Annotation of OpenXM_contrib/gmp/mpn/generic/mod_1.c, Revision 1.1.1.1
1.1 maekawa 1: /* mpn_mod_1(dividend_ptr, dividend_size, divisor_limb) --
2: Divide (DIVIDEND_PTR,,DIVIDEND_SIZE) by DIVISOR_LIMB.
3: Return the single-limb remainder.
4: There are no constraints on the value of the divisor.
5:
6: Copyright (C) 1991, 1993, 1994, Free Software Foundation, Inc.
7:
8: This file is part of the GNU MP Library.
9:
10: The GNU MP Library is free software; you can redistribute it and/or modify
11: it under the terms of the GNU Library General Public License as published by
12: the Free Software Foundation; either version 2 of the License, or (at your
13: option) any later version.
14:
15: The GNU MP Library is distributed in the hope that it will be useful, but
16: WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
17: or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Library General Public
18: License for more details.
19:
20: You should have received a copy of the GNU Library General Public License
21: along with the GNU MP Library; see the file COPYING.LIB. If not, write to
22: the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
23: MA 02111-1307, USA. */
24:
25: #include "gmp.h"
26: #include "gmp-impl.h"
27: #include "longlong.h"
28:
29: #ifndef UMUL_TIME
30: #define UMUL_TIME 1
31: #endif
32:
33: #ifndef UDIV_TIME
34: #define UDIV_TIME UMUL_TIME
35: #endif
36:
37: /* FIXME: We should be using invert_limb (or invert_normalized_limb)
38: here (not udiv_qrnnd). */
39:
40: mp_limb_t
41: #if __STDC__
42: mpn_mod_1 (mp_srcptr dividend_ptr, mp_size_t dividend_size,
43: mp_limb_t divisor_limb)
44: #else
45: mpn_mod_1 (dividend_ptr, dividend_size, divisor_limb)
46: mp_srcptr dividend_ptr;
47: mp_size_t dividend_size;
48: mp_limb_t divisor_limb;
49: #endif
50: {
51: mp_size_t i;
52: mp_limb_t n1, n0, r;
53: int dummy;
54:
55: /* Botch: Should this be handled at all? Rely on callers? */
56: if (dividend_size == 0)
57: return 0;
58:
59: /* If multiplication is much faster than division, and the
60: dividend is large, pre-invert the divisor, and use
61: only multiplications in the inner loop. */
62:
63: /* This test should be read:
64: Does it ever help to use udiv_qrnnd_preinv?
65: && Does what we save compensate for the inversion overhead? */
66: if (UDIV_TIME > (2 * UMUL_TIME + 6)
67: && (UDIV_TIME - (2 * UMUL_TIME + 6)) * dividend_size > UDIV_TIME)
68: {
69: int normalization_steps;
70:
71: count_leading_zeros (normalization_steps, divisor_limb);
72: if (normalization_steps != 0)
73: {
74: mp_limb_t divisor_limb_inverted;
75:
76: divisor_limb <<= normalization_steps;
77:
78: /* Compute (2**2N - 2**N * DIVISOR_LIMB) / DIVISOR_LIMB. The
79: result is a (N+1)-bit approximation to 1/DIVISOR_LIMB, with the
80: most significant bit (with weight 2**N) implicit. */
81:
82: /* Special case for DIVISOR_LIMB == 100...000. */
83: if (divisor_limb << 1 == 0)
84: divisor_limb_inverted = ~(mp_limb_t) 0;
85: else
86: udiv_qrnnd (divisor_limb_inverted, dummy,
87: -divisor_limb, 0, divisor_limb);
88:
89: n1 = dividend_ptr[dividend_size - 1];
90: r = n1 >> (BITS_PER_MP_LIMB - normalization_steps);
91:
92: /* Possible optimization:
93: if (r == 0
94: && divisor_limb > ((n1 << normalization_steps)
95: | (dividend_ptr[dividend_size - 2] >> ...)))
96: ...one division less... */
97:
98: for (i = dividend_size - 2; i >= 0; i--)
99: {
100: n0 = dividend_ptr[i];
101: udiv_qrnnd_preinv (dummy, r, r,
102: ((n1 << normalization_steps)
103: | (n0 >> (BITS_PER_MP_LIMB - normalization_steps))),
104: divisor_limb, divisor_limb_inverted);
105: n1 = n0;
106: }
107: udiv_qrnnd_preinv (dummy, r, r,
108: n1 << normalization_steps,
109: divisor_limb, divisor_limb_inverted);
110: return r >> normalization_steps;
111: }
112: else
113: {
114: mp_limb_t divisor_limb_inverted;
115:
116: /* Compute (2**2N - 2**N * DIVISOR_LIMB) / DIVISOR_LIMB. The
117: result is a (N+1)-bit approximation to 1/DIVISOR_LIMB, with the
118: most significant bit (with weight 2**N) implicit. */
119:
120: /* Special case for DIVISOR_LIMB == 100...000. */
121: if (divisor_limb << 1 == 0)
122: divisor_limb_inverted = ~(mp_limb_t) 0;
123: else
124: udiv_qrnnd (divisor_limb_inverted, dummy,
125: -divisor_limb, 0, divisor_limb);
126:
127: i = dividend_size - 1;
128: r = dividend_ptr[i];
129:
130: if (r >= divisor_limb)
131: r = 0;
132: else
133: i--;
134:
135: for (; i >= 0; i--)
136: {
137: n0 = dividend_ptr[i];
138: udiv_qrnnd_preinv (dummy, r, r,
139: n0, divisor_limb, divisor_limb_inverted);
140: }
141: return r;
142: }
143: }
144: else
145: {
146: if (UDIV_NEEDS_NORMALIZATION)
147: {
148: int normalization_steps;
149:
150: count_leading_zeros (normalization_steps, divisor_limb);
151: if (normalization_steps != 0)
152: {
153: divisor_limb <<= normalization_steps;
154:
155: n1 = dividend_ptr[dividend_size - 1];
156: r = n1 >> (BITS_PER_MP_LIMB - normalization_steps);
157:
158: /* Possible optimization:
159: if (r == 0
160: && divisor_limb > ((n1 << normalization_steps)
161: | (dividend_ptr[dividend_size - 2] >> ...)))
162: ...one division less... */
163:
164: for (i = dividend_size - 2; i >= 0; i--)
165: {
166: n0 = dividend_ptr[i];
167: udiv_qrnnd (dummy, r, r,
168: ((n1 << normalization_steps)
169: | (n0 >> (BITS_PER_MP_LIMB - normalization_steps))),
170: divisor_limb);
171: n1 = n0;
172: }
173: udiv_qrnnd (dummy, r, r,
174: n1 << normalization_steps,
175: divisor_limb);
176: return r >> normalization_steps;
177: }
178: }
179: /* No normalization needed, either because udiv_qrnnd doesn't require
180: it, or because DIVISOR_LIMB is already normalized. */
181:
182: i = dividend_size - 1;
183: r = dividend_ptr[i];
184:
185: if (r >= divisor_limb)
186: r = 0;
187: else
188: i--;
189:
190: for (; i >= 0; i--)
191: {
192: n0 = dividend_ptr[i];
193: udiv_qrnnd (dummy, r, r, n0, divisor_limb);
194: }
195: return r;
196: }
197: }
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