https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114052
--- Comment #5 from Jan Hubicka <hubicka at gcc dot gnu.org> --- So if I understand it right, you want to determine the property that if the loop header is executed then BB containing undefined behavior at that iteration will be executed, too. modref tracks if function will always return and if it can not determine it, it will set the side_effect flag. So you can check for that in modref summary. It uses finite_function_p which was originally done for pure/const detection and is implemented by looking at loop nest if all loops are known to be finite and also by checking for irreducible loops. In your setup you probably also want to check for volatile asms that are also possibly infinite. In mod-ref we get around by considering them to be side-effects anyway. There is also determine_unlikely_bbs which is trying to set profile_count to zero to as many basic blocks as possible by propagating from basic blocks containing undefined behaviour or cold noreturn call backward & forward. The backward walk can be used to determine the property hat executing header implies UB. It stops on all loops though. In this case it would be nice to walk through loops known to be finite...
