> Given that Apple has been making its own CPU cores for years now, I suspect overflowing checking on Apple CPUs is virtually free (aside from code size).

Never make guesses based on a particular programming language. In Apple's own C documentation (https://developer.apple.com/documentation/xcode/integer-over...) it is stated that "Overflows result in undefined behavior." and enabling wrapping behaviour "may adversely impact performance", indicating that overflow detection is in fact not "virtually free".