Basic GNU C implementations don't get auto-vectorised full stop. But with a little bit of effort Chacha20 can be made to vectorise. The implementation in here

Basic GNU C implementations don't get auto-vectorised full stop. But with a little bit of effort Chacha20 can be made to vectorise. The implementation in here is vectorised by GNU C: https://sourceforge.net/p/pbkdf2/code/ci/default/tree/pbkdf2.c If "ultra-efficient code" means what could be produced by a programmer highly skilled in some amd64 implementation (intel core2, amd bulldozer, ...) for that implementation then yes I doubt GNU C produces it. But the odds are GNU C's output runs faster than that's guru's code on other amd64 implementations.