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

[Roger Riggs]

On Mon, 5 Oct 2020 17:25:28 GMT, Christoph Dreis <github.com+6304496+dreis2211 at openjdk.org> wrote:

> Hi,
> 
> the following PR optimizes `new AtomicBoolean(boolean)` by avoiding the volatile write in case `false` is passed.
> Essentially, it changes the ternary operator to a simple `if` without the `else` that would cause the volatile write.
> The resulting bytecode seems to also benefit from the change:
>     Code:
>        0: aload_0
>        1: invokespecial #1                  // Method java/lang/Object."<init>":()V
>        4: aload_0
>        5: iload_1
>        6: ifeq          13
>        9: iconst_1
>       10: goto          14
>       13: iconst_0
>       14: putfield      #7                  // Field value:I
>       17: return
> 
> After:
>     Code:
>        0: aload_0
>        1: invokespecial #1                  // Method java/lang/Object."<init>":()V
>        4: iload_1
>        5: ifeq          13
>        8: aload_0
>        9: iconst_1
>       10: putfield      #7                  // Field value:I
>       13: return
> 
> A simple benchmark that returns `new AtomicBoolean(false)` shows the following results, that brings it on par to `new
> AtomicBoolean()`: MyBenchmark.empty                                         avgt   10     3,103 ±   0,246   ns/op
> MyBenchmark.explicitNew                                   avgt   10     2,966 ±   0,071   ns/op
> MyBenchmark.explicitOld                                   avgt   10     7,738 ±   0,321   ns/op
> 
> In case you think this is worthwhile I'd be happy if this is sponsored.
> Cheers,
> Christoph

Marked as reviewed by rriggs (Reviewer).

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

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