[llvm-dev] [RFC] Upstream development of support for yet-to-be-ratified RISC-V extensions

Jacob Lifshay via llvm-dev llvm-dev at lists.llvm.org
Fri Jan 17 08:07:25 PST 2020

On Fri, Jan 17, 2020, 07:33 James Y Knight via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> One way to resolve this would be to require _both_ -- you must specify
> your ISA options with explicit versions in the ISA string, but, doing so
> would print an error without a separate flag also being enabled, e.g.
> "error: ISA extension b0p92 is an experimental extension, and support may
> be removed in future LLVM releases. Pass
> -mallow-experimental-riscv-isa-extensions to enable."

I think this makes a lot of sense, it's also similar to how experimental
features are enabled on cargo (rust's package manager/build system) where
you have to
use "-Z unstable-features" as well as the flag for whatever feature you
wanted to use.


Example for using the unstable --out-dir option:
cargo +nightly build --out-dir=out -Z unstable-options

I'm probably going to shamelessly steal your idea when we (libre-riscv) add
our GPU extensions to PowerISA and/or RISC-V.


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