bpo-37249: add declaration of _PyObject_GetMethod

[jdemeyer]

https://bugs.python.org/issue37249

[mangrisano]

/cc @methane @msullivan @1st1

[jdemeyer]

I'm not actually convinced that _PyObject_GetMethod should be super-internal. For example, Cython literally copied the code of _PyObject_GetMethod from CPython because CPython does not expose it. So maybe we shouldn't actually make it internal.

[methane]

I expect further optimization which requires changing this API completely.

For general use, I want more high level API for calling method with vectercall.

So, if expected user is only Cython, please keep copying.

[jdemeyer]

So, if expected user is only Cython, please keep copying.

OK, got it.

[msullivan]

So, if expected user is only Cython, please keep copying.

I'm probably also going to copy it for mypyc at some point, but I think that falls in the same bucket.

[methane]

As I send to ML, it seems dllexport doesn't affect calling convention (and calling performance).

It may affects LTO. But I don't think LTO is important here because _PyObject_GetMethod is relatively heavy function.

[methane]

If there are at least two users (mypyc and Cython), I'm OK to move it to private/.

[jdemeyer]

I'm OK to move it to private/.

What do you mean here? There is a directory cpython/ for the default non-limited API and a directory internal/ for the API that is only defined when Py_BUILD_CORE is defined. Which of the two do you mean?

[methane]

I'm sorry, there is no private/ directory. As @vstinner said:

Python 3.8 now has a better separation between "private" and "internal" 
APIs:

* private are "_Py" symbols which are exported in the python DLL: they 
should not be used, but can be used techically

* internal are symbols defined in internal header files 
(Include/internal/). Some symbols use PyAPI_FUNC()/PyAPI_DATA(), some 
only use "extern".

So I meant put this API in Include/cpython/object.h, near to _PyObject_GetAttrId.

I will add new really internal API to implement per-opcode cache for LOAD_METHOD,
instead of changing existing _PyObject_GetMethod.

[jdemeyer]

So I meant put this API in Include/cpython/object.h, near to _PyObject_GetAttrId.

OK, done.