RFR: fix loop unswitching with -XX:-ShenandoahWriteBarrierMemBar

[Roman Kennke]

Am 31.01.2018 um 17:04 schrieb Roland Westrelin:
> 
>> Do we care anywhere else about deeper chain from that iff to the actual load? I.e. no other code is
>> broken due to the new graph shape?
> 
> Not as far as can tell.
> 
> Roland.
> 


With the patch, -XX:-ShenandoahWriteBarrierMemBar does not crash 
anymore, but it's significantly slower than with membar... like 75% 
slower. Which seems illogical.

tried with:
-XX:+UseShenandoahGC -XX:+UnlockDiagnosticVMOptions 
-XX:+UnlockExperimentalVMOptions -Xms4g -Xmx4g 
-XX:ShenandoahGCHeuristics=traversal -XX:ShenandoahFreeThreshold=17 
-XX:-ShenandoahWriteBarrierMemBar

on specjvm compiler.

Roman