Are local variables necessary?

Question

My understanding is the C calling convention places arguments on the stack prior to calling a function. These arguments could be accessed within the function through explicit stack parameters using EBP as a reference such as [EBP + 8] or [EBP + 12].

My question is if it could be accessed this way, why are local variables necessary? - couldn't a function just work with the arguments directly? Is it just for cases where the function has no parameters, but still initializes local variables for internal use?

Answer 1

Quite often, functions need to remember more data than just the parameters. For example, consider this function counting the number of set bits in its argument:

int popcount(int k)
{
    int count = 0;

    while (k != 0) {
        k &= k - 1;
        count++;
    }

    return (count);
}

In addition to the parameter, we also need to remember how many bits we saw. Thus, a local variable is required.