RFR: 8336856: Optimize String Concat [v42]

[Chen Liang]

On Mon, 12 Aug 2024 17:44:50 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

src/java.base/share/classes/java/lang/StringConcatHelper.java line 61:

> 59: 
> 60:         @ForceInline
> 61:         private String concat0(String value) {

I think we should make the base class abstract and add a `final class StringConcat1` with all these single-argument concat private methods, and that class is exposed via JLA instead.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20273#discussion_r1714231500