[llvm] [RISCV] Introduce a new tune feature string syntax and its parser (PR #168160)

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

RISC-V toolchain convention PR: https://github.com/riscv-non-isa/riscv-toolchain-conventions/pull/122

https://github.com/llvm/llvm-project/pull/168160