[issue33597] Compact PyGC_Head

[STINNER Victor]

STINNER Victor <vstinner at redhat.com> added the comment:

I asked if this change breaks the stable ABI. Steve Dower replied:

"Looks like it breaks the 3.7 ABI, which is certainly not allowed at this time. But it’s not a limited API structure, so no problem for 3.8."

https://mail.python.org/pipermail/python-dev/2018-May/153745.html

I didn't understand the answer. It breaks the ABI but it doesn't break the API?

It seems like PyObject.ob_refcnt is part of the "Py_LIMITED_API" and so an extension module using the stable API/ABI can access directly the field with no function call. For example, Py_INCREF() modifies directly the field at the ABI level.

*But* PyGC_Head is a strange thing since it's stored "before" the usual PyObject* pointer, so fields starting at PyObject* address are not affected by this change, no?

Hopefully, PyGC_Head seems to be excluded from PyGC_Head, and so it seems like the PR 7043 doesn't break the stable *ABI*.

Can someone please confirm my analysis?

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