Now look at a large code base, with lots of checks for inputs. With O0 it's usually straight-forward to read, but with heavy optimizations applied it can become

Now look at a large code base, with lots of checks for inputs. With O0 it's usually straight-forward to read, but with heavy optimizations applied it can become a real nightmare. And in that scenario all it takes is some reordering of the UB before the DCE pass can prove the code to be unreachable (which might not happen on the first DCE pass, because the value analysis/constant propagation didn't work its magic, yet). Also static program analyses can be both forward and backward. I'm afraid it's not nonsense, that's why it's a common misunderstanding ;)