What are the benefits to passing integral types by const ref

Question

The question: Is there benefit to passing an integral type by const reference as opposed to simply by value.

ie.

void foo(const int& n); // case #1
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Answer 1

Passing a built-in int type by const ref will actually be a minor de-optimization (generally). At least for a non-inline function. The compiler may have to actually pass a pointer that has to be de-referenced to get the value. You might think it could always optimize this away, but aliasing rules and the need to support separate compilation might force the compiler's hand.

However, for your secondary question:

For very small inline functions, will the compiler have to make a copy of the integer in case #2? By letting the compiler know we won't change the reference can it inline the function call without needless copying of the integer?

The compiler should be able to optimize away the copy or the dereference if semantics allow it, since in that situation the compiler has full knowledge of the state at the call site and the function implementation. It'll likely just load the value into a register have its way with it and just use the register for something else when it's done with the parameter. Of course,all this is very dependent on the actual implementation of the function.