I just don't think it makes much sense to say that an optimization can "introduce a UB operation". UB is a property of C programs: if a C program executes an op

I just don't think it makes much sense to say that an optimization can "introduce a UB operation". UB is a property of C programs: if a C program executes an operation that the standard says is UB, then no requirement is imposed on the compiler for what should happen. In contrast, optimizations operate solely on the compiler's internal representation of the program. If an optimization erroneously makes another decide that a branch is unreachable, or that a condition can be replaced with a constant true or false, then that's not "a UB operation", that's just a miscompilation. The latter set of optimizations is just commonly associated with UB, since C programs with UB often trigger those optimizations unexpectedly.