Determining maximum vector length supported by the CPU?

[Martin Reinecke]

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.

> 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