Re: [Emacs-diffs] master d826037 3/3: Remove the need for temacs.in

[Paul Eggert <eggert@cs.ucla.edu>]

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Daniel Colascione wrote:
> If the temacs.in scheme *were* to
> break, it would have to be in such a way that 1) some change in Emacs or
> the environment resulted in temacs.in_1 (before the change) and
> temacs.in_2 (after the change) having the same hash, but temacs_1 (before
> the change) and temacs_2 (after the change) being different in a way that
> breaks pdumper. Can you think of such a scenario?

Sure:

    cd src
    make LDFLAGS="something"
    rm temacs
    make LDFLAGS="something else" temacs

Here, temacs.in_1 and temacs.in_2 are identical (because temacs.in is built just 
once) but temacs_1 and temacs_2 can differ in a way that breaks the portable dumper.

None of the approaches proposed in this thread work reliably in the presence of 
all plausible linkers, even ignoring the problem of hash collisions. All the 
approaches have false alarms (where the Emacs loader incorrectly reports an 
incompatibility) and false negatives (where the Emacs loader incorrectly fails 
to report an incompatibility that hurts later execution), and these flaws are 
pretty much inherent to the current fingerprint strategy. That's OK: the point 
of the fingerprint is not to be perfect (it can't be); it's only to be good 
enough. However, nobody knows even which of the discussed approaches will be 
best in practice, because nobody has practical experience with any of these 
approaches in a realistic deployment (and also, because we probably don't agree 
100% on what constitutes "best".)

> If you want to address the build time issue, just rewrite the fingerprint
> in place.

Although I don't agree that this approach works reliably (in the sense described 
above), it's better than the old temacs.in approach and it should be good enough 
in practice, so I installed the attached patch to do that.


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From 5c2f94a182a1154766154040eb5b4b39275fd3b6 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Sun, 14 Apr 2019 16:31:24 -0700
Subject: [PATCH] =?UTF-8?q?Replace=20executable=E2=80=99s=20fingerprint=20?=
 =?UTF-8?q?in=20place?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

* admin/merge-gnulib (GNULIB_MODULES): Add memmem-simple.
(AVOIDED_MODULES): Add memchr.
* configure.ac (HAVE_PDUMPER): AC_SUBST it, too, for use in makefiles.
* lib/Makefile.in (libgnu_a_OBJECTS): Add fingerprint.o.
* lib/fingerprint.c: New file.
* lib/memmem.c, lib/str-two-way.h, m4/memmem.m4: New files,
copied from Gnulib.
* lib/fingerprint.h: Rename from src/fingerprint.h.
* lib-src/make-fingerprint.c: Include limits.h, sys/stat.h,
fingerprint.h, intprops.h, min-max.h.
(SSIZE_MAX): New macro, if not already defined.
(main): Without -r, Replace the fingerprint in the input file
instead of generating a fingerprint.c.
* lib/Makefile.in (libgnu_a_OBJECTS): Add fingerprint.o.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.
* src/Makefile.in (HAVE_PDUMPER, MAKE_PDUMPER_FINGERPRINT):
New macros.
(temacs$(EXEEXT)): Use them to replace the fingerprint instead
of precalculating it.
(mostlyclean, ctagsfiles1): Do not worry about fingerprint.c.
---
 admin/merge-gnulib         |   4 +-
 configure.ac               |   6 +-
 lib-src/make-fingerprint.c | 119 +++++++---
 lib/Makefile.in            |   2 +-
 lib/fingerprint.c          |  66 ++++++
 {src => lib}/fingerprint.h |   0
 lib/gnulib.mk.in           |  14 +-
 lib/memmem.c               |  71 ++++++
 lib/str-two-way.h          | 452 +++++++++++++++++++++++++++++++++++++
 m4/gnulib-comp.m4          |  22 +-
 m4/memmem.m4               | 154 +++++++++++++
 src/Makefile.in            |  30 ++-
 12 files changed, 880 insertions(+), 60 deletions(-)
 create mode 100644 lib/fingerprint.c
 rename {src => lib}/fingerprint.h (100%)
 create mode 100644 lib/memmem.c
 create mode 100644 lib/str-two-way.h
 create mode 100644 m4/memmem.m4

diff --git a/admin/merge-gnulib b/admin/merge-gnulib
index 055e791d62..4a69310d83 100755
--- a/admin/merge-gnulib
+++ b/admin/merge-gnulib
@@ -36,7 +36,7 @@ GNULIB_MODULES=
   filemode filevercmp flexmember fpieee fstatat fsusage fsync
   getloadavg getopt-gnu gettime gettimeofday gitlog-to-changelog
   ieee754-h ignore-value intprops largefile lstat
-  manywarnings memrchr minmax mkostemp mktime nstrftime
+  manywarnings memmem-simple memrchr minmax mkostemp mktime nstrftime
   pipe2 pselect pthread_sigmask putenv qcopy-acl readlink readlinkat regex
   sig2str socklen stat-time std-gnu11 stdalign stddef stdio
   stpcpy strtoimax symlink sys_stat sys_time
@@ -47,7 +47,7 @@ GNULIB_MODULES=
 
 AVOIDED_MODULES='
   btowc close dup fchdir fstat langinfo lock
-  malloc-posix mbrtowc mbsinit mkdir msvc-inval msvc-nothrow nl_langinfo
+  malloc-posix mbrtowc mbsinit memchr mkdir msvc-inval msvc-nothrow nl_langinfo
   openat-die opendir raise
   save-cwd select setenv sigprocmask stat stdarg stdbool
   threadlib tzset unsetenv utime utime-h
diff --git a/configure.ac b/configure.ac
index 1814a30cbc..cff9a27422 100644
--- a/configure.ac
+++ b/configure.ac
@@ -379,8 +379,12 @@ AC_DEFUN
 fi
 
 if test "$with_pdumper" = "yes"; then
-    AC_DEFINE(HAVE_PDUMPER, 1, [Define to build with portable dumper support])
+  AC_DEFINE([HAVE_PDUMPER], 1, [Define to build with portable dumper support])
+  HAVE_PDUMPER=yes
+else
+  HAVE_PDUMPER=no
 fi
+AC_SUBST([HAVE_PDUMPER])
 
 DUMPING=$with_dumping
 AC_SUBST(DUMPING)
diff --git a/lib-src/make-fingerprint.c b/lib-src/make-fingerprint.c
index 35bb8b98a0..dc21fc2aa9 100644
--- a/lib-src/make-fingerprint.c
+++ b/lib-src/make-fingerprint.c
@@ -25,14 +25,25 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 
 #include <config.h>
 
