[Python-Dev] PEP 590 discussion

[Jeroen Demeyer]

On 2019-04-10 18:25, Petr Viktorin wrote:
> Hello!
> I've had time for a more thorough reading of PEP 590 and the reference
> implementation. Thank you for the work!

And thank you for the review!

> I'd now describe the fundamental
> difference between PEP 580 and PEP 590 as:
> - PEP 580 tries to optimize all existing calling conventions
> - PEP 590 tries to optimize (and expose) the most general calling
> convention (i.e. fastcall)

And PEP 580 has better performance overall, even for METH_FASTCALL. See 
this thread:
https://mail.python.org/pipermail/python-dev/2019-April/156954.html

Since these PEPs are all about performance, I consider this a very 
relevant argument in favor of PEP 580.

> PEP 580 also does a number of other things, as listed in PEP 579. But I
> think PEP 590 does not block future PEPs for the other items.
> On the other hand, PEP 580 has a much more mature implementation -- and
> that's where it picked up real-world complexity.
About complexity, please read what I wrote in
https://mail.python.org/pipermail/python-dev/2019-March/156853.html

I claim that the complexity in the protocol of PEP 580 is a good thing, 
as it removes complexity from other places, in particular from the users 
of the protocol (better have a complex protocol that's simple to use, 
rather than a simple protocol that's complex to use).

As a more concrete example of the simplicity that PEP 580 could bring, 
CPython currently has 2 classes for bound methods implemented in C:
- "builtin_function_or_method" for normal C methods
- "method-descriptor" for slot wrappers like __eq__ or __add__

With PEP 590, these classes would need to stay separate to get maximal 
performance. With PEP 580, just one class for bound methods would be 
sufficient and there wouldn't be any performance loss. And this extends 
to custom third-party function/method classes, for example as 
implemented by Cython.

> PEP 590's METH_VECTORCALL is designed to handle all existing use cases,
> rather than mirroring the existing METH_* varieties.
> But both PEPs require the callable's code to be modified, so requiring
> it to switch calling conventions shouldn't be a problem.

Agreed.

> Jeroen's analysis from
> https://mail.python.org/pipermail/python-dev/2018-July/154238.html seems
> to miss a step at the top:
>
> a. CALL_FUNCTION* / CALL_METHOD opcode
>        calls
> b. _PyObject_FastCallKeywords()
>        which calls
> c. _PyCFunction_FastCallKeywords()
>        which calls
> d. _PyMethodDef_RawFastCallKeywords()
>        which calls
> e. the actual C function (*ml_meth)()
>
> I think it's more useful to say that both PEPs bridge a->e (via
> _Py_VectorCall or PyCCall_Call).

Not quite. For a builtin_function_or_method, we have with PEP 580:

a. call_function()
     calls
d. PyCCall_FastCall
     which calls
e. the actual C function

and with PEP 590 it's more like:

a. call_function()
     calls
c. _PyCFunction_FastCallKeywords
     which calls
d. _PyMethodDef_RawFastCallKeywords
     which calls
e. the actual C function

Level c. above is the vectorcall wrapper, which is a level that PEP 580 
doesn't have.

> The way `const` is handled in the function signatures strikes me as too
> fragile for public API.

That's a detail which shouldn't influence the acceptance of either PEP.

> Why not have a per-type pointer, and for types that need it (like
> PyTypeObject), make it dispatch to an instance-specific function?

That would be exactly https://bugs.python.org/issue29259

I'll let Mark comment on this.

> Minor things:
> - "Continued prohibition of callable classes as base classes" -- this
> section reads as a final. Would you be OK wording this as something
> other PEPs can tackle?
> - "PyObject_VectorCall" -- this looks extraneous, and the reference
> imlementation doesn't need it so far. Can it be removed, or justified?
> - METH_VECTORCALL is *not* strictly "equivalent to the currently
> undocumented METH_FASTCALL | METH_KEYWORD flags" (it has the
> ARGUMENTS_OFFSET complication).
> - I'd like to officially call this PEP "Vectorcall", see
> https://github.com/python/peps/pull/984

Those are indeed details which shouldn't influence the acceptance of 
either PEP. If you go with PEP 590, then we should discuss this further.

> Mark, what are your plans for next steps with PEP 590? If a volunteer
> wanted to help you push this forward, what would be the best thing to
> work on?

Personally, I think what we need now is a decision between PEP 580 and 
PEP 590 (there is still the possibility of rejecting both but I really 
hope that this won't happen). There is a lot of work that still needs to 
be done after either PEP is accepted, such as:
- finish and merge the reference implementation
- document everything
- use the protocol in more classes where it makes sense (for example, 
staticmethod, wrapper_descriptor)
- use this in Cython
- handle more issues from PEP 579

I volunteer to put my time into this, regardless of which PEP is 
accepted. Of course, I still think that PEP 580 is better, but I also 
want this functionality even if PEP 590 is accepted.

> Jeroen, is there something in PEPs 579/580 that PEP 590 blocks, or
> should address?

Well, PEP 580 is an extensible protocol while PEP 590 is not. But, 
PyTypeObject is extensible, so even with PEP 590 one can always extend 
that (for example, PEP 590 uses a type flag Py_TPFLAGS_METHOD_DESCRIPTOR 
where PEP 580 instead uses the structs for the C call protocol). But I 
guess that extending PyTypeObject will be harder to justify (say, in a 
future PEP) than extending the C call protocol.

Also, it's explicitly allowed for users of the PEP 580 protocol to 
extend the PyCCallDef structure with custom fields. But I don't have a 
concrete idea of whether that will be useful.


Kind regards,
Jeroen.