Annotation of OpenXM_contrib/gmp/mpn/generic/divmod_1.c, Revision 1.1.1.1
1.1 maekawa 1: /* mpn_divmod_1(quot_ptr, dividend_ptr, dividend_size, divisor_limb) --
2: Divide (DIVIDEND_PTR,,DIVIDEND_SIZE) by DIVISOR_LIMB.
3: Write DIVIDEND_SIZE limbs of quotient at QUOT_PTR.
4: Return the single-limb remainder.
5: There are no constraints on the value of the divisor.
6:
7: QUOT_PTR and DIVIDEND_PTR might point to the same limb.
8:
9: Copyright (C) 1991, 1993, 1994, 1996 Free Software Foundation, Inc.
10:
11: This file is part of the GNU MP Library.
12:
13: The GNU MP Library is free software; you can redistribute it and/or modify
14: it under the terms of the GNU Library General Public License as published by
15: the Free Software Foundation; either version 2 of the License, or (at your
16: option) any later version.
17:
18: The GNU MP Library is distributed in the hope that it will be useful, but
19: WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
20: or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Library General Public
21: License for more details.
22:
23: You should have received a copy of the GNU Library General Public License
24: along with the GNU MP Library; see the file COPYING.LIB. If not, write to
25: the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
26: MA 02111-1307, USA. */
27:
28: #include "gmp.h"
29: #include "gmp-impl.h"
30: #include "longlong.h"
31:
32: #ifndef UMUL_TIME
33: #define UMUL_TIME 1
34: #endif
35:
36: #ifndef UDIV_TIME
37: #define UDIV_TIME UMUL_TIME
38: #endif
39:
40: /* FIXME: We should be using invert_limb (or invert_normalized_limb)
41: here (not udiv_qrnnd). */
42:
43: mp_limb_t
44: #if __STDC__
45: mpn_divmod_1 (mp_ptr quot_ptr,
46: mp_srcptr dividend_ptr, mp_size_t dividend_size,
47: mp_limb_t divisor_limb)
48: #else
49: mpn_divmod_1 (quot_ptr, dividend_ptr, dividend_size, divisor_limb)
50: mp_ptr quot_ptr;
51: mp_srcptr dividend_ptr;
52: mp_size_t dividend_size;
53: mp_limb_t divisor_limb;
54: #endif
55: {
56: mp_size_t i;
57: mp_limb_t n1, n0, r;
58: int dummy;
59:
60: /* ??? Should this be handled at all? Rely on callers? */
61: if (dividend_size == 0)
62: return 0;
63:
64: /* If multiplication is much faster than division, and the
65: dividend is large, pre-invert the divisor, and use
66: only multiplications in the inner loop. */
67:
68: /* This test should be read:
69: Does it ever help to use udiv_qrnnd_preinv?
70: && Does what we save compensate for the inversion overhead? */
71: if (UDIV_TIME > (2 * UMUL_TIME + 6)
72: && (UDIV_TIME - (2 * UMUL_TIME + 6)) * dividend_size > UDIV_TIME)
73: {
74: int normalization_steps;
75:
76: count_leading_zeros (normalization_steps, divisor_limb);
77: if (normalization_steps != 0)
78: {
79: mp_limb_t divisor_limb_inverted;
80:
81: divisor_limb <<= normalization_steps;
82:
83: /* Compute (2**2N - 2**N * DIVISOR_LIMB) / DIVISOR_LIMB. The
84: result is a (N+1)-bit approximation to 1/DIVISOR_LIMB, with the
85: most significant bit (with weight 2**N) implicit. */
86:
87: /* Special case for DIVISOR_LIMB == 100...000. */
88: if (divisor_limb << 1 == 0)
89: divisor_limb_inverted = ~(mp_limb_t) 0;
90: else
91: udiv_qrnnd (divisor_limb_inverted, dummy,
92: -divisor_limb, 0, divisor_limb);
93:
94: n1 = dividend_ptr[dividend_size - 1];
95: r = n1 >> (BITS_PER_MP_LIMB - normalization_steps);
96:
97: /* Possible optimization:
98: if (r == 0
99: && divisor_limb > ((n1 << normalization_steps)
100: | (dividend_ptr[dividend_size - 2] >> ...)))