+#include <limits.h>
 #include <stdarg.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/stat.h>
+
 #include <sysstdio.h>
-#include <sha256.h>
+
+#include <fingerprint.h>
 #include <getopt.h>
+#include <intprops.h>
+#include <min-max.h>
+#include <sha256.h>
+
+#ifndef SSIZE_MAX
+# define SSIZE_MAX TYPE_MAXIMUM (ssize_t)
+#endif
 
 #ifdef WINDOWSNT
 /* Defined to be sys_fopen in ms-w32.h, but only #ifdef emacs, so this
@@ -54,41 +65,62 @@ main (int argc, char **argv)
           raw = true;
           break;
         case 'h':
-          printf ("make-fingerprint [-r] FILES...: compute a hash\n");
-          return 0;
+          printf ("make-fingerprint [-r] FILE: replace or compute a hash\n");
+          return EXIT_SUCCESS;
         default:
-          return 1;
+          return EXIT_FAILURE;
         }
     }
 
   struct sha256_ctx ctx;
   sha256_init_ctx (&ctx);
 
-  for (int i = optind; i < argc; ++i)
+  if (argc - optind != 1)
     {
-      FILE *f = fopen (argv[i], "r" FOPEN_BINARY);
-      if (!f)
-        {
-          fprintf (stderr, "%s: Error: could not open %s\n",
-                   argv[0], argv[i]);
-          return 1;
-        }
+      fprintf (stderr, "%s: missing or extra file operand\n", argv[0]);
+      return EXIT_FAILURE;
+    }
 
-      char buf[128*1024];
-      do
-        {
-          size_t chunksz = fread (buf, 1, sizeof (buf), f);
-          if (ferror (f))
-            {
-              fprintf (stderr, "%s: Error: could not read %s\n",
-                       argv[0], argv[i]);
-              return 1;
-            }
-          sha256_process_bytes (buf, chunksz, &ctx);
-        } while (!feof (f));
-      fclose (f);
+  FILE *f = fopen (argv[1], raw ? "r" FOPEN_BINARY : "r+" FOPEN_BINARY);
+  struct stat st;
+  if (!f || fstat (fileno (f), &st) != 0)
+    {
+      perror (argv[1]);
+      return EXIT_FAILURE;
     }
 
+  if (!S_ISREG (st.st_mode))
+    {
+      fprintf (stderr, "%s: Error: %s is not a regular file\n",
+	       argv[0], argv[1]);
+      return EXIT_FAILURE;
+    }
+
+  ptrdiff_t maxlen = min (min (TYPE_MAXIMUM (off_t), PTRDIFF_MAX),
+			  min (SIZE_MAX, SSIZE_MAX));
+  if (maxlen <= st.st_size)
+    {
+      fprintf (stderr, "%s: %s: file too big\n", argv[0], argv[1]);
+      return EXIT_FAILURE;
+    }
+
+  char *buf = malloc (st.st_size + 1);
+  if (!buf)
+    {
+      perror ("malloc");
+      return EXIT_FAILURE;
+    }
+
+  size_t chunksz = fread (buf, 1, st.st_size + 1, f);
+  if (ferror (f) || chunksz != st.st_size)
+    {
+      fprintf (stderr, "%s: Error: could not read %s\n",
+	       argv[0], argv[1]);
+      return EXIT_FAILURE;
+    }
+
+  sha256_process_bytes (buf, chunksz, &ctx);
+
   unsigned char digest[32];
   sha256_finish_ctx (&ctx, digest);
 
@@ -99,12 +131,37 @@ main (int argc, char **argv)
     }
   else
     {
-      puts ("#include \"fingerprint.h\"\n"
-	    "unsigned char const fingerprint[] =\n"
-	    "{");
-      for (int i = 0; i < 32; ++i)
-        printf ("\t0x%02X,\n", digest[i]);
-      puts ("};");
+      char *finger = memmem (buf, chunksz, fingerprint, sizeof fingerprint);
+      if (!finger)
+	{
+	  fprintf (stderr, "%s: %s: missing fingerprint\n", argv[0], argv[1]);
+	  return EXIT_FAILURE;
+	}
+      else if (memmem (finger + 1, buf + chunksz - (finger + 1),
+		       fingerprint, sizeof fingerprint))
+	{
+	  fprintf (stderr, "%s: %s: two occurrences of fingerprint\n",
+		   argv[0], argv[1]);
+	  return EXIT_FAILURE;
+	}
+
+      if (fseeko (f, finger - buf, SEEK_SET) != 0)
+	{
+	  perror (argv[1]);
+	  return EXIT_FAILURE;
+	}
+
+      if (fwrite (digest, 1, sizeof digest, f) != sizeof digest)
+	{
+	  perror (argv[1]);
+	  return EXIT_FAILURE;
+	}
+    }
+
+  if (fclose (f) != 0)
+    {
+      perror (argv[1]);
+      return EXIT_FAILURE;
     }
 
   return EXIT_SUCCESS;
diff --git a/lib/Makefile.in b/lib/Makefile.in
index f2d203564a..ac32c7070f 100644
--- a/lib/Makefile.in
+++ b/lib/Makefile.in
@@ -84,7 +84,7 @@ Makefile:
 # and building it would just waste time.
 not_emacs_OBJECTS = regex.o
 
