RFR: 8291065: Creating a VarHandle for a static field triggers class initialization [v5]

[Mandy Chung]

On Fri, 2 Jun 2023 02:12:30 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  12.668 ± 0.069  ns/op
>> VarHandleLazyStaticInvocation.lazyInvocation         avgt   30  12.683 ± 0.069  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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 11 additional commits since the last revision:
> 
>  - Compute base and offset right after linking, simplify code
>  - Merge branch 'master' into lazy-static-varhandle
>  - Fix exact swap
>    
>    Co-authored-by: Mandy Chung <mandy.chung at oracle.com>
>  - Remove export for removed package
>  - Merge branch 'master' into lazy-static-varhandle
>  - Test the creation performance of handles, lazy one indeed better
>  - Merge branch 'master' into lazy-static-varhandle
>  - copyright year
>  - Benchmarks. lazy initialize is SLOW
>  - nuke meaningless overrides
>  - ... and 1 more: https://git.openjdk.org/jdk/compare/4978eac7...27e18b7c

test/jdk/java/lang/invoke/VarHandles/LazyStaticTest.java line 42:

> 40:         assert initialized.isEmpty() : "Incorrect initial state";
> 41: 
> 42:         class Sample1 {

Nit: suggest to move this inner class outside of the method body.  I think it helps readability.   maybe name it as `Sample` or `Test` or sth.   I expect `Sample2` when I read `Sample1`.

test/jdk/java/lang/invoke/VarHandles/LazyStaticTest.java line 59:

> 57: 
> 58:         assert initialized.contains(Sample1.class) : "Sample1 class initialized after VarHandle use";
> 59:         assert Sample1.apple == 42 : "The value is not set correctly to Sample1.apple";

It may be good to extend the test case to invoke the lazy VH multiple times that will exercise in the case if lazy VH transitions to direct VH.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13821#discussion_r1214748270
PR Review Comment: https://git.openjdk.org/jdk/pull/13821#discussion_r1214752797