On Mon, 3 Jun 2024 17:07:28 GMT, Aleksey Shipilev <sh...@openjdk.org> 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 338: > >> 336: */ >> 337: public Object[] toArray() { >> 338: return getArray().length == 0 ? getArray() : getArray().clone(); > > Unfortunately, I think this is against the spec for this method, which > explicitly says the method must allocate a new array. Yes, this change would > be within the spirit of the spec ("You can modify the array", which you > cannot if the object is empty), but is against the letter of it. I think you're right, I'll remove the change to toArray(). ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/19527#discussion_r1624873832