Does Const-Correctness Enhance Optimization Performance?

Const-Correctness in Optimizations

While const-correctness enhances code readability and reduces errors, its impact on performance is limited contrary to popular belief.

Declaring a pointer-to-const or a reference-of-const alone does not provide the compiler with additional optimization opportunities. The const declaration merely specifies how an identifier should be used within its scope but does not guarantee that the underlying object is immutable.

Example:

<code class="c">int foo(const int *p) {
    int x = *p;
    bar(x);
    x = *p;
    return x;
}</code>

Even with the const declaration, the compiler cannot assume that p remains unaltered by bar() as it could point to a global integer that bar() has access to. If the compiler possesses sufficient knowledge about foo() and bar() and can prove bar() doesn't modify p, such optimization can still be achieved without const.

Similarly, const declarations by itself do not assist in caller function optimizations:

<code class="c">int x = 37;
foo(&x);
printf("%d\n", x);</code>

Foo() remains capable of modifying x, making it impossible for the compiler to optimize with solely const.

The primary benefit of const is error prevention rather than optimization. It restricts how an identifier is utilized within its scope but does not inform the compiler of anything it cannot already ascertain.

Pointer vs. Reference

Pointers and references differ despite having similar in-memory representations. While pointers store the address of a variable, references act as aliases, providing direct access to the underlying object. Additionally, references cannot be NULL while pointers can.

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