RFR: 8252857: AArch64: Shenandoah C1 CAS is not sequentially consistent [v2]

[Nick Gasson]

> C1 atomic operations are supposed to be sequentially consistent by
> default but the variant in the Shenandoah C1 barrier set assembler only
> provides a half-barrier when the CAS succeeds. Added a trailing full
> barrier and load-acquire to exactly match the non-Shenandoah C1 CAS
> implementation. This prevents any memory accesses following the CAS
> operation being observed before it.
> 
> Also adjusted the documentation for ShenandoahBarrierSetAssembler
> ::cmpxchg_oop as it currently claims to be sequentially consistent by
> default but it's not clear what the "default" values of acquire and
> release should be, and the comment for acquire/release implies to me
> that setting them to true would relax the ordering guarantees but
> actually it's the opposite. I tried to simplify this and make it less
> ambiguous.
> 
> One other thing I noticed when reading this: the comment
> 
>    // Step 4. CAS has failed because the value most recently fetched
>    // from addr (which is now held in tmp1) ...
> 
> is wrong on non-LSE systems because tmp1 is rscratch1 and that is
> clobbered by the call to __ cmpxchg() at the end of step3. Although it
> doesn't matter because the value in tmp1 is not used after that point.
> Adjusted the comment to clarify this.
> 
> Tested hotspot_all_no_apps, jdk_core plus jcstress with -jvmArgs
> '-XX:+UseShenandoahGC -XX:ShenandoahGCMode=iu -XX:TieredStopAtLevel=1'.

Nick Gasson has updated the pull request incrementally with one additional commit since the last revision:

  Only insert trailing barrier when is_c1_or_interpreter_only()

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/280/files
  - new: https://git.openjdk.java.net/jdk/pull/280/files/4768d96f..d2358620

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=280&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=280&range=00-01

  Stats: 25 lines in 2 files changed: 8 ins; 12 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/280.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/280/head:pull/280

PR: https://git.openjdk.java.net/jdk/pull/280