On Sat, 3 Sep 2016 at 16:43 Yury Selivanov <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. > >> > >> 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? -Brett > > Yury >
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