[Python] Why does Python 3.14 use ob_ref_local and ob_ref_shared for reference counts when GIL...

According to Include/object.h in Python's source code:

struct _object {
// ob_tid stores the thread id (or zero). It is also used by the GC and the
// trashcan mechanism as a linked list pointer and by the GC to store the
// computed "gc_refs" refcount.
uintptr_t ob_tid;
uint16_t ob_flags;
PyMutex ob_mutex; // per-object lock
uint8_t ob_gc_bits; // gc-related state
uint32_t ob_ref_local; // local reference count
Py_ssize_t ob_ref_shared; // shared (atomic) reference count
PyTypeObject *ob_type;
};


When I managed to use an external tool, pyobject, to inspect them:

C:\Users\admin>py314t
Python 3.14.0a5 experimental free-threading build (tags/v3.14.0a5:3ae9101, Feb 11 2025, 17:44:01) [MSC v.1942 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from pyobject import getrefcount_nogil,setrefcount_nogil
>>> class Aass
...
>>> a=A()
>>> getrefcount_nogil(a)
(1, 0) # the first is ob_ref_local and the second is ob_ref_shared
>>> def another_thread():
... b=a
... while Trueass
...
>>> from _thread import start_new_thread
>>> start_new_thread(another_thread,())
10824
>>> getrefcount_nogil(a)
(1, 5)
>>>


So, what is the function and implication of ob_ref_local and ob_ref_shared values that replace ob_refcnt?

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