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

[Thomas Schatzl]

Hi all,

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. 

StefanJ correctly pointed out that there must more than one reason to
be allowed to do this.

- we can skip the humongous liveness check because G1 never pushes oop*
with references outside of the collection set onto the task queue.
Humongous objects are always logically in old gen, so we should never
see such a reference here.
I added an additional assert to make that clear.

- the second reason for being able to exit early if the reference in
the oop* is in old gen is because then we know that that reference has
already been processed (as we try to only push references into the
collection set on the task queue), and the remembered set update etc
already done.

Note that in case of evacuation failure this check is not sufficient,
i.e. the reference is still a reference into the collection set, so G1
still does the update_rs() again the next time we get here. However
this is no problem either - naturally they are not references to
humongous objects, and we do not care too much about performance in
that rare case either (i.e. the additional update_rs() call is very
quick anyway).

> Just exit the method if we detect such a task.

New webrev at 
http://cr.openjdk.java.net/~tschatzl/8211853/webrev.0_to_1 (diff)
http://cr.openjdk.java.net/~tschatzl/8211853/webrev.1 (full)
with the added assert.

Passed hs-tier1-3 as expected.

Thanks,
  Thomas