This appears to use the stack in a strange way, at the expense of using registers. Was this perhaps compiled with "-O0" as an example of a wholly unoptimized program?

Yes, I think that's quite likely, as aggressively optimized programs can have esoteric assembly outputs! This output seems rather verbose (which is characteristic of unoptimized programs), but it's definitely possible to trace through it. As a cool aside, this link discusses some ways to make assembly output more readable!