RFR: 8333750: GenShen: Only instantiate young/old generations in generational mode [v2]

[Y. Srinivas Ramakrishna]

On Thu, 6 Jun 2024 21:12:50 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> * Other modes should not instantiate or touch young/old generations.
>> * The generation sizer has been put in its own file and moved out of shHeap
>> * The logic for aging regions has been decoupled from non-generational final update refs
>
> William Kemper has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix zero build

src/hotspot/share/gc/shenandoah/shenandoahBarrierSet.cpp line 138:

> 136:       // This is safe iff it is assured that each PLAB is a whole-number multiple of card-mark memory size and each
> 137:       // PLAB is aligned with the start of each card's memory range.
> 138:       // TODO: Assert this in retire_plab?

I recall seeing this comment in other code I recently touched. (I made the comment a bit more succinct there.) Looking at this here and at that place, I now feel this portion of the comment actually belongs in the implementation of `retire_plab()` where the filling & registration are done, rather than being repeated at each place where we call `retire_plab()` as is the case currently.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/444#discussion_r1631717951