Even if you use the GNU C vector extension to explicitly give the compiler ways of optimizing C, it is not very good at generating good vector code:

https://github.com/openzfs/zfs/pull/14234#issuecomment-13345...

A bug report has been filed with GCC for one of the issues. LLVM is much better here, but not perfect, or at least that has been my experience when trying to have the compiler generate assembly for an explicitly vectorized fletcher4 implementation.