RFR: 8312424: 80% throughput decrease in Arrays.hashCode(Object[]) after JDK-8312164

[Chen Liang]

On Thu, 20 Jul 2023 01:27:41 GMT, Glavo <duke at openjdk.org> wrote:

>> The changes to `Arrays.hashCode(Object[])` in JDK-8312164 caused its performance  is reduced by about 80%.
>> 
>> This PR reverts this change.
>
>> A similar performance decrease have been discussed here: [#14752 (comment)](https://github.com/openjdk/jdk/pull/14752#discussion_r1250888931)
>> 
>> Can you check whether adding `@ForceInline` on `Objects.equals` help with JIT? If it helps, then I think adding `@ForceInline` might be a better solution.
> 
> I added `@ForceInline` on `Objects.hashCode(Object)`, but the JMH results did not change.

@Glavo I have created a test case with polymorphic objects in an array: https://github.com/liachmodded/jdk/tree/explore/arrays-hash-poly
Can you see if you can produce the same outcome on your patch, and how the existing `Objects.equals` behave on the polymorphicObjects case?


Benchmark               (size)  Mode  Cnt     Score    Error  Units
ArraysHashCode.objects       1  avgt   15     0.770 ±  0.011  ns/op
ArraysHashCode.objects      10  avgt   15     4.704 ±  0.059  ns/op
ArraysHashCode.objects     100  avgt   15    54.610 ±  0.384  ns/op
ArraysHashCode.objects   10000  avgt   15  6053.742 ± 35.244  ns/op



Benchmark                          (size)  Mode  Cnt      Score      Error  Units
ArraysHashCode.polymorphicObjects       1  avgt   15      0.921 ±    0.069  ns/op
ArraysHashCode.polymorphicObjects      10  avgt   15     23.322 ±    1.076  ns/op
ArraysHashCode.polymorphicObjects     100  avgt   15    211.325 ±    2.200  ns/op
ArraysHashCode.polymorphicObjects   10000  avgt   15  81850.916 ± 2427.661  ns/op

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PR Comment: https://git.openjdk.org/jdk/pull/14944#issuecomment-1642998263