If you turned on specific optimization and warning flags it emitted a lot of useful information to stdout/stderr. It was quite helpful even to a compiler technology ignoramus like me.

It would identify specific loops and would provide reasons why it could not vectorize it, usually some sort of aliasing that it could not rule out. I would then rewrite the code if the rewrite was simple, to make it obvious that such aliasing wouldn't occur. If it wasn't aliasing it was some sort of a cost benefit model that my loop had not crossed.