Clang will if `i` is not referenced anywhere else. e.g.

    int i,n;
    for (i = 0; i < length; i++)
        n = something();
    return n;

It's also free to treat `i` as a different variable:

    int i,n = 0;
    for (i = 0; i < length; i++)
        something(n++); /* Compiler will merge the n and i variables */
    return n;

Or eliminate the loop completely if there are no side effects.

    int i, n = 0;
    for (i = 0; i < length; i++)
        n = i + 1; /* A very roundabout way to say "n = length" */
    return n;