Comment by dzaima

On clang, you can actually request that it gives a warning on missed vectorization of a given loop with "#pragma clang loop vectorize(enable)": https://godbolt.org/z/sP7drPqMT (and you can even make it an error).

There's even "#pragma clang loop vectorize(assume_safety)" to tell it that pointer aliasing won't be an issue (gcc has a similar "#pragma GCC ivdep"), which should get rid of most odd reasons for missed vectorization.