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

[Hamlin Li]

On Wed, 19 Mar 2025 06:50:25 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

>> Hi please consider.
>> 
>> Added case to turn off UseZvfh when no RVV. 
>> Which is the cause of the test issues, zvfh on but no rvv.
>> 
>> Also made all case identical and added no warning when default.
>> Move them to the common init, as the "UseExtension" is not C2 specific.
>> 
>> Manual tested and some random compiler tests.
>> 
>> Thanks, Robbin
>
> Robbin Ehn has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:
> 
>  - hwprobe deps
>  - Merge branch 'master' into maxvector_0
>  - Moved to common
>  - Disable UseZvfh when no RVV

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

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PR Comment: https://git.openjdk.org/jdk/pull/24094#issuecomment-2739589571