RFR (XS): 8211853: Avoid additional duplicate work when a reference in the task queue has already been evacuated

[Thomas Schatzl]

Hi,

  ping for some reviews...

Thomas

On Mon, 2018-10-08 at 16:58 +0200, Thomas Schatzl wrote:
> Hi all,
> 
>   can I have reviews for this tiny change that removes G1 doing some
> duplicate work?
> 
> Due to benign races we can sometimes get references that have already
> been evacuated from the task queue. 
> 
> Current code in G1ParScanThreadState::do_oop_evac() needlessly
> applies follow-up calculations (remembered set, humonguous liveness)
> on this already evacuated reference - that all has been done the
> first time around already. 
> 
> Just exit the method if we detect such a task.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8211853
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8211853/webrev/
> Testing:
> local jtreg run of gc passes, some benchmarks
> 
> Thanks,
>   Thomas
> 
>