[clang] [llvm] [RISCV][MC] Add MC support of Zibi experimental extension (PR #127463)

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

> It would be nice to see an actual encodings spec PDF to be able to verify that the opcode encoding in this PR is actually correct.

Thanks @arichardson ! We will release an v0.1 PDF in https://github.com/riscv/zibi this week and then update this PR. :-)

https://github.com/llvm/llvm-project/pull/127463