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

[David Holmes]

Hi Remi,

On 19/06/2020 6:03 pm, Remi Forax wrote:
> 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);
>    }

Surely you intended to read field to get the volatile read to pair with 
the volatile write of the initializer? Otherwise there is no 
happens-before ordering.

David

> 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