-libgnu_a_OBJECTS = $(gl_LIBOBJS) \
+libgnu_a_OBJECTS = fingerprint.o $(gl_LIBOBJS) \
   $(patsubst %.c,%.o,$(filter %.c,$(libgnu_a_SOURCES)))
 for_emacs_OBJECTS = $(filter-out $(not_emacs_OBJECTS),$(libgnu_a_OBJECTS))
 libegnu_a_OBJECTS = $(patsubst %.o,e-%.o,$(for_emacs_OBJECTS))
diff --git a/lib/fingerprint.c b/lib/fingerprint.c
new file mode 100644
index 0000000000..e55de9c6da
--- /dev/null
+++ b/lib/fingerprint.c
@@ -0,0 +1,66 @@
+/* Placeholder fingerprint for Emacs
+
+Copyright 2019 Free Software Foundation, Inc.
+
+This file is part of GNU Emacs.
+
+GNU Emacs 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.
+
+GNU Emacs 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 GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <config.h>
+
+#include "fingerprint.h"
+
+/* This random fingerprint was generated by the shell command:
+
+   shuf -i 0-255 -n 32 -r | awk '{printf "   0x%.02X,\n", $0}'
+
+   In the final Emacs executable, this random fingerprint is replaced
+   by a fingerprint of the temporary Emacs executable that was built
+   along the way.  */
+
+unsigned char const fingerprint[] =
+  {
+   0xDE,
+   0x86,
+   0xBB,
+   0x99,
+   0xFF,
+   0xF5,
+   0x46,
+   0x9A,
+   0x9E,
+   0x3F,
+   0x9F,
+   0x5D,
+   0x9A,
+   0xDF,
+   0xF0,
+   0x91,
+   0xBD,
+   0xCD,
+   0xC1,
+   0xE8,
+   0x0C,
+   0x16,
+   0x1E,
+   0xAF,
+   0xB8,
+   0x6C,
+   0xE2,
+   0x2B,
+   0xB1,
+   0x24,
+   0xCE,
+   0xB0,
+  };
diff --git a/src/fingerprint.h b/lib/fingerprint.h
similarity index 100%
rename from src/fingerprint.h
rename to lib/fingerprint.h
diff --git a/lib/gnulib.mk.in b/lib/gnulib.mk.in
index 1450df9f63..03160340c8 100644
--- a/lib/gnulib.mk.in
+++ b/lib/gnulib.mk.in
@@ -44,6 +44,7 @@
 #  --avoid=malloc-posix \
 #  --avoid=mbrtowc \
 #  --avoid=mbsinit \
+#  --avoid=memchr \
 #  --avoid=mkdir \
 #  --avoid=msvc-inval \
 #  --avoid=msvc-nothrow \
@@ -111,6 +112,7 @@
 #  largefile \
 #  lstat \
 #  manywarnings \
+#  memmem-simple \
 #  memrchr \
 #  minmax \
 #  mkostemp \
@@ -1053,7 +1055,6 @@ gl_GNULIB_ENABLED_03e0aaad4cb89ca757653bd367a6ccb7 = @gl_GNULIB_ENABLED_03e0aaad
 gl_GNULIB_ENABLED_2049e887c7e5308faad27b3f894bb8c9 = @gl_GNULIB_ENABLED_2049e887c7e5308faad27b3f894bb8c9@
 gl_GNULIB_ENABLED_21ee726a3540c09237a8e70c0baf7467 = @gl_GNULIB_ENABLED_21ee726a3540c09237a8e70c0baf7467@
 gl_GNULIB_ENABLED_260941c0e5dc67ec9e87d1fb321c300b = @gl_GNULIB_ENABLED_260941c0e5dc67ec9e87d1fb321c300b@
-gl_GNULIB_ENABLED_37f71b604aa9c54446783d80f42fe547 = @gl_GNULIB_ENABLED_37f71b604aa9c54446783d80f42fe547@
 gl_GNULIB_ENABLED_5264294aa0a5557541b53c8c741f7f31 = @gl_GNULIB_ENABLED_5264294aa0a5557541b53c8c741f7f31@
 gl_GNULIB_ENABLED_6099e9737f757db36c47fa9d9f02e88c = @gl_GNULIB_ENABLED_6099e9737f757db36c47fa9d9f02e88c@
 gl_GNULIB_ENABLED_682e609604ccaac6be382e4ee3a4eaec = @gl_GNULIB_ENABLED_682e609604ccaac6be382e4ee3a4eaec@
@@ -1963,6 +1964,17 @@ EXTRA_libgnu_a_SOURCES += lstat.c
 endif
 ## end   gnulib module lstat
 
+## begin gnulib module memmem-simple
+ifeq (,$(OMIT_GNULIB_MODULE_memmem-simple))
+
+
+EXTRA_DIST += memmem.c str-two-way.h
+
+EXTRA_libgnu_a_SOURCES += memmem.c
+
+endif
+## end   gnulib module memmem-simple
+
 ## begin gnulib module memrchr
 ifeq (,$(OMIT_GNULIB_MODULE_memrchr))
 
