RFR: 8368897: RISC-V: Cleanup RV_EXT_FEATURE_FLAGS & RV_NON_EXT_FEATURE_FLAGS [v4]

[Fei Yang]

On Tue, 7 Oct 2025 10:19:39 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> But why is not vector size/length called:
>> `VM_Version::non_ext::rvv_vlen()`  or similar ? Should it ?
>> 
>> Should VM_Version::non_ext and VM_Version::ext be private, so we create a public:
>> VM_Version::ziboz_block_size() ?
>> 
>> I added:
>> `static bool supports_fencei_barrier() { return ext_Zifencei.enabled(); }`
>> 
>> 
>> Was this wrong? Should I have used `VM_Version::ext_Zifencei.enabled()` in MASM ?
>> 
>> Can we agree to one consistent way?
>
>> But why is not vector size/length called: `VM_Version::non_ext::rvv_vlen()` or similar ? Should it ?
> 
> I think this should be in another pr.
> 
>> Should VM_Version::non_ext and VM_Version::ext be private, so we create a public: VM_Version::ziboz_block_size() ?
> 
> If you prefer `VM_Version::ziboz_block_size` as the interface, then I don't think it's necessary to have `VM_Version::non_ext` and `VM_Version::ext` in the implemention.
> 
>> I added: `static bool supports_fencei_barrier() { return ext_Zifencei.enabled(); }`
>> Was this wrong? Should I have used `VM_Version::ext_Zifencei.enabled()` in MASM ?
>> Can we agree to one consistent way?
> 
> As mentioned above, I think this should be in another pr: a consistent interface of VM_Version.

That reminds me of a previous discussion about non-extension features [1].
It doesn't seem to be necessary to have a `enabled()` check for non-extension features like `zicboz_block_size`, `mvendorid`, etc. So we might want to remove this check and give these features a default value (maybe -1 as suggested by @Hamlin-Li).

This is just like how the global flags are used. It will have a valid value if it could be detected from hwprobe interface at JVM startup. Hopefully, that will simplify the use sites of them a bit making it more consistent. In this repect, it works for me if we keep using the current naming, like `VM_Version::zicboz_block_size.value()`.

[1] https://github.com/openjdk/jdk/pull/27512#discussion_r2387421424

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PR Review Comment: https://git.openjdk.org/jdk/pull/27562#discussion_r2412752454