RFR: 8337511: Implement JEP 404: Generational Shenandoah (Experimental) [v7]

[Aleksey Shipilev]

On Wed, 13 Nov 2024 19:32:53 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> This PR merges JEP 404, a generational mode for the Shenandoah garbage collector. The JEP can be viewed here: https://openjdk.org/jeps/404. We would like to target JDK24 with this PR.
>
> William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 510 commits:
> 
>  - Merge branch 'merge-latest' into great-genshen-pr-redux
>  - Use new CompactHeader forwarding APIs in generational mode
>  - Merge remote-tracking branch 'jdk/master' into merge-latest
>  - Merge
>  - 8343649: Shenandoah: ShenandoahEvacInfo event does not follow JFR guidelines
>    
>    Reviewed-by: wkemper
>  - Merge
>  - 8343227: GenShen: Fold resource mark into management of preselected regions
>    
>    Reviewed-by: kdnilsen
>  - Merge openjdk/jdk tip into great-genshen-pr-redux
>  - Merge remote-tracking branch 'jdk/master' into merge-latest
>  - Merge remote-tracking branch 'jdk/master' into merge-latest
>  - ... and 500 more: https://git.openjdk.org/jdk/compare/889f9062...5e02b5d8

src/hotspot/share/gc/shenandoah/shenandoahScanRemembered.hpp line 361:

> 359:   // frequently than last byte.  This is true when number of clean cards is greater than number of dirty cards.
> 360:   static const uint16_t ObjectStartsInCardRegion = 0x80;
> 361:   static const uint16_t FirstStartBits           = 0x7f;

I see these are used to do operations against `uint8_t` (`....offsets.first`). Any reason why these should not be `uint8_t`?

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/21273#discussion_r1843980993