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.
> 
> Java atomics already have this distinction; 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.

@kimbarrett, tell me what you think. I can polish and test it further if this is sensible.

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PR Comment: https://git.openjdk.org/jdk/pull/29135#issuecomment-3728135411