Long long usage question

[Claus Fischer]

I need to write a function that takes four signed 32 bit values,
builds two products, and compares the size, thusly:

int prodcomp(long a, long b, long c, long d)
{
    long long p1 = (long long) a * (long long) b;
    long long p2 = (long long) c * (long long) d;
    return p1 < p2;
}


I'm assuming that most 32 bit CPU's have an instruction for
multiplying two 32 bit values.

Questions:

1. Does the compiler know that the cast results have their highest
   32 bits zeroed? Does it pick a fast instruction for that case?

2. If the answer is 'no', does it help to use

    long long p1 = (long long) a * b;

3. Is there a way of telling the compiler what I really want:
    long long p1 = a * b;
   but the multiplication performed in the long long realm?

4. Is there documentation covering how to best do such arithmetic
   in GNU C on glibc? I'm speaking of 32 bit arithmetics with
   occasional products and divisions leading in the 64 bit domain.


Thanks in advance for your kind answers

Claus

-- 
Claus Fischer <claus.fischer@clausfischer.com>
http://www.clausfischer.com/