RFR: 8297784: Optimize @Stable field for Method.isCallerSensitive [v2]

[Alan Bateman]

On Wed, 30 Nov 2022 08:43:54 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> [JDK-8271820](https://bugs.openjdk.org/browse/JDK-8271820) introduced the `@Stable private Boolean callerSensitive;` cache in `Method`. That field is essentially a tri-state `Boolean` that relies on its `null` value to serve as "not initialized" value for `@Stable`.
>> 
>> This works well when the holder `Method` instance is constant: JIT compilers fold `@Stable` well. But if such folding fails, we always dereference the full-blown reference to "boxed" `Boolean` from the field on every access. We can instead do a more lean tri-state primitive field to improve that non-optimized case without sacrificing optimized case.
>> 
>> I chose `byte` and `-1`, `0`, `1` to let the fastpath encode well on most (all?) architectures.
>> 
>> On adhoc benchmark like:
>> 
>> 
>> @State(Scope.Thread)
>> public class WrapperConstness {
>>     static final Method CONST;
>>     static Method NON_CONST;
>> 
>>     static {
>>         try {
>>             CONST = WrapperConstness.class.getDeclaredMethod("target");
>>             NON_CONST = CONST;
>>         } catch (NoSuchMethodException e) {
>>             throw new RuntimeException(e);
>>         }
>>     }
>> 
>>     public void target() {}
>> 
>>     @Benchmark
>>     public void nonConstant() throws Exception {
>>         NON_CONST.invoke(this);
>>     }
>> 
>>     @Benchmark
>>     public void constant() throws Exception {
>>         CONST.invoke(this);
>>     }
>> }
>> 
>> 
>> We have a minor improvement for non-const case, confirmed due to better `isCallerSensitive` access:
>> 
>> 
>> Benchmark                     Mode  Cnt  Score   Error  Units
>> 
>> # Baseline
>> WrapperConstness.constant     avgt   25  0.410 ± 0.010  ns/op
>> WrapperConstness.nonConstant  avgt   25  7.283 ± 0.025  ns/op
>> 
>> # Patched
>> WrapperConstness.constant     avgt    5  0.407 ± 0.008  ns/op
>> WrapperConstness.nonConstant  avgt    5  7.054 ± 0.027  ns/op  ; -3%
>> 
>> 
>> Additional testing:
>>  - [x] Ad-hoc benchmarks
>>  - [x] Linux x86_64 fastdebug `java/lang/reflect`
>>  - [x] Linux x86_64 fastdebug `tier1`, `tier2`
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add a newline

Marked as reviewed by alanb (Reviewer).

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PR: https://git.openjdk.org/jdk/pull/11422