Re: Using the GNU GMP Library for Bignums in Emacs

[Paul Eggert]

On 05/03/2018 10:49 AM, Eli Zaretskii wrote:
> > (format "%016x" (lognot 257)) => "3ffffffffffffefe"
> >
> > but the 16-hex-digit string should be "fffffffffffffefe".
> But a 16-hex-digit fffffffffffffefe is not a fixnum, right?

Exactly right. If we have bignums, this will be straightforward; it'll 
work like Common Lisp, Python, etc. and it'll be obvious what to do. The 
problems mentioned in this thread mainly come from the fact that Emacs 
does not have bignums now and that some code may depend on fixnum 
overflow wrapping around.

Here's another way to think about it. (format "%016x" (lognot 257)) is 
already nonportable, as it yields "000000003ffffefe" on 32-bit platforms 
and "3ffffffffffffefe" on 64-bit platforms. Once we add bignums, the 
same expression should yield "-000000000000102" on all platforms. Code 
intended to be portable between old and new regimes already needs to 
worry about nonportability in this area, and it shouldn't be hard to 
adjust (if it needs adjusting at all). Code intended to run only under 
the new regime will be able to safely assume the expression yields 
"-000000000000102" on all platforms, regardless of whether the 
underlying platform is 32- or 64- or some other number of bits, and in 
the long run this will reduce the number of portability issues in Elisp 
code.