RFR: 8352218: RISC-V: Zvfh requires RVV

[Fei Yang]

On Tue, 18 Mar 2025 09:04:07 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

src/hotspot/cpu/riscv/vm_version_riscv.cpp line 222:

> 220:   // UseZvbb (depends on RVV).
> 221:   if (UseZvbb && !UseRVV) {
> 222:     if (!FLAG_IS_DEFAULT(UseZvbb)) {

So we have two code paths to enable this flag: 1. through the command line; 2. through hwprobe. I think the issue 
here is related to case 2. I wonder if that could be handled in that code path, that is when we call `UPDATE_DEFAULT` [1].

Then we could only consider case 1 here and simplify the code removing this `FLAG_IS_DEFAULT` check. It's a bit confusing to me as people might think that a true value of `UseZvbb` will mean that `FLAG_IS_DEFAULT` is false. What do you think?

[1] https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/riscv/vm_version_riscv.hpp#L171

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PR Review Comment: https://git.openjdk.org/jdk/pull/24094#discussion_r2000841570