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

[Ludovic Henry]

On Tue, 27 Feb 2024 09:50:33 GMT, Fei Yang <fyang at openjdk.org> wrote:

>> Hamlin Li has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   add some comments
>
> src/hotspot/cpu/riscv/globals_riscv.hpp line 117:
> 
>> 115:   product(bool, UseZihintpause, false, EXPERIMENTAL,                             \
>> 116:           "Use Zihintpause instructions")                                        \
>> 117:   product(bool, UseZvfh, false, "Use Zvfh instructions")                         \
> 
> I don't think it's a good idea to have more and more experimental / disagnostic options for new RV extensions. I think it might be better to make use of the hwprobe syscall and take a vendor-specific approach. One example is https://github.com/openjdk/jdk/pull/17122 which adds instructions for RV Zcb extension.

We still want to give end users the ability to disable extensions if things are broken. The flags would be the only way if we rely on hwprobe + vendor specific flags (like https://github.com/openjdk/jdk/blob/master/src/hotspot/os_cpu/linux_riscv/vm_version_linux_riscv.cpp#L232-L260, and which I'm surprised not more hw vendors have added their own)

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PR Review Comment: https://git.openjdk.org/jdk/pull/17698#discussion_r1504360368