[PATCH] remove redundant initialization of volatile fields with default values

[Remi Forax]

Hi Sergei,
the problem is that you are changing the semantics if there are several fields.

By example with the code below, you have the guarantee that the code will print 4 (if it prints something),
if you remove the assignment field = false, the code can print 0 or 4.

  class A {
    int i = 4;
    volatile boolean field = false;
  }

thread 1:
  global = new A()

thread 2:
  var a = global;
  if (a != null) {
    System.out.println(a.i);
  }

regards,
Rémi

----- Mail original -----
> De: "Сергей Цыпанов" <sergei.tsypanov at yandex.ru>
> À: "core-libs-dev" <core-libs-dev at openjdk.java.net>
> Envoyé: Vendredi 19 Juin 2020 06:57:25
> Objet: [PATCH] remove redundant initialization of volatile fields with default values

> Hello,
> 
> while investigating an issue I've found out that assignment of default value to
> volatile fields slows down object instantiation.
> 
> Consider the benchmark:
> 
> @State(Scope.Thread)
> @OutputTimeUnit(TimeUnit.NANOSECONDS)
> @BenchmarkMode(value = Mode.AverageTime)
> @Fork(jvmArgsAppend = {"-Xms2g", "-Xmx2g"})
> public class VolatileFieldBenchmark {
>  @Benchmark
>  public Object explicitInit() {
>    return new ExplicitInit();
>  }
> 
>  @Benchmark
>  public Object noInit() {
>    return new NoInit();
>  }
> 
>  private static class ExplicitInit {
>    private volatile boolean field = false;
>  }
>  private static class NoInit {
>    private volatile boolean field;
>  }
> }
> 
> This gives the following results as of my machine:
> 
> Benchmark                            Mode  Cnt   Score   Error  Units
> VolatileFieldBenchmark.explicitInit  avgt   40  11.087 ± 0.140  ns/op
> VolatileFieldBenchmark.noInit        avgt   40   3.367 ± 0.131  ns/op
> 
> 
> I've looked into source code of java.base and found out several cases where the
> default value is assigned to volatile field.
> 
> Getting rid of such assignements demonstates improvement as of object
> instantiation, e.g. javax.security.auth.Subject:
> 
>           Mode  Cnt   Score   Error  Units
> before     avgt   40  35.933 ± 2.647  ns/op
> after      avgt   40  30.817 ± 2.384  ns/op
> 
> As of testing tier1 and tier2 are both ok after the changes.
> 
> Best regards,
> Sergey Tsypanov