RFR: 8339687: Rearrange reachabilityFence()s in jdk.test.lib.util.ForceGC

[David Holmes]

On Mon, 9 Sep 2024 16:25:15 GMT, Stuart Marks <smarks at openjdk.org> wrote:

>> @dholmes-ora Is this really possible? The `obj` ref is passed to the PhantomReference constructor, which stores it in a field, the constructed PhantomReference is returned, and it's then used in a reachabilityFence call below. So `obj` should remain reachable the entire time, right?
>
> (As an aside, I wasn't able to determine what any of the Reference classes do if they're created with a null reference. Possibly a spec bug?)

@stuart-marks  My recollection, which I can't confirm is that this pattern was discussed internally and there was a lot of uncertainty about what was actually needed. Interestingly there was zero discussion of this in the actual PR that added it - https://github.com/openjdk/jdk/pull/8979

Thinking it through now, I tend to agree with you that the RF for `ref` suffices to prevent `obj` from being elided

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PR Review Comment: https://git.openjdk.org/jdk/pull/20898#discussion_r1751000575