101: ...one division less... */
102:
103: for (i = dividend_size - 2; i >= 0; i--)
104: {
105: n0 = dividend_ptr[i];
106: udiv_qrnnd_preinv (quot_ptr[i + 1], r, r,
107: ((n1 << normalization_steps)
108: | (n0 >> (BITS_PER_MP_LIMB - normalization_steps))),
109: divisor_limb, divisor_limb_inverted);
110: n1 = n0;
111: }
112: udiv_qrnnd_preinv (quot_ptr[0], r, r,
113: n1 << normalization_steps,
114: divisor_limb, divisor_limb_inverted);
115: return r >> normalization_steps;
116: }
117: else
118: {
119: mp_limb_t divisor_limb_inverted;
120:
121: /* Compute (2**2N - 2**N * DIVISOR_LIMB) / DIVISOR_LIMB. The
122: result is a (N+1)-bit approximation to 1/DIVISOR_LIMB, with the
123: most significant bit (with weight 2**N) implicit. */
124:
125: /* Special case for DIVISOR_LIMB == 100...000. */
126: if (divisor_limb << 1 == 0)
127: divisor_limb_inverted = ~(mp_limb_t) 0;
128: else
129: udiv_qrnnd (divisor_limb_inverted, dummy,
130: -divisor_limb, 0, divisor_limb);
131:
132: i = dividend_size - 1;
133: r = dividend_ptr[i];
134:
135: if (r >= divisor_limb)
136: r = 0;
137: else
138: {
139: quot_ptr[i] = 0;
140: i--;
141: }
142:
143: for (; i >= 0; i--)
144: {
145: n0 = dividend_ptr[i];
146: udiv_qrnnd_preinv (quot_ptr[i], r, r,
147: n0, divisor_limb, divisor_limb_inverted);
148: }
149: return r;
150: }
151: }
152: else
153: {
154: if (UDIV_NEEDS_NORMALIZATION)
155: {
156: int normalization_steps;
157:
158: count_leading_zeros (normalization_steps, divisor_limb);
159: if (normalization_steps != 0)
160: {
161: divisor_limb <<= normalization_steps;
162:
163: n1 = dividend_ptr[dividend_size - 1];
164: r = n1 >> (BITS_PER_MP_LIMB - normalization_steps);
165:
166: /* Possible optimization:
167: if (r == 0
168: && divisor_limb > ((n1 << normalization_steps)
169: | (dividend_ptr[dividend_size - 2] >> ...)))
170: ...one division less... */
171:
172: for (i = dividend_size - 2; i >= 0; i--)
173: {
174: n0 = dividend_ptr[i];
175: udiv_qrnnd (quot_ptr[i + 1], r, r,
176: ((n1 << normalization_steps)
177: | (n0 >> (BITS_PER_MP_LIMB - normalization_steps))),
178: divisor_limb);
179: n1 = n0;
180: }
181: udiv_qrnnd (quot_ptr[0], r, r,
182: n1 << normalization_steps,
183: divisor_limb);
184: return r >> normalization_steps;
185: }
186: }
187: /* No normalization needed, either because udiv_qrnnd doesn't require
188: it, or because DIVISOR_LIMB is already normalized. */
189:
190: i = dividend_size - 1;
191: r = dividend_ptr[i];
192:
193: if (r >= divisor_limb)
194: r = 0;
195: else
196: {
197: quot_ptr[i] = 0;
198: i--;
199: }
200:
201: for (; i >= 0; i--)
202: {
203: n0 = dividend_ptr[i];
204: udiv_qrnnd (quot_ptr[i], r, r, n0, divisor_limb);
205: }
206: return r;
207: }
208: }
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