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

[Kim Barrett]

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.

I like this addition.  One optional suggestion.

src/hotspot/share/runtime/atomic.hpp line 273:

> 271:   bool compare_set(T compare_value, T new_value,
> 272:                    atomic_memory_order order = memory_order_conservative) {
> 273:     return AtomicAccess::cmpxchg(value_ptr(), compare_value, new_value, order) == compare_value;

I think I would prefer both this and (especially) the translated case be

return compare_exchange(compare_value, new_value, order) == compare_value;

rather than duplicating some of the respective `compare_exchange`
implementations. I won't block for this though.

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Marked as reviewed by kbarrett (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/29135#pullrequestreview-3643716862
PR Review Comment: https://git.openjdk.org/jdk/pull/29135#discussion_r2676010126