RFR: 8336856: Optimize String Concat [v3]

[Shaojin Wen]

On Mon, 22 Jul 2024 22:34:01 GMT, Shaojin Wen <duke at openjdk.org> wrote:

>> The current implementation of StringConcat is to mix the coder and length into a long. This operation will have some overhead for int/long/boolean types. We can separate the calculation of the coder from the calculation of the length, which can improve the performance in the scenario of concat int/long/boolean.
>> 
>> This idea comes from the suggestion of @l4es in the discussion of PR https://github.com/openjdk/jdk/pull/20253#issuecomment-2240412866
>
> Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   non-MH-based implementation

It is now implemented based on bytecode-spinning. Private member access is implemented by StringConcatHelp lookup, but there are several strange errors in the build, and I don't know why.

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PR Comment: https://git.openjdk.org/jdk/pull/20273#issuecomment-2244113274