--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,13 @@
+2008-03-01  Eric Blake  <ebb9@byu.net>
+
+	New module 'memchr2'.
+	* modules/memchr2: New file.
+	* modules/memchr2-tests: Likewise.
+	* lib/memchr2.h: Likewise.
+	* lib/memchr2.c: Likewise, based on memchr.c.
+	* tests/test-memchr2.c: New test.
+	* MODULES.html.sh (String handling): Add memchr2.
+
 2008-02-29  Bruno Haible  <bruno@clisp.org>
 
 	* modules/freadseek-tests: New file.

--- a/MODULES.html.sh
+++ b/MODULES.html.sh
@@ -1653,6 +1653,7 @@
 
   func_begin_table
   func_module bcopy
+  func_module memchr2
   func_module memmem
   func_module memmem-simple
   func_module mempcpy

new file mode 100644
--- /dev/null
+++ b/lib/memchr2.c
@@ -0,0 +1,194 @@
+/* Copyright (C) 1991, 1993, 1996, 1997, 1999, 2000, 2003, 2004, 2006,
+   2008 Free Software Foundation, Inc.
+
+   Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
+   with help from Dan Sahlin (dan@sics.se) and
+   commentary by Jim Blandy (jimb@ai.mit.edu);
+   adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
+   and implemented in glibc by Roland McGrath (roland@ai.mit.edu).
+   Extension to memchr2 implemented by Eric Blake (ebb9@byu.net).
+
+This program is free software: you can redistribute it and/or modify it
+under the terms of the GNU General Public License as published by the
+Free Software Foundation; either version 3 of the License, or any
+later version.
+
+This program 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 a copy of the GNU General Public License
+along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <config.h>
+
+#include "memchr2.h"
+
+#include <limits.h>
+#include <stdint.h>
+#include <string.h>
+
+/* Return the first address of either C1 or C2 (treated as unsigned
+   char) that occurs within N bytes of the memory region S.  If
+   neither byte appears, return NULL.  */
+void *
+memchr2 (void const *s, int c1_in, int c2_in, size_t n)
+{
+  const unsigned char *char_ptr;
+  const uintmax_t *longword_ptr;
+  uintmax_t longword1;
+  uintmax_t longword2;
+  uintmax_t magic_bits;
+  uintmax_t charmask1;
+  uintmax_t charmask2;
+  unsigned char c1;
+  unsigned char c2;
+  int i;
+
+  c1 = (unsigned char) c1_in;
+  c2 = (unsigned char) c2_in;
+
+  if (c1 == c2)
+    return memchr (s, c1, n);
+
+  /* Handle the first few characters by reading one character at a time.
+     Do this until CHAR_PTR is aligned on a longword boundary.  */
+  for (char_ptr = (const unsigned char *) s;
+       n > 0 && (size_t) char_ptr % sizeof longword1 != 0;
+       --n, ++char_ptr)
+    if (*char_ptr == c1 || *char_ptr == c2)
+      return (void *) char_ptr;
+
+  /* All these elucidatory comments refer to 4-byte longwords,
+     but the theory applies equally well to any size longwords.  */
+
+  longword_ptr = (const uintmax_t *) char_ptr;
+
+  /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
+     the "holes."  Note that there is a hole just to the left of
+     each byte, with an extra at the end:
+
+     bits:  01111110 11111110 11111110 11111111
+     bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
+
+     The 1-bits make sure that carries propagate to the next 0-bit.
+     The 0-bits provide holes for carries to fall into.  */
+
+  /* Set MAGIC_BITS to be this pattern of 1 and 0 bits.
+     Set CHARMASK to be a longword, each of whose bytes is C.  */
+
+  magic_bits = 0xfefefefe;
+  charmask1 = c1 | (c1 << 8);
+  charmask2 = c2 | (c2 << 8);
+  charmask1 |= charmask2 << 16;
+  charmask1 |= charmask2 << 16;
+#if 0xffffffffU < UINTMAX_MAX
+  magic_bits |= magic_bits << 32;
+  charmask1 |= charmask1 << 32;
+  charmask2 |= charmask2 << 32;
