RFR: 8320646: RISC-V: C2 VectorCastHF2F [v2]

[Fei Yang]

On Thu, 29 Feb 2024 15:13:52 GMT, Hamlin Li <mli at openjdk.org> wrote:

>>> That make sense to me. In the long run, we should depend on hwprobe + vendor specific code to auto-enable or disable certain RV extensions on certain hardwares. But I agree on that there should be some options of DIAGNOSTIC kind to help diagnose issues and find workarounds. I am OK to make this option an EXPERIMENTAL one at the current stage. But we should turn them into the DIAGNOSTIC kind in the future when we have the hardwares and hwprobe is capable to satisfy our needs. Mind you, we are lacking one option for https://github.com/openjdk/jdk/pull/17122 in that respect. Let me know if you going to add one.
>> 
>> Agreed on all of that! The lack of currently and widely available hardware with more of these extensions is what makes me want to keep them EXPERIMENTAL. But once we validated that these work on commercially available hardware, I would indeed move them out of EXPERIMENTAL, and I agree DIAGNOSTIC makes for a fine change.
>> 
>> Another question will be about backporting the EXPERIMENTAL -> DIAGNOSTIC. We can revisit later but I think we should be fairly proactive about that when the case arises. Do you see any issues that may come from it? Possibly we expect that's a breaking change as users would need to switch from `-XX:+UseExperimentalFeatures` to `-XX:+UseDiagnosticsFeatures`.
>
>> Mind you, we are lacking one option for https://github.com/openjdk/jdk/pull/17122 in that respect. Let me know if you going to add one.
> 
> @RealFYang I can do that. FYI, tested on lichipi and VF2, seems both does not support Zcb yet, tracked by https://bugs.openjdk.org/browse/JDK-8327058

> Another question will be about backporting the EXPERIMENTAL -> DIAGNOSTIC. We can revisit later but I think we should be fairly proactive about that when the case arises. Do you see any issues that may come from it? Possibly we expect that's a breaking change as users would need to switch from `-XX:+UseExperimentalFeatures` to `-XX:+UseDiagnosticsFeatures`.

I didn't see a big issue as they are EXPERIMENTAL options for now which are subject to change. User are not supposed to use them casually as they use normal product VM options. I am thinking that the backporting to LTS versions should happen only after they are turned into DIAGNOSTIC in JDK mainline some day.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17698#discussion_r1508555665