lenary wrote:
> > Not related to this PR, but I'd like to raise the question here:
> > For configurable cores, what is the best way to specify the features?
> > `-mcpu` is meant to support the base configuration, but how can we specify
> > the additional optional extensions? Apparently, failing back to `-march` is
> > silly.
> > My thought is: can we support `-march/-mcpu` where the values can be
> > `{cpu}(_ext)*`? Will such use be problematic?
>
> Unfortunately, we have to suggest our users to specify both `-march` and
> `-mcpu` when their processor includes additional optional extensions.
> Ideally, we hope that code generated using `-mcpu` alone would always be
> compatible with processors that have varying configurations.
This is the same trade-off that I would choose, but I understand why others
have gone a different way.
My hope would be that the combination of `-march=` and `-mtune=` would be
equivalent to `-mcpu=`, so if you added more features to `-march=`, you'd still
get the code generation you want (scheduling, optimisations, etc) but also the
additional instructions that you asked for. I think we've worked out how to
model this well in the RISC-V backend, but I haven't examined how `-mtune=` is
treated fully.
https://github.com/llvm/llvm-project/pull/144022
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