Vectors with runtime-defined length

[penzn]

Opening to document an item added to the agenda for meeting on April 28. This came out of a number of discussion in context of the SIMD proposal, for example WebAssembly/simd#29.

The gist of the proposal is to provide a set of vector operations with runtime-constant length, which is not known at compile time, see overview repository or slides.

This is by no means a proposal to modify existing SIMD, in fact, part of the motivation to ask for phase 0 repository is to move discussions on this idea away from the SIMD proposal. Wasm SIMD serves as inspiration for this, in a way, but it should move forward without waiting for this idea to flesh out.

Note that the proposal does not cover true vector instruction sets (as discussed #1151, for example), though are still relatively rare in consumer hardware.

[lars-t-hansen]

@penzn, on p9 and p14 of the slides the innermost "block" should be "loop". Also, I don't understand slide 14, because I don't understand the set_length operation. Apparently it returns some value, but what value?

[penzn]

@lars-t-hansen, I just got to your comment (time zone differences, sorry). Thank you for pointing out the loop typo - fixed that. I am running out of time before the meeting, might not be able to address the other comment in the PDF (though I will try) - it returns minimum between its parameter and available system length, and sets the internal vector length to that value.

[penzn]

Phase 1 proposal repository is set up: https://github.com/WebAssembly/flexible-vectors

With that, I am not sure if we should keep this issue open.

[tlively]

Probably not, let's move discussion over to the new repo.