[Python-Dev] C-level duck typing

[Stefan Behnel]

Dag Sverre Seljebotn, 16.05.2012 12:48:
> On 05/16/2012 11:50 AM, "Martin v. Löwis" wrote:
>>> Agreed in general, but in this case, it's really not that easy. A C
>>> function call involves a certain overhead all by itself, so calling into
>>> the C-API multiple times may be substantially more costly than, say,
>>> calling through a function pointer once and then running over a
>>> returned C
>>> array comparing numbers. And definitely way more costly than running over
>>> an array that the type struct points to directly. We are not talking
>>> about
>>> hundreds of entries here, just a few. A linear scan in 64 bit steps over
>>> something like a hundred bytes in the L1 cache should hardly be
>>> measurable.
>>
>> I give up, then. I fail to understand the problem. Apparently, you want
>> to do something with the value you get from this lookup operation, but
>> that something won't involve function calls (or else the function call
>> overhead for the lookup wouldn't be relevant).
> 
> In our specific case the value would be an offset added to the PyObject*,
> and there we would find a pointer to a C function (together with a 64-bit
> signature), and calling that C function (after checking the 64 bit
> signature) is our final objective.

I think the use case hasn't been communicated all that clearly yet. Let's
give it another try.

Imagine we have two sides, one that provides a callable and the other side
that wants to call it. Both sides are implemented in C, so the callee has a
C signature and the caller has the arguments available as C data types. The
signature may or may not match the argument types exactly (float vs.
double, int vs. long, ...), because the caller and the callee know nothing
about each other initially, they just happen to appear in the same program
at runtime. All they know is that they could call each other through Python
space, but that would require data conversion, tuple packing, calling,
tuple unpacking, data unpacking, and then potentially the same thing on the
way back. They want to avoid that overhead.

Now, the caller needs to figure out if the callee has a compatible
signature. The callee may provide more than one signature (i.e. more than
one C call entry point), perhaps because it is implemented to deal with
different input data types efficiently, or perhaps because it can
efficiently convert them to its expected input. So, there is a signature on
the caller side given by the argument types it holds, and a couple of
signature on the callee side that can accept different C data input. Then
the caller needs to find out which signatures there are and match them
against what it can efficiently call. It may even be a JIT compiler that
can generate an efficient call signature on the fly, given a suitable
signature on callee side.

An example for this is an algorithm that evaluates a user provided function
on a large NumPy array. The caller knows what array type it is operating
on, and the user provided function may be designed to efficiently operate
on arrays of int, float and double entries.

Does this use case make sense to everyone?

The reason why we are discussing this on python-dev is that we are looking
for a general way to expose these C level signatures within the Python
ecosystem. And Dag's idea was to expose them as part of the type object,
basically as an addition to the current Python level tp_call() slot.

Stefan