On 05/16/2012 02:16 PM, Stefan Behnel wrote:
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 tp_call() 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).
Here's some reasons for the generic proposal:
a) Avoid pre-mature PEP-ing. Look at PEP 3118 for instance; that would
almost certainly had been better if there had been a few years of
beta-testing in the wild among Cython and NumPy users.
I think PEP-ing the "nativecall" proposal soon (even in the unlikely
event that it would be accepted) is bound to give suboptimal results --
it needs to be tested in the wild on Cython and SciPy users for a few
years first. (Still, we can't ask those to recompile their Python.)
My proposal is then about allowing people to play with their own slots,
and deploy that to users, without having to create a PEP for their
specific usecase.
b) There's more than the "nativecall" we'd use this for in Cython.
Something like compiled abstract base classes/compiled multiple
inheritance/Go-style interfaces for instance. Some of those things we'd
like to use it for certainly will never be a PEP.
c) Get NumPy users off their PyObject_TypeCheck habit, which IMO is
damaging to the NumPy project (because you can't that easily play around
with different array libraries and new ideas -- NumPy is the only array
type you can ever have, because millions of code lines have been written
using its C API. My proposal provides a way of moving that API over to
accept any object implementing a NumPy-specified spec. We certainly
don't want to have a 20 nanosecond speed regression on every single call
they make to the NumPy C API, and you simply don't rewrite millions of
code lines.).
I think having millions of lines of "Python" code written in C, and not
Python, and considering 20 nanoseconds as "much", is perhaps not the
typical usecase on this list. Still, that's the world of scientific
computing with Python. Python-the-interpreter is just the "shell" around
the real stuff that all happens in C or Fortran.
(Cython is not just about scientific computing, as I'm sure Stefan has
told you all about. But in other situations I think there's less of a
need of "cross-talk" between extensions without going through the Python
API.)
I guess I don't get "if something needs to be fast on the C level, then
that one specific usecase should be in a PEP". And all we're asking for
is really that one bit in tp_flags.
Dag
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