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

[Pip Cet <pipcet@gmail.com>]

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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 <yamaoka@jpl.org> 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 #2: Minor-bignum-tweaks.patch --]
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diff --git a/src/data.c b/src/data.c
index a1215b9d6bf..eb15bc5bdda 100644
--- a/src/data.c
+++ b/src/data.c
@@ -3006,7 +3006,7 @@ arith_driver (enum arithop code, ptrdiff_t nargs, Lisp_Object *args)
 	    {
 	      mpz_t tem;
 	      mpz_init (tem);
-	      mpz_set_uintmax (tem, XUFIXNUM (val));
+	      mpz_set_intmax (tem, XFIXNUM (val));
 	      mpz_and (accum, accum, tem);
 	      mpz_clear (tem);
 	    }
@@ -3018,7 +3018,7 @@ arith_driver (enum arithop code, ptrdiff_t nargs, Lisp_Object *args)
 	    {
 	      mpz_t tem;
 	      mpz_init (tem);
-	      mpz_set_uintmax (tem, XUFIXNUM (val));
+	      mpz_set_intmax (tem, XFIXNUM (val));
 	      mpz_ior (accum, accum, tem);
 	      mpz_clear (tem);
 	    }
@@ -3030,7 +3030,7 @@ arith_driver (enum arithop code, ptrdiff_t nargs, Lisp_Object *args)
 	    {
 	      mpz_t tem;
 	      mpz_init (tem);
-	      mpz_set_uintmax (tem, XUFIXNUM (val));
+	      mpz_set_intmax (tem, XFIXNUM (val));
 	      mpz_xor (accum, accum, tem);
 	      mpz_clear (tem);
 	    }
@@ -3383,8 +3383,6 @@ ash_lsh_impl (Lisp_Object value, Lisp_Object count, bool lsh)
       mpz_init (result);
       if (XFIXNUM (count) >= 0)
 	mpz_mul_2exp (result, XBIGNUM (value)->value, XFIXNUM (count));
-      else if (lsh)
-	mpz_tdiv_q_2exp (result, XBIGNUM (value)->value, - XFIXNUM (count));
       else
 	mpz_fdiv_q_2exp (result, XBIGNUM (value)->value, - XFIXNUM (count));
       val = make_number (result);
@@ -3401,14 +3399,7 @@ ash_lsh_impl (Lisp_Object value, Lisp_Object count, bool lsh)
 
       if (XFIXNUM (count) >= 0)
 	mpz_mul_2exp (result, result, XFIXNUM (count));
-      else if (lsh)
-	{
-	  if (mpz_sgn (result) > 0)
-	    mpz_fdiv_q_2exp (result, result, - XFIXNUM (count));
-	  else
-	    mpz_fdiv_q_2exp (result, result, - XFIXNUM (count));
-	}
-      else /* ash */
+      else
 	mpz_fdiv_q_2exp (result, result, - XFIXNUM (count));
 
       val = make_number (result);
diff --git a/src/fns.c b/src/fns.c
index f6e68036413..5f4b455b503 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -56,15 +56,15 @@ DEFUN ("identity", Fidentity, Sidentity, 1, 1, 0,
 }
 
 DEFUN ("random", Frandom, Srandom, 0, 1, 0,
-       doc: /* Return a pseudo-random number.
-All integers representable in Lisp, i.e. between `most-negative-fixnum'
+       doc: /* Return a pseudo-random fixnum.
+All integers representable as fixnums, i.e. between `most-negative-fixnum'
 and `most-positive-fixnum', inclusive, are equally likely.
 
-With positive integer LIMIT, return random number in interval [0,LIMIT).
-With argument t, set the random number seed from the system's entropy
-pool if available, otherwise from less-random volatile data such as the time.
-With a string argument, set the seed based on the string's contents.
-Other values of LIMIT are ignored.
+With positive fixnum integer LIMIT, return random number in interval
+[0,LIMIT).  With argument t, set the random number seed from the
+system's entropy pool if available, otherwise from less-random
+volatile data such as the time.  With a string argument, set the seed
+based on the string's contents.  Other values of LIMIT are ignored.
 
 See Info node `(elisp)Random Numbers' for more details.  */)
   (Lisp_Object limit)