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

[Hannes Greule]

On Mon, 24 Nov 2025 13:05:40 GMT, Francesco Nigro <duke 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" />

I wonder if `MhUtil.findVarHandle(...)` should make VarHandles with invokeExact behavior by default, to catch such problems earlier in the JDK.

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PR Comment: https://git.openjdk.org/jdk/pull/28475#issuecomment-3574208351