diff --git a/lib/memmem.c b/lib/memmem.c
new file mode 100644
index 0000000000..12ae24f41b
--- /dev/null
+++ b/lib/memmem.c
@@ -0,0 +1,71 @@
+/* Copyright (C) 1991-1994, 1996-1998, 2000, 2004, 2007-2019 Free Software
+   Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   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, 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 <https://www.gnu.org/licenses/>.  */
+
+/* This particular implementation was written by Eric Blake, 2008.  */
+
+#ifndef _LIBC
+# include <config.h>
+#endif
+
+/* Specification of memmem.  */
+#include <string.h>
+
+#define RETURN_TYPE void *
+#define AVAILABLE(h, h_l, j, n_l) ((j) <= (h_l) - (n_l))
+#include "str-two-way.h"
+
+/* Return the first occurrence of NEEDLE in HAYSTACK.  Return HAYSTACK
+   if NEEDLE_LEN is 0, otherwise NULL if NEEDLE is not found in
+   HAYSTACK.  */
+void *
+memmem (const void *haystack_start, size_t haystack_len,
+        const void *needle_start, size_t needle_len)
+{
+  /* Abstract memory is considered to be an array of 'unsigned char' values,
+     not an array of 'char' values.  See ISO C 99 section 6.2.6.1.  */
+  const unsigned char *haystack = (const unsigned char *) haystack_start;
+  const unsigned char *needle = (const unsigned char *) needle_start;
+
+  if (needle_len == 0)
+    /* The first occurrence of the empty string is deemed to occur at
+       the beginning of the string.  */
+    return (void *) haystack;
+
+  /* Sanity check, otherwise the loop might search through the whole
+     memory.  */
+  if (__builtin_expect (haystack_len < needle_len, 0))
+    return NULL;
+
+  /* Use optimizations in memchr when possible, to reduce the search
+     size of haystack using a linear algorithm with a smaller
+     coefficient.  However, avoid memchr for long needles, since we
+     can often achieve sublinear performance.  */
+  if (needle_len < LONG_NEEDLE_THRESHOLD)
+    {
+      haystack = memchr (haystack, *needle, haystack_len);
+      if (!haystack || __builtin_expect (needle_len == 1, 0))
+        return (void *) haystack;
+      haystack_len -= haystack - (const unsigned char *) haystack_start;
+      if (haystack_len < needle_len)
+        return NULL;
+      return two_way_short_needle (haystack, haystack_len, needle, needle_len);
+    }
+  else
+    return two_way_long_needle (haystack, haystack_len, needle, needle_len);
+}
+
+#undef LONG_NEEDLE_THRESHOLD
diff --git a/lib/str-two-way.h b/lib/str-two-way.h
new file mode 100644
index 0000000000..9155e6b560
--- /dev/null
+++ b/lib/str-two-way.h
@@ -0,0 +1,452 @@
+/* Byte-wise substring search, using the Two-Way algorithm.
+   Copyright (C) 2008-2019 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   Written by Eric Blake <ebb9@byu.net>, 2008.
+
+   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, 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 <https://www.gnu.org/licenses/>.  */
+
+/* Before including this file, you need to include <config.h> and
+   <string.h>, and define:
+     RESULT_TYPE             A macro that expands to the return type.
+     AVAILABLE(h, h_l, j, n_l)
+                             A macro that returns nonzero if there are
+                             at least N_L bytes left starting at H[J].
+                             H is 'unsigned char *', H_L, J, and N_L
+                             are 'size_t'; H_L is an lvalue.  For
+                             NUL-terminated searches, H_L can be
+                             modified each iteration to avoid having
+                             to compute the end of H up front.
+
+  For case-insensitivity, you may optionally define:
+     CMP_FUNC(p1, p2, l)     A macro that returns 0 iff the first L
+                             characters of P1 and P2 are equal.
+     CANON_ELEMENT(c)        A macro that canonicalizes an element right after
+                             it has been fetched from one of the two strings.
+                             The argument is an 'unsigned char'; the result
+                             must be an 'unsigned char' as well.
+
+  This file undefines the macros documented above, and defines
+  LONG_NEEDLE_THRESHOLD.
+*/
+
+#include <limits.h>
+#include <stdint.h>
+
+/* We use the Two-Way string matching algorithm (also known as
+   Chrochemore-Perrin), which guarantees linear complexity with
+   constant space.  Additionally, for long needles, we also use a bad
+   character shift table similar to the Boyer-Moore algorithm to
+   achieve improved (potentially sub-linear) performance.
+
+   See http://www-igm.univ-mlv.fr/~lecroq/string/node26.html#SECTION00260,
+   https://en.wikipedia.org/wiki/Boyer-Moore_string_search_algorithm,
+   https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.34.6641&rep=rep1&type=pdf
+*/
+
+/* Point at which computing a bad-byte shift table is likely to be
+   worthwhile.  Small needles should not compute a table, since it
+   adds (1 << CHAR_BIT) + NEEDLE_LEN computations of preparation for a
+   speedup no greater than a factor of NEEDLE_LEN.  The larger the
+   needle, the better the potential performance gain.  On the other
+   hand, on non-POSIX systems with CHAR_BIT larger than eight, the
+   memory required for the table is prohibitive.  */
+#if CHAR_BIT < 10
+# define LONG_NEEDLE_THRESHOLD 32U
+#else
+# define LONG_NEEDLE_THRESHOLD SIZE_MAX
+#endif
+
+#ifndef MAX
+# define MAX(a, b) ((a < b) ? (b) : (a))
+#endif
+
+#ifndef CANON_ELEMENT
+# define CANON_ELEMENT(c) c
+#endif
+#ifndef CMP_FUNC
+# define CMP_FUNC memcmp
+#endif
+
+/* Perform a critical factorization of NEEDLE, of length NEEDLE_LEN.
+   Return the index of the first byte in the right half, and set
+   *PERIOD to the global period of the right half.
+
+   The global period of a string is the smallest index (possibly its
+   length) at which all remaining bytes in the string are repetitions
+   of the prefix (the last repetition may be a subset of the prefix).
+
+   When NEEDLE is factored into two halves, a local period is the
+   length of the smallest word that shares a suffix with the left half
+   and shares a prefix with the right half.  All factorizations of a
+   non-empty NEEDLE have a local period of at least 1 and no greater
+   than NEEDLE_LEN.
+
+   A critical factorization has the property that the local period
+   equals the global period.  All strings have at least one critical
+   factorization with the left half smaller than the global period.
+   And while some strings have more than one critical factorization,
+   it is provable that with an ordered alphabet, at least one of the
+   critical factorizations corresponds to a maximal suffix.
+
+   Given an ordered alphabet, a critical factorization can be computed
+   in linear time, with 2 * NEEDLE_LEN comparisons, by computing the
+   shorter of two ordered maximal suffixes.  The ordered maximal
+   suffixes are determined by lexicographic comparison while tracking
+   periodicity.  */
+static size_t
