RFR: 8160821: VarHandle accesses are penalized when argument conversion is required [v3]

[Hannes Greule]

On Tue, 2 Dec 2025 02:30:28 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>> JIT can fully optimize it in JMH benchmarks. I don't know why the IR in this test can't optimize it - I couldn't reproduce this CI failure locally on my linux-x64-debug profile, but this modified test passes on CI.
>
> I'd say it's a bad sign. Intermittent bugs manifest exactly in such a way.

> The performance is measured by the existing `org.openjdk.bench.java.lang.invoke.VarHandleExact` benchmark, which originally expects `generic_genericInvocation` to be much slower. Now it instead has a performance on par with the exact invocations.
> 
> The constant folding ability is verified with the new `VarHandleMismatchedTypeFold` IR test.

The benchmark doesn't consider such inexact getAndAdd calls (with a void return type), I think it should cover that too. This is a very common pattern that really must not regress.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/28585#discussion_r2580041626