unsigned long f(unsigned int n) {
    unsigned long total = 0;
    for (unsigned int i = 0; i < n; ++i)
        total += i;
    return total;
}

It may not be terribly useful to report this, as I can (and arguably should) write n(n-1)/2 manually, but clang does successfully transform this into:

        lea     eax, [rdi - 1]
        add     edi, -2
        imul    rdi, rax
        shr     rdi
        add     rdi, rax

whereas GCC tries instead to vectorize and process the loop as written.  Obviously, O(1) is preferable to O(n) here.

This applies to both C and C++.