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[glibc/history.git] / stdlib / divmod_1.c
blob0a8a77e12e0f043f21563f7f8e7399e0ba209e3a
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.
7 QUOT_PTR and DIVIDEND_PTR might point to the same limb.
9 Copyright (C) 1991, 1993, 1994, 1996 Free Software Foundation, Inc.
11 This file is part of the GNU MP Library.
13 The GNU MP Library is free software; you can redistribute it and/or modify
14 it under the terms of the GNU Lesser General Public License as published by
15 the Free Software Foundation; either version 2.1 of the License, or (at your
16 option) any later version.
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 Lesser General Public
21 License for more details.
23 You should have received a copy of the GNU Lesser 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. */
28 #include <gmp.h>
29 #include "gmp-impl.h"
30 #include "longlong.h"
32 #ifndef UMUL_TIME
33 #define UMUL_TIME 1
34 #endif
36 #ifndef UDIV_TIME
37 #define UDIV_TIME UMUL_TIME
38 #endif
40 /* FIXME: We should be using invert_limb (or invert_normalized_limb)
41 here (not udiv_qrnnd). */
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
56 mp_size_t i;
57 mp_limb_t n1, n0, r;
58 mp_limb_t dummy;
60 /* ??? Should this be handled at all? Rely on callers? */
61 if (dividend_size == 0)
62 return 0;
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. */
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)
74 int normalization_steps;
76 count_leading_zeros (normalization_steps, divisor_limb);
77 if (normalization_steps != 0)
79 mp_limb_t divisor_limb_inverted;
81 divisor_limb <<= normalization_steps;
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. */
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);
94 n1 = dividend_ptr[dividend_size - 1];
95 r = n1 >> (BITS_PER_MP_LIMB - normalization_steps);
97 /* Possible optimization:
98 if (r == 0
99 && divisor_limb > ((n1 << normalization_steps)
100 | (dividend_ptr[dividend_size - 2] >> ...)))
101 ...one division less... */
103 for (i = dividend_size - 2; i >= 0; i--)
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;
112 udiv_qrnnd_preinv (quot_ptr[0], r, r,
113 n1 << normalization_steps,
114 divisor_limb, divisor_limb_inverted);
115 return r >> normalization_steps;
117 else
119 mp_limb_t divisor_limb_inverted;
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. */
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);
132 i = dividend_size - 1;
133 r = dividend_ptr[i];
135 if (r >= divisor_limb)
136 r = 0;
137 else
139 quot_ptr[i] = 0;
140 i--;
143 for (; i >= 0; i--)
145 n0 = dividend_ptr[i];
146 udiv_qrnnd_preinv (quot_ptr[i], r, r,
147 n0, divisor_limb, divisor_limb_inverted);
149 return r;
152 else
154 if (UDIV_NEEDS_NORMALIZATION)
156 int normalization_steps;
158 count_leading_zeros (normalization_steps, divisor_limb);
159 if (normalization_steps != 0)
161 divisor_limb <<= normalization_steps;
163 n1 = dividend_ptr[dividend_size - 1];
164 r = n1 >> (BITS_PER_MP_LIMB - normalization_steps);
166 /* Possible optimization:
167 if (r == 0
168 && divisor_limb > ((n1 << normalization_steps)
169 | (dividend_ptr[dividend_size - 2] >> ...)))
170 ...one division less... */
172 for (i = dividend_size - 2; i >= 0; i--)
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;
181 udiv_qrnnd (quot_ptr[0], r, r,
182 n1 << normalization_steps,
183 divisor_limb);
184 return r >> normalization_steps;
187 /* No normalization needed, either because udiv_qrnnd doesn't require
188 it, or because DIVISOR_LIMB is already normalized. */
190 i = dividend_size - 1;
191 r = dividend_ptr[i];
193 if (r >= divisor_limb)
194 r = 0;
195 else
197 quot_ptr[i] = 0;
198 i--;
201 for (; i >= 0; i--)
203 n0 = dividend_ptr[i];
204 udiv_qrnnd (quot_ptr[i], r, r, n0, divisor_limb);
206 return r;