[aarch64-port-dev ] RFR(s): 8171449" [aarch64] store_klass needs to use store release

[Andrew Haley]

On 27/12/16 21:21, Kim Barrett wrote:
>> On Dec 27, 2016, at 5:40 AM, Andrew Haley <aph at redhat.com> wrote:
>> If we really did want to do this on AArch64, we could better by
>> zeroing the object first, writing the length field, then storing the
>> klass field with stlr.  One fence rather than two would be better than
>> what we do at the moment, which is storing the klass field then
>> zeroing the object. 
> 
> I think zeroing the object is intentionally delayed.
> 
> In the slow path, the allocation of a raw chunk of memory is done
> while holding the Heap_lock.  That lock prevents other Java threads
> from entering the slow path.
> 
> Part of that raw chunk allocation makes the chunk unavailable to other
> Java threads.  But it also makes the chunk visible to a concurrent GC.
> Hence the need to first zero out what will eventually be the klass
> field.

Absolutely so.  I was talking about the fast path.

Thank you for the explanation: none of what I'd heard before made any
sense!

Andrew.