Disable tracking of frozenset objects with immutables in the GC

[eendebakpt]

Feature or enhancement

Proposal:

Frozenset objects with immutable objects do not need to be tracked by the GC. A similar optimization is already in place for tuples (#48938, #140204).
In particular for longer running programs with immutable frozensets this is benefical.

Linked PRs

• gh-140232: Do not track frozenset objects with immutables #140234

[markshannon]

Why untrack eagerly, rather than lazily as we do for tuples?

Also, frozensets are not used that much, but are generated by the bytecode compiler. All compiler generated frozensets do not need tracking, so explicitly untracking them during unmarshalling might be more valuable.

[eendebakpt]

I used eager untracking to prevent having to do the check in the GC for each iteration, For the tuples there is a reason it has to be done in the GC (because tuples can be generated with NULL values and filled later).

Frozensets are not used a lot indeed, but I do expect usage of immutable datasets (including the frozenset) to increase with the free-threading build being used more.

Doing the untracking during unmarshalling is not something I considered. I think the bytecode compiler generated frozensets are already handled with the approach in the PR. Correct? If adding this to the unmarshalling, do you have a pointer where this should be roughly added in the codebase?