https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79621
--- Comment #3 from Jeffrey A. Law <law at redhat dot com> --- What a cool little bug. AFAICT, the underlying issue has been latent since the introduction of gimple-ssa-isolate-paths a few years back. We have this PHI at the start of BB4: e7_11 = PHI <e7_13(D)(3), e7_13(D)(5), e7_11(4), 0(6)> Note the 0 associated with the edge 6->4, which gets used in bb4 like this: _3 = b5.2_2 / e7_11; Which of course triggers isolation of the path with a division by zero. Also note there is a PHI arg for the edge 4->4. With that background in mind... Consider the sequencing of path isolation. First we duplicate bb4 so that we have a copy we can freely modify. The duplication (of course) includes duplicating the outgoing edges, including the edge 4->4. That potentially resizes the PHI. *But* the loop in gimple-ssa-isolate_paths.c isn't aware of that possibility and as a result may use the old PHI the next iteration of the loop and nothing good happens thereafter.