+critical_factorization (const unsigned char *needle, size_t needle_len,
+                        size_t *period)
+{
+  /* Index of last byte of left half, or SIZE_MAX.  */
+  size_t max_suffix, max_suffix_rev;
+  size_t j; /* Index into NEEDLE for current candidate suffix.  */
+  size_t k; /* Offset into current period.  */
+  size_t p; /* Intermediate period.  */
+  unsigned char a, b; /* Current comparison bytes.  */
+
+  /* Special case NEEDLE_LEN of 1 or 2 (all callers already filtered
+     out 0-length needles.  */
+  if (needle_len < 3)
+    {
+      *period = 1;
+      return needle_len - 1;
+    }
+
+  /* Invariants:
+     0 <= j < NEEDLE_LEN - 1
+     -1 <= max_suffix{,_rev} < j (treating SIZE_MAX as if it were signed)
+     min(max_suffix, max_suffix_rev) < global period of NEEDLE
+     1 <= p <= global period of NEEDLE
+     p == global period of the substring NEEDLE[max_suffix{,_rev}+1...j]
+     1 <= k <= p
+  */
+
+  /* Perform lexicographic search.  */
+  max_suffix = SIZE_MAX;
+  j = 0;
+  k = p = 1;
+  while (j + k < needle_len)
+    {
+      a = CANON_ELEMENT (needle[j + k]);
+      b = CANON_ELEMENT (needle[max_suffix + k]);
+      if (a < b)
+        {
+          /* Suffix is smaller, period is entire prefix so far.  */
+          j += k;
+          k = 1;
+          p = j - max_suffix;
+        }
+      else if (a == b)
+        {
+          /* Advance through repetition of the current period.  */
+          if (k != p)
+            ++k;
+          else
+            {
+              j += p;
+              k = 1;
+            }
+        }
+      else /* b < a */
+        {
+          /* Suffix is larger, start over from current location.  */
+          max_suffix = j++;
+          k = p = 1;
+        }
+    }
+  *period = p;
+
+  /* Perform reverse lexicographic search.  */
+  max_suffix_rev = SIZE_MAX;
+  j = 0;
+  k = p = 1;
+  while (j + k < needle_len)
+    {
+      a = CANON_ELEMENT (needle[j + k]);
+      b = CANON_ELEMENT (needle[max_suffix_rev + k]);
+      if (b < a)
+        {
+          /* Suffix is smaller, period is entire prefix so far.  */
+          j += k;
+          k = 1;
+          p = j - max_suffix_rev;
+        }
+      else if (a == b)
+        {
+          /* Advance through repetition of the current period.  */
+          if (k != p)
+            ++k;
+          else
+            {
+              j += p;
+              k = 1;
+            }
+        }
+      else /* a < b */
+        {
+          /* Suffix is larger, start over from current location.  */
+          max_suffix_rev = j++;
+          k = p = 1;
+        }
+    }
+
+  /* Choose the shorter suffix.  Return the index of the first byte of
+     the right half, rather than the last byte of the left half.
+
+     For some examples, 'banana' has two critical factorizations, both
+     exposed by the two lexicographic extreme suffixes of 'anana' and
+     'nana', where both suffixes have a period of 2.  On the other
+     hand, with 'aab' and 'bba', both strings have a single critical
+     factorization of the last byte, with the suffix having a period
+     of 1.  While the maximal lexicographic suffix of 'aab' is 'b',
+     the maximal lexicographic suffix of 'bba' is 'ba', which is not a
+     critical factorization.  Conversely, the maximal reverse
+     lexicographic suffix of 'a' works for 'bba', but not 'ab' for
+     'aab'.  The shorter suffix of the two will always be a critical
+     factorization.  */
+  if (max_suffix_rev + 1 < max_suffix + 1)
+    return max_suffix + 1;
+  *period = p;
+  return max_suffix_rev + 1;
+}
+
+/* Return the first location of non-empty NEEDLE within HAYSTACK, or
+   NULL.  HAYSTACK_LEN is the minimum known length of HAYSTACK.  This
+   method is optimized for NEEDLE_LEN < LONG_NEEDLE_THRESHOLD.
+   Performance is guaranteed to be linear, with an initialization cost
+   of 2 * NEEDLE_LEN comparisons.
+
+   If AVAILABLE does not modify HAYSTACK_LEN (as in memmem), then at
+   most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching.
+   If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 *
+   HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching.  */
+static RETURN_TYPE
+two_way_short_needle (const unsigned char *haystack, size_t haystack_len,
+                      const unsigned char *needle, size_t needle_len)
+{
+  size_t i; /* Index into current byte of NEEDLE.  */
+  size_t j; /* Index into current window of HAYSTACK.  */
+  size_t period; /* The period of the right half of needle.  */
+  size_t suffix; /* The index of the right half of needle.  */
+
+  /* Factor the needle into two halves, such that the left half is
+     smaller than the global period, and the right half is
+     periodic (with a period as large as NEEDLE_LEN - suffix).  */
+  suffix = critical_factorization (needle, needle_len, &period);
+
+  /* Perform the search.  Each iteration compares the right half
+     first.  */
+  if (CMP_FUNC (needle, needle + period, suffix) == 0)
+    {
+      /* Entire needle is periodic; a mismatch in the left half can
+         only advance by the period, so use memory to avoid rescanning
+         known occurrences of the period in the right half.  */
+      size_t memory = 0;
+      j = 0;
+      while (AVAILABLE (haystack, haystack_len, j, needle_len))
+        {
+          /* Scan for matches in right half.  */
+          i = MAX (suffix, memory);
+          while (i < needle_len && (CANON_ELEMENT (needle[i])
+                                    == CANON_ELEMENT (haystack[i + j])))
+            ++i;
+          if (needle_len <= i)
+            {
+              /* Scan for matches in left half.  */
+              i = suffix - 1;
+              while (memory < i + 1 && (CANON_ELEMENT (needle[i])
+                                        == CANON_ELEMENT (haystack[i + j])))
+                --i;
+              if (i + 1 < memory + 1)
+                return (RETURN_TYPE) (haystack + j);
+              /* No match, so remember how many repetitions of period
+                 on the right half were scanned.  */
+              j += period;
+              memory = needle_len - period;
+            }
+          else
+            {
+              j += i - suffix + 1;
+              memory = 0;
+            }
+        }
+    }
+  else
+    {
+      /* The two halves of needle are distinct; no extra memory is
+         required, and any mismatch results in a maximal shift.  */
+      period = MAX (suffix, needle_len - suffix) + 1;
+      j = 0;
+      while (AVAILABLE (haystack, haystack_len, j, needle_len))
+        {
+          /* Scan for matches in right half.  */
+          i = suffix;
+          while (i < needle_len && (CANON_ELEMENT (needle[i])
+                                    == CANON_ELEMENT (haystack[i + j])))
+            ++i;
+          if (needle_len <= i)
+            {
+              /* Scan for matches in left half.  */
+              i = suffix - 1;
+              while (i != SIZE_MAX && (CANON_ELEMENT (needle[i])
+                                       == CANON_ELEMENT (haystack[i + j])))
