1 /* mpn_gcd_1 -- mpn and 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 /* Does not work for U == 0 or V == 0. It would be tough to make it work for
35 V == 0 since gcd(x,0) = x, and U does not generally fit in an mp_limb_t.
36
37 The threshold for doing u%v when size==1 will vary by CPU according to
38 the speed of a division and the code generated for the main loop. Any
39 tuning for this is left to a CPU specific implementation. */
40
41 mp_limb_t
mpn_gcd_1(mp_srcptr up,mp_size_t size,mp_limb_t vlimb)42 mpn_gcd_1 (mp_srcptr up, mp_size_t size, mp_limb_t vlimb)
43 {
44 mp_limb_t ulimb;
45 unsigned long zero_bits, u_low_zero_bits;
46 int c;
47
48 ASSERT (size >= 1);
49 ASSERT (vlimb != 0);
50 ASSERT_MPN_NONZERO_P (up, size);
51
52 ulimb = up[0];
53
54 /* Need vlimb odd for modexact, want it odd to get common zeros. */
55 count_trailing_zeros (zero_bits, vlimb);
56 vlimb >>= zero_bits;
57
58 if (size > 1)
59 {
60 /* Must get common zeros before the mod reduction. If ulimb==0 then
61 vlimb already gives the common zeros. */
62 if (ulimb != 0)
63 {
64 count_trailing_zeros (u_low_zero_bits, ulimb);
65 zero_bits = MIN (zero_bits, u_low_zero_bits);
66 }
67
68 ulimb = MPN_MOD_OR_MODEXACT_1_ODD (up, size, vlimb);
69 if (ulimb == 0)
70 goto done;
71
72 count_trailing_zeros (c, ulimb);
73 ulimb >>= c;
74 }
75 else
76 {
77 /* size==1, so up[0]!=0 */
78 count_trailing_zeros (u_low_zero_bits, ulimb);
79 ulimb >>= u_low_zero_bits;
80 zero_bits = MIN (zero_bits, u_low_zero_bits);
81
82 /* make u bigger */
83 if (vlimb > ulimb)
84 MP_LIMB_T_SWAP (ulimb, vlimb);
85
86 /* if u is much bigger than v, reduce using a division rather than
87 chipping away at it bit-by-bit */
88 if ((ulimb >> 16) > vlimb)
89 {
90 ulimb %= vlimb;
91 if (ulimb == 0)
92 goto done;
93
94 count_trailing_zeros (c, ulimb);
95 ulimb >>= c;
96 }
97 }
98
99 vlimb = mpn_gcd_11 (ulimb, vlimb);
100
101 done:
102 return vlimb << zero_bits;
103 }
104