I think a good middle-ground would be to have more narrowly scoped behavior. Not quite implementation-defined (as I believe that requires the implementation to

I think a good middle-ground would be to have more narrowly scoped behavior. Not quite implementation-defined (as I believe that requires the implementation to be consistent in its treatment?) but for instance "Integer overflow will result in the integer taking an unspecified (and potentially changeable) valid value for that integer". I'm not sure whether that particular example sufficiently covers the requirements of the optimiser, but defining it as "May cause any of the following results, but not nasal demons" would perhaps be an improvement? DISCLAIMER: I haven't done any C programming in years, and I wasn't particularly good at it, even then.