[Python-Dev] PEP 575 (Unifying function/method classes) update

[Victor Stinner]

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* If tp_call is modified, tp_fastcall may be outdated.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:

1) 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.
2) 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 `tp_fastcall` slot, 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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