RFR: 8371381: [Shenandoah] Setting ergo flags should use FLAG_SET_ERGO [v3]

[Xiaolong Peng]

On Wed, 12 Nov 2025 19:33:12 GMT, Rui Li <duke at openjdk.org> wrote:

>> Setting ergo flags using `FLAG_SET_ERGO`, instead of `FLAG_SET_DEFAULT`, so we can have the right origin info.
>> 
>> 
>> For `ShenandoahEvacReserve` change: when we use shenandoah passive mode and degen is also turned off (`-XX:ShenandoahGCMode=passive -XX:-ShenandoahDegeneratedGC`), we set the ShenandoahEvacReserve to 0:  https://github.com/openjdk/jdk/blob/c6a8027b94bbcbde5f7dcabd0bff48b93bbb5a7f/src/hotspot/share/gc/shenandoah/mode/shenandoahPassiveMode.cpp#L40-L42
>> 
>> `FLAG_SET_DEFAULT` doesn't check the range but `FLAG_SET_ERGO` does so some of the jtreg would fail because of this change. Had to move this ergo setting to regular `FLAG_SET_DEFAULT`.
>> 
>> Testing: jtreg gc. GHA pending.
>
> Rui Li has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Move ShenandoahEvacReserve back to (1,100)

Marked as reviewed by xpeng (Committer).

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PR Review: https://git.openjdk.org/jdk/pull/28242#pullrequestreview-3460930727