[Python-Dev] PEP 575 (Unifying function/method classes) update (original) (raw)
Victor Stinner vstinner at redhat.com
Mon Jun 18 09:09:52 EDT 2018
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Hi,
I tried two options to add support for FASTCALL on calling an object: add a flag in tp_flags and reuse tp_call, or add a new tp_fastcall slot. I failed to implement correctly any of these two options.
There are multiple issues with tp_fastcall:
- ABI issue: it's possible to load a C extension using the old ABI, without tp_fastcall: it's not possible to write type->tp_fastcall on such type. This limitation causes different issues.
- If tp_call is modified, tp_fastcall may be outdated. Same if tp_fastcall is modified. What happens on "del obj.call" or "del type.call"?
- Many public functions of the C API still requires the tuple and dict to pass positional and keyword arguments, so a compatibility layer is required to types who only want to implement FASTCALL. Related issue: what is something calls tp_call with (args: tuple, kwargs: dict)? Crash or call a compatibility layer converting arguments to FASTCALL calling convention?
Reusing tp_call for FASTCALL cause similar or worse issues.
I abandoned my idea for two reasons:
- in the worst case, my changes caused a crash which is not accepted for an optimization. My first intent was to removed the property_descr_get() hack because its implementation is fragile and caused crashes.
- we implemented a lot of other optimizations which made calls faster without having to touch tp_call nor tp_fastcall. The benefit of FASTCALL for tp_call/tp_fastcall was not really significant.
Victor
2018-06-18 7:55 GMT+02:00 Jeroen Demeyer <J.Demeyer at ugent.be>:
On 2018-06-18 03:34, INADA Naoki wrote:
Victor had tried to add
tpfastcallslot, but he suspended his effort because it's benefit is not enough for it's complexity. https://bugs.python.org/issue29259 I has a quick look at that patch and it's really orthogonal to what I'm proposing. I'm proposing to use the slot instead of existing fastcall optimizations. Victor's patch was about adding fastcall support to classes that didn't support it before. Jeroen.
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