8195148: Collapse G1SATBCardTableModRefBS and G1SATBCardTableLoggingModRefBS into a single G1BarrierSet

[Kim Barrett]

> On Mar 1, 2018, at 9:15 PM, Kim Barrett <kim.barrett at oracle.com> wrote:
> 
>> On Feb 26, 2018, at 8:38 AM, Erik Österlund <erik.osterlund at oracle.com> wrote:
>> 
>> Hi,
>> 
>> G1 has two barrier sets: an abstract G1SATBCardTableModRefBS barrier set that is incomplete and you can't use, and a concrete G1SATBCardTableLoggingModRefBS barrier set is what is the one actually used all over the place. The inheritance makes this code more difficult to understand than it needs to be.
>> 
>> There should really not be an abstract G1 barrier set that is not used - it serves no purpose. There should be a single G1BarrierSet instead reflecting the actual G1 barriers used.
>> 
>> Webrev:
>> http://cr.openjdk.java.net/~eosterlund/8195148/webrev.00/
>> 
>> Bug:
>> https://bugs.openjdk.java.net/browse/JDK-8195148
>> 
>> Thanks,
>> /Erik
> 
> Mostly looks good.  One minor formatting nit, and one significant not so sure about this.
> 
> […]

> ------------------------------------------------------------------------------
> src/hotspot/share/gc/g1/g1BarrierSet.inline.hpp
> 32 inline void G1BarrierSet::write_ref_field_pre(T* field) {
> 
> The change here doesn't seem to have anything to do with the renaming.
> Rather, it looks like a separate bug fix?
> 
> The old code deferred the decode until after the null check, with the
> decoding benefitting from having already done the null check.  At
> first glance, the new code seems like it might not perform as well.
> 
> I do see why adding volatile is needed here though.

Without exploring too deeply or doing any testing, it seems to me the
only change that should be made here is to replace the use of
oopDesc::load_heap_oop with RawAccess<MO_VOLATILE>::oop_load, e.g. it
should be

  T heap_oop = RawAccess<MO_VOLATILE>::oop_load(field);
  if (!oopDesc::is_null(heap_oop)) {
    enqueue(oopDesc::decode_heap_oop_not_null(heap_oop));
  }

That's assuming the LoadProxy can resolve to a narrowOop.