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

Alex Bradbury via llvm-dev llvm-dev at lists.llvm.org
Thu Jan 23 06:58:27 PST 2020

On Wed, 22 Jan 2020 at 19:55, Chris Lattner via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> On Jan 21, 2020, at 5:00 AM, Alex Bradbury <asb at lowrisc.org> wrote:
> >> This all makes sense to me.
> >
> > That's correct, thanks for the feedback.
> >
> > I do like the idea from James of having the compiler always spit out a
> > note when enabling the experimental extension, warning of its
> > experimental nature. If we had such a warning and additionally
> > required a `-riscv-enable-experimental-extensions` or similar, then I
> > think there could be merit in including in the ISA string as Simon
> > suggests, especially as we're likely to start putting that string in
> > ELF output etc.
> Are you suggesting this behavior from Clang or from LLVM?  I think it would be a bad thing for LLVM to produce this warning: there isn’t a precedent for this, and it breaks the library-based design goals.  Having clang produce a warning could be done, but it would be very noisy (one warning for every .c file in a build) and I’m not sure how much value it provides.

That's a good point. It felt like there may be an opportunity to
educate users that they're enabling a feature that might mutate from
release to release, but hopefully the "experimental" string in the
flag name indicates that, and as you say there's not much precedent
for such noisy warnings. After all, you can have a really bad time by
setting -mstack-alignment and not understanding the consequences.

So I'm in favour of dropping the noisy warning idea.

Thanks again for the input, and thanks James for your clarification.



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