String(byte[], int, int, Charset) constructor and String.translateEscapes() miss bounds check elimination

First this was spotted by Amir Hadadi in https://stackoverflow.com/questions/70272651/missing-bounds-checking-elimination-in-string-constructor

It looks like in the following code

while (offset < sl) {
    int b1 = bytes[offset];
    if (b1 >= 0) {
        dst[dp++] = (byte)b1;
        offset++; // <---
        continue;
    }
    if ((b1 == (byte)0xc2 || b1 == (byte)0xc3) &&
            offset + 1 < sl) {
        int b2 = bytes[offset + 1];
        if (!isNotContinuation(b2)) {
            dst[dp++] = (byte)decode2(b1, b2);
            offset += 2;
            continue;
        }
    }
    // anything not a latin1, including the repl
    // we have to go with the utf16
    break;
}

bounds check elimination is not executed when accessing byte array via bytes[offset].

The reason, I guess, is that offset variable is modified within the loop (marked with arrow).

Possible fix for this could be changing:

while (offset < sl) ---> while (offset >= 0 && offset < sl)

However the best is to invest in C2 optimization to handle all such cases.

The following benchmark demonstrates good improvement:

@State(Scope.Thread)
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public class StringConstructorBenchmark {
  private byte[] array;

  @Setup
  public void setup() {
    String str = "Quizdeltagerne spiste jordbær med fløde, mens cirkusklovnen. Я"; // Latin1 ending with Russian
    array = str.getBytes(StandardCharsets.UTF_8);
  }

  @Benchmark
  public String newString() {
      return new String(array, 0, array.length, StandardCharsets.UTF_8);
  }
}

//baseline
Benchmark Mode Cnt Score Error Units
StringConstructorBenchmark.newString avgt 50 173,092 ± 3,048 ns/op

//patched
Benchmark Mode Cnt Score Error Units
StringConstructorBenchmark.newString avgt 50 126,908 ± 2,355 ns/op

The same is observed in String.translateEscapes() for the same String as in the benchmark above:

//baseline
Benchmark Mode Cnt Score Error Units
StringConstructorBenchmark.translateEscapes avgt 100 53,627 ± 0,850 ns/op

//patched
Benchmark Mode Cnt Score Error Units
StringConstructorBenchmark.translateEscapes avgt 100 48,087 ± 1,129 ns/op