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

[Fei Yang]

On Fri, 24 May 2024 11:39:20 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> src/hotspot/cpu/riscv/riscv_v.ad line 3097:
>> 
>>> 3095: %}
>>> 3096: 
>>> 3097: instruct vrotate_right_imm(vReg dst, vReg src, immI shift) %{
>> 
>> Question: Could we make use of the vector-scalar rotate variants (vrol_vx / vror_vx) in case `shift` is not a constant?
>
> Do you mean have another instruct like `instruct vrotate_right_imm(vReg dst, vReg src, Reg shift)`?
> Seems not, as in both vectorization or Vector API implementation, when it's not const, it will be put into a vector first, then match `vrotate_right(vReg dst, vReg src, vReg shift)`

Yeah. If that is the case, maybe we can save one vector register then? 
I mean let `instruct vrotate_right_reg(vReg dst, vReg src, Reg shift)` match something like this:
`match(Set dst (RotateRightV src (Replicate shift)))`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19325#discussion_r1613386212