[aarch64-port-dev ] RFR(s): 8171449" [aarch64] store_klass needs to use store release

[Kim Barrett]

I think where we've ended up with this discussion is that

(1) Derek's proposed change should be withdrawn.

(2) JDK-8171449 should be closed as not an issue.

(3) There is a FIXME comment in the aarch64 store_klass definition
suggesting a release_store might be needed.  That comment appears to
be incorrect.

(4) There are comments on various uses of store_klass in TLAB contexts
(for multiple platforms) saying the store_klass must be last due to
ordering constraints when using concurrent GCs.  Those comments appear
to be doubly incorrect; existing concurrent GCs don't impose such an
ordering constraint in TLAB contexts, and there are no membars (on
platforms where they are needed) to ensure the suggested ordering.

(5) There might be uses of store_klass that aren't in TLAB contexts.
Someone should look for these to determine whether there actually are
any, and whether they have any ordering constraints.  If any do have
real ordering constraints, then some code changes are likely needed to
actually provide that ordering (perhaps an additional ordered form of
store_klass).

(6) It would be *really* nice to have the contracts in this area
written down in some more explicit and accessible form than the code
and scattered through multiple email threads.

Does that look like a correct summary?