RFR: 8197454: Need Access decorator for storing oop into uninitialized location

[Kim Barrett]

> On Feb 12, 2018, at 9:16 AM, Erik Österlund <erik.osterlund at oracle.com> wrote:
> 
> Hi Kim,
> 
> Looks good.

Thanks, Erik.

> This decorator was renamed to have the ARRAYCOPY_ prefix after discovering it was only used by arraycopy. But if you have a different use case that needs to perform stores on uninitialized memory, then I support this change.

This came up while Access-orizing JNI.  The couple of places where this was needed for
JNI could instead do a raw store of NULL followed by a normally barriered oop_store.
It seemed clearer to make this change in order to be explicit about the semantics.

> Thanks,
> /Erik
> 
> On 2018-02-11 08:25, Kim Barrett wrote:
>> Please review this change to the Access API to support stores of oops
>> into uninitialized locations.  This change is needed to prevent such
>> stores from, for example, having the G1 pre-barrier applied to
>> whatever garbage happens to be in the location being stored into.
>> 
>> There was already support for stores to uninitialized locations in the
>> Access API, but only for array initialization.  This change
>> generalizes that mechanism, and renames it accordingly:
>> ARRAYCOPY_DEST_NOT_INITIALIZED => AS_DEST_NOT_INITIALIZED.
>> 
>> CR:
>> https://bugs.openjdk.java.net/browse/JDK-8197454
>> 
>> Webrev:
>> http://cr.openjdk.java.net/~kbarrett/8197454/open.00/
>> 
>> Testing:
>> Mach5 {hs,jdk}-tier{1,2,3}