RFR: Evac assist should touch marked objects only

[Roman Kennke]

Yes, go!

Am 23. August 2018 21:12:19 MESZ schrieb Aleksey Shipilev <shade at redhat.com>:
>http://cr.openjdk.java.net/~shade/shenandoah/evac-assist-unmarked/webrev.01/
>
>Our testing shows evac assist is still broken. It fails very
>intermittently with normal degens too.
>The failure comes from touching the dead object's size() during
>assists. This works fine when
>assisted object is above the TAMS, when all objects are implicitly
>alive, but fails when it is below
>the TAMS.
>
>Normal scanning code uses the bitmap for below-TAMS scans, but I think
>it is easier to terminate the
>assist as soon as first non-marked object is detected. Logging shows we
>still assist plenty. Maybe
>we would consider doing the bitmap scan in future.
>
>I believe degen-traversal workaround is not needed anymore, because it
>fails for the same reason:
>touching the dead object. Only the symptom differs: there, dead object
>appears doubly-forwarded.
>This patch resolves that by never touching the fwdptr for unmarked
>objects.
>
>I'd like to give this patch a ride in sh/jdk nightlies.
>
>Testing: tier3_gc_shenandoah (running), intermittently failing stress
>test
>
>Thanks,
>-Aleksey

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