[vectorIntrinsics] RFR: Optimize mem barriers for ByteBuffer cases [v6]

[Radoslaw Smogura]

On Tue, 3 Aug 2021 19:19:23 GMT, Paul Sandoz <psandoz at openjdk.org> wrote:

> > Hi all, I think that for this PR, I rather don't want to push more changes. It already stepped out from the original concern of optimizing mem-bars, to optimizing polluted cases.
> 
> Yeah, I am inclined to dial back to focus this PR on just the non-polluted cases.
> AFAICT I think we can get a set of minimal changes using casting and unsafe for accessing buffer state with no need to modify byte buffer code or branch on heao/direct. WDYT?
> 
> We can then expand later with further experiments for polluted cases, evaluating the costs and benefits.

Definitely, to all of the above (branching actually was added for polluted cases) - this went too big.

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PR: https://git.openjdk.java.net/panama-vector/pull/104