/* 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;

}