Comment by jcranmer

For the simple SIMD cases where the computation doesn't have any horizontal dependencies (except for a total reduction step), there's little need for any sort of intrinsic usage. (Although you may need some compiler hints to push it to autovectorize).

The real problem is that SIMD hardware sets tends to have lots of instructions that have mixing of horizontal lanes. The scope and performance of these mixing instructions varies greatly from implementation to implementation, and these kinds of instructions are hard for compilers to automatically pick up. It's the latter case that means you need the SIMD intrinsics to be exposed to use the hardware effectively.