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

[Tingjun Yuan]

On Fri, 7 Apr 2023 08:27:18 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.

I've just added the benchmark code, and here is the result tested on my computer.

Before change:


Benchmark                                           Mode  Cnt          Score          Error  Units
StringJoinWithIterable.joinWithArrayList            avgt   30  146820983.745 ±  6654877.812  ns/op
StringJoinWithIterable.joinWithIterable             avgt   30  145236859.688 ±  5683967.478  ns/op
StringJoinWithIterable.joinWithLinkedBlockingDeque  avgt   30  250139433.967 ± 14381158.389  ns/op
StringJoinWithIterable.joinWithLinkedHashSet        avgt   30  167718287.313 ±  7394905.875  ns/op
StringJoinWithIterable.joinWithLinkedTransferQueue  avgt   30  153245459.832 ±  7972716.070  ns/op


After change:


Benchmark                                           Mode  Cnt          Score          Error  Units
StringJoinWithIterable.joinWithArrayList            avgt   30  138343729.406 ±  5959140.498  ns/op
StringJoinWithIterable.joinWithIterable             avgt   30  149566458.195 ±  9755905.704  ns/op
StringJoinWithIterable.joinWithLinkedBlockingDeque  avgt   30  247384811.927 ± 14647765.134  ns/op
StringJoinWithIterable.joinWithLinkedHashSet        avgt   30  153244492.767 ±  4589884.301  ns/op
StringJoinWithIterable.joinWithLinkedTransferQueue  avgt   30  157162685.594 ±  4330658.539  ns/op



It seems that the improvement is not notable :-)

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

PR Comment: https://git.openjdk.org/jdk/pull/13383#issuecomment-1501022549