RFR: 8287818: Shenandoah: adapt nmethod arming from Loom

[Aleksey Shipilev]

On Mon, 6 Jun 2022 23:29:10 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> Loom implemented nmethod arming mechanism, which is very similar to Shenandoah's. Shenandoah can use it.
> 
> Test:
> 
> - [x] hotspot_gc_shenandoah on Linux x86_64 and Windows x64
> - [x] tier1 with Shenandoah GC on Linux x86_64 and Windows x64 
> - [x] hotspot_gc_shenandoah on AArch64

Looks mostly fine to me, but I have questions.

src/hotspot/share/gc/shenandoah/shenandoahBarrierSetNMethod.cpp line 72:

> 70:   ShenandoahNMethod::disarm_nmethod(nm);
> 71:   return true;
> 72: }

Nit: missing newline.

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?

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