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 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.
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.
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. */
37 #define UDIV_TIME UMUL_TIME
40 /* FIXME: We should be using invert_limb (or invert_normalized_limb)
41 here (not udiv_qrnnd). */
45 mpn_divmod_1 (mp_ptr quot_ptr,
46 mp_srcptr dividend_ptr, mp_size_t dividend_size,
47 mp_limb_t divisor_limb)
49 mpn_divmod_1 (quot_ptr, dividend_ptr, dividend_size, divisor_limb)
51 mp_srcptr dividend_ptr;
52 mp_size_t dividend_size;
53 mp_limb_t divisor_limb;
60 /* ??? Should this be handled at all? Rely on callers? */
61 if (dividend_size == 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;
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:
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);
112 udiv_qrnnd_preinv (quot_ptr[0], r, r,
113 n1 << normalization_steps,
114 divisor_limb, divisor_limb_inverted);
115 return r >> normalization_steps;
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;
129 udiv_qrnnd (divisor_limb_inverted, dummy,
130 -divisor_limb, 0, divisor_limb);
132 i = dividend_size - 1;
135 if (r >= divisor_limb)
145 n0 = dividend_ptr[i];
146 udiv_qrnnd_preinv (quot_ptr[i], r, r,
147 n0, divisor_limb, divisor_limb_inverted);
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:
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))),
181 udiv_qrnnd (quot_ptr[0], r, r,
182 n1 << normalization_steps,
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;
193 if (r >= divisor_limb)
203 n0 = dividend_ptr[i];
204 udiv_qrnnd (quot_ptr[i], r, r, n0, divisor_limb);