Robert Bradshaw, 23.07.2010 08:52:
> On Thu, Jul 22, 2010 at 11:10 PM, Stefan Behnel wrote:
>> Stefan Brunthaler started a thread on python-dev about a couple of patches
>> he has written for CPython as part of his PhD. It seems that he has
>> implemented inline caching for CPython.
>>
>> http://comments.gmane.org/gmane.comp.python.devel/115362
>>
>> http://en.wikipedia.org/wiki/Inline_caching
>>
>> Something like this might be interesting for Cython, too. He hasn't
>> published the patches yet, but they might well be worth a look when he does.
>
> Very cool. I have actually thought a lot about this, and I know
> unladen swallow did some stuff in this direction for module/builtin
> lookups. Ideally, there could be some kind of a hook called, or even a
> dirty bit, that gets set when an objects attribute set gets modified,
> so we wouldn't have to re-look things up most of the time.
I just had a funny idea. We could simply embed each call to a Python method
in a snippet of code that does a bit of caching. It would use a fixed size
static C array (say, size 4) of structs for each call node:
{Python type, underlying C function of method}
and add all types to that array that it finds as call target at runtime
that do not have a __dict__ (most builtin types but (sadly) no modules).
Then, before doing the CPython call dance, it would just quickly run
through the array to check if it finds the current type, and on success,
call the function for that method directly. The normal Python attribute
lookup and call would then become a fallback that would only be required
for dynamic Python objects. We could also add types to it that we deem
unoptimisable (e.g. because the method is not efficiently callable) and set
their method pointer to NULL, so that we can skip over any further
optimisation attempts in the future. We always hold the GIL when
manipulating the arrays, so static caching will work perfectly.
What do you think?
I'm not so sure yet about the ref-counting implications of keeping such a
static pointer to a type, that may also need some more consideration. A
borrowed reference will not work if the type is allowed to get garbage
collected, as the type's address may then end up being reused for another
type, leaving the method pointer invalid.
Stefan
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