RFR: 8336856: Optimize String Concat

[Chen Liang]

On Sun, 21 Jul 2024 12:25:36 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

For the question [here](https://github.com/openjdk/jdk/pull/20253#issuecomment-2242267516):
1. A sample of loading an otherwise inaccessible method handle from class data: https://github.com/openjdk/jdk/blob/c3226aaeb810521257e961be5763552c86ee5651/src/java.base/share/classes/java/lang/invoke/InnerClassLambdaMetafactory.java#L517-L518
2. In offline discussion with Claes, we are thinking of generating direct bytecode to avoid the overhead of creating a lot of intermediate method handles.

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PR Comment: https://git.openjdk.org/jdk/pull/20273#issuecomment-2243101199