RFR: 8318457: Use prefix-less prepend methods directly to reduce branches in String concat expressions [v3]

[Chen Liang]

On Thu, 19 Oct 2023 15:59:54 GMT, Claes Redestad <redestad at openjdk.org> wrote:

>> In #16006 using `StringConcatFactory` seem to induce a run-to-run variance. One idea to reduce this is to generate expression trees with fewer branches by looking up and using the prefix-less `prepend` methods directly for cases where constant is null/empty so that the prefixed prepender will always deal with a non-null prefix constant.
>
> Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:
> 
>   drop dropLookupMode

An alternative way to provide access control could be to call `MethodHandles.lookup()` in the anonymous `JavaLangAccess` implementation in `System`. This won't require an SM check as the lookup has full-privilege access, yet the lookup cannot access private members of `StringConcatHelper`.

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PR Comment: https://git.openjdk.org/jdk/pull/16244#issuecomment-1772087445