Is it possible to catch overflow in long long multiply ?

[Bradley Lucier]

This is the wrong list to ask such a question, but I'll answer it  
anyway since the answer might be of general interest.

There is a wonderful book "Hacker's Delight" by Henry S. Warren Jr.,

http://www.awprofessional.com/bookstore/product.asp? 
isbn=0201914654&redir=1&rl=1

In some ways it can be thought of as building on the HAKMEM memos of  
MIT; it has a Forward by Guy Steele.  The book has a lot of fast  
bit-twiddling (unbelievably, "twiddling" just passed my Mac's  
spell-checker) algorithms for operating at the machine word level and  
gives the answer to your question in Chapter 1.

Assuming that overflow of signed integer arithmetic wraps (and what gcc  
flag do I have to set to assume this?) then here is the algorithm to  
multiply x and y with overflow detection.

if y = 0 then
    result = 0
else if y = -1 then
   if x = the_minimum_negative_value then
       result = overflow
   else
       result = -x
else
   let z = x * y;
      if z / y = x then
         result = z
      else
         result = overflow

Brad