RFR: 8333457: GenShen: Move remembered set into new generational code [v2]

[William Kemper]

On Tue, 4 Jun 2024 01:24:17 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

>> William Kemper has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fix typo in comment
>
> I didn't follow all of the logic in ShenandoahMark::mark_through_ref, but otherwise looks good.
> 
> Left a few remarks, none of them super-important.
> 
> Thanks for making these changes, further separating out the generational code from the non-generational original.

@ysramakrishna , the change for `mark_through_ref` creates a specialization for the NON_GEN case (i.e., single gen modes). It's likely the compiler could generate an optimized function for the NON_GEN template parameter, but I wanted to be able to use the `ShenandoahGenerationalHeap` type here without a runtime check on the mode.

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PR Comment: https://git.openjdk.org/shenandoah/pull/443#issuecomment-2148211732