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

[Craig Topper via llvm-commits]

topperc wrote:

> That sounds like a great way for users to not notice they've made a typo in their -mtune string?

Or not notice that they upgraded the compiler and those pesky compiler developers deleted or renamed their option.

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