+                --i;
+              if (i == SIZE_MAX)
+                return (RETURN_TYPE) (haystack + j);
+              j += period;
+            }
+          else
+            j += i - suffix + 1;
+        }
+    }
+  return NULL;
+}
+
+/* Return the first location of non-empty NEEDLE within HAYSTACK, or
+   NULL.  HAYSTACK_LEN is the minimum known length of HAYSTACK.  This
+   method is optimized for LONG_NEEDLE_THRESHOLD <= NEEDLE_LEN.
+   Performance is guaranteed to be linear, with an initialization cost
+   of 3 * NEEDLE_LEN + (1 << CHAR_BIT) operations.
+
+   If AVAILABLE does not modify HAYSTACK_LEN (as in memmem), then at
+   most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching,
+   and sublinear performance O(HAYSTACK_LEN / NEEDLE_LEN) is possible.
+   If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 *
+   HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching, and
+   sublinear performance is not possible.  */
+static RETURN_TYPE
+two_way_long_needle (const unsigned char *haystack, size_t haystack_len,
+                     const unsigned char *needle, size_t needle_len)
+{
+  size_t i; /* Index into current byte of NEEDLE.  */
+  size_t j; /* Index into current window of HAYSTACK.  */
+  size_t period; /* The period of the right half of needle.  */
+  size_t suffix; /* The index of the right half of needle.  */
+  size_t shift_table[1U << CHAR_BIT]; /* See below.  */
+
+  /* Factor the needle into two halves, such that the left half is
+     smaller than the global period, and the right half is
+     periodic (with a period as large as NEEDLE_LEN - suffix).  */
+  suffix = critical_factorization (needle, needle_len, &period);
+
+  /* Populate shift_table.  For each possible byte value c,
+     shift_table[c] is the distance from the last occurrence of c to
+     the end of NEEDLE, or NEEDLE_LEN if c is absent from the NEEDLE.
+     shift_table[NEEDLE[NEEDLE_LEN - 1]] contains the only 0.  */
+  for (i = 0; i < 1U << CHAR_BIT; i++)
+    shift_table[i] = needle_len;
+  for (i = 0; i < needle_len; i++)
+    shift_table[CANON_ELEMENT (needle[i])] = needle_len - i - 1;
+
+  /* Perform the search.  Each iteration compares the right half
+     first.  */
+  if (CMP_FUNC (needle, needle + period, suffix) == 0)
+    {
+      /* Entire needle is periodic; a mismatch in the left half can
+         only advance by the period, so use memory to avoid rescanning
+         known occurrences of the period in the right half.  */
+      size_t memory = 0;
+      size_t shift;
+      j = 0;
+      while (AVAILABLE (haystack, haystack_len, j, needle_len))
+        {
+          /* Check the last byte first; if it does not match, then
+             shift to the next possible match location.  */
+          shift = shift_table[CANON_ELEMENT (haystack[j + needle_len - 1])];
+          if (0 < shift)
+            {
+              if (memory && shift < period)
+                {
+                  /* Since needle is periodic, but the last period has
+                     a byte out of place, there can be no match until
+                     after the mismatch.  */
+                  shift = needle_len - period;
+                }
+              memory = 0;
+              j += shift;
+              continue;
+            }
+          /* Scan for matches in right half.  The last byte has
+             already been matched, by virtue of the shift table.  */
+          i = MAX (suffix, memory);
+          while (i < needle_len - 1 && (CANON_ELEMENT (needle[i])
+                                        == CANON_ELEMENT (haystack[i + j])))
+            ++i;
+          if (needle_len - 1 <= i)
+            {
+              /* Scan for matches in left half.  */
+              i = suffix - 1;
+              while (memory < i + 1 && (CANON_ELEMENT (needle[i])
+                                        == CANON_ELEMENT (haystack[i + j])))
+                --i;
+              if (i + 1 < memory + 1)
+                return (RETURN_TYPE) (haystack + j);
+              /* No match, so remember how many repetitions of period
+                 on the right half were scanned.  */
+              j += period;
+              memory = needle_len - period;
+            }
+          else
+            {
+              j += i - suffix + 1;
+              memory = 0;
+            }
+        }
+    }
+  else
+    {
+      /* The two halves of needle are distinct; no extra memory is
+         required, and any mismatch results in a maximal shift.  */
+      size_t shift;
+      period = MAX (suffix, needle_len - suffix) + 1;
+      j = 0;
+      while (AVAILABLE (haystack, haystack_len, j, needle_len))
+        {
+          /* Check the last byte first; if it does not match, then
+             shift to the next possible match location.  */
+          shift = shift_table[CANON_ELEMENT (haystack[j + needle_len - 1])];
+          if (0 < shift)
+            {
+              j += shift;
+              continue;
+            }
+          /* Scan for matches in right half.  The last byte has
+             already been matched, by virtue of the shift table.  */
+          i = suffix;
+          while (i < needle_len - 1 && (CANON_ELEMENT (needle[i])
+                                        == CANON_ELEMENT (haystack[i + j])))
+            ++i;
+          if (needle_len - 1 <= i)
+            {
+              /* Scan for matches in left half.  */
+              i = suffix - 1;
+              while (i != SIZE_MAX && (CANON_ELEMENT (needle[i])
+                                       == CANON_ELEMENT (haystack[i + j])))
+                --i;
+              if (i == SIZE_MAX)
+                return (RETURN_TYPE) (haystack + j);
+              j += period;
+            }
+          else
+            j += i - suffix + 1;
+        }
+    }
+  return NULL;
+}
+
+#undef AVAILABLE
+#undef CANON_ELEMENT
+#undef CMP_FUNC
+#undef MAX
+#undef RETURN_TYPE
diff --git a/m4/gnulib-comp.m4 b/m4/gnulib-comp.m4
index f25a0e4081..f648b7a495 100644
--- a/m4/gnulib-comp.m4
+++ b/m4/gnulib-comp.m4
@@ -113,6 +113,7 @@ AC_DEFUN
   # Code from module localtime-buffer:
   # Code from module lstat:
   # Code from module manywarnings:
+  # Code from module memmem-simple:
   # Code from module memrchr:
   # Code from module minmax:
   # Code from module mkostemp:
@@ -195,6 +196,7 @@ AC_DEFUN
   gl_source_base='lib'
   gl_FUNC_ACL
   gl_FUNC_ALLOCA
+  gl___BUILTIN_EXPECT
   gl_BYTESWAP
   AC_CHECK_FUNCS_ONCE([readlinkat])
   gl_CLOCK_TIME
@@ -302,6 +304,11 @@ AC_DEFUN
     gl_PREREQ_LSTAT
   fi
   gl_SYS_STAT_MODULE_INDICATOR([lstat])
+  gl_FUNC_MEMMEM_SIMPLE
+  if test $HAVE_MEMMEM = 0 || test $REPLACE_MEMMEM = 1; then
+    AC_LIBOBJ([memmem])
+  fi
+  gl_STRING_MODULE_INDICATOR([memmem])
   gl_FUNC_MEMRCHR
   if test $ac_cv_func_memrchr = no; then
     AC_LIBOBJ([memrchr])
@@ -426,7 +433,6 @@ AC_DEFUN
   gl_UTIMENS
   AC_C_VARARRAYS
   gl_gnulib_enabled_260941c0e5dc67ec9e87d1fb321c300b=false
-  gl_gnulib_enabled_37f71b604aa9c54446783d80f42fe547=false
