RFR: 8332249: Micro-optimize Method.hashCode

[Chen Liang]

On Wed, 29 May 2024 15:03:15 GMT, Sean Gwizdak <duke at openjdk.org> wrote:

>> src/java.base/share/classes/java/lang/reflect/Method.java line 99:
>> 
>>> 97:     private Method              root;
>>> 98:     // Hash code of this object
>>> 99:     private int                 hash;
>> 
>> We can declare this field `@Stable`, like the `methodAccessor`.
>
> This was something that I tried and I observed a performance regression on the ARM platform that I was testing. From my understanding, `@Stable` tells the compiler to trust the contents of this field which is only given when the field holder is a known constant. This is generally true for cases like `Enums` but not usually for `Method`. 
> 
> 
> Benchmark                         Mode  Cnt  Score   Error  Units
> # Intel Skylake
> MethodHashCode.benchmarkHashCode  avgt    5  1.113 ± 1.146  ns/op
> # Arm Neoverse N1
> MethodHashCode.benchmarkHashCode  avgt    5  3.204 ± 0.001  ns/op

Interesting, don't know about hotspot internals so I can't diagnose the exact cause of this regression.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19433#discussion_r1619080123