8369207: _Copy_conjoint_{jshorts,jints,jlongs}_atomic on Linux/BSD AArch64 do not prevent tearing

[jcking]

Ensure the compiler cannot optimize the copy loops by replacing them with memcpy which is allowed to copy byte by byte potentially introducing tearing. Currently there is nothing preventing the compiler from doing this. We add volatile which prevents the compiler from making this optimization in the future.

Currently the code generates to ldp and stp, this change does have the side affect of forcing it to do ldr and str. If that seems unacceptable from a performance standpoint we can hand-roll assembly to do ldp and stp.

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Issue

• JDK-8369207: Copy_conjoint{jshorts,jints,jlongs}_atomic on Linux/BSD AArch64 do not prevent tearing (Bug - P4)

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[theRealAph]

Currently the code generates to ldp and stp, this change does have the side affect of forcing it to do ldr and str. If that seems unacceptable from a performance standpoint we can hand-roll assembly to do ldp and stp.

That would be much better.

[jcking]

Currently the code generates to ldp and stp, this change does have the side affect of forcing it to do ldr and str. If that seems unacceptable from a performance standpoint we can hand-roll assembly to do ldp and stp.

That would be much better.

#27642 is already going to move it to ldr and str, FYI.

[jcking]

Moving this back to draft until discussions in #27642 finish.