RFR: 8078743: AARCH64: Extend use of stlr to cater for volatile object stores

[Andrew Dinn]

The following webrev is a follow-on to the fix posted for JDK-8078263.
The earlier fix optimized *non-object* volatile puts/gets to use
stlr/ldar and elide the associated leading/trailing dmb instructions.
This one extends the previous optimization to cover volatile object puts.

  http://cr.openjdk.java.net/~adinn/8078743/webrev.03/

The fix involves identifying certain Ideal Graph configurations in which
generation of leading and trailing dmb can be avoided in favour of
generating an stlr instruction. As a consequence the fix is sensitive to
the current GC configs and also to whether the value being written is
null, potentially null or known notnull. I have tested it using 5 GC
configs:

  G1GC
  CMS+UseCondCardMark
  CMS-UseCondCardMark
  Parallel+UseCondCardMark
  Parallel-UseCondCardMark

The last two configs are much of a muchness as regards what the patch
code does since with Parallel (or Serial) GC the patch code does not
need to look at the code which does the card mark -- but I tested
against both just to be sure.

Testing involved

  i) eyeballing the code generated for normal and unsafe volatile puts
with null, possibly null and notnull values

  ii) exercising a large program (build and run sample project in
netbeans) and eyeballing the code generated for methods of ConcurrentHashMap

  iii) running the full jcstress suite

Comments and reviews very welcome

n.b. I have a 3rd patch queued which performs a similar optimization for
CAS operations (drop the dmbs in favour of an ldaxr/stlxr pair).

regards,


Andrew Dinn
-----------
Senior Principal Software Engineer
Red Hat UK Ltd
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