On Sat, 3 Sep 2016 at 17:27 Yury Selivanov <yselivanov...@gmail.com> wrote:
> > On 2016-09-03 5:19 PM, Brett Cannon wrote: > > > > > > On Sat, 3 Sep 2016 at 16:43 Yury Selivanov <yselivanov...@gmail.com > > <mailto:yselivanov...@gmail.com>> wrote: > > > > > > > > On 2016-09-03 4:15 PM, Christian Heimes wrote: > > > On 2016-09-04 00:03, Yury Selivanov wrote: > > >> > > >> On 2016-09-03 12:27 PM, Brett Cannon wrote: > > >>> Below is the `co_extra` section of PEP 523 with the update > > saying that > > >>> users are expected to put a tuple in the field for easier > > simultaneous > > >>> use of the field. > > >>> > > >>> Since the `co_extra` discussions do not affect CPython itself I'm > > >>> planning on landing the changes stemming from the PEP probably > > on Monday. > > >> Tuples are immutable. If you have multiple co_extra users then > > they > > >> will have to either mutate tuple (which isn't always possible, for > > >> instance, you can't increase size), or to replace it with > > another tuple. > > >> > > >> Creating lists is a bit more expensive, but item access speed > > should be > > >> in the same ballpark. > > >> > > >> Another question -- sorry if this was discussed before -- why > > do we want > > >> a PyObject* there at all? I.e. why don't we create a dedicated > > struct > > >> CoExtraContainer to manage the stuff in co_extra? My > > understanding is > > >> that the users of co_extra are C-level python optimizers and > > profilers, > > >> which don't need the overhead of CPython API. > > > > > > As Chris pointed out in another email, the overhead is only in the > > allocation, not the iteration/access if you use the PyTuple macros to > > get the size and index into the tuple the overhead is negligible. > > Yes, my point was that it's as cheap to use a list as a tuple for > co_extra. If we decide to store PyObject in co_extra. > > > >> > > >> This way my work to add an extra caching layer (which I'm very > much > > >> willing to continue to work on) wouldn't require another set of > > extra > > >> fields for code objects. > > > Quick idea before I go to bed: > > > > > > You could adopt a similar API to OpenSSL's > CRYPTO_get_ex_new_index() > > > API, > > > > > > https://www.openssl.org/docs/manmaster/crypto/CRYPTO_get_ex_new_index.html > > > > > > > > > static int code_index = 0; > > > > > > int PyCodeObject_NewIndex() { > > > return code_index++; > > > } > > > > > > A library like Pyjion has to acquire an index first. In further > > calls it > > > uses the index as offset into the new co_extra field. Libraries > > don't > > > have to hard-code their offset and two libraries will never > > conflict. > > > PyCode_New() can pre-populate co_extra with a PyTuple of size > > > code_index. This avoids most resizes if you load Pyjion early. For > > > code_index == 0 leaf the field NULL. > > > > Sounds like a very good idea! > > > > > > The problem with this is the pre-population. If you don't get your > > index assigned before the very first code object is allocated then you > > still have to manage the size of the tuple in co_extra. So what this > > would do is avoid the iteration but not the allocation overhead. > > > > If we open up the can of worms in terms of custom functions for this > > (which I was trying to avoid), then you end up with Py_ssize_t > > _PyCode_ExtraIndex(), PyObject * > > _PyCode_GetExtra(PyCodeObject *code, Py_ssize_t index), and int > > _PyCode_SetExtra(PyCodeObject *code, Py_ssize_t index, PyObject *data) > > which does all the right things for creating or resizing the tuple as > > necessary and which I think matches mostly what Nick had proposed > > earlier. But the pseudo-code for _PyCode_GetExtra() would be:: > > > > if co_extra is None: > > co_extra = (None,) * _next_extra_index; > > return None > > elif len(co_extra) < index - 1: > > ... pad out tuple > > return None > > else: > > return co_extra[index] > > > > Is that going to save us enough to want to have a custom API for this? > > But without that new API (basically what Christian proposed) you'd need > to iterate over the list in order to find the object that belongs to > Pyjion. Yes. > If we manage to implement my opcode caching idea, we'll have at > least two known users of co_extra. Without a way to claim a particular > index in co_extra you will have some overhead to locate your objects. > Two things. One, I would want any new API to start with an underscore so people know we can and will change its semantics as necessary. Two, Guido would have to re-accept the PEP as this is a shift in the use of the field if this is how people want to go.
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