RFR: 8291065: Creating a VarHandle for a static field triggers class initialization [v4]

[Mandy Chung]

On Thu, 1 Jun 2023 21:26:25 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       0.769 ±      0.003  ns/op
>> VarHandleLazyStaticInvocation.lazyInvocation         avgt   30       0.767 ±      0.002  ns/op
>> 
>> 
>> Benchmark                                            Mode  Cnt       Score        Error  Units
>> LazyStaticColdStart.methodHandleCreateEager            ss   10   73140.100 ±   7735.276  ns/op
>> LazyStaticColdStart.methodHandleCreateLazy             ss   10   50000.000 ±   8482.883  ns/op
>> LazyStaticColdStart.methodHandleInitializeCallEager    ss   10   91350.100 ±   9776.343  ns/op
>> LazyStaticColdStart.methodHandleInitializeCallLazy     ss   10  145540.200 ±  72894.865  ns/op
>> LazyStaticColdStart.varHandleCreateEager               ss   10   47069.900 ±   3513.909  ns/op
>> LazyStaticColdStart.varHandleCreateLazy                ss   10   24780.300 ±   5144.540  ns/op
>> LazyStaticColdStart.varHandleInitializeCallEager       ss   10   85479.700 ±  10816.983  ns/op
>> LazyStaticColdStart.varHandleInitializeCallLazy        ss   10  413630.100 ± 189370.448  ns/op
>
> Chen Liang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix exact swap
>   
>   Co-authored-by: Mandy Chung <mandy.chung at oracle.com>

`staticFieldBase` and `staticFieldOffset` can be called before initialization once a class has been linked.   They are unsafe operations and so the code has to ensure class initialization before it accesses a static field.

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PR Comment: https://git.openjdk.org/jdk/pull/13821#issuecomment-1572912754