RFR: 8352218: RISC-V: Zvfh requires RVV [v2]

[Robbin Ehn]

On Thu, 20 Mar 2025 08:36:49 GMT, Hamlin Li <mli at openjdk.org> wrote:

> Interesting suggestion and change. Thanks for asking, I was thinking the modification based on Fei's suggestion is a simple one.
> 
> Have several questions in my mind:
> 
> 1. in the while loop of `VM_Version::setup_cpu_available_features`, is there an order guarantee in cpu features? consider a situation, A depends on B, but B is after A in the while loop.

They are in list order. I did add a comment explaining that.

> 2. what if there are more than 2 cpu features in the dependency chain?

I assume independent dependencies, so you would have to fix that.

> 3. For consistency, should we disable the cpu feature in RVFeatureValue? e.g. in vm_version_riscv.cpp when `(UseZvbb && !UseRVV)` we should also call `RVFeatureValue::disable_feature`.

If the feature flag is identical to the use flag we may as well remove the feature flag.
The point was that they can be different, I think this piece of code here is wrong:

         /* Sync CPU features with flags */ \
         if (!flag) {
           disable_feature();
         }



As it is done before the flag is final, which means the flag may later be true.
We build the feature string before flags are final.
We build the feature string from RVFeatureValue enabled/disabled instead of UseFlags value.

So we should probably build the feature string last in VM_Version::initialize() when all flags are stable.
And change the function:
`bool feature_string()        { return _feature_string; }` to "return UseXXX" if it have a flag mapping.

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PR Comment: https://git.openjdk.org/jdk/pull/24094#issuecomment-2740373896