RFR: 8254146: Avoid unnecessary volatile write on new AtomicBoolean(false)

[Claes Redestad]

On Thu, 15 Oct 2020 09:17:09 GMT, Сергей Цыпанов <github.com+10835776+stsypanov at openjdk.org> wrote:

> What do you think?

For `AtomicBoolean`, this optimization piggy-backed on an existing test, just rewriting it so that no action is taken
when none is needed. So it's never a pessimization, and even improves bytecode size marginally. So it'll be a marginal
gain even if the current overhead of writing to a volatile field in a constructor is removed/reduced.

For `AtomicInteger/-Long`, we'd have to add a test that's not there, so we'd pessimize non-zero cases by adding some
bytecode and a branch to remove the overhead. An overhead that can already be avoided by calling the empty constructor
instead. So I think it's questionable to do this.

In the short term we could consider a point fix to replace existing use of `new AtomicInteger/-Long(0)` with `new
AtomicInteger/-Long()`, but in the long term we should really try and optimize our JITs so that writing to a volatile
field in constructors can be made cheaper.

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PR: https://git.openjdk.java.net/jdk/pull/510