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.