Can @Stable (or something similar) be made accessible? (original) (raw)

Remi Forax forax at univ-mlv.fr
Fri Jan 12 17:51:12 UTC 2018

Or you can use method handles ...

I've not tested but this should work: https://gist.github.com/forax/e49d63d50e5973f602f720ba3b6ea1b8

I have implemented two semantics, either do a call to get field value the first time (FIRST_WIN) and then for the subsequent calls always return this value as a constant or put a guard (if) that checks that the reference never change so instead of getting the field value, i always return the same constant (STABLE).

You may think that it doesn't work because the method handle is a captured value of a lambda that is stored in a field of the lambda proxy but the VM has a special case for that (fields of VM anonynous classes (as the lambda proxies) are always trusted, see [1]). So it's safe, from the perf POV to use lambdas has a mechanism to hidden the method handle incantations.

cheers, Rémi

[1] https://shipilev.net/jvm-anatomy-park/17-trust-nonstatic-final-fields/

----- Mail original -----

De: "Aleksey Shipilev" <shade at redhat.com> À: "Jason Greene" <jason.greene at redhat.com>, "core-libs-dev" <core-libs-dev at openjdk.java.net> Envoyé: Vendredi 12 Janvier 2018 11:37:37 Objet: Re: Can @Stable (or something similar) be made accessible?

On 01/12/2018 05:33 AM, Jason Greene wrote:

MethodHandle.invokeExact() can match the performance of direct invocation, but it requires constant folding to do so. Otherwise results are similar to Java reflection(e.g [1]).

While TrustFinalNonStaticFields can be used, it’s a sledgehammer where a scalpel is really be more appropriate. So, the easier way out would be enable TrustFinalNonStaticFields to be used with finer-grained control, that JEP 165 (Compiler Control) provides. Then you can tell which user-side classes/packages are safe for trusting, and which are not. IIRC, the reason why TrustFinalNonStaticFields is not on by default is because we cannot rely that everyone is not writing to final fields and expecting update to be visible. ...which makes this one partially go away: It seems odd that the much more aggressive facility (TrustFinalNonStaticFields) is simpler to use than a more targeted one. Thanks, -Aleksey

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