Determining maximum vector length supported by the CPU?

[Matthias Kretz]

On Mittwoch, 22. Mai 2019 11:17:57 CEST Richard Biener wrote:
> 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?).

AVX vs. AVX2 is a valid question. std::experimental::simd solves it by having 
the size depend on the element type. So I agree, `vector_size(auto)` seems 
like a possible solution. One could then take the sizeof if the number is 
important to know.

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