Re: Using the GNU GMP Library for Bignums in Emacs

[Helmut Eller]

On Sun, Apr 22 2018, Paul Eggert wrote:

> Most integer
> computation will likely be fixnum only, and the only slowdown there
> will be integer overflow checking that we currently aren't doing. With
> decent hardware and compiler, I'd guess this would cost us three
> machine instructions per Lisp arithmetic operation, including the
> conditional branch that is typically not taken. Hardly anybody will
> notice.

Out of curiousity: what's the quickest way to detect overflow on
multiplication in ANSI C?  E.g.

  int64_t x, y, result;
  result = x * y;
  if (<how to detect overflow?>)

Helmut