https://gcc.gnu.org/gcc-5/changes.html

A new set of built-in functions for arithmetics with overflow checking has been added: __builtin_add_overflow, __builtin_sub_overflow and __builtin_mul_overflow and for compatibility with clang also other variants. These builtins have two integral arguments (which don't need to have the same type), the arguments are extended to infinite precision signed type, +, - or * is performed on those, and the result is stored in an integer variable pointed to by the last argument. If the stored value is equal to the infinite precision result, the built-in functions return false, otherwise true. The type of the integer variable that will hold the result can be different from the types of the first two arguments.


void *
calloc (size_t x, size_t y)
{
  size_t sz;
  if (__builtin_mul_overflow (x, y, &sz))
    return NULL;
  void *ret = malloc (sz);
  if (ret) memset (res, 0, sz);
  return ret;
}

On e.g. i?86 or x86-64 the above will result in a mul instruction followed by a jump on overflow.


Clang had these for quite some time. This might be useful to implement the core.checkedint for GDC.