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annotate libtommath/bn_s_mp_mul_digs_fast.c @ 1820:e9854650d45b

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Clarify help text for dropbear -e environment option
author Matt Johnston <matt@ucc.asn.au>
date Thu, 19 Aug 2021 23:17:34 +0800
parents 1051e4eea25a
children
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1692
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1 #include "tommath_private.h"
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2 #ifdef BN_S_MP_MUL_DIGS_FAST_C
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3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
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4 /* SPDX-License-Identifier: Unlicense */
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5
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6 /* Fast (comba) multiplier
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7 *
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8 * This is the fast column-array [comba] multiplier. It is
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9 * designed to compute the columns of the product first
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10 * then handle the carries afterwards. This has the effect
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11 * of making the nested loops that compute the columns very
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12 * simple and schedulable on super-scalar processors.
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13 *
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14 * This has been modified to produce a variable number of
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15 * digits of output so if say only a half-product is required
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16 * you don't have to compute the upper half (a feature
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17 * required for fast Barrett reduction).
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18 *
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19 * Based on Algorithm 14.12 on pp.595 of HAC.
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20 *
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21 */
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22 mp_err s_mp_mul_digs_fast(const mp_int *a, const mp_int *b, mp_int *c, int digs)
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23 {
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24 int olduse, pa, ix, iz;
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25 mp_err err;
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26 mp_digit W[MP_WARRAY];
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27 mp_word _W;
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28
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29 /* grow the destination as required */
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30 if (c->alloc < digs) {
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31 if ((err = mp_grow(c, digs)) != MP_OKAY) {
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32 return err;
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33 }
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34 }
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35
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36 /* number of output digits to produce */
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37 pa = MP_MIN(digs, a->used + b->used);
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38
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39 /* clear the carry */
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40 _W = 0;
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41 for (ix = 0; ix < pa; ix++) {
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42 int tx, ty;
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43 int iy;
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44 mp_digit *tmpx, *tmpy;
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45
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46 /* get offsets into the two bignums */
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47 ty = MP_MIN(b->used-1, ix);
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48 tx = ix - ty;
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49
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50 /* setup temp aliases */
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51 tmpx = a->dp + tx;
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52 tmpy = b->dp + ty;
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53
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54 /* this is the number of times the loop will iterrate, essentially
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55 while (tx++ < a->used && ty-- >= 0) { ... }
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56 */
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57 iy = MP_MIN(a->used-tx, ty+1);
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58
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59 /* execute loop */
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60 for (iz = 0; iz < iy; ++iz) {
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61 _W += (mp_word)*tmpx++ * (mp_word)*tmpy--;
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62
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63 }
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64
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65 /* store term */
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66 W[ix] = (mp_digit)_W & MP_MASK;
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67
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68 /* make next carry */
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69 _W = _W >> (mp_word)MP_DIGIT_BIT;
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70 }
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71
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72 /* setup dest */
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73 olduse = c->used;
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74 c->used = pa;
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75
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76 {
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77 mp_digit *tmpc;
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78 tmpc = c->dp;
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79 for (ix = 0; ix < pa; ix++) {
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80 /* now extract the previous digit [below the carry] */
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81 *tmpc++ = W[ix];
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82 }
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83
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84 /* clear unused digits [that existed in the old copy of c] */
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85 MP_ZERO_DIGITS(tmpc, olduse - ix);
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86 }
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87 mp_clamp(c);
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88 return MP_OKAY;
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89 }
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90 #endif