RFR: 8367013: Add Atomic<T> to package/replace idiom of volatile var plus AtomicAccess:: operations

[Kim Barrett]

On Sun, 28 Sep 2025 11:10:41 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this change that adds the type Atomic<T>, to use as the type
> of a variable that is accessed (including writes) concurrently by multiple
> threads. This is intended to replace (most) uses of the current HotSpot idiom
> of declaring a variable volatile and accessing that variable using functions
> from the AtomicAccess class.
> https://github.com/openjdk/jdk/blame/528f93f8cb9f1fb9c19f31ab80c8a546f47beed2/doc/hotspot-style.md#L138-L147
> 
> This change replaces https://github.com/openjdk/jdk/pull/27462. Differences are
> 
> * Substantially restructured `Atomic<T>`, to be IDE friendly. It's
> operationally the same, with the same API, hence uses and gtests didn't need
> to change in that respect. Thanks to @stefank for raising this issue, and for
> some suggestions toward improvements.
> 
> * Changed how fetch_then_set for atomic translated types is handled, to avoid
> having the function there at all if it isn't usable, rather than just removing
> it via SFINAE, leaving an empty overload set.
> 
> * Added more gtests.
> 
> Testing: mach5 tier1-6, GHA sanity tests

The implementation of Atomic<T> leans heavily on AtomicAccess. But note
that some of the Atomic<T> function names differ from the corresponding
AtomicAccess function names. Without the AtomicAccess:: prefix, the names
from that class are in some cases subtle and easy to overlook, which is not a
benefit when dealing with the complexities of lock-free code.

This PR includes gtests for the new type. These gtests only test functional
behavior, not concurrent access behavior.

This PR also includes conversions for a few uses. This provides examples of
what the new code will look like. It also provides some additional testing for
this change. The plan is that other uses of the volatile + AtomicAccess
idiom will be converted incrementally. It is expected that there will be some
remaining direct uses of AtomicAccess, in places where conversion to
Atomic<T> isn't appropriate. An example is the BitMap::par_xxx suite of
functions.

This PR contains a sequence of commits, each covering different aspects of the
change. It might help to focus on particular commits when reviewing.

The HotSpot Style Guide needs to be updated for JDK-8367014 and for this
change. That will be done in a followup.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/27539#issuecomment-3342949371