The problem is that, if we assume that integers don’t have provenance, some far distant part of the code could guess the integer and do an int2ptr. If you can

The problem is that, if we assume that integers don’t have provenance, some far distant part of the code could guess the integer and do an int2ptr. If you can prove that nothing in the entire program could possibly do this for the entire lifetime of the original object, then sure, you could remove the ptr2int. But compiler optimizations usually work one function at a time. In some cases it might be feasible to prove this anyway, like if (a) you have a function that doesn’t call any other functions and (b) the object in question is a local variable that will go out of scope at the end of the function, making any further accesses UB regardless. But in most cases it’s not feasible.