_ Git - BearSSL/blob - src/int/i32_sub.c

   1 /*
   2  * Copyright (c) 2016 Thomas Pornin <pornin@bolet.org>
   3  *
   4  * Permission is hereby granted, free of charge, to any person obtaining
   5  * a copy of this software and associated documentation files (the
   6  * "Software"), to deal in the Software without restriction, including
   7  * without limitation the rights to use, copy, modify, merge, publish,
   8  * distribute, sublicense, and/or sell copies of the Software, and to
   9  * permit persons to whom the Software is furnished to do so, subject to
  10  * the following conditions:
  11  *
  12  * The above copyright notice and this permission notice shall be
  13  * included in all copies or substantial portions of the Software.
  14  *
  15  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
  16  * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
  17  * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
  18  * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
  19  * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
  20  * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
  21  * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  22  * SOFTWARE.
  23  */
  24
  25 #include "inner.h"
  26
  27 /* see inner.h */
  28 uint32_t
  29 br_i32_sub(uint32_t *a, const uint32_t *b, uint32_t ctl)
  30 {
  31         uint32_t cc;
  32         size_t u, m;
  33
  34         cc = 0;
  35         m = (a[0] + 63) >> 5;
  36         for (u = 1; u < m; u ++) {
  37                 uint32_t aw, bw, naw;
  38
  39                 aw = a[u];
  40                 bw = b[u];
  41                 naw = aw - bw - cc;
  42
  43                 /*
  44                  * Carry is 1 if naw > aw. Carry is 1 also if naw == aw
  45                  * AND the carry was already 1.
  46                  */
  47                 cc = (cc & EQ(naw, aw)) | GT(naw, aw);
  48                 a[u] = MUX(ctl, naw, aw);
  49         }
  50         return cc;
  51 }