RFR(M) 8222992: Shenandoah: Pre-evacuate all roots

[Aleksey Shipilev]

On 4/26/19 2:50 PM, Zhengyu Gu wrote:
> Since we switched to strong to-space invariant, if we pre-evacuate all roots, the roots should only
> contain to-space references, until next evacuation cycle. Therefore, we can avoid a couple of update
> roots calls in regular paths. Also, make code easy to reason.
> 
> The change may prolong final_mark pause. It is temporary. I intent to move several pre-evacuated
> roots to concurrent phase, in followup RFEs.
> 
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8222992
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8222992/webrev.00/

I'll take a more extensive look next week. Some brief observations here:

*) There is a plain store in ShenandoahBarrierSet::AccessBarrier<decorators,
BarrierSetT>::oop_load_not_in_heap. First, I wonder if it even makes sense to do it here -- it feels
awkward to have writes in "loads". Second, shouldn't that be a CAS, on the off-chance something else
is storing the another value?

*) I wonder if verify_roots() and the associated closure belongs to Verifier (not necessarily
protected by ShenandoahVerify) or shenandoahAsserts. In fact, maybe we should just strategically
rely on Verifier to catch non-forwarded roots?

*) In final-marking, the assert(task_queues()->is_empty(), "Should be empty"); is gone, why?

*) Can the per-line DEBUG_ONLY in shenandoahPhaseTimings be turned into #ifdef DEBUG block?

*) Typo here, "Verfiy":

  322   DEBUG_ONLY(f(verify_roots,                        "Verfiy Roots"))                    \

-Aleksey