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LibJS: Stop making shapes unique
We previously had a concept of unique shapes, which meant that they couldn't be shared between multiple objects. Object shapes became unique in three situations: - They were the shape of the global object. - They had more than 100 properties added to them. - They had one or more properties deleted from them. Unfortunately, unique shapes presented an annoying problem for inline caches, and we added a "unique shape serial number" for being able to tell that a unique shape had been mutated. This patch gets rid of the concept of unique shapes, simplifying all the caching code, since inline caches can now simply perform a shape check and then we're good. To make this possible, we now have the concept of delete transitions, which occur when a property is deleted from a shape. Note that this patch by itself introduces a performance regression in some situtations, since we now create a lot more shapes, and marking their property keys can be very heavy. This will be addressed in a subsequent patch.
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8 changed files with 63 additions and 214 deletions
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@ -12,18 +12,6 @@ namespace JS {
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JS_DEFINE_ALLOCATOR(Shape);
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Shape* Shape::create_unique_clone() const
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{
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auto new_shape = heap().allocate_without_realm<Shape>(m_realm);
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new_shape->m_unique = true;
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new_shape->m_prototype = m_prototype;
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ensure_property_table();
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new_shape->ensure_property_table();
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(*new_shape->m_property_table) = *m_property_table;
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new_shape->m_property_count = new_shape->m_property_table->size();
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return new_shape;
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}
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Shape* Shape::get_or_prune_cached_forward_transition(TransitionKey const& key)
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{
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if (!m_forward_transitions)
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@ -39,6 +27,21 @@ Shape* Shape::get_or_prune_cached_forward_transition(TransitionKey const& key)
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return it->value;
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}
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GCPtr<Shape> Shape::get_or_prune_cached_delete_transition(StringOrSymbol const& key)
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{
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if (!m_delete_transitions)
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return nullptr;
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auto it = m_delete_transitions->find(key);
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if (it == m_delete_transitions->end())
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return nullptr;
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if (!it->value) {
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// The cached delete transition has gone stale (from garbage collection). Prune it.
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m_delete_transitions->remove(it);
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return nullptr;
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}
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return it->value.ptr();
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}
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Shape* Shape::get_or_prune_cached_prototype_transition(Object* prototype)
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{
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if (!m_prototype_transitions)
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@ -105,6 +108,17 @@ Shape::Shape(Shape& previous_shape, StringOrSymbol const& property_key, Property
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{
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}
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Shape::Shape(Shape& previous_shape, StringOrSymbol const& property_key, TransitionType transition_type)
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: m_realm(previous_shape.m_realm)
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, m_previous(&previous_shape)
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, m_property_key(property_key)
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, m_prototype(previous_shape.m_prototype)
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, m_property_count(previous_shape.m_property_count - 1)
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, m_transition_type(transition_type)
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{
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VERIFY(transition_type == TransitionType::Delete);
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}
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Shape::Shape(Shape& previous_shape, Object* new_prototype)
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: m_realm(previous_shape.m_realm)
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, m_previous(&previous_shape)
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@ -132,6 +146,12 @@ void Shape::visit_edges(Cell::Visitor& visitor)
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for (auto& it : *m_forward_transitions)
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it.key.property_key.visit_edges(visitor);
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}
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// FIXME: The delete transition keys should be weak, but we have to mark them for now in case they go stale.
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if (m_delete_transitions) {
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for (auto& it : *m_delete_transitions)
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it.key.visit_edges(visitor);
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}
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}
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Optional<PropertyMetadata> Shape::lookup(StringOrSymbol const& property_key) const
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@ -159,6 +179,7 @@ void Shape::ensure_property_table() const
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u32 next_offset = 0;
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Vector<Shape const&, 64> transition_chain;
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transition_chain.append(*this);
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for (auto shape = m_previous; shape; shape = shape->m_previous) {
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if (shape->m_property_table) {
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*m_property_table = *shape->m_property_table;
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@ -167,7 +188,6 @@ void Shape::ensure_property_table() const
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}
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transition_chain.append(*shape);
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}
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transition_chain.append(*this);
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for (auto const& shape : transition_chain.in_reverse()) {
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if (!shape.m_property_key.is_valid()) {
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@ -180,48 +200,29 @@ void Shape::ensure_property_table() const
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auto it = m_property_table->find(shape.m_property_key);
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VERIFY(it != m_property_table->end());
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it->value.attributes = shape.m_attributes;
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} else if (shape.m_transition_type == TransitionType::Delete) {
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auto remove_it = m_property_table->find(shape.m_property_key);
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VERIFY(remove_it != m_property_table->end());
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auto removed_offset = remove_it->value.offset;
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m_property_table->remove(remove_it);
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for (auto& it : *m_property_table) {
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if (it.value.offset > removed_offset)
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--it.value.offset;
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}
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--next_offset;
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}
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}
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}
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void Shape::add_property_to_unique_shape(StringOrSymbol const& property_key, PropertyAttributes attributes)
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NonnullGCPtr<Shape> Shape::create_delete_transition(StringOrSymbol const& property_key)
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{
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VERIFY(is_unique());
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VERIFY(m_property_table);
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VERIFY(!m_property_table->contains(property_key));
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m_property_table->set(property_key, { static_cast<u32>(m_property_table->size()), attributes });
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VERIFY(m_property_count < NumericLimits<u32>::max());
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++m_property_count;
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++m_unique_shape_serial_number;
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}
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void Shape::reconfigure_property_in_unique_shape(StringOrSymbol const& property_key, PropertyAttributes attributes)
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{
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VERIFY(is_unique());
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VERIFY(m_property_table);
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auto it = m_property_table->find(property_key);
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VERIFY(it != m_property_table->end());
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it->value.attributes = attributes;
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m_property_table->set(property_key, it->value);
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++m_unique_shape_serial_number;
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}
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void Shape::remove_property_from_unique_shape(StringOrSymbol const& property_key, size_t offset)
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{
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VERIFY(is_unique());
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VERIFY(m_property_table);
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if (m_property_table->remove(property_key))
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--m_property_count;
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for (auto& it : *m_property_table) {
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VERIFY(it.value.offset != offset);
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if (it.value.offset > offset)
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--it.value.offset;
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}
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++m_unique_shape_serial_number;
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if (auto existing_shape = get_or_prune_cached_delete_transition(property_key))
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return *existing_shape;
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auto new_shape = heap().allocate_without_realm<Shape>(*this, property_key, TransitionType::Delete);
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if (!m_delete_transitions)
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m_delete_transitions = make<HashMap<StringOrSymbol, WeakPtr<Shape>>>();
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m_delete_transitions->set(property_key, new_shape.ptr());
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return new_shape;
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}
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void Shape::add_property_without_transition(StringOrSymbol const& property_key, PropertyAttributes attributes)
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