When compiler decides something is UB aka "result of this code is not defined and could be any" it selects the most performant version of undefined behavior - doing nothing by optimizing code away.
The compiler is not free to remove accesses to something marked volatile - its defined as a side-effect.
Volatile means something else may be acting here. Something else may install anything into the register at any time - and every time you access.
The compiler is required to preserve the order of accesses. In almost every C compiler, today, there are almost no optimisations the moment a volatile is introduced, for this reason.
If code has undefined behavior, the entire execution path that leads to that UB has no assigned semantics in the C model. So there are no volatile accesses in this code according to the C abstract machine - the entire execution path is UB, so it can be assumed it doesn't happen at all.
> An object that has volatile-qualified type may be modified in ways unknown to the implementation or have other unknown side effects. Therefore any expression referring to such an object shall be evaluated strictly according to the rules of the abstract machine
The execution path has unknown side effects, and so the execution path must be strictly followed. That's uh... The entire point of that section in the C standard. Its why volatile is called out, in the semantic model for the abstract machine.
Otherwise... Why call it out, at all? It must be strictly followed, not lazily, as in other areas of the standard.
UB supersedes volatile, once the compiler hits UB then all bets are off. Compilers can and do optimize out UB branches, which is almost never what you want... yet here we are.
>> The moment you enter a compilation unit (assuming no link optimizations) with a state which at some point will run into undefined behavior all bets are of. [...] Yes, UB can "time travel"
> Close, but not quite. This is a common misconception in the reverse direction.
> Abstractly, what UB can do is performing the inverse of the preceding instructions, effectively making the abstract machine run in reverse. However, this is only equivalent to "time-traveling" until you get to the point of the last side effect (where "side effect" here refers to predefined operations in the standard that interact with the external world, such as I/O and volatile accesses), because only everything since that point can be optimized away under the as-if rule without altering the externally visible effects of the program.
> As a concrete, practical example, this means the following: if you do fflush(stdout); return INT_MAX + 1; the compiler cannot omit the fflush() call merely because the subsequent statement had undefined behavior. That is, the UB cannot time-travel to before the flush. What the program can do is to write garbage to the file afterward, or attempt to overwrite what you wrote in the file to revert it to its previous state, but the fflush() must still occur before anything wild happens. If nobody observes the in-between state, then the end result can look like time-travel, but if the system blocks on fflush() and the user terminates the program while it's blocked, there is no opportunity for UB.
The print example has no defined order of accesses, function parameters can be evaluated in any order. But further, the entire problem with UB is that it supercedes the regular guarantees that you get (like with volatile) when it's encountered. Yes gcc and clang do the obvious thing that makes the most sense in this example, but what people are trying to tell you is that they could just not do that and they would still be complying with the standard. For example, you can imagine a more serious example of UB that causes the program to fail to compile completely, and then do you emit the correct number of in order reads of volatile variables? Obviously not.
Function parameters cannot be evaluated in any order, when one of them is a volatile.
> The initialization shall occur in initializer list order, each initializer provided for a particular subobject overriding any previously listed initializer for the same subobject
And what I am trying to tell people, is the standard has expectations around the volatile keyword, that the compilers took into account when designing how they would work - it isn't just kindness, its compliance. But no one is actually talking about the quotes from the standard, and just quoting themselves and their own understandings.
That quote doesn't have anything to do with parameter evaluation order. There is no order for function parameter evaluation.
And no, there is no exception for undefined behavior. There can't be, otherwise the behavior would be... defined. It's in the name. Again, what do you think the compiler emits when the undefined behavior causes the program to not compile altogether?
Volatile means something else may be acting here. Something else may install anything into the register at any time - and every time you access.
The compiler is required to preserve the order of accesses. In almost every C compiler, today, there are almost no optimisations the moment a volatile is introduced, for this reason.