Re: [RFC] Implementing detection of saturation and rounding arithmetic

[Liu Hao via Gcc]

在 5/12/21 5:13 PM, Tamar Christina via Gcc 写道:

int f (int a, int b)
{
     int res;
     if (__builtin_add_overflow (a, b, &res))
       {
           if (res >= 0)
             return INT_MAX;
           else
             return INT_MIN;
       }
     return res;
}

Should be recognized as saturating as well.  But yeah, following the same 
approach we
would rewrite the sequence to something like res = .ADD_SAT (a, b);

Is this a correct saturating addition implementation?




This should be

  int f (int a, int b)
  {
    int res;
    if (__builtin_add_overflow (a, b, &res))
      {
        if (a >= 0)      /* changed from `res` to `a`  */
          return INT_MAX;
        else
          return INT_MIN;
      }
    return res;
  }

which can be optimized further as

  int f (int a, int b)
  {
    int res;
    if (__builtin_add_overflow (a, b, &res))
      res = (a >> sizeof(int) * CHAR_BIT - 1) ^ INT_MAX;
    return res;
  }


--
Best regards,
Liu Hao

If the addition has overflowed, you get a positive result or zero for the sum of two negative numbers (or a negative one for two positive numbers); and it is not straightforward to write it this way.

OpenPGP_signature
Description: OpenPGP digital signature