new module 'thread-optim'

[Bruno Haible]

The glibc 2.32 announcement says:

  Using weak references to libpthread functions such as pthread_create
  or pthread_key_create to detect the singled-threaded nature of a
  program is an obsolescent feature.  Future versions of glibc will
  define pthread_create within libc.so.6 itself, so such checks will
  always flag the program as multi-threaded.  Applications should check
  the __libc_single_threaded variable declared in
  <sys/single_threaded.h> instead.

See also
<https://www.gnu.org/software/libc/manual/html_node/Single_002dThreaded.html>.

So, this means, the pthread_in_use() macro will evaluate to 1 always
in some future versions of glibc.

The new __libc_single_threaded variable is not a full replacement for
this. In these future glibc versions,
  - It will not be possible to skip the initialization of locks,
    only the lock/unlock operations - because some part of the program
    may create a thread, and then __libc_single_threaded changes from 1 to 0.
  - Optimization of TLS (pthread_getspecific and pthread_setspecific
    functions) is hairy as well.

Let me add support for this technique to some gnulib modules.
'regex' could use the same technique as well. May I leave this to you, Paul?


2020-08-08  Bruno Haible  <bruno@clisp.org>

        localename: Use module 'thread-optim'.
        * lib/localename.c: Include thread-optim.h.
        (struniq): Use IF_MT macro.
        * modules/localename (Depends-on): Add thread-optim.

        clean-temp: Use module 'thread-optim'.
        * lib/clean-temp.c: Include thread-optim.h.
        (register_temporary_file, unregister_temporary_file, create_temp_dir,
        register_temp_file, unregister_temp_file, register_temp_subdir,
        unregister_temp_subdir, cleanup_temp_dir_contents, register_fd,
        close_temp, fclose_variant_temp): Use IF_MT macro.
        * modules/clean-temp (Depends-on): Add thread-optim.

        fatal-signal: Use module 'thread-optim'.
        * lib/fatal-signal.c: Include thread-optim.h.
        (at_fatal_signal, block_fatal_signals, unblock_fatal_signals): Use IF_MT
        macro.
        * modules/fatal-signal (Depends-on): Add thread-optim.

0001-New-module-thread-optim.patch
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0002-fatal-signal-Use-module-thread-optim.patch
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0003-clean-temp-Use-module-thread-optim.patch
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0004-localename-Use-module-thread-optim.patch
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