kc3-lang/libtommath/s_mp_sub.c

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• Hash : bcda8fc6
  Author :
  Date : 2019-10-29T22:38:49
  

  simplifications: remove unnecessary optimization * these double checks are not necessary * the compiler will move the early return outside of the called function, basically the functions is partially inlined * however lto/amalgamation needed for the optimization

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• s_mp_sub.c

• #include "tommath_private.h"
  #ifdef S_MP_SUB_C
  /* LibTomMath, multiple-precision integer library -- Tom St Denis */
  /* SPDX-License-Identifier: Unlicense */
  
  /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
  mp_err s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
  {
     int oldused = c->used, min = b->used, max = a->used, i;
     mp_digit u;
     mp_err err;
  
     /* init result */
     if ((err = mp_grow(c, max)) != MP_OKAY) {
        return err;
     }
  
     c->used = max;
  
     /* set carry to zero */
     u = 0;
     for (i = 0; i < min; i++) {
        /* T[i] = A[i] - B[i] - U */
        c->dp[i] = (a->dp[i] - b->dp[i]) - 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 = c->dp[i] >> (MP_SIZEOF_BITS(mp_digit) - 1u);
  
        /* Clear carry from T[i] */
        c->dp[i] &= 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 */
        c->dp[i] = a->dp[i] - u;
  
        /* U = carry bit of T[i] */
        u = c->dp[i] >> (MP_SIZEOF_BITS(mp_digit) - 1u);
  
        /* Clear carry from T[i] */
        c->dp[i] &= MP_MASK;
     }
  
     /* clear digits above used (since we may not have grown result above) */
     s_mp_zero_digs(c->dp + c->used, oldused - c->used);
  
     mp_clamp(c);
     return MP_OKAY;
  }
  
  #endif
  

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