[Cython] CEP1000: Native dispatch through callables

Dag Sverre Seljebotn d.s.seljebotn at astro.uio.no
Fri Apr 13 10:52:07 CEST 2012

On 04/13/2012 01:38 AM, Robert Bradshaw wrote:
> On Thu, Apr 12, 2012 at 3:34 PM, Dag Sverre Seljebotn
> <d.s.seljebotn at astro.uio.no>  wrote:
>> On 04/13/2012 12:11 AM, Dag Sverre Seljebotn wrote:
>>> Travis Oliphant recently raised the issue on the NumPy list of what
>>> mechanisms to use to box native functions produced by his Numba so that
>>> SciPy functions can call it, e.g. (I'm making the numba part up):
>>> @numba # Compiles function using LLVM
>>> def f(x):
>>> return 3 * x
>>> print scipy.integrate.quad(f, 1, 2) # do many callbacks natively!
>>> Obviously, we want something standard, so that Cython functions can also
>>> be called in a fast way.
>>> This is very similar to CEP 523
>>> (http://wiki.cython.org/enhancements/nativecall), but rather than
>>> Cython-to-Cython, we want something that both SciPy, NumPy, numba,
>>> Cython, f2py, fwrap can implement.
>>> Here's my proposal; Travis seems happy to implement something like it
>>> for numba and parts of SciPy:
>>> http://wiki.cython.org/enhancements/nativecall
>> I'm sorry. HERE is the CEP:
>> http://wiki.cython.org/enhancements/cep1000
>> Since writing that yesterday, I've moved more in the direction of wanting a
>> zero-terminated list of overloads instead of providing a count, and have the
>> fast protocol jump over the header (since version is available elsewhere),
>> and just demand that the structure is sizeof(void*)-aligned in the first
>> place rather than the complicated padding.
> Great idea to coordinate with the many other projects here. Eventually
> this could maybe even be a PEP.
> Somewhat related, I'd like to add support for Go-style interfaces.
> These would essentially be vtables of pre-fetched function pointers,
> and could play very nicely with this interface.

Yep; but you agree that this can be done in isolation without 
considering vtables first?

> Have you given any thought as to what happens if __call__ is
> re-assigned for an object (or subclass of an object) supporting this
> interface? Or is this out of scope?

Out-of-scope, I'd say. Though you can always write an object that 
detects if you assign to __call__...

> Minor nit: I don't think should_dereference is worth branching on, if
> one wants to save the allocation one can still use a variable-sized
> type and point to oneself. Yes, that's an extra dereference, but the
> memory is already likely close and it greatly simplifies the logic.
> But I could be wrong here.

Those minor nits are exactly what I seek; since Travis will have the 
first implementation in numba<->SciPy, I just want to make sure that 
what he does will work efficiently work Cython.

Can we perhaps just require that the information is embedded in the object?

I must admit that when I wrote that I was mostly thinking of JIT-style 
code generation, where you only use should_dereference for 
code-generation. But yes, by converting the table to a C structure you 
can do without a JIT.

> Also, I'm not sure the type registration will scale, especially if
> every callable type wanted to get registered. (E.g. currently closures
> and generators are new types...) Where to draw the line? (Perhaps
> things could get registered lazily on the first __nativecall__ lookup,
> as they're likely to be looked up again?)

Right... if we do some work to synchronize the types for Cython modules 
generated by the same version of Cython, we're left with 3-4 types for 
Cython, right? Then a couple for numba and one for f2py; so on the order 
of 10?

An alternative is do something funny in the type object to get across 
the offset-in-object information (abusing the docstring, or introduce 
our own flag which means that the type object has an additional 
non-standard field at the end).


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