(memxor_common_alignment): New function.

(memxor_different_alignment): New function.
(memxor): Optimized to do word-operations rather than byte
operations.

Rev: nettle/memxor.c:1.2

memxor.c

 /* memxor.c
  *
- * $Id$
  */
 
-/* XOR LEN bytes starting at SRCADDR onto DESTADDR.  Result undefined
-   if the source overlaps with the destination.
-   Return DESTADDR. */
+/* nettle, low-level cryptographics library
+ *
+ * Copyright (C) 1991, 1993, 1995 Free Software Foundation, Inc.
+ * Copyright (C) 2010 Niels Mller
+ *  
+ * The nettle library is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation; either version 2.1 of the License, or (at your
+ * option) any later version.
+ * 
+ * The nettle library 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 Lesser General Public
+ * License for more details.
+ * 
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with the nettle library; see the file COPYING.LIB.  If not, write to
+ * the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
+ * MA 02111-1307, USA.
+ */
+
+/* Implementation inspired by memcmp in glibc, contributed to the FSF by Torbjorn Granlund.
+ */
 
 #if HAVE_CONFIG_H
 # include "config.h"
 #endif
 
+#include <limits.h>
+
 #include "memxor.h"
 
+#if 1
+typedef unsigned long int word_t;
+
+#if SIZEOF_LONG & (SIZEOF_LONG - 1)
+#error Word size must be a power of two
+#endif
+
+#define ALIGN_OFFSET(p) ((uintptr_t) (p) % sizeof(word_t))
+
+#ifndef WORDS_BIGENDIAN
+#define MERGE(w0, sh_1, w1, sh_2) (((w0) >> (sh_1)) | ((w1) << (sh_2)))
+#else
+#define MERGE(w0, sh_1, w1, sh_2) (((w0) << (sh_1)) | ((w1) >> (sh_2)))
+#endif
+
+#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)
+{
+  size_t i;
+  /* FIXME: Unroll four times, like memcmp? */
+  i = n & 1;
+  if (i)
+    dst[0] ^= src[0];
+
+  for (; i < n; i += 2)
+    {
+      dst[i] ^= src[i];
+      dst[i+1] ^= src[i+1];
+    }
+}
+
+/* XOR *un-aligned* src-area onto aligned dst area. n is numer 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 uint8_t *src, size_t n)
+{
+  size_t i;
+  int shl, shr;
+  const word_t *src_word;
+  unsigned offset = ALIGN_OFFSET (src);
+  word_t s0, s1;
+
+  shl = CHAR_BIT * offset;
+  shr = CHAR_BIT * (sizeof(word_t) - offset);
+
+  src_word = (const word_t *) (src - offset);
+
+  /* FIXME: Unroll four times, like memcmp? */
+  i = n & 1;
+  s0 = src_word[i];
+  if (i)
+    {
+      s1 = src_word[0];
+      s0 = src_word[1];
+      dst[0] ^= MERGE (s1, shl, s0, shr);
+    }
+
+  for (; i < n; i += 2)
+    {
+      s1 = src_word[i+1];
+      dst[i] ^= MERGE(s0, shl, s1, shr);
+      s0 = src_word[i+2];
+      dst[i+1] ^= MERGE(s1, shl, s0, shr);
+    }
+}
+
+/* XOR LEN bytes starting at SRCADDR onto DESTADDR. Result undefined
+   if the source overlaps with the destination. Return DESTADDR. */
+uint8_t *
+memxor(uint8_t *dst, const uint8_t *src, size_t n)
+{
+  uint8_t *orig_dst = dst;
+
+  if (n >= WORD_T_THRESH)
+    {
+      size_t left_over;
+
+      /* There are at least some bytes to compare.  No need to test
+	 for N == 0 in this alignment loop.  */
+      while (ALIGN_OFFSET (dst))
+	{
+	  *dst++ ^= *src++;
+	  n--;
+	}
+      if (ALIGN_OFFSET (src))
+	memxor_different_alignment ((word_t *) dst, src, n / sizeof(word_t));
+      else
+	memxor_common_alignment ((word_t *) dst, (const word_t *) src, n / sizeof(word_t));
+
+      left_over = n % sizeof(word_t);
+      dst += n - left_over;
+      src += n - left_over;
+      n = left_over;
+    }
+  for (; n > 0; n--)
+    *dst++ ^= *src++;
+
+  return orig_dst;
+}
+#else
 uint8_t *
 memxor(uint8_t *dst, const uint8_t *src, size_t n)
 {
@@ -22,6 +147,7 @@ memxor(uint8_t *dst, const uint8_t *src, size_t n)
 
   return dst;
 }
+#endif
 
 uint8_t *
 memxor3(uint8_t *dst, const uint8_t *a, const uint8_t *b, size_t n)