gh-92810: Reduce memory usage by ABCMeta.__subclasscheck__ (alternative2) by dolfinus · Pull Request #150540 · python/cpython

• Issue: _abc._abc_subclasscheck has very poor performance and (I think) a memory leak #92810

Alternative implementation of #131914 which does not check __subclasses__ recursively.
To handle cases like:

class Number(ABC): ...

class Real(Number): ...

class Integral(Real): ...

Integral.register(int)

assert issubclass(int, Number) is True

which previously were implemented via recursive __subclasses__ check, method cls.register(subclass) is calling super(cls).register(subclass) recursively ("bubble-up" registration to all the parents).

For benchmark from #131914:

sudo ./python -m pyperf system tune
taskset -c 0 ./python benchmark.py --metaclass abc.ABCMeta --rounds 3 --classes 5000
taskset -c 0 ./python benchmark.py --metaclass _py_abc.ABCMeta --rounds 3 --classes 5000

Memory increment is measured during isinstance() / issubclass() calls, not during preparation, like class creation or registration where actual registry allocation is performed. So memory usage in tables below is almost always 0.

Timing drop in _py_abc implementation for 2 first rows is due to if subclass in cls._abc_registry: check added to match _abc.c implementation.

Just to check that nothing is broken:

Flamegraphs for _py_abc impl and test issubclass_uncle (the most time and memory consuming case on main):
main_vs_pr150540.tar.gz