RFR: 8253180: ZGC: Implementation of JEP 376: ZGC: Concurrent Thread-Stack Processing [v7]

[Aleksey Shipilev]

On Mon, 28 Sep 2020 16:41:28 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

>> src/hotspot/share/compiler/oopMap.cpp line 319:
>> 
>>> 317: #ifdef ASSERT
>>> 318:         if ((((uintptr_t)loc & (sizeof(oop)-1)) != 0) ||
>>> 319:             !Universe::heap()->is_in_or_null((oop)NativeAccess<AS_NO_KEEPALIVE>::oop_load(&val))) {
>> 
>> This NativeAccess call causes the troubles with Shenandoah GC. It triggers evacuation of the object referenced by the
>> stack location during a safepoint, which we'd rather avoid. Why is it needed?
>
> We call frame::oops_do from the GC to make bad oops good. But the oops_do logic assers the oop is good *before* the
> closure is applied. Every time I do something, I run i to issues with this assert. It seems like an invalid assumption
> to me, that oops are meant to always be good, even before they are fixed. I vote for removing this assert. It has
> caused a lot of pain. Any takers?

I think the previous assert was intentionally weaker: `is_in` checks that argument points to a committed area in the
heap, which can include the oops not yet fixed. It does not check if oop is valid as far as GC is concerned. So maybe
reverting `NativeAccess::oop_load` is enough?

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PR: https://git.openjdk.java.net/jdk/pull/296