bug#32463: 27.0.50; (logior -1) => 4611686018427387903

[Pip Cet <pipcet@gmail.com>]

On Sat, Aug 18, 2018 at 10:58 PM Paul Eggert <eggert@cs.ucla.edu> wrote:
 My bigger concern is memory management, along with integer overflow in size or
> bitcount calculation. Copies are made of bignums when not needed, behavior is
> dicey if memory is exhausted during bignum computation, and I'm afraid C-g will
> have problems when bignums get large.

Even if memory isn't exhausted, creating a bignum larger than 16 GB
(our most-positive-bignum) results in an immediate crash with external
libgmp (Linux, x86_64), and that appears not to be easy to fix without
modifying gmp.

> I don't have a good handle on this stuff
> yet. I have put in some sanity checks (e.g., see check_bignum_size in emacs.c)
> but I suspect more are needed. In particular, Fexpt will need to be careful as
> it is a good way to explode a bignum's size.

That and left shifts are probably the ones to worry about for now.
Creating a large bignum by repeated multiplication will require at
least some intermediate bignums, which need to be allocated and copied
and thus probably alert the user to something going on.