[Python-Dev] Update on PEP 523 and adding a co_extra field to code objects

[Brett Cannon]

On Tue, 30 Aug 2016 at 10:49 Antoine Pitrou <solipsis at pitrou.net> wrote:

> On Tue, 30 Aug 2016 17:35:35 +0000
> Brett Cannon <brett at python.org> wrote:
> > >
> > > Perhaps a list would work indeed.  Realistically, if there are at most
> > > 2-3 users of the field at any given time (and most probably only one or
> > > zero), a simple type check (by pointer equality) on each list item may
> > > be sufficient.
> > >
> >
> > Let's see what Maciej says, but we could standardize on switching the
> field
> > to a list when a conflict of usage is detected so the common case in the
> > frame eval function is checking for your own type, and if that fails then
> > doing a PyList_CheckExact() and look for your object, otherwise make a
> list
> > and move over to that for everyone to use. A little bit more code, but
> it's
> > simple code and takes care of conflicts only when it calls for it.
>
> That's a bit obscure and confusing, though (I *think* the weakref module
> uses a similar kludge in some place).  If you want to iterate on it you
> have to write some bizarre macro to share the loop body between the two
> different code-paths (list / non-list), or some equally tedious
> function-pointer-based code.


I don't quite follow where the complexity you're suggesting comes from. The
frame evaluation function in Pyjion would just do:

  if (co_extra == NULL) {
      // No one using the field.
      co_extra = pyjion_cache = PyPyjion_New();
  }
  else if (!is_pyjion_object(co_extra)) {
    // Someone other than us is using the field.
    if (PyList_CheckExact(co_extra)) {
      // Field is already a list.
      ... look for object ...
      if (ob_found != NULL) {
        // We're in the list.
        pyjion_cache = ob_found;
      }
      else {
        // Not in the list, so add ourselves.
        pyjion_cache = PyPyjion_New();
        co_extra.append(pyjion_cache);
      }
    }
    else {
      // Someone else in the field, not a list (yet).
      other_ob = co_extra;
      co_extra = PyList_New();
      co_extra.append(other_ob);
      pyjion_cache = PyPyjion_New();
      co_extra.append(pyjion_cache);
    }
  }
  else {
    // We're in the field.
    pyjion_cache = co_extra;
  }


>
> Why not make it always a list?  List objects are reasonably cheap in
> memory and access time... (unlike dicts)
>

Because I would prefer to avoid any form of unnecessary performance
overhead for the common case.
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