Comment by ynik
2 months ago
Under your interpretation, neither gcc nor clang are POSIX compliant. Because in practice all these optimizing compilers will reorder memory accesses without bothering to prove that the pointers involved are valid -- the compiler just assumes that the pointers are valid, which is justified because otherwise the program would have undefined behavior.
I am not so sure. Assuming the program does not do anything undefined is sort of the worst possible take on leaving the behavior undefined in the first place. I mean the behavior was left undefined so that "something" could be done. the language standard just does not know what that "something" is. Hell the compiler could do nothing and that would make more sense.
But to make optimizations pretending it is an invariant, it can't happen, when the specification clearly says it could happen. That's wild, and I would argue out of specification.
Undefined Behavior has a very specific meaning in the C and (respectively) C++ standards. And it is this.
I think you may be misunderstand how optimization works. It's not "AHA! Undefined Behavior, I'll poke at that!".
It's more that compilers are allowed to assume that some things just "don't happen"... and can optimize based on that. And it does make sense... if you were told to optimize a thing given certain rules, how would you do it?
Actually you are right, what I said about reordering is nonsense. The compiler will definitely reorder non-aliasing accesses. There are much weaker properties that are preserved.