RFR: 8309599: WeakHandle and OopHandle release should clear obj pointer

[Roman Kennke]

On Tue, 26 Sep 2023 12:47:42 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> This change makes WeakHandle and OopHandle release null out the obj pointer, at the cost of making the release function non-const and some changes that propagated from that.  This enables ObjectMonitor code to test for null to see if the obj was already released, and seems like the right thing to do.  See comments from related PR in the bug report.
> Tested with tier1-4.

Not a full review yet, but it looks like it achieves the same as https://github.com/openjdk/jdk/pull/13721 but differently. I opened that PR then for other reasons, and it has been rejected then, on the grounds that those reasons are not valid (which I agree). Since then I have found at least one other use of the same cleaning mechanism, which is ZGC support for Lilliput, but we decided to follow another, better route (full obj->OM mapping). Maybe you find inspiration by that old PR ;-) It seems to get away with less intrusions in unrelated code.

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PR Comment: https://git.openjdk.org/jdk/pull/15920#issuecomment-1735534464