Optimizations themselves (except for perhaps -ffast-math) can't cause undefined behavior: the undefined behavior was already there. They can just change the program from behaving expectedly to behaving unexpectedly. The problem is that so many snippets, which have historically been obvious or even idiomatic, contain UB that has almost never resulted in unexpected behavior. Modern optimizing compilers have only been catching up to these in recent years.
There have been more than a few compiler bugs that have introduced UB and then that was subsequently optimized, leading to very incorrect program behavior.
A compiler bug cannot introduce UB by definition. UB is a contract between the coder and the C language standard. UB is solely determined by looking at your code, the standard, and the input data; it is independent of the compiler. If the compiler converts UB-free code into misbehavior, then that's a compiler bug / miscompilation, not an introduction of UB.
A compiler bug is a compiler bug, UB or not. You might as well just say "There have been more than a few compiler bugs, leading to very incorrect program behavior."
The whole thread is about how UB is not like other kinds of bugs. Having a compiler optimization erroneously introduce a UB operation means that downstream the program can be radically altered in ways (as discussed in thread) that don't happen in systems without the notion of a UB.
While it's technically true that any compiler bug (in any system) introduces bizarre, incorrect behavior into a program, UB just supercharges the things that can go wrong due to downstream optimizations. And incidentally, makes things much, much harder to diagnose.
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.
LLVM IR has operations that have UB for some inputs. It also has poison values that act...weird. They have all the same implications of source-level UB, so I see no need to make a distinction. The compiler doesn't.
