RFR: 8291065: Creating a VarHandle for a static field triggers class initialization [v8]

[ExE Boss]

On Sun, 4 Jun 2023 13:23:46 GMT, Chen Liang <liach at openjdk.org> wrote:

>> Also fixed the bug with NPE in `IndirectVarHandle::isAccessModeSupported`.
>> 
>> A few implementation-detail methods in VarHandle are now documented with the implied constraints to avoid subtle problems in the future. Changed `IndirectVarHandle` to call `asDirect` lazily to accomodate the lazy VarHandle changes. Also changed VarHandleBaseTest to report the whole incorrect type of exception caught than swallow it and leaving only a message.
>> 
>> Current problems:
>> - [ ] The lazy var handle is quite slow on the first invocation.
>>    - As seen in the benchmark, users can first call `Lookup::ensureInitialized` to create an eager handle.
>>    - After that, the lazy handle has a performance on par with the regular var handle.
>> - [ ] The class-loading-based test is not in a unit test
>>    - The test frameworks don't seem to offer fine-grained control for class-loading detection or reliable unloading
>> 
>> 
>> Benchmark                                            Mode  Cnt  Score   Error  Units
>> VarHandleLazyStaticInvocation.initializedInvocation  avgt   30  0.817 ± 0.012  ns/op
>> VarHandleLazyStaticInvocation.lazyInvocation         avgt   30  0.805 ± 0.007  ns/op
>> 
>> 
>> Benchmark                                            Mode  Cnt     Score     Error  Units
>> LazyStaticColdStart.methodHandleCreateEager            ss   10    50.980 ±   9.454  us/op
>> LazyStaticColdStart.methodHandleCreateLazy             ss   10    24.350 ±   6.701  us/op
>> LazyStaticColdStart.methodHandleInitializeCallEager    ss   10    65.140 ±   7.552  us/op
>> LazyStaticColdStart.methodHandleInitializeCallLazy     ss   10   118.360 ±  20.320  us/op
>> LazyStaticColdStart.varHandleCreateEager               ss   10    49.500 ±   4.277  us/op
>> LazyStaticColdStart.varHandleCreateLazy                ss   10    26.690 ±   5.157  us/op
>> LazyStaticColdStart.varHandleInitializeCallEager       ss   10    87.930 ±  12.643  us/op
>> LazyStaticColdStart.varHandleInitializeCallLazy        ss   10  1057.120 ± 189.810  us/op
>
> Chen Liang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Improve subsequent VH call performance, thanks exeboss

src/java.base/share/classes/java/lang/invoke/LazyInitializingVarHandle.java line 125:

> 123:                 cache[i] = callTarget;
> 124:             }
> 125:         }

Unlike calling `Unsafe::ensureClassInitialized(…)` (which performs its own synchronisation) or writing `true` to `this.initialized` (which is a benign race), updating the lambda forms this way is probably not quite correct to do under a race and probably needs some synchronisation.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13821#discussion_r1216862767