RFR: 8332548: GenShen: Factor generational mode out of gc helpers

[Y. Srinivas Ramakrishna]

On Tue, 21 May 2024 17:29:19 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

>> Some of the generational mode support in `shenandoahConcurrentGC.cpp` and `shenandoahDegeneratedGC.cpp` can be factored into generational mode specific files and coalesced under fewer generational mode checks.
>
> src/hotspot/share/gc/shenandoah/shenandoahDegeneratedGC.cpp line 175:
> 
>> 173:         // and this degenerated cycle. These pointers need to be included the 'read' table
>> 174:         // used to scan the remembered set during the STW mark which follows here.
>> 175:         _generation->merge_write_table();
> 
> Interesting. I am trying to understand this diff (the change in nesting looks curious because I didn't see anything change above. As a result, this specifically generational work(?) may have been pulled into the `else {` arm of the if-then-else at lines 145-149, introducing a bug?
> 
> Or is this intentional and I am missing some subtle reasoning here?

Or may be just an issue with how this displays in the diffs. Worth a quick check.

> src/hotspot/share/gc/shenandoah/shenandoahDegeneratedGC.cpp line 286:
> 
>> 284: 
>> 285:       if (heap->mode()->is_generational()) {
>> 286:         ShenandoahGenerationalHeap::heap()->complete_degenerated_cycle();
> 
> Not a change that you made, but I wondered about the semantics of "complete" in the name "op_cleanup_complete". It appears to use a phase timer around its work. Was the idea to include the "complete_degenerated_cycle" work inside that phase for timers associated with whatever `ShenandoahPhaseTimings::degen_gc_cleanup_complete` wanted to track, or is it good as is to exclude that generational work which may be is separately tracked?

(fwiw, I didn't see any obvious phase timers around the work in the generational `complete_degenerated_cycle`. Should there be any?)

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/436#discussion_r1608723088
PR Review Comment: https://git.openjdk.org/shenandoah/pull/436#discussion_r1608685213