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 still think this is out of scope for python-dev. If this is something
you want to be able to do for Python 2.4 as well, then you don't need
any change to Python - you can do whatever you come up with for all
Python versions, no need to (or point in) changing Python 3.4 (say).
We can go ahead and hijack tp_flags bit 22 to make things work in
existing versions. But what if Python 3.8 then starts using that bit for
something else?
As this is apparently only relevant to speed fanatics, too, I suggest
that you check how fast PyPI works for you. Supposedly, they have very
efficient lookup procedures, supported by the JIT. If this doesn't work
for some reason, I suggest that you'll have to trade speed for
convenience: a compile-time fixed layout will beat any dynamic lookup
any time. Just define a common base class, and have all interesting
types inherit from it.
Did you mean PyPy? Me and Stefan are Cython developers, so that's kind
of our angle... And I'm a Cython developer because it solves a practical
need (in my case in scientific computation), not because I think it's
that beautiful. PyPy won't work for me (let's not go down that road now...)
Defining a common base class is what NumPy already does, and Cython
would be forced to without this proposal. We just think it has
significant disadvantages and were looking for something else.
Dag
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