restrict qualifier on member functions (restrict this pointer)

Question

Note: To clarify, the question is not about the use of the restrict keyword in general, but specifically about applying it to member functions

Answer 1

I belive what you guys are missing is that an argument to a member function could also alias parts or an object. Here's an example

struct some_class {
    int some_value;

    void compute_something(int& result) {
        result = 2*some_value;
        ...
        result -= some_value;
    }
}

One would probably expect that to compile to

*(this + offsetof(some_value)) -> register1
2*register1 -> register2
...
register2 - register1 -> result

That code, unfortunately, would be wrong if someone passes a reference to some_value for result. Thus, a compiler would actually need to generate to following

*(this + offsetof(some_value)) -> register1
2*register1 -> register2
register2 -> result

...
*(this + offsetof(some_value)) -> register1
result -> register2
register2 - register1 -> register2
register2 -> result

which is obviously way less efficient. Note that unless compute_something is inlines, the compiler has no way of knowing whether result may alias some_value or not, so it has to assume the worst case, no matter no smart or dumb it is. So there a definite and very real advantage to restrict, even if applied to the this pointer.