RFR: 8287818: Shenandoah: adapt nmethod arming from Loom [v2]

[Aleksey Shipilev]

On Thu, 23 Jun 2022 18:58:45 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> src/hotspot/share/gc/shenandoah/shenandoahCodeRoots.cpp line 129:
>> 
>>> 127: 
>>> 128: void ShenandoahCodeRoots::arm_nmethods() {
>>> 129:   BarrierSet::barrier_set()->barrier_set_nmethod()->arm_all_nmethods();
>> 
>> Question: in `ShenandoahBarrierSet::on_thread_attach`, we check `barrier_set_nmethod() != NULL`, do we need to do the same check here, or?
>
> Only in `passive` mode, Shenandoah does not install nmethod barrier, but `passive` mode never calls `arm_nmethod()`, etc. nmethod entry barrier related methods.

All right, would you mind adding at least the assert here, so that we don't have a spurious SEGV if that was ever violated?

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PR: https://git.openjdk.org/jdk/pull/9048