Xtensa is an unusual beast because its USP (at least, back when it was owned by tensilica) was that you could easily add extensions. Not just off the shelf ones

Xtensa is an unusual beast because its USP (at least, back when it was owned by tensilica) was that you could easily add extensions. Not just off the shelf ones - ones you defined yourself. They had some automation that would generate a toolchain for you to use with your shiny new instructions. Most CPU architectures exist to allow programs written on once implementation to work on another, with Xtensa it's kind of the opposite - it exists to allow each chip to have its own special sauce. Honestly I was a bit surprised that espressif used it without defining their own extension of some kind, if you're not doing that then you might as well use something better known. Edit: ajross* points out that this SIMD extension is such a one, not an off the shelf one. So I guess that explains it. * https://news.ycombinator.com/item?id=40267977 https://news.ycombinator.com/item?id=40267977