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LibJS: Introduce "dictionary" mode for object shapes

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This is similar to "unique" shapes, which were removed in commit
3d92c26445.

The key difference is that dictionary shapes don't have a serial number,
but instead have a "cacheable" flag.

Shapes become dictionaries after 64 transitions have occurred, at which
point no further transitions occur.

As long as properties are only added to a dictionary shape, it remains
cacheable. (Since if we've cached the shape pointer in an IC somewhere,
we know the IC is still valid.)

Deleting a property from a dictionary shape causes it to become an
uncacheable dictionary.

Note that deleting a property from a non-dictionary shape still performs
a delete transition.

This fixes an issue on Discord where Object.freeze() would eventually
OOM us, since they add more than 16000 properties to a single object
before freezing it.

It also yields a 15% speedup on Octane/pdfjs.js :^)
This commit is contained in:
Andreas Kling 2023-12-16 11:34:01 +01:00
parent 8255a1a5ee
commit 1e90379008
4 changed files with 94 additions and 4 deletions
Download patch file Download diff file Expand all files Collapse all files

49
Userland/Libraries/LibJS/Runtime/Shape.cpp
View file

@ -12,6 +12,32 @@ namespace JS {
JS_DEFINE_ALLOCATOR(Shape);
NonnullGCPtr<Shape> Shape::create_cacheable_dictionary_transition()
{
auto new_shape = heap().allocate_without_realm<Shape>(m_realm);
new_shape->m_dictionary = true;
new_shape->m_cacheable = 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;
}
NonnullGCPtr<Shape> Shape::create_uncacheable_dictionary_transition()
{
auto new_shape = heap().allocate_without_realm<Shape>(m_realm);
new_shape->m_dictionary = true;
new_shape->m_cacheable = 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)
@ -241,4 +267,27 @@ FLATTEN void Shape::add_property_without_transition(PropertyKey const& property_
add_property_without_transition(property_key.to_string_or_symbol(), attributes);
}
void Shape::set_property_attributes_without_transition(StringOrSymbol const& property_key, PropertyAttributes attributes)
{
VERIFY(is_dictionary());
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);
}
void Shape::remove_property_without_transition(StringOrSymbol const& property_key, u32 offset)
{
VERIFY(is_uncacheable_dictionary());
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;
}
}
}