Clang rewrites my tolower64() into something rather more clever, and it adds a 2x unrolled version. (gcc keeps the object code much closer to what I wrote.) But

Clang rewrites my tolower64() into something rather more clever, and it adds a 2x unrolled version. (gcc keeps the object code much closer to what I wrote.) But from my point of view the important thing is that Clang retains the masked load and store for short string fragments. Its autovectorizer is not able to generate masked loads and stores. I used Clang 11 for copybytes64() because it is unable to recognize the memcpy() idiom, whereas Clang 16 does turn it into memcpy() - which is slower!