RFR: 8255299: Drop explicit zeroing at instantiation of Atomic* objects [v2]

[Сергей Цыпанов]

> As discussed in https://github.com/openjdk/jdk/pull/510 there is never a reason to explicitly instantiate any instance of `Atomic*` class with its default value, i.e. `new AtomicInteger(0)` could be replaced with `new AtomicInteger()` which is faster:
> @State(Scope.Thread)
> @OutputTimeUnit(TimeUnit.NANOSECONDS)
> @BenchmarkMode(value = Mode.AverageTime)
> public class AtomicBenchmark {
>   @Benchmark
>   public Object defaultValue() {
>     return new AtomicInteger();
>   }
>   @Benchmark
>   public Object explicitValue() {
>     return new AtomicInteger(0);
>   }
> }
> THis benchmark demonstrates that `explicitValue()` is much slower:
> Benchmark                      Mode  Cnt   Score   Error  Units
> AtomicBenchmark.defaultValue   avgt   30   4.778 ± 0.403  ns/op
> AtomicBenchmark.explicitValue  avgt   30  11.846 ± 0.273  ns/op
> So meanwhile https://bugs.openjdk.java.net/browse/JDK-8145948 is still in progress we could trivially replace explicit zeroing with default constructors gaining some performance benefit with no risk.
> 
> I've tested the changes locally, both tier1 and tier 2 are ok. 
> 
> Could one create an issue for tracking this?

Сергей Цыпанов has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains one additional commit since the last revision:

  8255299: Drop explicit zeroing at instantiation of Atomic* objects

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/818/files
  - new: https://git.openjdk.java.net/jdk/pull/818/files/c1fb362f..7dc646d0

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=818&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=818&range=00-01

  Stats: 4576 lines in 201 files changed: 2659 ins; 1135 del; 782 mod
  Patch: https://git.openjdk.java.net/jdk/pull/818.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/818/head:pull/818

PR: https://git.openjdk.java.net/jdk/pull/818