GH-139922: Tail calling for MSVC (VS 2026)

[chris-eibl]

So much has happened on main that I started fresh over and sqash merged the needed things from #139962 onto latest main (mostly because I wanted to use it for my own experiments on main again).

Based on @markshannon's work in #142228 I've factored out _Py_VectorCallInstrumentation_StackRefSteal, because then @Fidget-Spinner's #138115 is not needed in this PR and can be done in a follow-up. An alternative would be to keep it and pass tstate into the Py_*_StackRefSteal functions.

• Issue: Tail calling interpreter for MSVC #139922

[Fidget-Spinner]

In the future, please communicate if you want to supercede a PR for other contributors. It's often considered not good etiquette to take over another person's PR unless they explicitly allow it. However, I'm more than happy to let you take over the PR in this case.

[chris-eibl]

It needed lots of trial and error to come up with this unintuive minimal change needed to convince MSVC that the local variable passed by reference into result here does not escape the tail call.

Using it as an return value in PyMapping_GetOptionalItem2 is more explicit, but might have other drawbacks?

[chris-eibl]

In the future, please communicate if you want to supercede a PR for other contributors. It's often considered not good etiquette to take over another person's PR unless they explicitly allow it. However, I'm more than happy to let you take over the PR in this case.

Sorry, wasn't ment as superseding, more as a follow up and maybe just a draft for new discussions. Thought about you're gonna take whatever deems appropriate back into your original PR ...

If this here is going to fly, I'd like to include you and Brandt as authors for the commits, but don't know how atm ...

[hugovk]

If this here is going to fly, I'd like to include you and Brandt as authors for the commits, but don't know how atm ...

Co-authored-by is the way:

https://docs.github.com/en/pull-requests/committing-changes-to-your-project/creating-and-editing-commits/creating-a-commit-with-multiple-authors

[Fidget-Spinner]

If this here is going to fly, I'd like to include you and Brandt as authors for the commits, but don't know how atm ...

Oh I'm happy to be superseded. If you'd like to attribute commit history though, you can just push a trivial commit with the co-author info like Hugo pointed out, and when I squash and merge I'll use that.

[Fidget-Spinner]

Try pulling main in, I fixed a JIT bug there that should turn the JIT CI green again.

[chris-eibl]

Co-authored-by is the way:

Done

[chris-eibl]

https://github.com/python/cpython/actions/runs/20437258430/job/58722460806 failed inbetween
test_extractall_none_gid (test.test_tarfile.NoneInfoExtractTests_Tar.test_extractall_none_gid) ... Windows fatal exception: access violation
(but got green when rerunning the failed test) even though pulling in main.

I've found this: #143073

Hopefully, this will fix it ...

[Fidget-Spinner]

Just ignore the JIT problems for now. The new JIT executor management has a known bug #143026

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @Fidget-Spinner for commit da1adaf 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F143068%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-refleak-buildbots label again.

[Fidget-Spinner]

Benchmark results are within noise for the default interpreter https://github.com/facebookexperimental/free-threading-benchmarking/blob/main/results/bm-20251222-3.15.0a3%2B-da1adaf/bm-20251222-vultr-x86_64-chris%252deibl-msvc_tail_call_new-3.15.0a3%2B-da1adaf-vs-base.md

Refleak bots are green.

[Fidget-Spinner]

Once again:

• This PR is the successor of one that has been up for 2 months already #139962 . We've had enough discussion there to get a sensing of what we want.
• Refleak tests are green, performance results are within noise for the default (computed goto) interpreter.
• This PR adds CI for the new configuration, which is also green.
• I have discussed this feature with the other core devs a few months back on our bi-weekly perf meetings. Everyone was in agreement that adding scopes to MSVC is fine as it isn't that invasive.
• This PR is the minimal changeset required to get TC working on MSVC.
• It uses the techniques suggested by Mark in the other PR (just adding scopes and commenting on them). So I expect he should be okay with the overall design I hope. I understand he might disagree on some of the smaller changes.
• It LGTM, from my review of the changes.

Therefore, I'm merging this PR.

Any follow-ups can be done on future PRs. Any discussion about the design can be done on the issue itself.