RFR: 8350960: RISC-V: Add riscv backend for Float16 operations - vectorization [v2]

[Fei Yang]

On Tue, 13 May 2025 13:26:34 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> src/hotspot/cpu/riscv/riscv_v.ad line 382:
>> 
>>> 380:   ins_encode %{
>>> 381:     assert(UseZvfh, "must");
>>> 382:     BasicType bt = Matcher::vector_element_basic_type(this);
>> 
>> Question: What is `bt` calculated here? Seems there isn't one for HF16 in `enum BasicType` definition in file src/hotspot/share/utilities/globalDefinitions.hpp. I only see `T_FLOAT` and `T_DOUBLE`, which I don't think is usable here as we need to set SEW=16 for this instruction.
>
> No, it uses T_SHORT instead, in Float16.java it also uses a short as underlying payload.
> And if you check the generated assembly code, you'll find some code like `vsetivli        t0,16,e16,m1,tu,mu`.
> 
> To avoid confusion, I will add an assertion here so that it can be understood later.

Thanks for the answer. I see that is also reflected on the C2 source code [1].
Why not save this `Matcher::vector_element_basic_type(this)` call then? I mean:

assert(Matcher::vector_element_basic_type(this) == T_SHORT, "must");
__ vsetvli_helper(T_SHORT, Matcher::vector_length(this));


[1] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/vectornode.cpp#L63

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PR Review Comment: https://git.openjdk.org/jdk/pull/25181#discussion_r2087822782