File: memxor.c

package info (click to toggle)
nettle 3.5.1+really3.4.1-1
  • links: PTS, VCS
  • area: main
  • in suites: sid
  • size: 8,012 kB
  • sloc: ansic: 51,527; asm: 12,157; sh: 3,632; makefile: 846; cpp: 71; awk: 7
file content (161 lines) | stat: -rw-r--r-- 4,084 bytes parent folder | download | duplicates (4)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
/* memxor.c

   Copyright (C) 2010, 2014 Niels Möller

   This file is part of GNU Nettle.

   GNU Nettle is free software: you can redistribute it and/or
   modify it under the terms of either:

     * the GNU Lesser General Public License as published by the Free
       Software Foundation; either version 3 of the License, or (at your
       option) any later version.

   or

     * the GNU General Public License as published by the Free
       Software Foundation; either version 2 of the License, or (at your
       option) any later version.

   or both in parallel, as here.

   GNU Nettle is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   General Public License for more details.

   You should have received copies of the GNU General Public License and
   the GNU Lesser General Public License along with this program.  If
   not, see http://www.gnu.org/licenses/.
*/

/* Implementation inspired by memcmp in glibc, contributed to the FSF
   by Torbjorn Granlund.
 */

#if HAVE_CONFIG_H
# include "config.h"
#endif

#include <assert.h>
#include <limits.h>

#include "memxor.h"
#include "memxor-internal.h"

#define WORD_T_THRESH 16

/* XOR word-aligned areas. n is the number of words, not bytes. */
static void
memxor_common_alignment (word_t *dst, const word_t *src, size_t n)
{
  /* FIXME: Require n > 0? */
  /* FIXME: Unroll four times, like memcmp? Probably not worth the
     effort. */

  if (n & 1)
    {
      n--;
      dst[n] ^= src[n];
    }
  while (n >= 2)
    {
      n -= 2;
      dst[n+1] ^= src[n+1];
      dst[n] ^= src[n];
    }
}

/* XOR *un-aligned* src-area onto aligned dst area. n is number of
   words, not bytes. Assumes we can read complete words at the start
   and end of the src operand. */
static void
memxor_different_alignment (word_t *dst, const unsigned char *src, size_t n)
{
  int shl, shr;
  const word_t *src_word;
  unsigned offset = ALIGN_OFFSET (src);
  word_t s0, s1;

  assert (n > 0);
  shl = CHAR_BIT * offset;
  shr = CHAR_BIT * (sizeof(word_t) - offset);

  src_word = (const word_t *) ((uintptr_t) src & -sizeof(word_t));

  /* Read top offset bytes, in native byte order. */
  READ_PARTIAL (s0, (unsigned char *) &src_word[n], offset);
#ifdef WORDS_BIGENDIAN
  s0 <<= shr; /* FIXME: Eliminate this shift? */
#endif

  /* Do n-1 regular iterations */
  if (n & 1)
    s1 = s0;
  else
    {
      n--;
      s1 = src_word[n];
      dst[n] ^= MERGE (s1, shl, s0, shr);
    }

  assert (n & 1);
  while (n > 2)
    {
      n -= 2;
      s0 = src_word[n+1];
      dst[n+1] ^= MERGE(s0, shl, s1, shr);
      s1 = src_word[n]; /* FIXME: Overread on last iteration */
      dst[n] ^= MERGE(s1, shl, s0, shr);
    }
  assert (n == 1);
  /* Read low wordsize - offset bytes */
  READ_PARTIAL (s0, src, sizeof(word_t) - offset);
#ifndef WORDS_BIGENDIAN
  s0 <<= shl; /* FIXME: eliminate shift? */
#endif /* !WORDS_BIGENDIAN */

  dst[0] ^= MERGE(s0, shl, s1, shr);
}

/* Performance, Intel SU1400 (x86_64): 0.25 cycles/byte aligned, 0.45
   cycles/byte unaligned. */

/* XOR LEN bytes starting at SRCADDR onto DESTADDR. Result undefined
   if the source overlaps with the destination. Return DESTADDR. */
void *
memxor(void *dst_in, const void *src_in, size_t n)
{
  unsigned char *dst = dst_in;
  const unsigned char *src = src_in;

  if (n >= WORD_T_THRESH)
    {
      unsigned i;
      unsigned offset;
      size_t nwords;
      /* There are at least some bytes to compare.  No need to test
	 for N == 0 in this alignment loop.  */
      for (i = ALIGN_OFFSET(dst + n); i > 0; i--)
	{
	  n--;
	  dst[n] ^= src[n];
	}
      offset = ALIGN_OFFSET(src + n);
      nwords = n / sizeof (word_t);
      n %= sizeof (word_t);

      if (offset)
	memxor_different_alignment ((word_t *) (dst+n), src+n, nwords);
      else
	memxor_common_alignment ((word_t *) (dst+n),
				 (const word_t *) (src+n), nwords);
    }
  while (n > 0)
    {
      n--;
      dst[n] ^= src[n];
    }

  return dst;
}