[Cython] Treatment of right-hand side after assignment

[Robert Bradshaw]

On Mon, Apr 10, 2017 at 5:57 AM, Clemens Hofreither
<chofreither at gmail.com> wrote:
>
> On Sun, Apr 9, 2017 at 8:00 AM, Robert Bradshaw <robertwb at gmail.com> wrote:
> > On Sun, Apr 2, 2017 at 9:42 AM, Clemens Hofreither <chofreither at gmail.com>
> > wrote:
> >>
> >> Hey all,
> >>
> >> (I hope this list isn't dead.)
> >>
> >> I'm reading up on the Cython codebase because I'm trying to implement
> >> a small feature (more on that later). There's one thing that really
> >> confuses me currently, and that's the distinction between
> >> generate_post_assignment_code and generate_disposal_code. From the
> >> comments, it appears that exactly one of these should be called, and
> >> which one it is depends on whether a reference from the rhs was
> >> absorbed into the lhs.
> >>
> >> So much for the theory, but in practice it seems to work differently.
> >> I'm looking at NameNode.generate_assignment_code() and
> >> IndexNode.generate_assignment_code(), which should do similar things
> >> (generate code for an assignment) except that the lhs has different
> >> structure.
> >>
> >> Yet they treat the cleanup of the rhs completely differently.
> >> IndexNode always calls rhs.generate_disposal_code. NameNode almost
> >> always calls rhs.generate_post_assignment_code.
> >>
> >> Can anyone shed light on why this cleanup is handled differently
> >> depending on the target of the assignment?
> >
> >
> > It's been a while since I've looked at this, but the best explanation is
> > that generate_post_assignment_code is used when the rhs is "moved" into the
> > lhs whereas generate_post_assignment_code is what happens for a "copy." This
> > is mostly relevant for temps, e.g. when a temp holds a reference to a Python
> > object, it can be transferred to the assignee (e.g. with NameNodes) or a new
> > reference can be created for the assignee (e.g. with IndexNodes) which
> > changes whether the temp lhs needs to be decref'ed.
> >
> > _______________________________________________
> > cython-devel mailing list
> > cython-devel at python.org
> > https://mail.python.org/mailman/listinfo/cython-devel
> >
>
> Thanks for your answer. I understood as much about the difference with
> temporaries and copies.
>
> My question was why the RIGHT-hand side is treated differently in
> NameNode.generate_assignment_code() and
> IndexNode.generate_assignment_code(). My understanding is that
> NameNode/IndexNode here refers to the type of the left-hand side. But
> I don't see why whether or not the RHS is a temporary should depend on
> the type of the LHS. Therefore it seems confusing that the code in
> these two methods uses different cleanup for the RHS.


One can generally "transfer" ownership to a NameNode, but this is not
the case with an IndexNode (as it depends on what the [] operator was
overloaded to do). One could always incref the NameNode and use
generate_disposal_code, but using generate_post_assignment_code saves
an unnecessary incref/decref pair.

- Robert