[llvm] [RISCV] enable VTYPE before whole RVVReg move (PR #117866)

[Piyou Chen via llvm-commits]

BeMg wrote:

> For the code itself, this is working in terms of COPY instructions. I'm not sure this is (or isn't) safe. Are we absolutely sure we don't have any code anywhere that moves a COPY between vector register allocation and lowering? I have not done this due diligence, but that seems like a strong claim to make. To be clear, the code structure bits are addressable. I suggest using this patch as a test vehicle to check the performance impact _before_ bothering to get the patch "just right".

I created another patch (https://github.com/llvm/llvm-project/pull/118252) that performs the same task during the COPY Pseudo expanding hook to handle any COPY pseudo generated after insertvsetvl. And current patch could be an optimization to reduce redundant VSETVL by inserting it with analysis.

https://github.com/llvm/llvm-project/pull/117866