RFR: 8345465: Fix performance regression on x64 after JDK-8345120

[Jorn Vernee]

On Wed, 4 Dec 2024 08:40:31 GMT, Per Minborg <pminborg at openjdk.org> wrote:

> This PR proposes to fix a performance regression (on x64 platforms) for quad-string words introduced by [JDK-8345120](https://bugs.openjdk.org/browse/JDK-8345120).
> 
> The PR also fixes a performance regression in the benchmarks caused by using the wrong type for `MemorySegment`.
> 
> Regrettably, this PR uses different code paths for various architectures. This gives optimum performance for all platforms at the expense of slightly more code complexity.

What are 'quad-string words'? FWIW, the word 'word' is somewhat ambiguous (although most people seem to interpret it as the size of an address), so you might want to specify in terms of bits/bytes instead.

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PR Comment: https://git.openjdk.org/jdk/pull/22539#issuecomment-2517554161