RFR: 8336831: Optimize StringConcatHelper.simpleConcat [v5]

[Claes Redestad]

On Tue, 23 Jul 2024 12:12:41 GMT, Chen Liang <liach at openjdk.org> wrote:

>> src/java.base/share/classes/java/lang/String.java line 2991:
>> 
>>> 2989:         }
>>> 2990:         if (isEmpty()) {
>>> 2991:             return str;
>> 
>> This case should probably be reflected more precisely in the specification, or do `return new String(str);` to avoid changing semantics for this corner-case.
>
> The spec for concat only asked for equal representation instead of the existence of a new identity. Thus this should be fine.

Yes, this isn't beholden to JLS 15.18.1, and it's already specified that `foo.concat("")` returns `foo` - so why shouldn't `"".concat(foo)` return `foo`? But still it's an observable semantic change so some care needs to be taken - possibly even a CSR is warranted. Doing `return new String(str)` to retain behavior avoids that headache for a fringe case.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/20253#discussion_r1687990148