> There was a real alternative being considered at the time: integrating ISPC-like semantics natively in the language.

I think this is the best solution for truely portable SIMD. Sure it doesn't cover everything, but it makes autovec explicit, guaranteed and more powerfull.

One of the biggest problems with "portable" SIMD libraries, is that when it's used for simple things, often autovec is better, as it has access to the direct ISA semantics and can much easier do things like unrolling.