SIMD can be pretty architecture specific. For example, does the CPU support AVX-512 or SSE3? So, you have to have a few code paths if you're going to support a wide variety of hardware.

I don't have an answer to your question it just occurs to me that maybe the Linux kernel doesn't allow a lot of SIMD for this reason or maybe they require a fall back/slow code path to be available if you're submitting a patch that includes SIMD operations?

Curious if anyone knows the answer to this.