+  if (8 < sizeof longword1)
+    for (i = 64; i < sizeof longword1 * 8; i *= 2)
+      {
+	magic_bits |= magic_bits << i;
+	charmask1 |= charmask1 << i;
+	charmask2 |= charmask2 << i;
+      }
+#endif
+  magic_bits = (UINTMAX_MAX >> 1) & (magic_bits | 1);
+
+  /* Instead of the traditional loop which tests each character,
+     we will test a longword at a time.  The tricky part is testing
+     if *any of the four* bytes in the longword in question are zero.  */
+  while (n >= sizeof longword1)
+    {
+      /* We tentatively exit the loop if adding MAGIC_BITS to
+	 LONGWORD fails to change any of the hole bits of LONGWORD.
+
+	 1) Is this safe?  Will it catch all the zero bytes?
+	 Suppose there is a byte with all zeros.  Any carry bits
+	 propagating from its left will fall into the hole at its
+	 least significant bit and stop.  Since there will be no
+	 carry from its most significant bit, the LSB of the
+	 byte to the left will be unchanged, and the zero will be
+	 detected.
+
+	 2) Is this worthwhile?  Will it ignore everything except
+	 zero bytes?  Suppose every byte of LONGWORD has a bit set
+	 somewhere.  There will be a carry into bit 8.  If bit 8
+	 is set, this will carry into bit 16.  If bit 8 is clear,
+	 one of bits 9-15 must be set, so there will be a carry
+	 into bit 16.  Similarly, there will be a carry into bit
+	 24.  If one of bits 24-30 is set, there will be a carry
+	 into bit 31, so all of the hole bits will be changed.
+
+	 The one misfire occurs when bits 24-30 are clear and bit
+	 31 is set; in this case, the hole at bit 31 is not
+	 changed.  If we had access to the processor carry flag,
+	 we could close this loophole by putting the fourth hole
+	 at bit 32!
+
+	 So it ignores everything except 128's, when they're aligned
+	 properly.
+
+	 3) But wait!  Aren't we looking for C, not zero?
+	 Good point.  So what we do is XOR LONGWORD with a longword,
+	 each of whose bytes is C.  This turns each byte that is C
+	 into a zero.  */
+
+      longword1 = *longword_ptr ^ charmask1;
+      longword2 = *longword_ptr++ ^ charmask2;
+
+      /* Add MAGIC_BITS to LONGWORD.  */
+      if ((((longword1 + magic_bits)
+
+	    /* Set those bits that were unchanged by the addition.  */
+	    ^ ~longword1)
+
+	   /* Look at only the hole bits.  If any of the hole bits
+	      are unchanged, most likely one of the bytes was a
+	      zero.  */
+	   & ~magic_bits) != 0
+	  || (((longword2 + magic_bits) ^ ~longword2) & ~magic_bits) != 0)
+	{
+	  /* Which of the bytes was C?  If none of them were, it was
+	     a misfire; continue the search.  */
+
+	  const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
+
+	  if (cp[0] == c1 || cp[0] == c2)
+	    return (void *) cp;
+	  if (cp[1] == c1 || cp[1] == c2)
+	    return (void *) &cp[1];
+	  if (cp[2] == c1 || cp[2] == c2)
+	    return (void *) &cp[2];
+	  if (cp[3] == c1 || cp[3] == c2)
+	    return (void *) &cp[3];
+	  if (4 < sizeof longword1 && (cp[4] == c1 || cp[4] == c2))
+	    return (void *) &cp[4];
+	  if (5 < sizeof longword1 && (cp[5] == c1 || cp[5] == c2))
+	    return (void *) &cp[5];
+	  if (6 < sizeof longword1 && (cp[6] == c1 || cp[6] == c2))
+	    return (void *) &cp[6];
+	  if (7 < sizeof longword1 && (cp[7] == c1 || cp[7] == c2))
+	    return (void *) &cp[7];
+	  if (8 < sizeof longword1)
+	    for (i = 8; i < sizeof longword1; i++)
+	      if (cp[i] == c1 || cp[i] == c2)
+		return (void *) &cp[i];
+	}
+
+      n -= sizeof longword1;
+    }
+
+  char_ptr = (const unsigned char *) longword_ptr;
+
+  while (n-- > 0)
+    {
+      if (*char_ptr == c1 || *char_ptr == c2)
+	return (void *) char_ptr;
+      ++char_ptr;
+    }
+
+  return 0;
+}

