gh-118335: Configure Tier 2 interpreter at build time

[gvanrossum]

The code for Tier 2 is now only compiled when configured with --enable-experimental-jit[=yes|interpreter]. We drop support for PYTHON_UOPS and -Xuops, but you can disable the interpreter or JIT at runtime by setting PYTHON_JIT=0. You can also build it without enabling it by default using --enable-experimental-jit=yes-off; enable with PYTHON_JIT=1.

@mdboom See note in issue about benchmarking setup.

@markshannon An alternative approach would be to keep the APIs but make them always fail. In particular, there are some PyUnstable APIs that will only exist when _Py_TIER2 is defined. Thoughts?

EDIT: In particular, we could arguably keep offering PyUnstable_{Get,Set}Optimizer, so 3rd parties could create and activate their own optimizers, like in your original design (which I cannot find back -- you had something where 3rd parties could register optimizations and parameters indicating the cost and benefit, or something). However, the rest of the optimizer machinery is now dependent on executors (starting with _PyOptimizer_Optimize, ). If we wanted to keep supporting both optimizers and executors, we'd have to do something different in ENTER_EXECUTOR too. (Possibly in both cases the key issue is that GOTO_TIER_TWO(executor) assumes JIT or Tier 2 IR.)

EDIT 2: Another consideration is whether we'd still want the runtime -Xuops flag and the PYTHON_UOPS env var, when enabled at build time. Or perhaps at least PYTHON_UOPS=0 to disable at runtime (which is a simple change in pylifecycle.c). I'd use that when debugging tests with PYTHON_LLTRACE=N, since the debug output from the test infrastructure might overwhelm that for the test proper.

Yet another option would be to leave everything as it was, except change the default with the build-time flag.

• Issue: Configure Tier 2 interpreter at build time #118335

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📚 Documentation preview 📚: https://cpython-previews--118339.org.readthedocs.build/

[gvanrossum]

WASI build is failing, just like it did before when I had an option to disable Tier 2.

I'm away from my Windows laptop so I can't debug the Windows JIT failures yet -- this seems to be in test_opcache.TestRacesDoNotCrash.test_store_attr_with_hint.

[gvanrossum]

There's also this failure (a hang in test_multiprocessing, apparently): https://github.com/python/cpython/actions/runs/8857124270/job/24324215355?pr=118339

PS. I've updated the discussion in the initial comment.

[gvanrossum]

Benchmark says 1% faster (in Tier 1 mode).

[gvanrossum]

Looks like Windows JIT builds now pass. WASI build still fails. I wonder if there's something in the WASI build similar to the Windows problem (which had to do with _Py_TIER2 never getting set in a place where it was tested).

[mdboom]

This may already be on your radar, but a reminder to also update the Windows build.bat script to support this 4-way option.

[gvanrossum]

WASI bliss at last.

@brandtbucher Do you have time to review this?

@mdboom I peeked at your new-flags branch in the benchmarking repo. We lose the ability to benchmark Tier 2 on Windows (for now) -- is that reflected in the diff there? (As an only occasional user of the UI there I rely on the script to let me know if I've selected an impossible combination.)

[brandtbucher]

@brandtbucher Do you have time to review this?

Yep, looking at it now.

[brandtbucher]

Thanks for taking the time to tackle this. Performing the surgery after-the-fact definitely seems like a tricky task.

[gvanrossum]

@brandtbucher I believe I've done everything from your review that I gave a thumbs up, and for everything else I stated a reason.

@mdboom -- At Brandt's suggestion I've shortened the "default off" flag to --enable-experimental-jit=yes-off. I think this has no repercussions for your new-flags branch of the benchmarking repo, but you may want to double-check.

[brandtbucher]

@brandtbucher I believe I've done everything from your review that I gave a thumbs up, and for everything else I stated a reason.

Yep!

[gvanrossum]

Okay, I've got the Windows flag stuff sorted out. It's a little different from Unix, because you apparently can't say "--experimental-jit=yes-off" while also supporting "--experimental-jit"; it treats the "=" as a space. Rather than becoming a .BAT file CLI parsing expert I chose the following option names:

• --experimental-jit
• --experimental-jit-off (build JIT, but default off)
• --experimental-interpreter
• --experimental-interpreter-off (undocumented, for Mark :-)