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Ok, I pushed a new commit "Add Py_TPFLAGS_HAVE_FASTCALL flag" in my Git branch.
(By the way, I'm using the Git branch as a working copy, it can change anytime to get a nice list of commits. If you want reliable changes, use patches attached to this issue.)
Serhiy Storchaka!:
> For compatibility with extensions built with older Pythons you should define new type flag and read tp_fastcall only if the flag is set. See for example Py_TPFLAGS_HAVE_FINALIZE.
Wait, there is maybe an issue with the stable API in my design :-/
Let's say that a builtin type NEW defines tp_fastcall and uses fastcall_wrapper() for tp_call. A type OLD inherits from NEW and don't override tp_call (not tp_fastcall). Type OLD comes from an extension compiled with Python 3.6 and use the stable ABI. Does the type OLD contain the field tp_fastcall in memory? Is it possible to copy the tp_fastcall filed from type NEW into the type OLD?
If not, the fastcall_wrapper() should be smarter to not get tp_fastcall from the type OLD but follow the MRO to find the first type with the Py_TPFLAGS_HAVE_FASTCALL flag. Is it possible that this short loop to find has a cost on performances?