On Sun, Apr 2, 2017 at 9:42 AM, Clemens Hofreither <chofreit...@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.
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