   gl_gnulib_enabled_cloexec=false
   gl_gnulib_enabled_dirfd=false
   gl_gnulib_enabled_euidaccess=false
@@ -450,13 +456,6 @@ AC_DEFUN
       func_gl_gnulib_m4code_open
     fi
   }
-  func_gl_gnulib_m4code_37f71b604aa9c54446783d80f42fe547 ()
-  {
-    if ! $gl_gnulib_enabled_37f71b604aa9c54446783d80f42fe547; then
-      gl___BUILTIN_EXPECT
-      gl_gnulib_enabled_37f71b604aa9c54446783d80f42fe547=true
-    fi
-  }
   func_gl_gnulib_m4code_cloexec ()
   {
     if ! $gl_gnulib_enabled_cloexec; then
@@ -652,9 +651,6 @@ AC_DEFUN
   if test $HAVE_READLINKAT = 0; then
     func_gl_gnulib_m4code_03e0aaad4cb89ca757653bd367a6ccb7
   fi
-  if test $ac_use_included_regex = yes; then
-    func_gl_gnulib_m4code_37f71b604aa9c54446783d80f42fe547
-  fi
   if test $ac_use_included_regex = yes; then
     func_gl_gnulib_m4code_21ee726a3540c09237a8e70c0baf7467
   fi
@@ -666,7 +662,6 @@ AC_DEFUN
   fi
   m4_pattern_allow([^gl_GNULIB_ENABLED_])
   AM_CONDITIONAL([gl_GNULIB_ENABLED_260941c0e5dc67ec9e87d1fb321c300b], [$gl_gnulib_enabled_260941c0e5dc67ec9e87d1fb321c300b])
-  AM_CONDITIONAL([gl_GNULIB_ENABLED_37f71b604aa9c54446783d80f42fe547], [$gl_gnulib_enabled_37f71b604aa9c54446783d80f42fe547])
   AM_CONDITIONAL([gl_GNULIB_ENABLED_cloexec], [$gl_gnulib_enabled_cloexec])
   AM_CONDITIONAL([gl_GNULIB_ENABLED_dirfd], [$gl_gnulib_enabled_dirfd])
   AM_CONDITIONAL([gl_GNULIB_ENABLED_euidaccess], [$gl_gnulib_enabled_euidaccess])
@@ -916,6 +911,7 @@ AC_DEFUN
   lib/lstat.c
   lib/md5.c
   lib/md5.h
+  lib/memmem.c
   lib/memrchr.c
   lib/minmax.h
   lib/mkostemp.c
@@ -959,6 +955,7 @@ AC_DEFUN
   lib/stdio.in.h
   lib/stdlib.in.h
   lib/stpcpy.c
+  lib/str-two-way.h
   lib/strftime.h
   lib/string.in.h
   lib/strtoimax.c
@@ -1049,6 +1046,7 @@ AC_DEFUN
   m4/manywarnings.m4
   m4/mbstate_t.m4
   m4/md5.m4
+  m4/memmem.m4
   m4/memrchr.m4
   m4/minmax.m4
   m4/mkostemp.m4
diff --git a/m4/memmem.m4 b/m4/memmem.m4
new file mode 100644
index 0000000000..af2d5bbcf5
--- /dev/null
+++ b/m4/memmem.m4
@@ -0,0 +1,154 @@
+# memmem.m4 serial 25
+dnl Copyright (C) 2002-2004, 2007-2019 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+dnl Check that memmem is present and functional.
+AC_DEFUN([gl_FUNC_MEMMEM_SIMPLE],
+[
+  dnl Persuade glibc <string.h> to declare memmem().
+  AC_REQUIRE([AC_USE_SYSTEM_EXTENSIONS])
+
+  AC_REQUIRE([gl_HEADER_STRING_H_DEFAULTS])
+  AC_CHECK_FUNCS([memmem])
+  if test $ac_cv_func_memmem = yes; then
+    HAVE_MEMMEM=1
+  else
+    HAVE_MEMMEM=0
+  fi
+  AC_CHECK_DECLS_ONCE([memmem])
+  if test $ac_cv_have_decl_memmem = no; then
+    HAVE_DECL_MEMMEM=0
+  else
+    dnl Detect https://sourceware.org/bugzilla/show_bug.cgi?id=12092.
+    dnl Also check that we handle empty needles correctly.
+    AC_CACHE_CHECK([whether memmem works],
+      [gl_cv_func_memmem_works_always],
+      [AC_RUN_IFELSE([AC_LANG_PROGRAM([[
+#include <string.h> /* for memmem */
+#define P "_EF_BF_BD"
+#define HAYSTACK "F_BD_CE_BD" P P P P "_C3_88_20" P P P "_C3_A7_20" P
+#define NEEDLE P P P P P
+]], [[
+    int result = 0;
+    if (memmem (HAYSTACK, strlen (HAYSTACK), NEEDLE, strlen (NEEDLE)))
+      result |= 1;
+    /* Check for empty needle behavior.  */
+    {
+      const char *haystack = "AAA";
+      if (memmem (haystack, 3, NULL, 0) != haystack)
+        result |= 2;
+    }
+    return result;
+    ]])],
+        [gl_cv_func_memmem_works_always=yes],
+        [gl_cv_func_memmem_works_always=no],
+        [dnl glibc 2.9..2.12 and cygwin 1.7.7 have issue #12092 above.
+         dnl Also empty needles work on glibc >= 2.1 and cygwin >= 1.7.0.
+         dnl uClibc is not affected, since it uses different source code.
+         dnl Assume that it works on all other platforms (even if not linear).
+         AC_EGREP_CPP([Lucky user],
+           [
+#ifdef __GNU_LIBRARY__
+ #include <features.h>
+ #if ((__GLIBC__ == 2 && ((__GLIBC_MINOR > 0 && __GLIBC_MINOR__ < 9) \
+                          || __GLIBC_MINOR__ > 12)) \
+      || (__GLIBC__ > 2)) \
+     || defined __UCLIBC__
+  Lucky user
+ #endif
+#elif defined __CYGWIN__
+ #include <cygwin/version.h>
+ #if CYGWIN_VERSION_DLL_COMBINED > CYGWIN_VERSION_DLL_MAKE_COMBINED (1007, 7)
+  Lucky user
+ #endif
+#else
+  Lucky user
+#endif
+           ],
+           [gl_cv_func_memmem_works_always="guessing yes"],
+           [gl_cv_func_memmem_works_always="guessing no"])
+        ])
+      ])
+    case "$gl_cv_func_memmem_works_always" in
+      *yes) ;;
+      *)
+        REPLACE_MEMMEM=1
+        ;;
+    esac
+  fi
+  gl_PREREQ_MEMMEM
+]) # gl_FUNC_MEMMEM_SIMPLE
+
+dnl Additionally, check that memmem has linear performance characteristics
+AC_DEFUN([gl_FUNC_MEMMEM],
+[
+  AC_REQUIRE([gl_FUNC_MEMMEM_SIMPLE])
+  if test $HAVE_DECL_MEMMEM = 1 && test $REPLACE_MEMMEM = 0; then
+    AC_CACHE_CHECK([whether memmem works in linear time],
+      [gl_cv_func_memmem_works_fast],
+      [AC_RUN_IFELSE([AC_LANG_PROGRAM([[
+#include <signal.h> /* for signal */
+#include <string.h> /* for memmem */
+#include <stdlib.h> /* for malloc */
+#include <unistd.h> /* for alarm */
+static void quit (int sig) { _exit (sig + 128); }
+]], [[
+    int result = 0;
+    size_t m = 1000000;
+    char *haystack = (char *) malloc (2 * m + 1);
+    char *needle = (char *) malloc (m + 1);
+    /* Failure to compile this test due to missing alarm is okay,
+       since all such platforms (mingw) also lack memmem.  */
+    signal (SIGALRM, quit);
+    alarm (5);
+    /* Check for quadratic performance.  */
+    if (haystack && needle)
+      {
+        memset (haystack, 'A', 2 * m);
+        haystack[2 * m] = 'B';
+        memset (needle, 'A', m);
+        needle[m] = 'B';
+        if (!memmem (haystack, 2 * m + 1, needle, m + 1))
+          result |= 1;
+      }
+    /* Free allocated memory, in case some sanitizer is watching.  */
+    free (haystack);
+    free (needle);
+    return result;
+    ]])],
+        [gl_cv_func_memmem_works_fast=yes], [gl_cv_func_memmem_works_fast=no],
+        [dnl Only glibc >= 2.9 and cygwin > 1.7.0 are known to have a
+         dnl memmem that works in linear time.
+         AC_EGREP_CPP([Lucky user],
+           [
+#include <features.h>
+#ifdef __GNU_LIBRARY__
+ #if ((__GLIBC__ == 2 && __GLIBC_MINOR__ >= 9) || (__GLIBC__ > 2)) \
+     && !defined __UCLIBC__
+  Lucky user
+ #endif
+#endif
+#ifdef __CYGWIN__
+ #include <cygwin/version.h>
+ #if CYGWIN_VERSION_DLL_COMBINED > CYGWIN_VERSION_DLL_MAKE_COMBINED (1007, 0)
+  Lucky user
+ #endif
+#endif
+           ],
+           [gl_cv_func_memmem_works_fast="guessing yes"],
+           [gl_cv_func_memmem_works_fast="guessing no"])
+        ])
+      ])
+    case "$gl_cv_func_memmem_works_fast" in
+      *yes) ;;
+      *)
+        REPLACE_MEMMEM=1
+        ;;
+    esac
+  fi
+]) # gl_FUNC_MEMMEM
+
+# Prerequisites of lib/memmem.c.
+AC_DEFUN([gl_PREREQ_MEMMEM], [:])
diff --git a/src/Makefile.in b/src/Makefile.in
index 2348c8dae4..3aab5270a4 100644
--- a/src/Makefile.in
+++ b/src/Makefile.in
@@ -332,6 +332,7 @@ UNEXEC_OBJ =
 
