RFR: 8356875: RISC-V: extension flag UseZvfh should depends on UseZfh

[Fei Yang]

On Tue, 13 May 2025 13:53:35 GMT, Hamlin Li <mli at openjdk.org> wrote:

> Hi,
> Can you help to review this simple patch?
> If we don't enable such dependency, then there will be situation that when Zvfh is enabled but Zfh is disabled, and optimizations depending on Zvfh will be disabled (e.g. in C2) because Zfh related IR node are considered not supported.
> 
> Thanks!
> 
> By [rvv spec](https://github.com/riscvarchive/riscv-v-spec/blob/master/v-spec.adoc#185-zvfh-vector-extension-for-half-precision-floating-point), `The Zvfh extension depends on the Zve32f and Zfhmin extensions.`
> But I think in logic and JVM code, we should let `Zvfh` depends on `Zfh`.

src/hotspot/cpu/riscv/vm_version_riscv.cpp line 251:

> 249:       warning("Cannot enable UseZvfh on cpu without Zfh support.");
> 250:       FLAG_SET_DEFAULT(UseZvfh, false);
> 251:     }

For QEMU, Zvfh also means Zvfhmin and Zfhmin. So there is no Zfh if you only enable Zvfh when start QEMU.
That means hwprobe syscall will detect Zvfh and Zfhmin but no Zfh. Is there a similar issue for this case?

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PR Review Comment: https://git.openjdk.org/jdk/pull/25213#discussion_r2087858752