Does const-correctness give the compiler more room for optimization?

Question

I know that it improves readability and makes the program less error-prone, but how much does it improve the performance?

And on a side note, what's the major difference between a reference and a const pointer? I would assume they're stored in the memory differently, but how so?

Answer 1

[Edit: OK so this question is more subtle than I thought at first.]

Declaring a pointer-to-const or reference-of-const never helps any compiler to optimize anything. (Although see the Update at the bottom of this answer.)

The const declaration only indicates how an identifier will be used within the scope of its declaration; it does not say that the underlying object can not change.

Example:

int foo(const int *p) {
    int x = *p;
    bar(x);
    x = *p;
    return x;
}

The compiler cannot assume that *p is not modified by the call to bar(), because p could be (e.g.) a pointer to a global int and bar() might modify it.

If the compiler knows enough about the caller of foo() and the contents of bar() that it can prove bar() does not modify *p, then it can also perform that proof without the const declaration.

But this is true in general. Because const only has an effect within the scope of the declaration, the compiler can already see how you are treating the pointer or reference within that scope; it already knows that you are not modifying the underlying object.

So in short, all const does in this context is prevent you from making mistakes. It does not tell the compiler anything it does not already know, and therefore it is irrelevant for optimization.

What about functions that call foo()? Like:

int x = 37;
foo(&x);
printf("%d\n", x);

Can the compiler prove that this prints 37, since foo() takes a const int *?

No. Even though foo() takes a pointer-to-const, it might cast the const-ness away and modify the int. (This is not undefined behavior.) Here again, the compiler cannot make any assumptions in general; and if it knows enough about foo() to make such an optimization, it will know that even without the const.

The only time const might allow optimizations is cases like this:

const int x = 37;
foo(&x);
printf("%d\n", x);

Here, to modify x through any mechanism whatsoever (e.g., by taking a pointer to it and casting away the const) is to invoke Undefined Behavior. So the compiler is free to assume you do not do that, and it can propagate the constant 37 into the printf(). This sort of optimization is legal for any object you declare const. (In practice, a local variable to which you never take a reference will not benefit, because the compiler can already see whether you modify it within its scope.)

To answer your "side note" question, (a) a const pointer is a pointer; and (b) a const pointer can equal NULL. You are correct that the internal representation (i.e. an address) is most likely the same.

[update]

As Christoph points out in the comments, my answer is incomplete because it does not mention restrict.

Section 6.7.3.1 (4) of the C99 standard says:

During each execution of B, let L be any lvalue that has &L based on P. If L is used to access the value of the object X that it designates, and X is also modified (by any means), then the following requirements apply: T shall not be const-qualified. ...

(Here B is a basic block over which P, a restrict-pointer-to-T, is in scope.)

So if a C function foo() is declared like this:

foo(const int * restrict p)

...then the compiler may assume that no modifications to *p occur during the lifetime of p -- i.e., during the execution of foo() -- because otherwise the Behavior would be Undefined.

So in principle, combining restrict with a pointer-to-const could enable both of the optimizations that are dismissed above. Do any compilers actually implement such an optimization, I wonder? (GCC 4.5.2, at least, does not.)

Note that restrict only exists in C, not C++ (not even C++0x), except as a compiler-specific extension.