explore: take advantage of modern CRuby GC features

[flavorjones]

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After reading Peter's Adventures in Ruby: Garbage Collection in Ruby - Peter Zhu:

Can we use rb_gc_mark_movable instead of rb_gc_mark?

I think we can.

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And after peeking at Gradual Write-Barrier Insertion into a Ruby Interpreter:

Can we take advantage of WB protection?

I need to learn more here, but:

a) it doesn't look like we can make Node or Document objects use WB, because they are T_DATA and so are always created with wb_protected=false; and (from the linked doc above) "WB-unprotected objects can not become WB-protected objects."

b) we've already (long ago) removed RB_ARRAY_PTR in favor of rb_ary_entry so I'm not sure we're unprotecting anything we don't need or intend to unprotect

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Things I'd like to do before we start working on this, though:

• ship v1.11.0 (see milestone)
• maybe ship v1.12.0 with Nokogumbo (see #2064)
• figure out whether our Valgrind suppressions are actual problems or not (I'm starting to suspect they might be)
• investigate and close all the existing memory issues (see label topic/memory)
• resurrect the GC memory test suite, and add more coverage to it (https://github.com/sparklemotion/nokogiri/issues/1603)

So anyone who's interested in this topic, I'm open to learning, but I'm unlikely to merge anything until and unless all of the above are addressed first.

[flavorjones]

cc @tenderlove to tell me how I'm incomplete or wrong here

[larskanis]

I migrated ruby-pg from Data_Wrap_Struct to TypedData_Wrap_Struct a.k. typed-data recently here: https://github.com/ged/ruby-pg/pull/349

This changed all T_DATA objects to typed-data, implements the GC.compact callbacks and removed all rb_gc_mark calls.

I did not make use of write barriers so far, since I didn't estimate a big performance advantage, but more complexity.

[flavorjones]

We implemented TypedData_Wrap_Struct in #2579 which I think took care of all the compaction support. Given Lars's comment above about write barriers, I'm going to deprioritize this work and close this issue.

If there's GC-related work anybody thinks we should do, please comment and we can re-open or open a new sisue.