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vstinner

opened on Sep 16, 2024

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In the limited C API 3.14 and newer, I propose to change Py_REFCNT() implementation to an opaque function call to hide implementation details. I made a similar change for Py_INCREF() and Py_DECREF() in Python 3.12.

The problem is that with Free Threading (PEP 703), the implementation of this functions becomes less trivial than just getting the object member PyObject.ob_refcnt:

static inline Py_ssize_t _Py_REFCNT(PyObject *ob) {
    uint32_t local = _Py_atomic_load_uint32_relaxed(&ob->ob_ref_local);
    if (local == _Py_IMMORTAL_REFCNT_LOCAL) {
        return _Py_IMMORTAL_REFCNT;
    }
    Py_ssize_t shared = _Py_atomic_load_ssize_relaxed(&ob->ob_ref_shared);
    return _Py_STATIC_CAST(Py_ssize_t, local) +
           Py_ARITHMETIC_RIGHT_SHIFT(Py_ssize_t, shared, _Py_REF_SHARED_SHIFT);
}

_Py_atomic_load_uint32_relaxed() and _Py_atomic_load_ssize_relaxed() must now be called. But I would prefer to not "leak" such implementation detail into the limited C API.

cc @colesbury @Fidget-Spinner @encukou

Linked PRs

• gh-124127: Make Py_REFCNT() opaque in limited C API 3.14 #124128

Reacted by Petr Viktorin

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1. vstinner

   added

   topic-C-API

   on Sep 16, 2024
2. vstinner

   added 4 commits that reference this issue on Sep 16, 2024

   pythongh-124127: Make Py_REFCNT() opaque in limited C API 3.14

   fcb803b

   pythongh-124127: Make Py_REFCNT() opaque in limited C API 3.14

   85d5f88

   pythongh-124127: Make Py_REFCNT() opaque in limited C API 3.14

   ce0e2bb

   pythongh-124127: Make Py_REFCNT() opaque in limited C API 3.14

   e4b3731
3. vstinner commented on Sep 16, 2024

   

   vstinner

   on Sep 16, 2024

   MemberAuthor

   More actions

   Previous issue about Py_TYPE(): #120600
4. vstinner

   added a commit that references this issue on Sep 24, 2024

   gh-124127: Make Py_REFCNT() opaque in limited C API 3.14 (#124128)

   9d344fa
5. vstinner commented on Sep 24, 2024

   

   vstinner

   on Sep 24, 2024

   MemberAuthor

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   Implemented in 9d344fa.
6. vstinner

   closed this as completedon Sep 24, 2024
7. vstinner commented on Sep 24, 2024

   

   vstinner

   on Sep 24, 2024

   MemberAuthor

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   Functions implemented as opaque function calls in limited C API 3.14:

   • Py_TYPE() (since 3.14)
   • Py_REFCNT() (since 3.14), Py_SET_REFCNT() (since 3.13)
   • Py_INCREF(), Py_XINCREF() (since 3.12)
   • Py_DECREF(), Py_XDECREF() (since 3.12)

   Py_SET_TYPE() is not implemented as an opaque function call since it should not be used in the limited C API, it's only useful with static types.

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