[ruby-core:117432] [Ruby master Feature#20408] Add heap_live_slots to GC.stat_heap

Issue #20408 has been reported by mk (Matthias K=E4ppler).

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Feature #20408: Add heap_live_slots to GC.stat_heap
https://bugs.ruby-lang.org/issues/20408

* Author: mk (Matthias K=E4ppler)
* Status: Open
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`GC.stat` exposes a `heap_live_slots` metric that returns the number of liv=
e objects occupying eden slots.

Because the slot size was fixed to `RVALUE_SIZE`, this allowed for some bas=
ic approximations of how many bytes were live/in-use (`heap_live_slots * RV=
ALUE_SIZE`).
This could furthermore be "embellished" by adding malloc'ed memory for thos=
e objects that were larger than a 40B slot (we track this via a custom CRub=
y patch).

However, with the addition of Variable Width Allocations (https://bugs.ruby=
-lang.org/issues/18239), this does not work anymore because we don't know h=
ow `heap_live_slots` distributes over all pools that use different slot siz=
es.

This could be addressed by also adding `heap_live_slots` (and for symmetry:=
 `heap_free_slots`) to `GC.stat_heap`.
The sum of all live slots per pool times its respective slot size should eq=
ual `GC.stat[:heap_live_slots]`.



--=20
https://bugs.ruby-lang.org/
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