On Mon, 3 Jun 2024 16:47:20 GMT, jengebr <[email protected]> wrote:
> Improve `java/util/concurrent/CopyOnWriteArrayList` by eliminating needless
> cloning of Object[0] instances. This cloning is intended to prevent callers
> from changing array contents, but many `CopyOnWriteArrayList`s are allocated
> to size zero, or are otherwise maintained empty, so cloning is unnecessary.
>
> Results from the included JMH benchmark:
> Before:
>
> Benchmark Mode Cnt
> Score Error Units
> CopyOnWriteArrayListBenchmark.clear avgt 5
> 74.487 ± 1.793 ns/op
> CopyOnWriteArrayListBenchmark.clearEmpty avgt 5
> 27.918 ± 0.759 ns/op
> CopyOnWriteArrayListBenchmark.createInstanceArray avgt 5
> 16.656 ± 0.375 ns/op
> CopyOnWriteArrayListBenchmark.createInstanceArrayEmpty avgt 5
> 15.415 ± 0.489 ns/op
> CopyOnWriteArrayListBenchmark.createInstanceCollection avgt 5
> 21.608 ± 0.363 ns/op
> CopyOnWriteArrayListBenchmark.createInstanceCollectionEmpty avgt 5
> 15.374 ± 0.260 ns/op
> CopyOnWriteArrayListBenchmark.createInstanceDefault avgt 5
> 15.688 ± 0.350 ns/op
>
>
> After:
>
> Benchmark Mode Cnt
> Score Error Units
> CopyOnWriteArrayListBenchmark.clear avgt 5
> 75.365 ± 2.092 ns/op
> CopyOnWriteArrayListBenchmark.clearEmpty avgt 5
> 20.803 ± 0.539 ns/op
> CopyOnWriteArrayListBenchmark.createInstanceArray avgt 5
> 16.808 ± 0.582 ns/op
> CopyOnWriteArrayListBenchmark.createInstanceArrayEmpty avgt 5
> 12.980 ± 0.418 ns/op
> CopyOnWriteArrayListBenchmark.createInstanceCollection avgt 5
> 21.627 ± 0.173 ns/op
> CopyOnWriteArrayListBenchmark.createInstanceCollectionEmpty avgt 5
> 12.864 ± 0.408 ns/op
> CopyOnWriteArrayListBenchmark.createInstanceDefault avgt 5
> 12.931 ± 0.255 ns/op
src/java.base/share/classes/java/util/concurrent/CopyOnWriteArrayList.java line
103:
> 101: private static final long serialVersionUID = 8673264195747942595L;
> 102:
> 103: private static final Object[] EMPTY_ELEMENTDATA = {};
Looks like readObject() ought to also be changed to use the EMPTY_ELEMENTDATA
if the length of the read array is 0? And perhaps `clone()`.
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/19527#discussion_r1625968254