 DUMPING=@DUMPING@
 CHECK_STRUCTS = @CHECK_STRUCTS@
+HAVE_PDUMPER = @HAVE_PDUMPER@
 
 # 'make' verbosity.
 AM_DEFAULT_VERBOSITY = @AM_DEFAULT_VERBOSITY@
@@ -627,19 +628,25 @@ LIBEGNU_ARCHIVE =
 $(LIBEGNU_ARCHIVE): $(config_h)
 	$(MAKE) -C $(dir $@) all
 
-FINGERPRINTED = $(LIBXMENU) $(ALLOBJS) $(LIBEGNU_ARCHIVE) $(EMACSRES)
-fingerprint.c: $(FINGERPRINTED) $(libsrc)/make-fingerprint$(EXEEXT)
-	$(AM_V_GEN)$(libsrc)/make-fingerprint$(EXEEXT) $(FINGERPRINTED) >$@.tmp
-	$(AM_V_at)mv $@.tmp $@
+ifeq ($(HAVE_PDUMPER),yes)
+  MAKE_PDUMPER_FINGERPRINT = $(libsrc)/make-fingerprint$(EXEEXT)
+else
+  MAKE_PDUMPER_FINGERPRINT =
+endif
 
 ## We have to create $(etc) here because init_cmdargs tests its
 ## existence when setting Vinstallation_directory (FIXME?).
 ## This goes on to affect various things, and the emacs binary fails
 ## to start if Vinstallation_directory has the wrong value.
-temacs$(EXEEXT): fingerprint.o $(charsets) $(charscript)
-	$(AM_V_CCLD)$(CC) -o $@ $(ALL_CFLAGS) $(TEMACS_LDFLAGS) $(LDFLAGS) \
-	  $(ALLOBJS) fingerprint.o \
-	  $(LIBEGNU_ARCHIVE) $(W32_RES_LINK) $(LIBES)
+temacs$(EXEEXT): $(LIBXMENU) $(ALLOBJS) $(LIBEGNU_ARCHIVE) $(EMACSRES) \
+  $(charsets) $(charscript) $(MAKE_PDUMPER_FINGERPRINT)
+	$(AM_V_CCLD)$(CC) -o $@.tmp \
+	  $(ALL_CFLAGS) $(TEMACS_LDFLAGS) $(LDFLAGS) \
+	  $(ALLOBJS) $(LIBEGNU_ARCHIVE) $(W32_RES_LINK) $(LIBES)
+ifeq ($(HAVE_PDUMPER),yes)
+	$(AM_V_at)$(MAKE_PDUMPER_FINGERPRINT) $@.tmp
+endif
+	$(AM_V_at)mv $@.tmp $@
 	$(MKDIR_P) $(etc)
 ifeq ($(DUMPING),unexec)
   ifneq ($(PAXCTL_notdumped),)
@@ -676,7 +683,7 @@ .PHONY:
 
 mostlyclean:
 	rm -f temacs$(EXEEXT) core ./*.core \#* ./*.o
-	rm -f dmpstruct.h fingerprint.c
+	rm -f dmpstruct.h
 	rm -f emacs.pdmp
 	rm -f ../etc/DOC
 	rm -f bootstrap-emacs$(EXEEXT) $(bootstrap_pdmp)
@@ -716,10 +723,9 @@ ETAGS =
 ${ETAGS}: FORCE
 	$(MAKE) -C $(dir $@) $(notdir $@)
 
-# Remove macuvs.h and fingerprint.c since they'd cause `src/emacs`
+# Remove macuvs.h since it'd cause `src/emacs`
 # to be built before we can get TAGS.
-ctagsfiles1 = $(filter-out ${srcdir}/macuvs.h ${srcdir}/fingerprint.c, \
-                           $(wildcard ${srcdir}/*.[hc]))
+ctagsfiles1 = $(filter-out ${srcdir}/macuvs.h, $(wildcard ${srcdir}/*.[hc]))
 ctagsfiles2 = $(wildcard ${srcdir}/*.m)
 
 ## In out-of-tree builds, TAGS are generated in the build dir, like
-- 
2.17.1