[PATCH] D80802: [RISCV] Upgrade RVV MC to v0.9.

[Ferran Pallarès Roca via Phabricator via llvm-commits]

fpallares added a comment.

Apologies we didn't identify this earlier but with the change of the mask register layout (`MLEN=1`) the overlap constraints involving the mask register were modified:

//**RVV-0.8, Section 5.3. Vector Masking:**//

> The destination vector register group for a masked vector instruction can only overlap the source mask register (v0) when LMUL=1. Otherwise, an illegal instruction exception is raised.

//**RVV-0.9, Section 5.3. Vector Masking:**//

> The destination vector register group for a masked vector instruction cannot overlap the source mask register (v0), unless the destination vector register is being written with a mask value (e.g., comparisons) or the scalar result of a reduction. Otherwise, an illegal instruction exception is raised.

The change was introduced in this commit <https://github.com/riscv/riscv-v-spec/commit/9a77e128b5e96ca984ad50b7cd9330c841321efb>.

>From my understanding, with this change an instruction such as the following should be rejected in RVV-0.9:

  vadd.vv	v0, v1, v2, v0.t

Also note that `vadc`/`vsbc` already have this behaviour.


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  https://reviews.llvm.org/D80802/new/

https://reviews.llvm.org/D80802