John Fine wrote: > Andrew Haley wrote: >> Chris Thiel wrote >>> Is there a reason that the equality and inequality expressions (and only >>> those expressions) are simplified? I understand that signed integer >>> overflow is undefined. >> I'm more than a little mystified by this question. Unless the target is >> using something truly weird like saturating arithmetic we can always >> guarantee that a+1 != a, but we cannot guarantee that a+1 > a. > The optimizer is always allowed to know the behaviors that result in > THIS compiler from undefined behavior. If the optimizer can predict the > run time answer with complete certainty, it can use that answer. Yes. > You could create a compiler with saturating arithmetic, so the most > positive integer plus one is still the most positive integer. Then the > optimizer of THAT compiler could safely assume a+1 >= a but couldn't > assume a+1>a nor a+1!=a. Yes. > But for THIS compiler the most positive integer plus one is the most > negative integer. The compiler knows that even though the standard > doesn't know it and a well written program shouldn't know it. So THIS > compiler can assume a+1!=a. Which it is doing. What, then, is your point? Andrew.
