They are free to optimize away access to any result that is not used based on code elimination, and these annotations limit what can be eliminated.
However, basing code elimination on UB, as pointed out earlier, would basically eliminate all code if you were rigorous enough because you basically cannot eliminate UB in C code, which is not in any way useful.
The standard defines it, it's `volatile` I believe. But it does not help with the examples above as far as I understand (removing the log, removing the early return, time-travel...).
? May we remove '*p = 0;', whether we remove the malloc+free or not?
Sure, it does not solve the question when arbitrarily removing NULL pointer checks is OK.
It is true that when the compiler is inlining code or expanding a macro it may have a NULL check that is spurious in environments that do not map page 0 based on the observation that the pointer was dereferenced previously.
And this assumption is incorrect in environments that do map page 0 causing wrong code generation.