RFR: 8252853: AArch64: gc/shenandoah/TestVerifyJCStress.java fails intermittently with C1

[Nick Gasson]

This test fails occasionally when run with -XX:TieredStopAtLevel=1 on
AArch64 with the error

  java.util.concurrent.ExecutionException: java.lang.NullPointerException: Cannot assign field "prev" because "node" is
  null
      at java.base/java.util.concurrent.FutureTask.report(FutureTask.java:122)
      at java.base/java.util.concurrent.FutureTask.get(FutureTask.java:191)
      at TestVerifyJCStress.main(TestVerifyJCStress.java:109)
      [..snip..]
  Caused by: java.lang.NullPointerException: Cannot assign field "prev" because "node" is null
      at java.base/java.util.concurrent.locks.StampedLock.acquireWrite(StampedLock.java:1221)
      at java.base/java.util.concurrent.locks.StampedLock.writeLockInterruptibly(StampedLock.java:536)
      at TestVerifyJCStress$Test.WLI_Us(TestVerifyJCStress.java:133)

The particular configuration that fails is -XX:ShenandoahGCMode=iu. In
this mode the Shenandoah C1 CAS implementation calls into
ShenandoahBarrierSetAssembler::storeval_barrier() which performs a VM
leaf call in the slow path (via satb_write_barrier_pre()) but only saves
R0-R4 and V0 (for the interpreter, I guess). Instead it needs to
preserve all the caller saved registers as some of these might hold live
values over the CAS in the C1 generated code.

Tested jtreg hotspot_all_no_apps, jdk_core, plus jcstress with -jvmArgs
'-XX:+UseShenandoahGC -XX:ShenandoahGCMode=iu -XX:TieredStopAtLevel=1'
which would also fail in the same way.

-------------

Commit messages:
 - 8252853: AArch64: gc/shenandoah/TestVerifyJCStress.java fails intermittently with C1

Changes: https://git.openjdk.java.net/jdk/pull/68/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=68&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252853
  Stats: 15 lines in 2 files changed: 5 ins; 7 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/68.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/68/head:pull/68

PR: https://git.openjdk.java.net/jdk/pull/68