But at the time C89 invented 'undefined behaviour', C compilers didn't (couldn't) do that. This modern interpretation simply didn't occur to anyone at the time.

But at the time C89 invented 'undefined behaviour', C compilers didn't (couldn't) do that. This modern interpretation simply didn't occur to anyone at the time. If it had, people would have screamed bloody murder, and the language simply would not have passed. Compare dmr's criticism of the 'noalias' proposal: “a license for the compiler to undertake aggressive optimizations that are completely legal by the committee's rules, but make hash of apparently safe programs”. In the accompanying Rationale, the committee explicitly calls out places where the standard intentionally diverged from existing practice; they did not do so for UB. They thought “the most serious semantic change” was mandating value-preserving rather than unsigned-preserving integer promotion.