Fwd: Scalar replacement issue in JDK 14.0.1

[Сергей Цыпанов]

Hello,

while looking into an issue I've found out that scalar replacement is not working in trivial case on JDK 14.0.1.

This benchmark illustrates the issue:

@State(Scope.Thread)
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@Fork(jvmArgsAppend = {"-Xms2g", "-Xmx2g"})
public class StringCompositeKeyBenchmark {
  @Benchmark
  public Object compositeKey(Data data) {
    return data.keyObjectMap.get(new Key(data.code, data.locale));
  }

  @State(Scope.Thread)
  public static class Data {
    private final String code = "code1";
    private final Locale locale = Locale.getDefault();

    private final HashMap<Key, Object> keyObjectMap = new HashMap<>();

    @Setup
    public void setUp() {
      keyObjectMap.put(new Key(code, locale), new Object());
    }
  }

  private static final class Key {
    private final String code;
    private final Locale locale;

    private Key(String code, Locale locale) {
      this.code = code;
      this.locale = locale;
    }

    @Override
    public boolean equals(Object o) {
      if (this == o) return true;
      if (o == null || getClass() != o.getClass()) return false;

      Key key = (Key) o;

      if (!code.equals(key.code)) return false;
      return locale.equals(key.locale);
    }

    @Override
    public int hashCode() {
      return 31 * code.hashCode() + locale.hashCode();
    }
  }
}

When I run this on JDK 11 (JDK 11.0.7, OpenJDK 64-Bit Server VM, 11.0.7+10-post-Ubuntu-2ubuntu218.04) I get this output:

Benchmark Mode Cnt Score Error Units
StringCompositeKeyBenchmark.compositeKey avgt 10 5.510 ± 0.121 ns/op
StringCompositeKeyBenchmark.compositeKey:·gc.alloc.rate avgt 10 ≈ 10⁻⁴ MB/sec
StringCompositeKeyBenchmark.compositeKey:·gc.alloc.rate.norm avgt 10 ≈ 10⁻⁶ B/op
StringCompositeKeyBenchmark.compositeKey:·gc.count avgt 10 ≈ 0 counts

As I understand Java runtime erases object allocation here and we don't use additional memory.

Same run on JDK 14 (JDK 14.0.1, Java HotSpot(TM) 64-Bit Server VM, 14.0.1+7) demonstrate object allocation per each method call:

Benchmark Mode Cnt Score Error Units
StringCompositeKeyBenchmark.compositeKey avgt 10 7.958 ± 1.360 ns/op
StringCompositeKeyBenchmark.compositeKey:·gc.alloc.rate avgt 10 1937.551 ± 320.718 MB/sec
StringCompositeKeyBenchmark.compositeKey:·gc.alloc.rate.norm avgt 10 24.001 ± 0.001 B/op
StringCompositeKeyBenchmark.compositeKey:·gc.churn.G1_Eden_Space avgt 10 1879.111 ± 596.770 MB/sec
StringCompositeKeyBenchmark.compositeKey:·gc.churn.G1_Eden_Space.norm avgt 10 23.244 ± 5.509 B/op
StringCompositeKeyBenchmark.compositeKey:·gc.churn.G1_Survivor_Space avgt 10 0.267 ± 0.750 MB/sec
StringCompositeKeyBenchmark.compositeKey:·gc.churn.G1_Survivor_Space.norm avgt 10 0.003 ± 0.009 B/op
StringCompositeKeyBenchmark.compositeKey:·gc.count avgt 10 23.000 counts
StringCompositeKeyBenchmark.compositeKey:·gc.time avgt 10 44.000 ms

At the same time in more trivial scenario like

@Benchmark
public int compositeKey(Data data) {
  return new Key(data.code, data.locale).hashCode();
}

scalar replacement again eliminates allocation of object.

So I'm curious whether this is normal behaviour or a bug?

Regards,
Sergey Tsypanov