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LibJS: Stop making shapes unique

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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.
This commit is contained in:
Andreas Kling 2023-12-11 14:29:40 +01:00
parent ef86cf4646
commit 3d92c26445
8 changed files with 63 additions and 214 deletions
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101
Userland/Libraries/LibJS/Runtime/Shape.cpp
View file

@ -12,18 +12,6 @@ namespace JS {
JS_DEFINE_ALLOCATOR(Shape);
Shape* Shape::create_unique_clone() const
{
auto new_shape = heap().allocate_without_realm<Shape>(m_realm);
new_shape->m_unique = true;
new_shape->m_prototype = m_prototype;
ensure_property_table();
new_shape->ensure_property_table();
(*new_shape->m_property_table) = *m_property_table;
new_shape->m_property_count = new_shape->m_property_table->size();
return new_shape;
}
Shape* Shape::get_or_prune_cached_forward_transition(TransitionKey const& key)
{
if (!m_forward_transitions)
@ -39,6 +27,21 @@ Shape* Shape::get_or_prune_cached_forward_transition(TransitionKey const& key)
return it->value;
}
GCPtr<Shape> Shape::get_or_prune_cached_delete_transition(StringOrSymbol const& key)
{
if (!m_delete_transitions)
return nullptr;
auto it = m_delete_transitions->find(key);
if (it == m_delete_transitions->end())
return nullptr;
if (!it->value) {
// The cached delete transition has gone stale (from garbage collection). Prune it.
m_delete_transitions->remove(it);
return nullptr;
}
return it->value.ptr();
}
Shape* Shape::get_or_prune_cached_prototype_transition(Object* prototype)
{
if (!m_prototype_transitions)
@ -105,6 +108,17 @@ Shape::Shape(Shape& previous_shape, StringOrSymbol const& property_key, Property
{
}
Shape::Shape(Shape& previous_shape, StringOrSymbol const& property_key, TransitionType transition_type)
: m_realm(previous_shape.m_realm)
, m_previous(&previous_shape)
, m_property_key(property_key)
, m_prototype(previous_shape.m_prototype)
, m_property_count(previous_shape.m_property_count - 1)
, m_transition_type(transition_type)
{
VERIFY(transition_type == TransitionType::Delete);
}
Shape::Shape(Shape& previous_shape, Object* new_prototype)
: m_realm(previous_shape.m_realm)
, m_previous(&previous_shape)
@ -132,6 +146,12 @@ void Shape::visit_edges(Cell::Visitor& visitor)
for (auto& it : *m_forward_transitions)
it.key.property_key.visit_edges(visitor);
}
// FIXME: The delete transition keys should be weak, but we have to mark them for now in case they go stale.
if (m_delete_transitions) {
for (auto& it : *m_delete_transitions)
it.key.visit_edges(visitor);
}
}
Optional<PropertyMetadata> Shape::lookup(StringOrSymbol const& property_key) const
@ -159,6 +179,7 @@ void Shape::ensure_property_table() const
u32 next_offset = 0;
Vector<Shape const&, 64> transition_chain;
transition_chain.append(*this);
for (auto shape = m_previous; shape; shape = shape->m_previous) {
if (shape->m_property_table) {
*m_property_table = *shape->m_property_table;
@ -167,7 +188,6 @@ void Shape::ensure_property_table() const
}
transition_chain.append(*shape);
}
transition_chain.append(*this);
for (auto const& shape : transition_chain.in_reverse()) {
if (!shape.m_property_key.is_valid()) {
@ -180,48 +200,29 @@ void Shape::ensure_property_table() const
auto it = m_property_table->find(shape.m_property_key);
VERIFY(it != m_property_table->end());
it->value.attributes = shape.m_attributes;
} else if (shape.m_transition_type == TransitionType::Delete) {
auto remove_it = m_property_table->find(shape.m_property_key);
VERIFY(remove_it != m_property_table->end());
auto removed_offset = remove_it->value.offset;
m_property_table->remove(remove_it);
for (auto& it : *m_property_table) {
if (it.value.offset > removed_offset)
--it.value.offset;
}
--next_offset;
}
}
}
void Shape::add_property_to_unique_shape(StringOrSymbol const& property_key, PropertyAttributes attributes)
NonnullGCPtr<Shape> Shape::create_delete_transition(StringOrSymbol const& property_key)
{
VERIFY(is_unique());
VERIFY(m_property_table);
VERIFY(!m_property_table->contains(property_key));
m_property_table->set(property_key, { static_cast<u32>(m_property_table->size()), attributes });
VERIFY(m_property_count < NumericLimits<u32>::max());
++m_property_count;
++m_unique_shape_serial_number;
}
void Shape::reconfigure_property_in_unique_shape(StringOrSymbol const& property_key, PropertyAttributes attributes)
{
VERIFY(is_unique());
VERIFY(m_property_table);
auto it = m_property_table->find(property_key);
VERIFY(it != m_property_table->end());
it->value.attributes = attributes;
m_property_table->set(property_key, it->value);
++m_unique_shape_serial_number;
}
void Shape::remove_property_from_unique_shape(StringOrSymbol const& property_key, size_t offset)
{
VERIFY(is_unique());
VERIFY(m_property_table);
if (m_property_table->remove(property_key))
--m_property_count;
for (auto& it : *m_property_table) {
VERIFY(it.value.offset != offset);
if (it.value.offset > offset)
--it.value.offset;
}
++m_unique_shape_serial_number;
if (auto existing_shape = get_or_prune_cached_delete_transition(property_key))
return *existing_shape;
auto new_shape = heap().allocate_without_realm<Shape>(*this, property_key, TransitionType::Delete);
if (!m_delete_transitions)
m_delete_transitions = make<HashMap<StringOrSymbol, WeakPtr<Shape>>>();
m_delete_transitions->set(property_key, new_shape.ptr());
return new_shape;
}
void Shape::add_property_without_transition(StringOrSymbol const& property_key, PropertyAttributes attributes)