> Compilers rather works in terms of UBs being constraints (on the program), which they can then leverage for optimisations. All the misbehaviour is emergent behaviour from the compiler assuming UBs don’t happen (because that’s what an UB is).

I think a good way to view this would be that optimization passes have invariants. The passes transform code from one shape to another while ensuring that the output from running the code remains the same. But in order for the transformation to be valid certain invariants must be upheld, and if they are not then the result of the pass will have different output (UB).