Note for reviewers: The PR is split in two commits in an attempt to make it easier to review. The first commit simply moves Modules/gcmodule.c to Python/gc.c without changes. The second commit pulls out the Python package back into Modules/gcmodule.c and fixes things up.

I recommend reviewing the second commit: fa068b5 "Split up gcmodule.c".

Here's the summary of changes beyond simply moving functions from one file to another:

• Move logic for freezing, unfreezing, getting all GC objects, etc. to Python/gc.c. See changes to pycore_gc.h. This avoids requiring many of the GC list operations in Modules/gcmodule.c.
• Move the DEBUG_STATS, etc. constants to pycore_gc.h and prefix them as e.g., _PyGC_DEBUG_STATS. The Python constants are not renamed (i.e., gc.DEBUG_STATS). They are used in both Modules/gcmodule.c and Python/gc.c.

Looks okay to me. I'd suggest it's time to revise that comment at the top of the file. It's not so useful anymore and someone can use git log to see contributors and the history. Maybe add a link to https://devguide.python.org/internals/garbage-collector/ instead of links to those mailing list posts. E.g. you could put this at the top of the gc.c file instead:

//  This implements the reference cycle garbage collector.
//  The Python module inteface to the collector is in gcmodule.c.
//  See https://devguide.python.org/internals/garbage-collector/

Looks okay to me. I'd suggest it's time to revise that comment at the top of the file. It's not so useful anymore and someone can use git log to see contributors and the history. Maybe add a link to https://devguide.python.org/internals/garbage-collector/ instead of links to those mailing list posts. E.g. you could put this at the top of the gc.c file instead:

//  This implements the reference cycle garbage collector.
//  The Python module inteface to the collector is in gcmodule.c.
//  See https://devguide.python.org/internals/garbage-collector/

Related to this although not a blocker for this PR: let's ensure we document the nogil parts also in https://devguide.python.org/internals/garbage-collector/

@pablogsal - that makes sense. Should I revert the style changes and put them out as a subsequent PR?

Nah, don't worry, I agree it was a good opportunity but I was being careful that we end putting a lot more of other stylistic changes or similar

Great - would one of the reviewers please merge the PR?

False positive, the builder is out of disk space