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