That's impossible in the general case, since it's trivially equivalent to the halting problem (take the UB-detecting compiler, have it perform UB iff the progra

That's impossible in the general case, since it's trivially equivalent to the halting problem (take the UB-detecting compiler, have it perform UB iff the program has no UB, run it on its own source code). What we can do is a combination of formal methods, heuristics and sanitizers. Plus software engineering techniques, such as running tests to high coverage with sanitizers, fuzzing, and so on.