RFR: 8352969: G1: Improve testability of optional collections [v6]

[Albert Mingkun Yang]

On Mon, 1 Sep 2025 04:48:28 GMT, Guanqiang Han <ghan at openjdk.org> wrote:

>> This PR introduces a new diagnostic flag EvacuateAllOptionalRegions to force G1 GC to evacuate all optional regions regardless of the predicted pause time. The motivation is to allow testing and validation of optional region evacuation behavior without being constrained by the remaining pause time.
>
> Guanqiang Han has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix format error

src/hotspot/share/gc/g1/g1_globals.hpp line 373:

> 371:           "the same or more than this number of code roots to be used.")    \
> 372:                                                                             \
> 373:   develop(bool, EvacuateAllOptionalRegions, false,                          \

Since the neighboring flags all have `G1` prefix, I suggesting doing the same for this flag as well.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26880#discussion_r2313944193