In C++, can I declare a reference so as to indicate that nothing will ever modify it?

Question

If I do

typedef void Cb();

int foo(int const& a, Cb cb) {
  int x = a;
  cb();
  return x - a;
}

and compile with g++ -O3 -save-temps -c foo.cpp, I see that the subtraction is preserved, whereas if cb(); is commented out, the entire function optimizes to

xorl    %eax, %eax

Is there something I can do to the specification of the parameter a so that the subtraction will be optimized out regardless of the call to cb(), and without forcing a to be a unique reference (ie, that it may be referred to elsewhere, but that via none of those references will it be modified)?

Answer 1

There's the __restrict extension, you can try this on gcc.godbolt.org:

typedef void Cb();

int foo(const int & __restrict a, Cb cb) {
  int x = a;
  cb();
  return x - a;
}

Curiously, only clang does the optimization, gcc doesn't do it.

---

Notice that restrict-like aliasing is being considered to be part of the C++ standard:

• N3988: Towards restrict-like aliasing semantics—Finkel, Tong, Carruth, Nelson, Vandevoorde, Wong

Maybe in the future you can do it by the standard.

Answer 2

Doing the suggested optimization would be incorrect because the rest of the code might be:

static int var;

void func()
{
    var++;
}

// ...
foo(var, func);

I'm not aware of any compiler-specific attribute you can set to say that cb() will not modify a.