RFR: 8339616: GenShen: Introduce new state to distinguish promote-in-place phase as distinct from concurrent evacuation

[Kelvin Nilsen]

On Fri, 20 Sep 2024 21:04:50 GMT, William Kemper <wkemper at openjdk.org> wrote:

> The generational mode for Shenandoah is able to promote regions in place (effectively moving an entire region of live objects, without evacuating any objects). It does this during a normal evacuation phase. However, in some cases, Shenandoah may choose to skip the evacuation phase, _even when there are entire regions to promote_. Prior to this PR, Shenandoah would essentially force itself into an evacuation phase, with nothing to evacuate. Though this was expedient, it cause all manner of knock-on effects and unintended consequences with barriers and asserts and the state of the heap. Here, we have reverted that business and created a path for this special "only doing in place promotions" cycle that does not cause the heap to expect to evacuate objects or find forwarding pointers in them.

Marked as reviewed by kdnilsen (Committer).

src/hotspot/share/gc/shenandoah/shenandoahConcurrentGC.cpp line 539:

> 537: 
> 538:   // Ultimately, the same closure handles in-place promotions for tenured regions. No objects will be moved.
> 539:   // The threads will still participate in the oom-evac-protocol, although no memory will be allocated for

Since there is no allocation for promote-in-place promotions, it seems there would be no need for these threads to participate in oom-evac protocol.  (But maybe they still monitor the conditions that would indicate oom-evac just so we can reuse the same code.)

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PR Review: https://git.openjdk.org/shenandoah/pull/503#pullrequestreview-2319381049
PR Review Comment: https://git.openjdk.org/shenandoah/pull/503#discussion_r1769270584