[Python-Dev] C-level duck typing (original) (raw)

Stefan Behnel stefan_ml at behnel.de
Wed May 16 14:16:05 CEST 2012

Stefan Behnel, 16.05.2012 13:13:

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 tpcall() slot.

... and to finish the loop that I started here (sorry for being verbose):

The proposal that Dag referenced describes a more generic way to make this kind of extension to type objects from user code. Basically, it allows implementers to say "my type object has capability X", in a C-ish kind of way. And the above C signature protocol would be one of those capabilities.

Personally, I wouldn't mind making the specific signature extension a proposal instead of asking for a general extension mechanism for arbitrary capabilities (although that still sounds tempting).

Stefan

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