RFR: 8336831: Optimize StringConcatHelper.simpleConcat [v5]

[Claes Redestad]

On Fri, 19 Jul 2024 21:42:09 GMT, Shaojin Wen <duke at openjdk.org> wrote:

>> Currently simpleConcat is implemented using mix and prepend, but in this simple scenario, it can be implemented in a simpler way and can improve performance.
>
> Shaojin Wen has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Update src/java.base/share/classes/java/lang/String.java
>    
>    Co-authored-by: Chen Liang <liach at openjdk.org>
>  - add comments

No, my idea is to rework and have SCF spin hidden classes using the new classfile API which will emit code or less in the style you are doing here for `simpleConcat`, but generalized. This may or may not work out beautifully. I'm coming back from a short vacation now and will work on having something to present on this topic at JVMLS two and a half weeks from now.

FTR I don't mind if you get this optimization in here. I'll just rebase my prototype, and it shouldn't alter things too much.

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PR Comment: https://git.openjdk.org/jdk/pull/20253#issuecomment-2240728186