Avoid libgmp aborts by imposing limits

libgmp calls ‘abort’ when given numbers too big for its
internal data structures.  The numeric limit is large and
platform-dependent; with 64-bit GMP 6.1.2 it is around
2**2**37.  Work around the problem by refusing to call libgmp
functions with arguments that would cause an abort.  With luck
libgmp will have a better way to do this in the future.
Also, introduce a variable integer-width that lets the user
control how large bignums can be.  This currently defaults
to 2**16, i.e., it allows bignums up to 2**2**16.  This
should be enough for ordinary computation, and should
help Emacs to avoid thrashing or hanging.
Problem noted by Pip Cet (Bug#32463#71).
* doc/lispref/numbers.texi, etc/NEWS:
Document recent bignum changes, including this one.
Improve documentation for bitwise operations, in the light
of bignums.
* src/alloc.c (make_number): Enforce integer-width.
(integer_overflow): New function.
(xrealloc_for_gmp, xfree_for_gmp):
Move here from emacs.c, as it's memory allocation.
(init_alloc): Initialize GMP here, rather than in emacs.c.
(integer_width): New var.
* src/data.c (GMP_NLIMBS_MAX, NLIMBS_LIMIT): New constants.
(emacs_mpz_size, emacs_mpz_mul)
(emacs_mpz_mul_2exp, emacs_mpz_pow_ui): New functions.
(arith_driver, Fash, expt_integer): Use them.
(expt_integer): New function, containing integer code
that was out of place in floatfns.c.
(check_bignum_size, xmalloc_for_gmp): Remove.
* src/emacs.c (main): Do not initialize GMP here.
* src/floatfns.c (Fexpt): Use expt_integer, which
now contains integer code moved from here.
* src/lisp.h (GMP_NUMB_BITS): Define if gmp.h doesn’t.

1 files changed, 1 insertions, 23 deletions

diff --git a/src/floatfns.c b/src/floatfns.c
index 7c52a0a9a20..ea9000b90a0 100644
--- a/src/floatfns.c
+++ b/src/floatfns.c
@@ -210,29 +210,7 @@ DEFUN ("expt", Fexpt, Sexpt, 2, 2, 0,
   /* Common Lisp spec: don't promote if both are integers, and if the
      result is not fractional.  */
   if (INTEGERP (arg1) && NATNUMP (arg2))
-    {
-      unsigned long exp;
-      if (TYPE_RANGED_FIXNUMP (unsigned long, arg2))
-        exp = XFIXNUM (arg2);
-      else if (MOST_POSITIVE_FIXNUM < ULONG_MAX && BIGNUMP (arg2)
-               && mpz_fits_ulong_p (XBIGNUM (arg2)->value))
-        exp = mpz_get_ui (XBIGNUM (arg2)->value);
-      else
-        xsignal3 (Qrange_error, build_string ("expt"), arg1, arg2);
-
-      mpz_t val;
-      mpz_init (val);
-      if (FIXNUMP (arg1))
-        {
-          mpz_set_intmax (val, XFIXNUM (arg1));
-          mpz_pow_ui (val, val, exp);
-        }
-      else
-        mpz_pow_ui (val, XBIGNUM (arg1)->value, exp);
-      Lisp_Object res = make_number (val);
-      mpz_clear (val);
-      return res;
-    }
+    return expt_integer (arg1, arg2);
 
   return make_float (pow (XFLOATINT (arg1), XFLOATINT (arg2)));
 }