Comment by AnimalMuppet
5 hours ago
Which is, if I understand correctly, the entire point of volatile. Don't use it if you don't want that behavior.
And in fact, in the example given, if there is something (another thread or whatever) that can change the value of x, then you don't know what either number will be. Well, in that circumstance, without volatile, it may print the same number both times, but you still don't know what the number will be (unless the read gets optimized away entirely).
If that behavior is the entire point, then I think the bigger point is that the spec should reflect that and not call it undefined.
I suspect that many undefined behaviors reflect the inability of the standard committee to come to a consensus on the nuances involved. “Punt to the implementers” is a way to allow every tool vendor to select their own expected behavior in those cases.
You seem to be operating under the assumption "undefined behavior" means "the compiler authors can decide what to do." That's not what it means. It means "any program that causes this behavior to be triggered is not a valid C program, the programmer knows this and did not submit an invalid program, and the programmer explicitly prevented this from happening elsewhere in ways automated analysis cannot detect. Proceed with compilation knowing this branch is impossible."
The spelling for compiler authors getting to choose a behavior is "implementation defined", as the other comment mentions.
Then it should be "implementation defined" rather than "undefined".