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diff --git a/src/gl/tests/thread-optim.h b/src/gl/tests/thread-optim.h
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+/* Optimization of multithreaded code.
+
+   Copyright (C) 2020-2021 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 <https://www.gnu.org/licenses/>.  */
+
+/* Written by Bruno Haible <bruno@clisp.org>, 2020.  */
+
+#ifndef _THREAD_OPTIM_H
+#define _THREAD_OPTIM_H
+
+/* This file defines a way to optimize multithreaded code for the single-thread
+   case, based on the variable '__libc_single_threaded', defined in
+   glibc >= 2.32.  */
+
+/* Typical use: In a block or function, use
+
+     bool mt = gl_multithreaded ();
+     ...
+     if (mt)
+       if (pthread_mutex_lock (&lock)) abort ();
+     ...
+     if (mt)
+       if (pthread_mutex_unlock (&lock)) abort ();
+
+   The gl_multithreaded () invocation determines whether the program currently
+   is multithreaded.
+
+   if (mt) STATEMENT executes STATEMENT in the multithreaded case, and skips
+   it in the single-threaded case.
+
+   The code between the gl_multithreaded () invocation and any use of the
+   variable 'mt' must not create threads or invoke functions that may
+   indirectly create threads (e.g. 'dlopen' may, indirectly through C++
+   initializers of global variables in the shared library being opened,
+   create threads).
+
+   The lock here is meant to synchronize threads in the same process.  The
+   same optimization cannot be applied to locks that synchronize different
+   processes (e.g. through shared memory mappings).  */
+
+#if HAVE_SYS_SINGLE_THREADED_H /* glibc >= 2.32 */
+# include <sys/single_threaded.h>
+# define gl_multithreaded()  !__libc_single_threaded
+#else
+# define gl_multithreaded()  1
+#endif
+
+#endif /* _THREAD_OPTIM_H */