RFR: 8338930: StringConcatFactory static string concatenation strategy

[Claes Redestad]

On Thu, 22 Aug 2024 11:50:02 GMT, Shaojin Wen <duke at openjdk.org> wrote:

> This is a follow-up to PR #20273, which improves performance when the number of parameters exceeds 20.
> 
> When the number of parameters is large, the possibility of reuse will be lower, so we can use the static concat method and write the length and coder directly into the bytecode to solve the performance regression problem.

- Can we try to optimize the default generation shape further before we reach for this sort of specialization?
- How well-motivated is it to increase complexity to optimize huge expressions, which can be assumed to be rare and less likely to be part of the critical path?
- If we do this I think we need to have a separate control flag than `inlineThreshold`
- Perhaps `hardCoded` would better describe what's going on than `static`: we're hard-coding all the constants and the values derived from them into the concat bytecode. As a result the concatenator becomes `static`, but that's is a side-effect of hard-coding rather than injecting constants.
- Trivial optimizations such as passing `true` to `methodType` and using `MethodTypeDescImpl.ofValidated` could be split out to a separate PR to keep this patch cleaner and easier to review/discuss.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/20675#issuecomment-2308789379