1 /* mpn_preinv_divrem_1 -- mpn by limb division with pre-inverted divisor. 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 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 /* Don't bloat a shared library with unused code. */ 40 #if USE_PREINV_DIVREM_1 41 42 /* Same test here for skipping one divide step as in mpn_divrem_1. 43 44 The main reason for a separate shift==0 case is that not all CPUs give 45 zero for "n0 >> GMP_LIMB_BITS" which would arise in the general case 46 code used on shift==0. shift==0 is also reasonably common in mp_bases 47 big_base, for instance base==10 on a 64-bit limb. 48 49 Under shift!=0 it would be possible to call mpn_lshift to adjust the 50 dividend all in one go (into the quotient space say), rather than 51 limb-by-limb in the loop. This might help if mpn_lshift is a lot faster 52 than what the compiler can generate for EXTRACT. But this is left to CPU 53 specific implementations to consider, especially since EXTRACT isn't on 54 the dependent chain. 55 56 If size==0 then the result is simply xsize limbs of zeros, but nothing 57 special is done for that, since it wouldn't be a usual call, and 58 certainly never arises from mpn_get_str which is our main caller. */ 59 60 mp_limb_t 61 mpn_preinv_divrem_1 (mp_ptr qp, mp_size_t xsize, 62 mp_srcptr ap, mp_size_t size, mp_limb_t d_unnorm, 63 mp_limb_t dinv, int shift) 64 { 65 mp_limb_t ahigh, qhigh, r; 66 mp_size_t i; 67 mp_limb_t n1, n0; 68 mp_limb_t d; 69 70 ASSERT (xsize >= 0); 71 ASSERT (size >= 1); 72 ASSERT (d_unnorm != 0); 73 #if WANT_ASSERT 74 { 75 int want_shift; 76 mp_limb_t want_dinv; 77 count_leading_zeros (want_shift, d_unnorm); 78 ASSERT (shift == want_shift); 79 invert_limb (want_dinv, d_unnorm << shift); 80 ASSERT (dinv == want_dinv); 81 } 82 #endif 83 /* FIXME: What's the correct overlap rule when xsize!=0? */ 84 ASSERT (MPN_SAME_OR_SEPARATE_P (qp+xsize, ap, size)); 85 86 ahigh = ap[size-1]; 87 d = d_unnorm << shift; 88 qp += (size + xsize - 1); /* dest high limb */ 89 90 if (shift == 0) 91 { 92 /* High quotient limb is 0 or 1, and skip a divide step. */ 93 r = ahigh; 94 qhigh = (r >= d); 95 r = (qhigh ? r-d : r); 96 *qp-- = qhigh; 97 size--; 98 99 for (i = size-1; i >= 0; i--) 100 { 101 n0 = ap[i]; 102 udiv_qrnnd_preinv (*qp, r, r, n0, d, dinv); 103 qp--; 104 } 105 } 106 else 107 { 108 r = 0; 109 if (ahigh < d_unnorm) 110 { 111 r = ahigh << shift; 112 *qp-- = 0; 113 size--; 114 if (size == 0) 115 goto done_integer; 116 } 117 118 n1 = ap[size-1]; 119 r |= n1 >> (GMP_LIMB_BITS - shift); 120 121 for (i = size-2; i >= 0; i--) 122 { 123 ASSERT (r < d); 124 n0 = ap[i]; 125 udiv_qrnnd_preinv (*qp, r, r, 126 ((n1 << shift) | (n0 >> (GMP_LIMB_BITS - shift))), 127 d, dinv); 128 qp--; 129 n1 = n0; 130 } 131 udiv_qrnnd_preinv (*qp, r, r, n1 << shift, d, dinv); 132 qp--; 133 } 134 135 done_integer: 136 for (i = 0; i < xsize; i++) 137 { 138 udiv_qrnnd_preinv (*qp, r, r, CNST_LIMB(0), d, dinv); 139 qp--; 140 } 141 142 return r >> shift; 143 } 144 145 #endif /* USE_PREINV_DIVREM_1 */ 146