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/* 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;
}
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