RFR(S) Contended Locking fast exit bucket (8073165)

[Aleksey Shipilev]

On 03/13/2015 07:50 AM, David Holmes wrote:
>> - remove old mfence() support
> 
> So I had expected to see this change conditional on the runtime
> processor type. But then I checked Dave's blog and saw this was
> determined 5+ years ago (!!!) so those older processors are no longer a
> consideration. I do hope the performance tests have been re-run since
> then though ;-)

MFENCE still has stronger ordering guarantees, and LOCK'ed instructions
are faster for regular accesses. Note that we also observed the
performance effects when doing blind "lock addl %(rsp), 0":
  https://bugs.openjdk.java.net/browse/JDK-8050147
  http://hg.openjdk.java.net/jdk9/hs-comp/hotspot/rev/51e474c55d20
  http://shipilev.net/blog/2014/on-the-fence-with-dependencies/

Thanks,
-Aleksey.