Re: Determining maximum vector length supported by the CPU?

[Richard Biener <richard dot guenther at gmail dot com>]

On Wed, May 22, 2019 at 10:36 AM Martin Reinecke
<martin@mpa-garching.mpg.de> wrote:
>
> Hi Matthias!
>
> > I agree, we need more information from the compiler. Esp. whether the user
> > specified `-mprefer-avx128` or `-mprefer-vector-width=none/128/256/512`.
> > OTOH `-msve-vector-bits=N` is reported as __ARM_FEATURE_SVE_BITS. So that's
> > covered.
>
> Almost ... except that I'd need a platform-agnostic definition. The
> point is that the code does not care about the underlying hardware at
> all, only for the vector length supported by it.

And then you run into AVX + SSE vs. AVX2 + SSE cases where the (optimal) length
depends on the component type...

I wonder if we'd want to have a 'auto' length instead ;)

I suppose exposing a __BIGGEST_VECTOR__ might be possible (not for SVE
though?).

> > Related: PR83875 - because while we're adding things in that area, it'd be
> > nice if they worked with target clones as well.
>
> Yes, this is a problem I've come across as well in the past.
> (https://gcc.gnu.org/ml/gcc-help/2018-10/msg00118.html)
>
> > Are you aware of std::experimental::simd? It didn't make GCC 9.1, but you
> > can easily patch your (installed) libstdc++ using https://github.com/VcDevel/
> > std-simd.
>
> This looks extremely interesting! I have to look at it in more detail,
> but this might be the way to go in the future.
> However, the code I'm working on may be incorporated into numpy/scipy at
> some point, and the minimum required compilers for these packages are
> pretty old. I can't expect more than vanilla C++11 support there.
>
> Cheers,
>   Martin
>