RFR: 8359435: AArch64: add support for 8.5 SB instruction

[Evgeny Astigeevich]

On Fri, 13 Jun 2025 14:00:08 GMT, Evgeny Astigeevich <eastigeevich at openjdk.org> wrote:

> Speculation Barrier (SB) instruction can be used instead of a pair of DSB, ISB if supported. It should have better performance than DSB+ISB (https://developer.arm.com/documentation/102825/0100):
> 
>> ... a DSB+ISB sequence is expected to have a significantly greater impact on performance than an SB ...
> 
> CPUs supporting it:
> - Apple M2+
> - Neoverse-N2
> - Neoverse-V2
> 
> Tested:
> - Gtests passed

The case is to use it for spin pauses instead of `ISB` on Neoverse-N2/V2.
There is data SB-based spin pauses are less disruptive then ISB-based one on them, so performance is better:
- https://github.com/mysql/mysql-server/pull/611
- https://github.com/facebook/folly/pull/2390

There are discussions regarding using it for spin pauses:
- https://github.com/gperftools/gperftools/pull/1594
- https://github.com/haproxy/haproxy/pull/2974

Do you think it is better to have a PR combining this PR and use of SB for spin pauses?

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PR Comment: https://git.openjdk.org/jdk/pull/25801#issuecomment-2970848883