RFR: 8372410: ThreadFactory used by Thread.Builder::factory uses non-optimal VarHandle

[Jorn Vernee]

On Mon, 24 Nov 2025 17:17:37 GMT, Chen Liang <liach at openjdk.org> wrote:

>> This is boosting VirtualThread allocations while using https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/Thread.Builder.OfVirtual.html#name(java.lang.String,long)
>> 
>> The pattern has been discovered on https://github.com/quarkusio/quarkus/issues/51201 and narrowed down in https://github.com/franz1981/java-puzzles/commit/5cb4216e8d56cdc6c3c23629419158550cbd9068
>> 
>> which, on my machine (Ryzen 7950 w fixed frequency at 4.2 GHz):
>> 
>> Benchmark                                                                Mode  Cnt   Score   Error  Units
>> VarHandlePolymorphicSignatureSensitivity.rightVarHandle                  avgt   10   1.852 ± 0.007  ns/op
>> VarHandlePolymorphicSignatureSensitivity.rightVarHandleConcatIndy        avgt   10   9.891 ± 0.103  ns/op
>> VarHandlePolymorphicSignatureSensitivity.wrongVarHandle                  avgt   10  10.430 ± 0.045  ns/op
>> VarHandlePolymorphicSignatureSensitivity.wrongVarHandleConcatIndy        avgt   10  19.312 ± 0.619  ns/op
>> 
>> where the `right` indy shows:
>> <img width="1915" height="469" alt="image" src="https://github.com/user-attachments/assets/4c836389-492b-4cc6-94a3-849f8cf6ea09" />
>> 
>> vs the `wrong` one:
>> <img width="1915" height="469" alt="image" src="https://github.com/user-attachments/assets/73765060-8e95-42a2-a03a-4f53f927a8b8" />
>
> This is not the first time this VarHandle problem is hit - see #16451

> This has bitten me lot of times @liach 😢 Including others while reading/writing oop fields into arrays too; it would be nice to get an option to fail fast with an exception enabled via diagnostic args eg EnableStrictPolymorphicSignature Although Javac should be able to detect it too, thinking about it twice

This can be done using [`VarHandle::withInvokeExactBehavior`](https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/lang/invoke/VarHandle.html#withInvokeExactBehavior()) on a per var handle basis. C2 can constant fold the type-check away as well.

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PR Comment: https://git.openjdk.org/jdk/pull/28475#issuecomment-3572364123