Scalar replacement issue in JDK 14.0.1

[David Holmes]

Hi Sergie,

This seems like a question for hotspot-compiler-dev rather than 
core-libs-dev.

Thanks,
David

On 26/06/2020 10:06 pm, Сергей Цыпанов wrote:
> 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
>