[mypyc] Reduce impact of immortality on reference counting performance #18459
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For example, module namespace dictionaries could reasonably be immortal. Still treat lists as non-immportal, since the performance hit be too high otherwise.
I'm not convinced that it's any better. Getting the address of None requires a memory read, so we'd have an extra memory read in the slow path.
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Fixes mypyc/mypyc#1044.
The addition of object immortality in Python 3.12 (PEP 683) introduced an extra immortality check to incref and decref operations. Objects with a specific reference count are treated as immortal, and their reference counts are never updated.
It turns out that this slowed down the performance of certain workloads a lot (up to 70% increase in runtime, compared to 3.11). This PR reduces the impact of immortality via a few optimizations:
None).The second change should be clear. We generally depend on CPython implementation details to improve performance, and this seems safe to do here as well.
The first change could turn immortal objects into non-immortal ones. For native classes this is a decision we can arguably make -- native classes don't properly support immortality, and they can't be shared between subinterpreters. As discussed in PEP 683, skipping immortality checks here is acceptable even in cases where somebody tries to make a native instance immortal, but this could have some performance or memory use impact. The performance gains make this a good tradeoff.
Since lists are mutable, they can't be safely shared between subinterpreters, so again not dealing with immortality is acceptable. It could reduce performance in some use cases by deimmortalizing lists, but this potential impact seems marginal compared to faster incref and decref operations on lists, which are some of the more common objects in Python programs.
This speeds up self check by about 1.5% on Python 3.13. This speeds up the richards benchmark by 30-35% (!) on 3.13, and also some other benchmarks see smaller improvements.