1/* mpn_gcd_11 -- limb greatest common divisor.
2
3Copyright 1994, 1996, 2000, 2001, 2009, 2012, 2019 Free Software Foundation,
4Inc.
5
6This file is part of the GNU MP Library.
7
8The GNU MP Library is free software; you can redistribute it and/or modify
9it under the terms of either:
10
11  * the GNU Lesser General Public License as published by the Free
12    Software Foundation; either version 3 of the License, or (at your
13    option) any later version.
14
15or
16
17  * the GNU General Public License as published by the Free Software
18    Foundation; either version 2 of the License, or (at your option) any
19    later version.
20
21or both in parallel, as here.
22
23The GNU MP Library is distributed in the hope that it will be useful, but
24WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
25or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
26for more details.
27
28You should have received copies of the GNU General Public License and the
29GNU Lesser General Public License along with the GNU MP Library.  If not,
30see https://www.gnu.org/licenses/.  */
31
32#include "gmp-impl.h"
33#include "longlong.h"
34
35mp_limb_t
36mpn_gcd_11 (mp_limb_t u, mp_limb_t v)
37{
38  ASSERT (u & v & 1);
39
40  /* In this loop, we represent the odd numbers ulimb and vlimb
41     without the redundant least significant one bit. This reduction
42     in size by one bit ensures that the high bit of t, below, is set
43     if and only if vlimb > ulimb. */
44
45  u >>= 1;
46  v >>= 1;
47
48  while (u != v)
49    {
50      mp_limb_t t;
51      mp_limb_t vgtu;
52      int c;
53
54      t = u - v;
55      vgtu = LIMB_HIGHBIT_TO_MASK (t);
56
57      /* v <-- min (u, v) */
58      v += (vgtu & t);
59
60      /* u <-- |u - v| */
61      u = (t ^ vgtu) - vgtu;
62
63      count_trailing_zeros (c, t);
64      /* We have c <= GMP_LIMB_BITS - 2 here, so that
65
66	   ulimb >>= (c + 1);
67
68	 would be safe. But unlike the addition c + 1, a separate
69	 shift by 1 is independent of c, and can be executed in
70	 parallel with count_trailing_zeros. */
71      u = (u >> 1) >> c;
72    }
73  return (u << 1) + 1;
74}
75