1 /* mpn_mod_34lsub1 -- remainder modulo 2^(GMP_NUMB_BITS*3/4)-1. 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 2000-2002 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 36 #include "gmp-impl.h" 37 38 39 /* Calculate a remainder from {p,n} divided by 2^(GMP_NUMB_BITS*3/4)-1. 40 The remainder is not fully reduced, it's any limb value congruent to 41 {p,n} modulo that divisor. 42 43 This implementation is only correct when GMP_NUMB_BITS is a multiple of 44 4. 45 46 FIXME: If GMP_NAIL_BITS is some silly big value during development then 47 it's possible the carry accumulators c0,c1,c2 could overflow. 48 49 General notes: 50 51 The basic idea is to use a set of N accumulators (N=3 in this case) to 52 effectively get a remainder mod 2^(GMP_NUMB_BITS*N)-1 followed at the end 53 by a reduction to GMP_NUMB_BITS*N/M bits (M=4 in this case) for a 54 remainder mod 2^(GMP_NUMB_BITS*N/M)-1. N and M are chosen to give a good 55 set of small prime factors in 2^(GMP_NUMB_BITS*N/M)-1. 56 57 N=3 M=4 suits GMP_NUMB_BITS==32 and GMP_NUMB_BITS==64 quite well, giving 58 a few more primes than a single accumulator N=1 does, and for no extra 59 cost (assuming the processor has a decent number of registers). 60 61 For strange nailified values of GMP_NUMB_BITS the idea would be to look 62 for what N and M give good primes. With GMP_NUMB_BITS not a power of 2 63 the choices for M may be opened up a bit. But such things are probably 64 best done in separate code, not grafted on here. */ 65 66 #if GMP_NUMB_BITS % 4 == 0 67 68 #define B1 (GMP_NUMB_BITS / 4) 69 #define B2 (B1 * 2) 70 #define B3 (B1 * 3) 71 72 #define M1 ((CNST_LIMB(1) << B1) - 1) 73 #define M2 ((CNST_LIMB(1) << B2) - 1) 74 #define M3 ((CNST_LIMB(1) << B3) - 1) 75 76 #define LOW0(n) ((n) & M3) 77 #define HIGH0(n) ((n) >> B3) 78 79 #define LOW1(n) (((n) & M2) << B1) 80 #define HIGH1(n) ((n) >> B2) 81 82 #define LOW2(n) (((n) & M1) << B2) 83 #define HIGH2(n) ((n) >> B1) 84 85 #define PARTS0(n) (LOW0(n) + HIGH0(n)) 86 #define PARTS1(n) (LOW1(n) + HIGH1(n)) 87 #define PARTS2(n) (LOW2(n) + HIGH2(n)) 88 89 #define ADD(c,a,val) \ 90 do { \ 91 mp_limb_t new_c; \ 92 ADDC_LIMB (new_c, a, a, val); \ 93 (c) += new_c; \ 94 } while (0) 95 96 mp_limb_t 97 mpn_mod_34lsub1 (mp_srcptr p, mp_size_t n) 98 { 99 mp_limb_t c0, c1, c2; 100 mp_limb_t a0, a1, a2; 101 102 ASSERT (n >= 1); 103 ASSERT (n/3 < GMP_NUMB_MAX); 104 105 a0 = a1 = a2 = 0; 106 c0 = c1 = c2 = 0; 107 108 while ((n -= 3) >= 0) 109 { 110 ADD (c0, a0, p[0]); 111 ADD (c1, a1, p[1]); 112 ADD (c2, a2, p[2]); 113 p += 3; 114 } 115 116 if (n != -3) 117 { 118 ADD (c0, a0, p[0]); 119 if (n != -2) 120 ADD (c1, a1, p[1]); 121 } 122 123 return 124 PARTS0 (a0) + PARTS1 (a1) + PARTS2 (a2) 125 + PARTS1 (c0) + PARTS2 (c1) + PARTS0 (c2); 126 } 127 128 #endif 129