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


From: Pip Cet
Subject: bug#32463: 27.0.50; (logior -1) => 4611686018427387903
Date: Fri, 17 Aug 2018 05:59:14 +0000

Can you try the attached patch? It fixes a few things that I reported
to Tom yesterday:

---


None of these are necessarily bugs, but:

* I find the behavior of `lsh', `logand', `logior', and `logxor', when
given negative arguments, surprising. I think it would make most sense
to treat negative numbers as the infinite bitstream consisting of all
ones to the left of the specified value:

(logand -1 -1) would be interpreted as ...1111111 & ...1111111 =
...1111111, so it would be -1 (rather than 2 * most-positive-fixnum +
1).
(lsh (- (lsh -1 64) 1) -1) would be ...1110111...1111 shifted to the
right by one digit, an odd number, rather than the even number
currently produced. (I believe lsh and ash should behave identically.)

* the documentation of `random' still refers to representable integers
* I think we should rename `random' to `random-fixnum' and add a Lisp
function `random' which accepts positive fixnum, bignum, and float
arguments, doing the right thing for each. `cl-random' similarly needs
updating, or documentation of its current 32-bit nature.
* there appears to be a most-positive-bignum; on x86-64, it consists
of 0x7fffffff 8-byte words of one bits, 16 GiB. Operating on integers
larger than that will currently abort emacs with an error message:
"gmp: overflow in mpz type".
* long-running bignum operations appear not to be interruptible.

Please consider something like the attached patch?

Thanks!
On Fri, Aug 17, 2018 at 3:31 AM Katsumi Yamaoka <address@hidden> wrote:
>
> Hi,
>
> What do I have to do to get -1 by `(logior -1)' ?  Otherwise,
> is it just a bug?  Setting `binary-as-unsigned' has no effect.
>
> I'm using an old input method sj3-egg[1] but it got not to work.
> The following Lisp snippet shows what it does first when opening
> the connection to the sj3 server:
>
> (with-temp-buffer
>   (set-buffer-multibyte nil)
>   (let ((pt (point-min)))
>     (insert "\377\377\377\376")
>     (logior
>      (lsh (- (logxor (char-after pt) 128) 128) 24)
>      (lsh (char-after (+ pt 1)) 16)
>      (lsh (char-after (+ pt 2)) 8)
>      (lsh (char-after (+ pt 3)) 0))))
>
> It should return -2, but 4611686018427387902 now.
> (Oh, it's doubled most-positive-fixnum!)
>
> Thanks.
>
> [1] http://www.jpl.org/ftp/pub/elisp/sj3-egg-0.8.5.tar.gz
>
> In GNU Emacs 27.0.50 (build 1, x86_64-unknown-cygwin, GTK+ Version 3.22.28)
>  of 2018-08-17 built on localhost
> Windowing system distributor 'The Cygwin/X Project', version 11.0.12000000
>
>
>

Attachment: Minor-bignum-tweaks.patch
Description: Text Data


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