RFR: 8291065: Creating a VarHandle for a static field triggers class initialization [v7]

[ExE Boss]

On Sat, 3 Jun 2023 00:16:23 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 incrementally with one additional commit since the last revision:
> 
>   Remove lazy init of MH, for all invocation of the Lazy VH needs this MH

src/java.base/share/classes/java/lang/invoke/LazyInitializingVarHandle.java line 106:

> 104:         UNSAFE.ensureClassInitialized(refc);
> 105:         this.initialized = true;
> 106:     }

This should probably have a fast‑path when `this.initialized` is `true`, so that it can be better constant folded by the **JIT**:
Suggestion:

    private void ensureInitialized() {
        if (this.initialized) {
            return;
        }
        UNSAFE.ensureClassInitialized(refc);
        this.initialized = true;
    }

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/13821#discussion_r1215132431