RFR: 8334554: RISC-V: verify & fix perf of string comparison [v3]

[Hamlin Li]

On Tue, 2 Jul 2024 13:13:38 GMT, Ludovic Henry <luhenry at openjdk.org> wrote:

>> Maybe we can create a class for vector register groups and pass references instead? That would help eliminate the problem of passing too many parameters for vector registers.
>
> Generally speaking, it would be great for C2 to be more register-group-aware. So if we specify `vReg_V2m2` it includes both `v2` and `v3` for example, and marking a `vReg_V2m2` as TEMP marks both `v2` and `v3` as TEMP implicitly. 
> 
> We already have the concept of `operand` ([`vRef_V2`](https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/riscv/riscv.ad#L3439-L3447) for example), but that expresses a `vRef_V2` is any of the register in the list `[v2_reg]` (aka `v2_reg` only). I don't know that there is a concept for a `vRef_V2m2` being a group of register (aka all register in `[v2_reg, v3_reg]`).

I can do further investigation on it, I kindly like what @luhenry suggested above, if we can implement it. 
But anyway I think it will be in another pr.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/19825#discussion_r1662583063