RFR: 8367103: RISC-V: store cpu features in a bitmap [v5]

[Hamlin Li]

On Mon, 22 Sep 2025 10:24:06 GMT, Ludovic Henry <luhenry at openjdk.org> wrote:

>> Hamlin Li has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   simplify names
>
> src/hotspot/cpu/riscv/vm_version_riscv.hpp line 275:
> 
>> 273:    public:
>> 274:     enum RVFeatureIndex {
>> 275:       #define DECLARE_RV_FEATURE_ENUM(NAME, PRETTY, LINUX_BIT, CPU_FEATURE_INDEX, FSTRING, FLAGF) CPU_##NAME=(CPU_FEATURE_INDEX),
> 
> Instead of explicitly setting the feature index, can't we use the implicit numbering of the enum and remove the `cpu feature index` from `RV_EXT_FEATURE_FLAGS` entirely? The only other use I can see of `CPU_FEATURE_INDEX` is at https://github.com/openjdk/jdk/pull/27152/files#diff-7b173d6e5834de13749c8333192fef5a874628a67b90a5d8d06235d507542ac4R39 and I wonder if we can't use the enum `CPU_#NAME` there directy.

Yes, it'll be good if we can just remove the explicit numbering of the enum, but seems there is no good way to pass in the enum value to the constructor of `NAME##RVExtFeatureValue` in `DEF_RV_EXT_FEATURE`. We could do it with an static member of RVExtFeatureValue, increment it in RVExtFeatureValue constructor, but seems it's not straight and clear.
So my preference is to keep the explicit enum value in `RV_EXT_FEATURE_FLAGS`.
How do you think about it?

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PR Review Comment: https://git.openjdk.org/jdk/pull/27152#discussion_r2367768118