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

[Andy Moreton]

On Tue 21 Aug 2018, Paul Eggert wrote:

>> Does this limit apply to bignum values in lisp objects, or to
>> intermediate values inside libgmp, which may require extra space ?
>> The documentation for `integer-width' should make this clear.
>
> integer-width applies only to Lisp objects. I'm not sure we should be exposing
> internal details of the interpreter to the Lisp user, which includes the sizes
> of its internal temporaries. If a need shows up for it we can do it, but I
> don't see the need.

The current documentation is uninformative:

  Maximum number of bits in bignums.
  Integers outside the fixnum range are limited to absolute values less
  than 2**N, where N is this variable’s value.  N should be nonnegative.

This says plenty abut fixnums, but does not mention that anything larger
than fixnum range is represented as a bignum. For users who are new to
all of this, it is important to state that.

I think it is worth mentioning that intermediate computations not visible
to lisp may use slightly larger bignums, and to make it clear that this
is a soft limit intended to prevent misbehaviour of the runtime.

    AndyM