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Title: co_annotations branch caused a crash in stackeffect() in compile.c
Type: crash Stage: patch review
Components: Interpreter Core Versions: Python 3.10
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: larry
Priority: normal Keywords:

Created on 2021-04-19 23:00 by larry, last changed 2022-04-11 14:59 by admin.

Messages (2)
msg391413 - (view) Author: Larry Hastings (larry) * (Python committer) Date: 2021-04-19 23:00
I'm working on a branch to implement PEP 649:

Inada Naoki discovered a crash in that branch, discussed here, including steps to reproduce:

valgrind showed me what the problem was.  stackeffect() allocates a "stack" variable, used to store pushed/popped context while iterating over the basic blocks of the function being assembled.  Most of the time, the stack is way bigger than it needs to be--we allocate 4 or 5 entries and it only uses 1 or 2.  But, somehow, in the co_annotations branch, the "stack" was occasionally *way too small*.  As in, it allocated 66 entries (!) but used 150 (!!).

I don't understand exactly how stackeffect works, so I don't know under what circumstances it would go so deep, much less what would cause it to so severely underestimate how many entries it needed.  I *did* make modifications to code generation in compile.c, so it *could* be my bug--but my changes were all much earlier in the process, and AFAIK I never touched any of the code under assemble().

Well, not until I worked around this problem, anyway.  My fix: if "stack" is too small, double the size and realloc().  Certainly it makes the problem go away.  That's checked in to my branch here:

But it doesn't address the underlying bug, whatever it is.

If anybody who understands stackeffect() could take a look and figure it out?  That would be neat-o keen.
msg391414 - (view) Author: Larry Hastings (larry) * (Python committer) Date: 2021-04-19 23:00
(Sorry, the name of the function is stackdepth(), not stackeffect().)
Date User Action Args
2022-04-11 14:59:44adminsetgithub: 88057
2021-04-19 23:00:38larrysetmessages: + msg391414
2021-04-19 23:00:10larrycreate