1 /* mpn_divexact_1 -- mpn by limb exact division.
2
3 THE FUNCTIONS IN THIS FILE ARE FOR INTERNAL USE ONLY. THEY'RE ALMOST
4 CERTAIN TO BE SUBJECT TO INCOMPATIBLE CHANGES OR DISAPPEAR COMPLETELY IN
5 FUTURE GNU MP RELEASES.
6
7 Copyright 2000-2003, 2005, 2013 Free Software Foundation, Inc.
8
9 This file is part of the GNU MP Library.
10
11 The GNU MP Library is free software; you can redistribute it and/or modify
12 it under the terms of either:
13
14 * the GNU Lesser General Public License as published by the Free
15 Software Foundation; either version 3 of the License, or (at your
16 option) any later version.
17
18 or
19
20 * the GNU General Public License as published by the Free Software
21 Foundation; either version 2 of the License, or (at your option) any
22 later version.
23
24 or both in parallel, as here.
25
26 The GNU MP Library is distributed in the hope that it will be useful, but
27 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
28 or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
29 for more details.
30
31 You should have received copies of the GNU General Public License and the
32 GNU Lesser General Public License along with the GNU MP Library. If not,
33 see https://www.gnu.org/licenses/. */
34
35 #include "gmp-impl.h"
36 #include "longlong.h"
37
38
39
40 /* Divide a={src,size} by d=divisor and store the quotient in q={dst,size}.
41 q will only be correct if d divides a exactly.
42
43 A separate loop is used for shift==0 because n<<GMP_LIMB_BITS doesn't
44 give zero on all CPUs (for instance it doesn't on the x86s). This
45 separate loop might run faster too, helping odd divisors.
46
47 Possibilities:
48
49 mpn_divexact_1c could be created, accepting and returning c. This would
50 let a long calculation be done piece by piece. Currently there's no
51 particular need for that, and not returning c means that a final umul can
52 be skipped.
53
54 Another use for returning c would be letting the caller know whether the
55 division was in fact exact. It would work just to return the carry bit
56 "c=(l>s)" and let the caller do a final umul if interested.
57
58 When the divisor is even, the factors of two could be handled with a
59 separate mpn_rshift, instead of shifting on the fly. That might be
60 faster on some CPUs and would mean just the shift==0 style loop would be
61 needed.
62
63 If n<<GMP_LIMB_BITS gives zero on a particular CPU then the separate
64 shift==0 loop is unnecessary, and could be eliminated if there's no great
65 speed difference.
66
67 It's not clear whether "/" is the best way to handle size==1. Alpha gcc
68 2.95 for instance has a poor "/" and might prefer the modular method.
69 Perhaps a tuned parameter should control this.
70
71 If src[size-1] < divisor then dst[size-1] will be zero, and one divide
72 step could be skipped. A test at last step for s<divisor (or ls in the
73 even case) might be a good way to do that. But if this code is often
74 used with small divisors then it might not be worth bothering */
75
76 void
mpn_divexact_1(mp_ptr dst,mp_srcptr src,mp_size_t size,mp_limb_t divisor)77 mpn_divexact_1 (mp_ptr dst, mp_srcptr src, mp_size_t size, mp_limb_t divisor)
78 {
79 mp_size_t i;
80 mp_limb_t c, h, l, ls, s, s_next, inverse, dummy;
81 unsigned shift;
82
83 ASSERT (size >= 1);
84 ASSERT (divisor != 0);
85 ASSERT (MPN_SAME_OR_SEPARATE_P (dst, src, size));
86 ASSERT_MPN (src, size);
87 ASSERT_LIMB (divisor);
88
89 if ((divisor & 1) == 0)
90 {
91 count_trailing_zeros (shift, divisor);
92 divisor >>= shift;
93 }
94 else
95 shift = 0;
96
97 binvert_limb (inverse, divisor);
98 divisor <<= GMP_NAIL_BITS;
99
100 if (shift != 0)
101 {
102 c = 0;
103
104 s = src[0];
105
106 for (i = 1; i < size; i++)
107 {
108 s_next = src[i];
109 ls = ((s >> shift) | (s_next << (GMP_NUMB_BITS-shift))) & GMP_NUMB_MASK;
110 s = s_next;
111
112 SUBC_LIMB (c, l, ls, c);
113
114 l = (l * inverse) & GMP_NUMB_MASK;
115 dst[i - 1] = l;
116
117 umul_ppmm (h, dummy, l, divisor);
118 c += h;
119 }
120
121 ls = s >> shift;
122 l = ls - c;
123 l = (l * inverse) & GMP_NUMB_MASK;
124 dst[size - 1] = l;
125 }
126 else
127 {
128 s = src[0];
129
130 l = (s * inverse) & GMP_NUMB_MASK;
131 dst[0] = l;
132 c = 0;
133
134 for (i = 1; i < size; i++)
135 {
136 umul_ppmm (h, dummy, l, divisor);
137 c += h;
138
139 s = src[i];
140 SUBC_LIMB (c, l, s, c);
141
142 l = (l * inverse) & GMP_NUMB_MASK;
143 dst[i] = l;
144 }
145 }
146 }
147