[llvm-dev] New x86-64 micro-architecture levels

[Richard Biener via llvm-dev]

On Wed, Jul 22, 2020 at 10:58 AM Florian Weimer via Gcc <gcc at gcc.gnu.org> wrote:
>
> * Dongsheng Song:
>
> > I fully agree these names (100/101, A/B/C/D) are not very intuitive, I
> > recommend using isa tags by year (e.g. x64_2010, x64_2014) like the
> > python's platform tags (e.g. manylinux2010, manylinux2014).
>
> I started out with a year number, but that was before the was Level A.
> Too many new CPUs only fall under level A unfortunately because they do
> not even have AVX.  This even applies to some new server CPU designs
> released this year.
>
> I'm concerned that putting a year into the level name suggests that
> everything main-stream released after that year supports that level, and
> that's not true.  I think for manylinux, it's different, and it actually
> works out there.  No one is building a new GNU/Linux distribution that
> is based on glibc 2.12 today, for example.  But not so much for x86
> CPUs.
>
> If you think my worry is unfounded, then a year-based approach sounds
> compelling.

I think the main question is whether those levels are supposed to be
an implementation detail hidden from most software developer or
if people are expected to make concious decisions between
-march=x86-100 and -march=x86-101.  Implementation detail
for system integrators, that is.

If it's not merely an implementation detail then names without
any chance of providing false hints (x86-2014 - oh, it will
run fine on the CPU I bought in 2015; or, x86-avx2 - ah, of
course I want avx2) is better.  But this also means this feature
should come with extensive documentation on how it is
supposed to be used.  For example we might suggest ISVs
provide binaries for all architecture levels or use IFUNCs
or other runtime CPU selection capabilities.  It's also required
to provide a (extensive?) list of SKUs that fall into the respective
categories (probably up to CPU vendors to amend those).
Since this is a feature crossing multiple projects - at least
glibc and GCC - sharing the source of said documentation
would be important.

So for the bike-shedding I indeed think x86-10{0,1,2,3}
or x86-{A,B,C,..}, eventually duplicating as x86_64- as
suggested by Jan is better than x86-2014 or x86-avx2.

Richard.

> Thanks,
> Florian
>