IMO what's needed is ISPC like guided autovec with a lot of hinting support to control codegen (e.g. hint for generating an unrolled version only or an unrolled and non-unrolled version).

Basically something like #pragma omp SIMD, but actually designed for the SIMD model, not parallel one, that erros when vectorization isn't possible.

Ideally it would support things like reductions, scans, reference of elements from other iterations (e.g. out[i] = in[i-1]+in[i+1]), full gather scatter, early break, conditional execution control (masking or also a fast-path, when no active elements), latency vs throughput sensitive (don't unroll or unroll to max without spilling), data dependent termination (fault-only-first load or page aligned for thigs like strlen), ...