RFR: 8374878: Add Atomic<T>::compare_set

[Aleksey Shipilev]

On Fri, 9 Jan 2026 09:47:18 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Following up on [JDK-8367013](https://bugs.openjdk.org/browse/JDK-8367013) improvement, there is an opportunity to rewrite some of our low-level `cas(oldv, newv) == oldv` patterns to more straight-forward compare-and-set helper method. This is useful when you do not actually care about the result that used to be in memory or that is currently in memory, you only care if CAS succeeded or not.
> 
> Java atomics already have this duality; arguably due to historical timeline of having compare-and-set before introducing compare-and-exchange.
> 
> Found this when converting Epsilon to Atomic<T> ([JDK-8374876](https://bugs.openjdk.org/browse/JDK-8374876)), where this method would simplify the code a bit. I looked around current uses of `compare_exchange` and rewrote them to `compare_set` to show what simplifications are possible.
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, `all`

First run of Linux x86_64 server fastdebug, `all` yields no failures. I polished the patch a little, and now running the second iteration. If it is still clean, and I have no other comments, I'll integrate.

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PR Comment: https://git.openjdk.org/jdk/pull/29135#issuecomment-3729965567