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Not only that, the code was wrong. The specification is quite clear that correct programs don't cause UB to be executed at run time. If your wrong code now prod
by chowells 15d ago
Not only that, the code was wrong. The specification is quite clear that correct programs don't cause UB to be executed at run time. If your wrong code now produces wrong results, that's because it's wrong. That your compiler allowed you to get away with it for decades is a compiler bug, not a feature.
Do I fully believe all of the above? Not exactly. But compiler authors do. Does it make a really good argument to never use C or C++? Yes. If only we had 50 years of optimization work in any language with better semantics.
- saghm 14d agoYeah, my slightly more verbose take is that a language that requires you to not ever make any mistakes in order to have a program behave in a predictable way is not a particularly good choice of language if you the ability to pick something else.
- JoshTriplett 14d agoThe mistake was declaring infinite loops to be UB in the first place.
- throwaway786678 14d ago> That your compiler allowed you to get away with it for decades is a compiler bug That UB was added in C++11.
- chowells 14d agoThat sounds like 1.5 decades to me...
- TuxSH 14d agoFWIW while(true) / for(;;) (or any other loop condition that is a true constant-expression) is NOT UB in C, only C++. In any case stuff like __asm__ __volatile__("" ::: "memory") prevent such optimizations in the rare case you do need branch-to-self.
- classified 14d ago> don't cause UB to be executed at run time. The insidious thing about UB is that it doesn't necessarily have to be executed to wreck your program. UB is not primarily about runtime behavior, it's about how the compiler interprets your code. The behavior that is undefined is your compiler's behavior.
- chowells 14d agoYou're right. I didn't enumerate every way UB can make a program go wrong. I'm sure I don't even know every way that can happen. But "UB doesn't happen at run time" is an assumption that lots of "optimizations" are based on.