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

[Aleksey Shipilev]

On Sat, 14 Jun 2025 16:00:55 GMT, Andrew Haley <aph at openjdk.org> wrote:

> > So, it would be tad less confusing if we had a dependent RFE for using SB in spin pauses, so it was obvious why do we need it.
> 
> Huh? The least confusing is when the SB support goes in the PR where it is used. That really is obvious, without any dependency chain.

I am flexible to have it either way. 

One of the drawbacks of piling up the instruction support and the feature that uses these instructions: if there is _ever_ a second feature that depends on the same instruction support, we would effectively bind two commits (commit A: instruction support + feature A; commit B: feature B) together with an accidental dependency. Which gets extra funky if you ever go with bisects, backouts, backports. Atomic commits rule, and I personally strive to do them, even if there is a window when some code appears dead momentarily.

But as I said, I would not quibble here. SB looks like something that we would solely use for spin-wait hints.

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PR Comment: https://git.openjdk.org/jdk/pull/25801#issuecomment-2975611656