RFR: 8314480: Memory ordering spec updates in java.lang.ref

[Hans Boehm]

On Mon, 13 Nov 2023 22:31:16 GMT, Brent Christian <bchristi at openjdk.org> wrote:

> Classes in the `java.lang.ref` package would benefit from an update to bring the spec in line with how the VM already behaves. The changes would focus on _happens-before_ edges at some key points during reference processing.
> 
> A couple key things we want to be able to say are:
> - `Reference.reachabilityFence(x)` _happens-before_ reference processing occurs for 'x'.
> - `Cleaner.register()` _happens-before_ the Cleaner thread runs the registered cleaning action.
> 
> This will bring Cleaner in line (or close) with the memory visibility guarantees made for finalizers in [JLS 17.4.5](https://docs.oracle.com/javase/specs/jls/se18/html/jls-17.html#jls-17.4.5):
> _"There is a happens-before edge from the end of a constructor of an object to the start of a finalizer (§12.6) for that object."_

src/java.base/share/classes/java/lang/ref/Reference.java line 546:

> 544:      * <a href="package-summary.html#reachability"><em>strongly reachable</em></a>,
> 545:      * regardless of any other actions of the program that might otherwise cause
> 546:      * the object to become unreachable; thus, the object is not

Delete "regardless of any other actions of the program that might otherwise cause the object to become unreachable" ? I have no idea what actions would qualify there. And it's more misleading thann informative, since IMO the real concern here is compiler dead variable elimination.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/16644#discussion_r1406844556