1 /* mpn_gcd_11 -- limb greatest common divisor.
2
3 Copyright 1994, 1996, 2000, 2001, 2009, 2012, 2019 Free Software Foundation,
4 Inc.
5
6 This file is part of the GNU MP Library.
7
8 The GNU MP Library is free software; you can redistribute it and/or modify
9 it 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
15 or
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
21 or both in parallel, as here.
22
23 The GNU MP Library is distributed in the hope that it will be useful, but
24 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
25 or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
26 for more details.
27
28 You should have received copies of the GNU General Public License and the
29 GNU Lesser General Public License along with the GNU MP Library. If not,
30 see https://www.gnu.org/licenses/. */
31
32 #include "gmp-impl.h"
33 #include "longlong.h"
34
35 mp_limb_t
mpn_gcd_11(mp_limb_t u,mp_limb_t v)36 mpn_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