The create is called portable_simd.

There is no reason a portable_simd relu_dot implemention should need to specify the SIMD width.

But the design and documentation of portable_simd makes the fixed size syntactically easy/the default and the width agnostic code harder.

Counter question: why shouldn't it be portable?

It's definitely a 80% solution where you occasionally need to drop down to intrinsics (at zero runtime perf cost) for CPU specific instructions.

But just having vector types, arithmetic, swizzling, loads and stores will go a long way for basic tasks.

And with generics you can write code that is type and width agnostic. No need to rewrite your code of you want to go from SSE to AVX512, just change from f32x4 to f32x16 (or use generics) and you are done.