RFR: 8315247: GenShen: Condition calls to post-write barrier code generation by a flag

[Y. Srinivas Ramakrishna]

On Mon, 28 Aug 2023 22:00:09 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> (or you could say, if they turn off ShenandoahCardBarrier, that would mean we have no remembered set and always scan all of old-gen.  but that would be a lot more work for us, and of no value, I think...)
>
> maybe just fatal out in ShenandoahArguments if mode->is_generational() and !ShenandoahCardBarrier.  So the user has to specify ShenandoahCardBarrier if they specify ShenandoahGCMode == generational.

I'm going to leave it visible on command-line like the other `Shenandoah*Barrier` flags, but force it off in all modes except generational where we force it on. Since these aren't manageable flags,, jcmd should not be able to enable them after start up either. 

If the concern is that we would be silently ignoring user request to explicitly enable it in non-generational mode, or disable it in generational mode, we can log a warning upon such an attempt and still do the right thing.

Let me test these various flag combos and ensure that they do the right thing. I'll update the PR once finished w/the changes and testing.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/313#discussion_r1308019915