[immibis]

In the original vision of C, UB was behaviour defined by the platform the code ran on, rather than the language itself. It was done this way so that the C language could be reasonably close to assembly on any platform, even if that platform's assembly was slightly different. A good example is shifts greater than the value's width: some processors give 0 (the mathematically correct result), some ignore the upper bits (the result that requires the fewest transistors) and some trap (the cautious result).

It was only much later that optimizing compilers began using it as an excuse to do things like time travel, and then everyone tried to show off how much of an intellectual they were by saying everyone else was stupid for not knowing this could happen all along.