RFR: 8254082: AbstractStringBuilder.insert(int dstOffset, CharSequence s, int start, int end) is missing fast-path for String

[Сергей Цыпанов]

Original mail: 
https://mail.openjdk.java.net/pipermail/core-libs-dev/2020-September/069197.html

Hello,

while working with `StringBuilder.insert()` I've spotted that its delegate 
`AbstractStringBuilder.insert()` is missing
a fast-path for the most frequent case when its argument is `String`.

Previously they did similart optimization for 
`StirngBuilder.append(CharSequence, int, int)`,
see https://bugs.openjdk.java.net/browse/JDK-8224986

I'd like to contribute a trivial patch that brings improvement for the case 
when SB's content is Latin1
and inserted String is Latin1 as well.

To measure improvement I've used simple benchmark:
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@Fork(jvmArgsAppend = {"-Xms2g", "-Xmx2g"})
public class StringBuilderInsertBenchmark {

  @Benchmark
  public StringBuilder insert(Data data) {
    String string = data.string;
    return new StringBuilder().append("ABC").insert(1, string, 1, data.length + 
1);
  }

  @State(Scope.Thread)
  public static class Data {
    String string;

    @Param({"true", "false"})
    private boolean latin;

    @Param({"8", "64", "128", "1024"})
    private int length;

    @Setup
    public void setup() {
      String alphabet = latin
        ? "abcdefghijklmnopqrstuvwxyz"        // English
        : "абвгдеёжзиклмнопрстуфхцчшщьыъэюя"; // Russian

      string = new RandomStringGenerator().randomString(alphabet, length + 2);
    }
  }
}

public final class RandomStringGenerator {

  public String randomString(String alphabet, int length) {
    char[] chars = alphabet.toCharArray();

    ThreadLocalRandom random = ThreadLocalRandom.current();

    char[] array = new char[length];
    for (int i = 0; i < length; i++) {
      array[i] = chars[random.nextInt(chars.length)];
    }

    return new String(array);
  }
}
Which gives

            (latin)  (length)       original       patched    Units
insert         true         8    24.2 ±  0.1   22.2 ±  0.0    ns/op
insert         true        64    53.8 ±  0.2   36.1 ±  0.1    ns/op
insert         true       128    80.9 ±  0.2   44.6 ±  0.0    ns/op
insert         true      1024   365.4 ±  0.5  109.8 ±  3.9    ns/op

insert        false         8    33.5 ±  0.5   32.3 ±  0.2    ns/op
insert        false        64    73.2 ±  0.3   73.2 ±  0.2    ns/op
insert        false       128   103.9 ±  0.6  103.3 ±  0.1    ns/op
insert        false      1024   576.5 ±  4.8  569.5 ±  2.0    ns/op
Patch is attached. As of tests tier1 and tier2 are ok.

With best regards,
Sergey Tsypanov

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

Commit messages:
 - 8254082: Add fast-path for String into AbstractStringBuilder.insert(int, 
CharSequence, int, int)

Changes: https://git.openjdk.java.net/jdk/pull/402/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=402&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254082
  Stats: 23 lines in 1 file changed: 13 ins; 4 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/402.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/402/head:pull/402

PR: https://git.openjdk.java.net/jdk/pull/402