failing branch elimination, how to diagnose

For the backend I am writing, branch elimination is not working with the following error message:

Pre-computing triangle chains.
Pre-merging due to unanalyzable fallthrough: %bb.1 ('L.2') -> %bb.2 ('return')
     %bb.2 ('return') -> Mid chain!

Obviously I am not constructing the chain correctly. How do I figure out where I went wrong?