RFR: 8189871: Refactor GC barriers to use declarative semantics

[Per Liden]

On 11/10/2017 07:39 PM, Aleksey Shipilev wrote:
> On 11/10/2017 05:29 PM, Erik Österlund wrote:
>> 1) The barrier resolver loads the current barrier set, and checks the "name" of it (the enum value).
>> 2) Each "name" for concrete barriers that you listed in barrierSetConfig.hpp is asked for...
>> 3) ...the BarrierSet::GetType of that enum "name", and...
>> 4) The AccessBarrier of that resulting BarrierSet (your ShenandoahBarrierSet) will be called.
> 
> So, I tried a simpler exercise with Epsilon, and it seems to work:
>   http://cr.openjdk.java.net/~shade/epsilon/gc-barriers-declarative.patch
> 
> A few comments from that exercise:
> 
>   *) After the recent BS cleanup, the EpsilonBarrierSet has only a few leftovers [1]. With current
> patch, write_ref_field_work seems to be gone. But write_ref_array_work and write_region_work are
> still used. This removal is deliberately not handled in current patch, right?

Correct, those will be removed in one of Erik's later patches.

> 
>   *) What is the meaning of AccesssBarrier::Raw like these?
>    typedef BarrierSet::AccessBarrier<decorators, BarrierSetT> Raw;

That's a convenience typedef for use when you want to call the super 
class to do raw accesses (i.e. access without GC barriers, other than 
potentially encode/decode oops), which you'd typically do inside 
AccessBarrier functions for a specific BarrierSet, where those calls are 
sandwiched in-between the GC specific barrier logic.

> 
>    I am asking because it is not unclear if BS should typedef this. Epsilon seems to work fine
> without the declaration. G1SATBCardTableLoggingModRefBS::AccessBarrier has it, but
> CardTableModRefBS::AccessBarrier has not.

In Epsilon you don't needed it since you have no barriers. All you are 
doing is just raw accesses which is the default behavior offered by the 
super class.

cheers,
Per

> 
> Thanks,
> -Aleksey
> 
> [1]
> http://hg.openjdk.java.net/jdk/sandbox/file/b2b4df384c83/src/hotspot/share/gc/epsilon/epsilonBarrierSet.hpp
>