RFR: 8320999: RISC-V: C2 RotateLeftV [v2]

[Fei Yang]

On Fri, 24 May 2024 14:53:15 GMT, Ludovic Henry <luhenry at openjdk.org> wrote:

>> Yes, I think there should be quite a few places where we could make use of vector-scalar variants, which would save us one vector register and one vmv.v.x instruction. @zifeihan has already handle some cases in vector logic instructions: https://github.com/openjdk/jdk/pull/18999. And He is currently working on handling more vector arithmetic instructions.
>> 
>> (One example: https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/riscv/riscv_v.ad#L523)
>
> I would also favor using `.vi` or `.vx` variants over `.vv` variants where possible. This would reduce the vector register pressure and remove an unnecessary instruction.
> 
> @Hamlin-Li  in your example, we could instead have:
> 
>   ... ...
>   0x00002aaac560c594:   vle32.v v2,(a4)
>   0x00002aaac560c598:   vsetivli        t0,8,e32,m1,tu,mu
>   0x00002aaac560c59c:   vror.vx v2,v2,a3

And for your case, this would help save the `vmv.v.x v1,a3` instruction if you do `vror.vv v2,v2,a3` instead of `vror.vv v2,v2,v1`. Right?

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PR Review Comment: https://git.openjdk.org/jdk/pull/19325#discussion_r1613614979