RFR: 8336856: Optimize String Concat [v39]

[Claes Redestad]

On Mon, 12 Aug 2024 16:52:53 GMT, Shaojin Wen <duke at openjdk.org> wrote:

>> src/java.base/share/classes/java/lang/StringConcatHelper.java line 83:
>> 
>>> 81: 
>>> 82:         @ForceInline
>>> 83:         private final String concat(char value) {
>> 
>> These mostly help avoid a bit of class spinning on startup, right? I think it's fine to add these now, but we should perhaps consider ways to pre-generate concat shapes more deliberately (e.g. using `GenerateJLIClassesHelper/-Plugin` to pre-spin concat classes when jlinking an image) rather than manually stamping out code here. If small arity concats benefit in throughput tests from a subtly different code shape then that should be reflected in the code generator.
>
> The current implementation generates length and coder methods, which slows down the startup. The built-in concat1 is to improve the startup speed. 1 parameter is a very common scenario. The current implementation does not need to add more classes. It is a lightweight implementation, so I keep it. In addition, I also need to learn how GenerateJLIClassesHelper works.

Right, what you have is OK here. I'll think a bit about a follow-up to integrate with the plugin to support arbitrary concat class pre-generation in jlink. This is something I had hoped to get done in the previous implementation, but I ran into a few road blocks there and then had to work on other things. With this implementation doing pre-generation will likely be way more straightforward.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20273#discussion_r1714135667