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, 2001, 2002, 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 the GNU Lesser General Public License as published by 13 the Free Software Foundation; either version 3 of the License, or (at your 14 option) any later version. 15 16 The GNU MP Library is distributed in the hope that it will be useful, but 17 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY 18 or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public 19 License for more details. 20 21 You should have received a copy of the GNU Lesser General Public License 22 along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */ 23 24 #include "gmp.h" 25 #include "gmp-impl.h" 26 #include "longlong.h" 27 28 29 /* Don't bloat a shared library with unused code. */ 30 #if USE_PREINV_DIVREM_1 31 32 /* Same test here for skipping one divide step as in mpn_divrem_1. 33 34 The main reason for a separate shift==0 case is that not all CPUs give 35 zero for "n0 >> GMP_LIMB_BITS" which would arise in the general case 36 code used on shift==0. shift==0 is also reasonably common in mp_bases 37 big_base, for instance base==10 on a 64-bit limb. 38 39 Under shift!=0 it would be possible to call mpn_lshift to adjust the 40 dividend all in one go (into the quotient space say), rather than 41 limb-by-limb in the loop. This might help if mpn_lshift is a lot faster 42 than what the compiler can generate for EXTRACT. But this is left to CPU 43 specific implementations to consider, especially since EXTRACT isn't on 44 the dependent chain. 45 46 If size==0 then the result is simply xsize limbs of zeros, but nothing 47 special is done for that, since it wouldn't be a usual call, and 48 certainly never arises from mpn_get_str which is our main caller. */ 49 50 mp_limb_t 51 mpn_preinv_divrem_1 (mp_ptr qp, mp_size_t xsize, 52 mp_srcptr ap, mp_size_t size, mp_limb_t d_unnorm, 53 mp_limb_t dinv, int shift) 54 { 55 mp_limb_t ahigh, qhigh, r; 56 mp_size_t i; 57 mp_limb_t n1, n0; 58 mp_limb_t d; 59 60 ASSERT (xsize >= 0); 61 ASSERT (size >= 1); 62 ASSERT (d_unnorm != 0); 63 #if WANT_ASSERT 64 { 65 int want_shift; 66 mp_limb_t want_dinv; 67 count_leading_zeros (want_shift, d_unnorm); 68 ASSERT (shift == want_shift); 69 invert_limb (want_dinv, d_unnorm << shift); 70 ASSERT (dinv == want_dinv); 71 } 72 #endif 73 /* FIXME: What's the correct overlap rule when xsize!=0? */ 74 ASSERT (MPN_SAME_OR_SEPARATE_P (qp+xsize, ap, size)); 75 76 ahigh = ap[size-1]; 77 d = d_unnorm << shift; 78 qp += (size + xsize - 1); /* dest high limb */ 79 80 if (shift == 0) 81 { 82 /* High quotient limb is 0 or 1, and skip a divide step. */ 83 r = ahigh; 84 qhigh = (r >= d); 85 r = (qhigh ? r-d : r); 86 *qp-- = qhigh; 87 size--; 88 89 for (i = size-1; i >= 0; i--) 90 { 91 n0 = ap[i]; 92 udiv_qrnnd_preinv (*qp, r, r, n0, d, dinv); 93 qp--; 94 } 95 } 96 else 97 { 98 r = 0; 99 if (ahigh < d_unnorm) 100 { 101 r = ahigh << shift; 102 *qp-- = 0; 103 size--; 104 if (size == 0) 105 goto done_integer; 106 } 107 108 n1 = ap[size-1]; 109 r |= n1 >> (GMP_LIMB_BITS - shift); 110 111 for (i = size-2; i >= 0; i--) 112 { 113 ASSERT (r < d); 114 n0 = ap[i]; 115 udiv_qrnnd_preinv (*qp, r, r, 116 ((n1 << shift) | (n0 >> (GMP_LIMB_BITS - shift))), 117 d, dinv); 118 qp--; 119 n1 = n0; 120 } 121 udiv_qrnnd_preinv (*qp, r, r, n1 << shift, d, dinv); 122 qp--; 123 } 124 125 done_integer: 126 for (i = 0; i < xsize; i++) 127 { 128 udiv_qrnnd_preinv (*qp, r, r, CNST_LIMB(0), d, dinv); 129 qp--; 130 } 131 132 return r >> shift; 133 } 134 135 #endif /* USE_PREINV_DIVREM_1 */ 136