Compilers will always be terrible at vectorizing code because the required transformation is architectural. It would require the compiler to understand your cod

Compilers will always be terrible at vectorizing code because the required transformation is architectural. It would require the compiler to understand your code well enough to replace the scalar algorithms and data structures with new ones that are semantically equivalent in all contexts with all necessary invariants preserved (e.g. how memory is organized). The code transformation would be very non-local. Compilers can't generally rewrite your scalar code as vector code for the same reason they can't rewrite your b-tree as a skip list.