueprint : class_blueprints) {