GH-124567: Reduce overhead of debug build for GC. Should help CI performance

[markshannon]

The doctests CI takes ages and it uses a debug build.

Arguably debug builds should be good for debugging, i.e. lots of asserts. If you want performance don't use a debug build.

However, the incremental GC does adds a lot of extra sanity checks and it is a pain if CI jobs take half an hour or more.

So, this PR adds an extra debug option to reduce the amount of checking done in a normal debug build, but keeps the extra sanity checks if needed for debugging the GC.

• Issue: Incremental GC causes a significant slowdown for Sphinx #124567

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📚 Documentation preview 📚: https://cpython-previews--126777.org.readthedocs.build/

[markshannon]

Reduces the time it takes CI to run the Doc/Doctest job by ~40%.

[pablogsal]

Not opposing to this but this will mean that debug builds that are redistributed won't have GC list validation so linked list corruption will be more difficult to debug when users leverage debug builds of the interpreter.

[markshannon]

This only removes the extra asserts added for the incremental GC, the original list validations from 3.13 are still present.

[pablogsal]

This only removes the extra asserts added for the incremental GC, the original list validations from 3.13 are still present.

Sorry I was confused by the diff

[brandtbucher]

Looks good. I'm guessing validate_list is the cheapest (or most useful) check?