[llvm] [RISCV][GISel] Instruction select for vector G_ADD, G_SUB (PR #74114)

[Jiahan Xie via llvm-commits]

jiahanxie353 wrote:

To implement `RISCVInstructionSelector::getRegClassForTypeOnBank`, I'm creating a mapping function that takes in an `LLT` and output `RegClass`.

Let's take `vscale x 1 x s32` as an example `LLT`, the expected `RegClass` is `RISCV::VRRegClassID` (the reason being we treat `LMUL` <= 1 all as `LMUL`=1 since they have similar behavior, if I remember our discussion from yesterday correctly? @michaelmaitland ).

I was trying to use `getElementType` to extract `s32`, and using `getNumElements()` to extract that `1`, which raises an exception. And I believe it's because `getNumElements()` extracts `vscale` and `1` at the same time. Since `vscale` is unknown at compile time, so it can't calculate the exact `NumElements`.

I'm wondering if I'm on the right track, and is there a generic function to extract `n` out of `vscale x n x m`?

https://github.com/llvm/llvm-project/pull/74114