[Python-Dev] Tweak to PEP 523 for storing a tuple in co_extra (original) (raw)

Yury Selivanov yselivanov.ml at gmail.com
Sat Sep 3 20:27:11 EDT 2016

On 2016-09-03 5:19 PM, Brett Cannon wrote:

On Sat, 3 Sep 2016 at 16:43 Yury Selivanov <yselivanov.ml at gmail.com_ _<mailto:yselivanov.ml at 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 coextra 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 coextra 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 coextra 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 coextra? My understanding is >> that the users of coextra 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 CRYPTOgetexnewindex() > API, > https://www.openssl.org/docs/manmaster/crypto/CRYPTOgetexnewindex.html > > > static int codeindex = 0; > > int PyCodeObjectNewIndex() { > return codeindex++; > } > > A library like Pyjion has to acquire an index first. In further calls it > uses the index as offset into the new coextra field. Libraries don't > have to hard-code their offset and two libraries will never conflict. > PyCodeNew() can pre-populate coextra with a PyTuple of size > codeindex. This avoids most resizes if you load Pyjion early. For > codeindex == 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 coextra. 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 Pyssizet PyCodeExtraIndex(), PyObject * PyCodeGetExtra(PyCodeObject *code, Pyssizet index), and int PyCodeSetExtra(PyCodeObject *code, Pyssizet 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 PyCodeGetExtra() would be:: if coextra is None: coextra = (None,) * nextextraindex; return None elif len(coextra) < index - 1: ... pad out tuple return None else: return coextra[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. 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.

Yury

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