[IshKebab]

Minor nit: RISC-V Vector isn't SIMD. It's actually like ARM's Scalable Vector Extension. Unlike traditional SIMD the code is agnostic to the register width and different hardware can run the same code with different widths.

There is also a traditional SIMD extension (P I think?) but it isn't finished. Most focus has been on the vector extension.

I am wondering how and if Rust will support these vector processing extensions.

[camel-cdr]

> RISC-V Vector isn't SIMD

Isn't SIMD a subset of vector processors?

To that matter, can anybody here provide a proper and useful distinction between the two, that is SIMD and vector ISAs?

You imply it's because it's vector length agnostic, but you could take e.g. the SSE encoding, and apart from a few instructions, make it operate on SIMD registers of any length. Wouldn't that also be vector length agnostic, as long as software can query the vector length? I think most people wouldn't call this a vector ISA, and how is this substantially different from dispatching to different implementations for SSE AVX and AVX512?

I've also seen people say it's about the predication, which would make AVX512 a vector isa.

I've seen others say it's about resource usage and vector chaining, but that is just an implementation detail and can be used or not used on traditional SIMD ISAs to the same extend as on vector ISAs.

[janwas]

I agree that SIMD and vector are basically interchangeable at a certain level.

There is still a difference in the binutils, because SSE4 and AVX2 and AVX-512 have different instruction encodings per length.

But yes, it is possible to write VL-agnostic code for both SIMD and vector, and indeed the same user code written with Highway works on both SIMD and RISC-V.

[Findecanor]

RISC-V's vector extension will have at least 128 bits in application processors, so I think you could set VLEN=128 and just use SIMD algorithms.

The P extension is intended more for embedded microcontrollers for which the V extension would be too expensive. It reuses the GPRs at whatever width they are at (32 or 64 bits).

[camel-cdr]

That or you can detect the vector length and specialize for it, just like it's already done on x86 with VLEN 128, 256, and 512 for sse, avx, and avx512.