RFR: 8367717: Cleanup atomic_copy64 [v2]

[Kim Barrett]

On Mon, 6 Oct 2025 14:23:11 GMT, Justin King <jcking at openjdk.org> wrote:

> This partially fixes https://bugs.openjdk.org/browse/JDK-8369207 as well, but this only does it for `jlong`. I can wait for this to go in, and then update `jshort` and `jint` to use `AtomicAccess` as well or we can do it all in this PR.

@jcking - I noticed that problem too, while reviewing this change. I don't care
whether it is fixed in this PR or in yours.

Note that your's currently doesn't perform atomic accesses on the "from"
value. I think that's wrong, since the documentation for these functions
doesn't indicate atomicity only for the store.  The comment in copy.hpp says
"atomicity: atomic or non-atomic on the copy unit." I take that to mean both
source and destination.

Your PR is also not addressing other platforms with similar issues, such as in
copy_ppc.hpp. 

Also, this all begs the question of why we have four different copies of these
functions - bsd_aarch64, bsd_zero, linux_aarch64, and linux_zero. All of these
should be identical.

Basically, I think the Copy class is kind of a mess, and that discourages its
use. Probably this needs an overall plan of attack (broken up into reviewable
chunks, but with an overarching goal), rather than piecemeal fixups.

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PR Comment: https://git.openjdk.org/jdk/pull/27642#issuecomment-3374841553