1 /* mpn_gcd_22 -- double limb greatest common divisor.
2
3 Copyright 1994, 1996, 2000, 2001, 2009, 2012, 2019 Free Software Foundation, Inc.
4
5 This file is part of the GNU MP Library.
6
7 The GNU MP Library is free software; you can redistribute it and/or modify
8 it under the terms of either:
9
10 * the GNU Lesser General Public License as published by the Free
11 Software Foundation; either version 3 of the License, or (at your
12 option) any later version.
13
14 or
15
16 * the GNU General Public License as published by the Free Software
17 Foundation; either version 2 of the License, or (at your option) any
18 later version.
19
20 or both in parallel, as here.
21
22 The GNU MP Library is distributed in the hope that it will be useful, but
23 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
24 or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
25 for more details.
26
27 You should have received copies of the GNU General Public License and the
28 GNU Lesser General Public License along with the GNU MP Library. If not,
29 see https://www.gnu.org/licenses/. */
30
31 #include "gmp-impl.h"
32 #include "longlong.h"
33
34 #if GMP_NAIL_BITS > 0
35 #error Nails not supported.
36 #endif
37
38 mp_double_limb_t
mpn_gcd_22(mp_limb_t u1,mp_limb_t u0,mp_limb_t v1,mp_limb_t v0)39 mpn_gcd_22 (mp_limb_t u1, mp_limb_t u0, mp_limb_t v1, mp_limb_t v0)
40 {
41 mp_double_limb_t g;
42 ASSERT (u0 & v0 & 1);
43
44 /* Implicit least significant bit */
45 u0 = (u0 >> 1) | (u1 << (GMP_LIMB_BITS - 1));
46 u1 >>= 1;
47
48 v0 = (v0 >> 1) | (v1 << (GMP_LIMB_BITS - 1));
49 v1 >>= 1;
50
51 while (u1 || v1) /* u1 == 0 can happen at most twice per call */
52 {
53 mp_limb_t vgtu, t1, t0;
54 sub_ddmmss (t1, t0, u1, u0, v1, v0);
55 vgtu = LIMB_HIGHBIT_TO_MASK(t1);
56
57 if (UNLIKELY (t0 == 0))
58 {
59 int c;
60 if (t1 == 0)
61 {
62 g.d1 = (u1 << 1) | (u0 >> (GMP_LIMB_BITS - 1));
63 g.d0 = (u0 << 1) | 1;
64 return g;
65 }
66 count_trailing_zeros (c, t1);
67
68 /* v1 = min (u1, v1) */
69 v1 += (vgtu & t1);
70 /* u0 = |u1 - v1| */
71 u0 = (t1 ^ vgtu) - vgtu;
72 ASSERT (c < GMP_LIMB_BITS - 1);
73 u0 >>= c + 1;
74 u1 = 0;
75 }
76 else
77 {
78 int c;
79 count_trailing_zeros (c, t0);
80 c++;
81 /* V <-- min (U, V).
82
83 Assembly version should use cmov. Another alternative,
84 avoiding carry propagation, would be
85
86 v0 += vgtu & t0; v1 += vtgu & (u1 - v1);
87 */
88 add_ssaaaa (v1, v0, v1, v0, vgtu & t1, vgtu & t0);
89 /* U <-- |U - V|
90 No carry handling needed in this conditional negation,
91 since t0 != 0. */
92 u0 = (t0 ^ vgtu) - vgtu;
93 u1 = t1 ^ vgtu;
94 if (UNLIKELY (c == GMP_LIMB_BITS))
95 {
96 u0 = u1;
97 u1 = 0;
98 }
99 else
100 {
101 u0 = (u0 >> c) | (u1 << (GMP_LIMB_BITS - c));
102 u1 >>= c;
103 }
104 }
105 }
106 while ((v0 | u0) & GMP_LIMB_HIGHBIT)
107 { /* At most two iterations */
108 mp_limb_t vgtu, t0;
109 int c;
110 sub_ddmmss (vgtu, t0, 0, u0, 0, v0);
111 if (UNLIKELY (t0 == 0))
112 {
113 g.d1 = u0 >> (GMP_LIMB_BITS - 1);
114 g.d0 = (u0 << 1) | 1;
115 return g;
116 }
117
118 /* v <-- min (u, v) */
119 v0 += (vgtu & t0);
120
121 /* u <-- |u - v| */
122 u0 = (t0 ^ vgtu) - vgtu;
123
124 count_trailing_zeros (c, t0);
125 u0 = (u0 >> 1) >> c;
126 }
127
128 g.d0 = mpn_gcd_11 ((u0 << 1) + 1, (v0 << 1) + 1);
129 g.d1 = 0;
130 return g;
131 }
132