new file mode 100644
--- /dev/null
+++ b/lib/memchr2.h
@@ -0,0 +1,31 @@
+/* Scan memory for the first of two bytes.
+   Copyright (C) 2008 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program 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 a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* Return the first address of either C1 or C2 (treated as unsigned
+   char) that occurs within N bytes of the memory region S.  If
+   neither byte appears, return NULL.  */
+
+extern void *memchr2 (void const *s, int c1, int c2, size_t n);
+
+#ifdef __cplusplus
+}
+#endif

new file mode 100644
--- /dev/null
+++ b/modules/memchr2
@@ -0,0 +1,24 @@
+Description:
+memchr2() function: scan memory for the first of two bytes.
+
+Files:
+lib/memchr2.h
+lib/memchr2.c
+
+Depends-on:
+stdint
+memchr
+
+configure.ac:
+
+Makefile.am:
+lib_SOURCES += memchr2.h memchr2.c
+
+Include:
+"memchr2.h"
+
+License:
+GPL
+
+Maintainer:
+Eric Blake

new file mode 100644
--- /dev/null
+++ b/modules/memchr2-tests
@@ -0,0 +1,10 @@
+Files:
+tests/test-memchr2.c
+
+Depends-on:
+
+configure.ac:
+
+Makefile.am:
+TESTS += test-memchr2
+check_PROGRAMS += test-memchr2

new file mode 100644
--- /dev/null
+++ b/tests/test-memchr2.c
@@ -0,0 +1,89 @@
+/*
+ * Copyright (C) 2008 Free Software Foundation
+ * Written by Eric Blake
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program 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 a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <config.h>
+
+#include "memchr2.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define ASSERT(expr) \
+  do									     \
+    {									     \
+      if (!(expr))							     \
+	{								     \
+	  fprintf (stderr, "%s:%d: assertion failed\n", __FILE__, __LINE__); \
+	  abort ();							     \
+	}								     \
+    }									     \
+  while (0)
+
+int
+main ()
+{
+  size_t n = 0x100000;
+  char *input = malloc (n);
+  ASSERT (input);
+
+  input[0] = 'a';
+  input[1] = 'b';
+  memset (input + 2, 'c', 1024);
+  memset (input + 1026, 'd', n - 1028);
+  input[n - 2] = 'e';
+  input[n - 1] = 'a';
+
+  ASSERT (memchr2 (input, 'a', 'b', n) == input);
+  ASSERT (memchr2 (input, 'b', 'a', n) == input);
+
+  ASSERT (memchr2 (input, 'a', 'b', 0) == NULL);
+  ASSERT (memchr2 (NULL, 'a', 'b', 0) == NULL);
+
+  ASSERT (memchr2 (input, 'b', 'd', n) == input + 1);
+  ASSERT (memchr2 (input + 2, 'b', 'd', n - 2) == input + 1026);
+
+  ASSERT (memchr2 (input, 'd', 'e', n) == input + 1026);
+  ASSERT (memchr2 (input, 'e', 'd', n) == input + 1026);
+
+  ASSERT (memchr2 (input + 1, 'a', 'e', n - 1) == input + n - 2);
+  ASSERT (memchr2 (input + 1, 'e', 'a', n - 1) == input + n - 2);
+
+  ASSERT (memchr2 (input, 'f', 'g', n) == NULL);
+  ASSERT (memchr2 (input, 'f', '\0', n) == NULL);
+
+  ASSERT (memchr2 (input, 'a', 'a', n) == input);
+  ASSERT (memchr2 (input + 1, 'a', 'a', n - 1) == input + n - 1);
+  ASSERT (memchr2 (input, 'f', 'f', n) == NULL);
+
+  /* Check that a very long haystack is handled quickly if one of the
+     two bytes is found near the beginning.  */
+  {
+    size_t repeat = 10000;
+    for (; repeat > 0; repeat--)
+      {
+	ASSERT (memchr2 (input, 'c', 'e', n) == input + 2);
+	ASSERT (memchr2 (input, 'e', 'c', n) == input + 2);
+	ASSERT (memchr2 (input, 'c', '\0', n) == input + 2);
+	ASSERT (memchr2 (input, '\0', 'c', n) == input + 2);
+      }
+  }
+
+  free (input);
+
+  return 0;
+}