Well that's much more interesting. Is there anything written publicly about the experience? Any tooling used to help get the implementation right beyond testing and writing it carefully?

I've found a little here https://ffmpeg.org/developer.html#SIMD_002fDSP-1

The context is I'm a compiler developer who really liked working side by side with old school assembly developers in a past role. I'm painfully aware that the tribal knowledge of building stuff out of asm is hard to find written down and always curious about the directions in which things like C can be extended to narrow the gap.