Wouldn't the compiler be allowed to elide the memory accesses that are being used to poke the memory region? If you are doing the poking before the memory has been initialized, then I'm not sure there are any guarantees that the compiler will read from the actual memory region (since it's uninitialized). And if you do writes to do the poking then there might be similar issues if the result of the write is never used. Even for reads you would have to make sure that the result gets used. Also not sure if casting to volatile is allowed if the object itself is not volatile. This is all true regardless of UB.

It seems to me like it is impossible to do poking in a way that could not be elided by the compiler, because removing poking never alters the semantics of the program (stack overflow or guard pages are not part of the semantics of C, I believe).

Of course, in practice you'll probably get the right behavior, but I'm personally always interested in language lawyery questions. I do believe that there are cases of security issues introduced by compilers eliding memset operations on password buffers, because the compiler can tell that the result of memset is never used and so it doesn't actually clear the memory. These sorts of things are tricky to get right.