RFR: Uses keep_alive_barrier to keep deduped char array alive

[Zhengyu Gu]

> Also, the whole block:
> 
> 384 if (UseShenandoahGC) {
> 385 oopDesc::bs()->keep_alive_barrier(existing_value);
> 386 existing_value = 
> (typeArrayOop)oopDesc::bs()->read_barrier(existing_value);
> 387 } else {
>   388       // Enqueue the reference to make sure it is kept alive. Concurrent mark might
>   389       // otherwise declare it dead if there are no other strong references to this object.
>   390       G1SATBCardTableModRefBS::enqueue(existing_value);
> 391 } Should be just: oopDesc::bs()->keep_alive_barrier(existing_value);
> 
> For G1 and Shenandoah under conc-mark, this enqueues the object in SATB, 
> for Shenandoah under conc-partial it does the WB. As I said above, I 
> don't think the RB is needed. I would be very surprised.
> 
Ok, let me clear my thoughts here.

If we do SATB barrier, that will ensure existing_value alive.

However, if we do keep_alive_barrier, the existing_value could be 
evacuated, but we still hold old oop here, right? then we can end up 
dead oop after recycle?

-Zhengyu



> Roman