[Python-Dev] Optionally using GMP to implement long if available

[Victor Stinner]

About 31, 32, 63 or 64 bits: I guess that you want to avoid integer overflow. 
Intel has an "overflow" flag, changed for all instructions. For other CPUs, 
you can use emulate this flag. Example with the type int that I used in my 
GMP patch:

Add:
  int a, b, sum;
  sum = a + b;
  exact = ((a < 0) ^ (b < 0)) || ((a < 0) == (sum < 0));

Substract:
  int a, b, diff;
  diff = a + b;
  exact = ((a < 0) == (b < 0)) || ((a < 0) == (diff < 0));

Multiply:
  int a, b, product;
  if (a == 0 || b == 0) {
     product = 0;  /* exact */
  } else if (a != INT_MIN || (b == 1)) {
     product = a * b;
     exact = (product / b) == a);
  } else {
     /* INT_MIN * -1 = -INT_MIN: overflow */
  }

Divide:
  int a, b, q;
  if (a != INT_MIN || b != -1) {
     q = a / b;   /* exact */
  } else {
     /* INT_MIN / -1 = -INT_MIN: overflow */
  }

Checking overflow may costs more than using a smaller base. Only a benchmark 
can answer ;-)

-- 
Victor Stinner aka haypo
http://www.haypocalc.com/blog/