kc3-lang/libtommath/bn_s_mp_sub.c

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• Hash : 8714ee55
  Author :
  Date : 2015-11-25T21:59:46
  

  remove register let the compiler find the best register optimization

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• bn_s_mp_sub.c

• #include <tommath_private.h>
  #ifdef BN_S_MP_SUB_C
  /* LibTomMath, multiple-precision integer library -- Tom St Denis
   *
   * LibTomMath is a library that provides multiple-precision
   * integer arithmetic as well as number theoretic functionality.
   *
   * The library was designed directly after the MPI library by
   * Michael Fromberger but has been written from scratch with
   * additional optimizations in place.
   *
   * The library is free for all purposes without any express
   * guarantee it works.
   *
   * Tom St Denis, tstdenis82@gmail.com, http://libtom.org
   */
  
  /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
  int
  s_mp_sub (mp_int * a, mp_int * b, mp_int * c)
  {
    int     olduse, res, min, max;
  
    /* find sizes */
    min = b->used;
    max = a->used;
  
    /* init result */
    if (c->alloc < max) {
      if ((res = mp_grow (c, max)) != MP_OKAY) {
        return res;
      }
    }
    olduse = c->used;
    c->used = max;
  
    {
      mp_digit u, *tmpa, *tmpb, *tmpc;
      int i;
  
      /* alias for digit pointers */
      tmpa = a->dp;
      tmpb = b->dp;
      tmpc = c->dp;
  
      /* set carry to zero */
      u = 0;
      for (i = 0; i < min; i++) {
        /* T[i] = A[i] - B[i] - U */
        *tmpc = (*tmpa++ - *tmpb++) - u;
  
        /* U = carry bit of T[i]
         * Note this saves performing an AND operation since
         * if a carry does occur it will propagate all the way to the
         * MSB.  As a result a single shift is enough to get the carry
         */
        u = *tmpc >> ((mp_digit)((CHAR_BIT * sizeof(mp_digit)) - 1));
  
        /* Clear carry from T[i] */
        *tmpc++ &= MP_MASK;
      }
  
      /* now copy higher words if any, e.g. if A has more digits than B  */
      for (; i < max; i++) {
        /* T[i] = A[i] - U */
        *tmpc = *tmpa++ - u;
  
        /* U = carry bit of T[i] */
        u = *tmpc >> ((mp_digit)((CHAR_BIT * sizeof(mp_digit)) - 1));
  
        /* Clear carry from T[i] */
        *tmpc++ &= MP_MASK;
      }
  
      /* clear digits above used (since we may not have grown result above) */
      for (i = c->used; i < olduse; i++) {
        *tmpc++ = 0;
      }
    }
  
    mp_clamp (c);
    return MP_OKAY;
  }
  
  #endif
  
  /* $Source$ */
  /* $Revision$ */
  /* $Date$ */
  

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