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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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22
Userland/Libraries/LibJS/Runtime/Shape.h
View file

@ -48,20 +48,17 @@ public:
Put,
Configure,
Prototype,
Delete,
};
Shape* create_put_transition(StringOrSymbol const&, PropertyAttributes attributes);
Shape* create_configure_transition(StringOrSymbol const&, PropertyAttributes attributes);
Shape* create_prototype_transition(Object* new_prototype);
[[nodiscard]] NonnullGCPtr<Shape> create_delete_transition(StringOrSymbol const&);
void add_property_without_transition(StringOrSymbol const&, PropertyAttributes);
void add_property_without_transition(PropertyKey const&, PropertyAttributes);
bool is_unique() const { return m_unique; }
static FlatPtr is_unique_offset() { return OFFSET_OF(Shape, m_unique); }
Shape* create_unique_clone() const;
Realm& realm() const { return m_realm; }
Object* prototype() { return m_prototype; }
@ -78,22 +75,17 @@ public:
void set_prototype_without_transition(Object* new_prototype) { m_prototype = new_prototype; }
void remove_property_from_unique_shape(StringOrSymbol const&, size_t offset);
void add_property_to_unique_shape(StringOrSymbol const&, PropertyAttributes attributes);
void reconfigure_property_in_unique_shape(StringOrSymbol const& property_key, PropertyAttributes attributes);
[[nodiscard]] u64 unique_shape_serial_number() const { return m_unique_shape_serial_number; }
static FlatPtr unique_shape_serial_number_offset() { return OFFSET_OF(Shape, m_unique_shape_serial_number); }
private:
explicit Shape(Realm&);
Shape(Shape& previous_shape, StringOrSymbol const& property_key, PropertyAttributes attributes, TransitionType);
Shape(Shape& previous_shape, StringOrSymbol const& property_key, TransitionType);
Shape(Shape& previous_shape, Object* new_prototype);
virtual void visit_edges(Visitor&) override;
Shape* get_or_prune_cached_forward_transition(TransitionKey const&);
Shape* get_or_prune_cached_prototype_transition(Object* prototype);
[[nodiscard]] GCPtr<Shape> get_or_prune_cached_delete_transition(StringOrSymbol const&);
void ensure_property_table() const;
@ -103,6 +95,7 @@ private:
OwnPtr<HashMap<TransitionKey, WeakPtr<Shape>>> m_forward_transitions;
OwnPtr<HashMap<GCPtr<Object>, WeakPtr<Shape>>> m_prototype_transitions;
OwnPtr<HashMap<StringOrSymbol, WeakPtr<Shape>>> m_delete_transitions;
GCPtr<Shape> m_previous;
StringOrSymbol m_property_key;
GCPtr<Object> m_prototype;
@ -110,11 +103,6 @@ private:
PropertyAttributes m_attributes { 0 };
TransitionType m_transition_type { TransitionType::Invalid };
bool m_unique { false };
// Since unique shapes never change identity, inline caches use this incrementing serial number
// to know whether its property table has been modified since last time we checked.
u64 m_unique_shape_serial_number { 0 };
};
}