RFR: Simplify parallel synchronizer roots iterator

[Roman Kennke]

The parallel synchronizer roots iterator is a bit too complicated for no
good reason (I think, correct me if I'm wrong). It's doing two CASes per
loop iteration, once over the CHAINED_MARKER/CLAIMED_MARKER in the
block, and once more over the _cur field in the
ParallelObjectSynchronizerIterator. I think doing a very simple CAS-loop
over the _cur field is good enough, and maybe even improves performance
slightly:

This is what I get with gc-bench's roots.Sync benchmark:

baseline:
[15,840s][info][gc,stats]     S: Synchronizer Roots   =     0,16 s (a
=    17334 us) (n =     9) (lvls, us =    11523,    13867,    14062,   
16992,    25422)
[15,841s][info][gc,stats]     UR: Synchronizer Roots  =     0,11 s (a
=    12206 us) (n =     9) (lvls, us =     9707,    10352,    11719,   
11914,    15709)

patched:
[15,463s][info][gc,stats]     S: Synchronizer Roots   =     0,15 s (a
=    16147 us) (n =     9) (lvls, us =    11523,    12500,    15430,   
15820,    25440)
[15,463s][info][gc,stats]     UR: Synchronizer Roots  =     0,10 s (a
=    11111 us) (n =     9) (lvls, us =     9160,     9414,     9766,   
11523,    13777)


http://cr.openjdk.java.net/~rkennke/simplify-sync-roots/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/simplify-sync-roots/webrev.00/>


As a nice bonus, this reduces diffs between upstream jdk9 and shenandoah
jdk9 (removed CLAIMED_MARKER and cas_set_object())

Testing: hotspot_gc_shenandoah, specjvm-derby, gc-bench/roots.Sync

Note: gc-basher fails in hotspot_gc_shenandoah run, but doesn't seem
related to that change.

Ok?


Roman