[vector] RFC: Add scalable shapes for Arm Scalable Vector Extension

[Ningsheng Jian (Arm Technology China)]

Hi Alejandro,

Thanks for the comment!

>
> The scalable shape is just a convenient way to avoid having to code the 16 vector
> lengths that SVE supports. We should always return a concrete VL shape if the
> maximum VL happens to be 128, 256 or 512 (which honestly, should be 99% of
> the time).
>

Yes, that's how we currently implemented. SVE has very good compatibility. :-)

> If this is going to be a contentious issue, I think we might as well bite the bullet
> and write the 16 versions, which are going to be almost identical. They could
> even be some thin layers over the current scalable vector class.
>

That's fine and should not affect our hotspot internal support much, but needs additional lots of lines of code. However, my other concern is that may be misleading of encouraging the use of explicit sizes, since those shapes are public available to users. I think current vector length agnostic way is better fit for SVE (and even other arches?), and we are trying to resolve the reshaping issues.

Thanks,
Ningsheng
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