RFR: 8305774: String.join(CharSequence, Iterable) can be optimized if Iterable is a Collection [v2]

[Chen Liang]

On Sun, 9 Apr 2023 02:28:37 GMT, Tingjun Yuan <duke at openjdk.org> wrote:

>> In the current implementation of `String.join(CharSequence, Iterable)`, the temp array `elems` is always initialized with a length of 8. It will cause many array recreations when the `Iterable` contains more than 8 elements. Furthermore, it's very common that an `Iterable` is also a `Collection`. So if the `Iterable` is an instance of `Collection`, the initial length of the array can be `((Collection<?>)elements).size()`. It will not change the current behavior even if the `Collection` is modified asynchronously.
>> 
>> I don't know whether this change requires a CSR request.
>
> Tingjun Yuan has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add benchmark

Converting array to iterable almost guarantees performance degradation, with the extra iterator allocation. Often even forEach is better.

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PR Comment: https://git.openjdk.org/jdk/pull/13383#issuecomment-1501413441