RFR: 8336856: Optimize String Concat [v39]

[Claes Redestad]

On Thu, 8 Aug 2024 11:43:07 GMT, Shaojin Wen <duke at openjdk.org> wrote:

>> This PR implements the same algorithm as the current generateMHInlineCopy based on bytecode to improve startup performance.
>
> Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix comments

The unbound `CallSite`s should put up the concat classes for GC as soon as they go out of scope, so this doesn't even lean on the instigating classloader being GC'd. Excellent!

I think we should try to create a jtreg test from this. @DanHeidinga suggested offline that we could consider using `j.l.instrument.Instrumentation::getAllLoadedClasses()` - which should enumerate all classes, including hidden ones. Perhaps that's easy to mix in with your test above. 

But having verified that classes *can* be unloaded manually then perhaps we can file an RFE to have such a test worked out later and move ahead with this PR without such in-depth testing. I think such a test might have some challenges to get exactly right since you need to set it up and tune it so that unloading actually happens reliably on all platforms.

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PR Comment: https://git.openjdk.org/jdk/pull/20273#issuecomment-2284174629