RFR(xs): JDK-8247605 Avoid array allocation when concatenating with empty string

[Tagir Valeev]

Hello!

Please review the following small change in StringConcatHelper::simpleConcat
https://bugs.openjdk.java.net/browse/JDK-8247605
https://cr.openjdk.java.net/~tvaleev/webrev/8247605/r1/

This change improves the concatenation of empty string and non-empty
string/object by reusing the internal byte[] array.

Simple benchmark (StringConcat::concatEmptyRight and
StringConcat::concatEmptyLeft) shows the significant performance
improvement in both cases when an empty string is prefixed or
suffixed. Before patch:

Benchmark                      (intValue)  Mode  Cnt   Score   Error  Units
StringConcat.concatEmptyLeft         4711  avgt   30  18,416 ± 0,081  ns/op
StringConcat.concatEmptyRight        4711  avgt   30  18,439 ± 0,068  ns/op
StringConcat.concatEmptyLeft:·gc.alloc.rate.norm
4711  avgt   10    48,003 ±   0,001    B/op
StringConcat.concatEmptyRight:·gc.alloc.rate.norm
4711  avgt   10    48,003 ±   0,001    B/op


After patch:
Benchmark                      (intValue)  Mode  Cnt  Score   Error  Units
StringConcat.concatEmptyLeft         4711  avgt   30  7,846 ± 0,069  ns/op
StringConcat.concatEmptyRight        4711  avgt   30  6,800 ± 0,082  ns/op
StringConcat.concatEmptyLeft:·gc.alloc.rate.norm
4711  avgt   10    24,001 ±   0,001    B/op
StringConcat.concatEmptyRight:·gc.alloc.rate.norm
4711  avgt   10    24,001 ±   0,001    B/op

Note that the allocation and array copying time is O(N) where N is the
length of non-empty string, so for longer strings, the difference
could be bigger. No performance degradation was noticed by me in other
StringConcat benchmarks.

Previous discussion:
http://mail.openjdk.java.net/pipermail/core-libs-dev/2020-June/067217.html

With best regards,
Tagir Valeev.