Comment by lelanthran

> What stage is the "just make the compiler define the undefined" stage?

It can be left as implementation defined, which means that the compiler can't simply do arbitrary things, it needs to document what it would do.

Take, for example, signed-integer overflow: currently a compiler can simply refuse to emit the code in one spot while emitting it in another spot in the same compilation unit! Making it IB means that the compiler vendor will be forced to define what happens when a signed-integer overflows, rather than just saying, as they do now, "you cannot do that, and if you do we can ignore it, correct it, replace it or simply travel back in time and corrupt your program".

> Somehow I built an entire lisp interpreter in freestanding C that actually managed to pass UBSan just by following the above logic. I was actually surprised at first: I expected it to crash and burn, but it didn't. So if I can do it, then anyone can do it too.

Same here; I built a few non-trivial things that passed the first attempt at tooling (valgrind, UBsan with tests, fuzzing, etc) with no UB issues found.