RFR: JDK-8180450: secondary_super_cache does not scale well [v4]

[Vladimir Ivanov]

On Mon, 18 Mar 2024 10:36:20 GMT, Andrew Haley <aph at openjdk.org> wrote:

> I'm unclear why you are presenting this as a Diagnostic feature?

>From a user perspective, there should be no reason to specify `HashSecondarySupers` or `UseSecondarySuperCache` unless you diagnose a performance regression or looking for a workaround for a crash. There should be no other reason to override default value and switch to the old implementation once `HashSecondarySupers` is supported on a platform.

IMO diagnostic flags are well justified here. Experimental flag doesn't cut it since the feature is turned on by default and making the flags product puts unnecessary obligations on the VM for something so obscure (from a user perspective). 

Some other VM flags followed the same practice (e.g., `UseVtableBasedCHA`) and so far it worked well.

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PR Comment: https://git.openjdk.org/jdk/pull/18309#issuecomment-2005653912