[committed] libstdc++: Use __libc_single_threaded for locale initialization

[Jonathan Wakely via Gcc-patches]

Most initialization of locales and facets happens before main() during
startup, when the program is likely to only have one thread. By using
the new __gnu_cxx::__is_single_threaded() function instead of checking
__gthread_active_p() we can avoid using pthread_once or atomics for the
common case.

That said, I'm not sure why we don't just use a local static variable
instead, as __cxa_guard_acquire() already optimizes for the
single-threaded case:

  static const bool init = (_S_initialize_once(), true);

I'll revisit that for GCC 12.

libstdc++-v3/ChangeLog:

        * src/c++98/locale.cc (locale::facet::_S_get_c_locale())
        (locale::id::_M_id() const): Use __is_single_threaded.
        * src/c++98/locale_init.cc (locale::_S_initialize()):
        Likewise.

Tested powerpc64le-linux. Committed to trunk.


commit e253d36214015ed10ffd335e3628ccaac22dd5c7
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Tue Nov 24 12:29:30 2020

    libstdc++: Use __libc_single_threaded for locale initialization
    
    Most initialization of locales and facets happens before main() during
    startup, when the program is likely to only have one thread. By using
    the new __gnu_cxx::__is_single_threaded() function instead of checking
    __gthread_active_p() we can avoid using pthread_once or atomics for the
    common case.
    
    That said, I'm not sure why we don't just use a local static variable
    instead, as __cxa_guard_acquire() already optimizes for the
    single-threaded case:
    
      static const bool init = (_S_initialize_once(), true);
    
    I'll revisit that for GCC 12.
    
    libstdc++-v3/ChangeLog:
    
            * src/c++98/locale.cc (locale::facet::_S_get_c_locale())
            (locale::id::_M_id() const): Use __is_single_threaded.
            * src/c++98/locale_init.cc (locale::_S_initialize()):
            Likewise.

diff --git a/libstdc++-v3/src/c++98/locale.cc b/libstdc++-v3/src/c++98/locale.cc
index 06422412039c..9b3fc3515152 100644
--- a/libstdc++-v3/src/c++98/locale.cc
+++ b/libstdc++-v3/src/c++98/locale.cc
@@ -214,7 +214,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   locale::facet::_S_get_c_locale()
   {
 #ifdef __GTHREADS
-    if (__gthread_active_p())
+    if (!__gnu_cxx::__is_single_threaded())
       __gthread_once(&_S_once, _S_initialize_once);
     else
 #endif
@@ -515,7 +515,7 @@ namespace {
 #endif
 
 #ifdef __GTHREADS
-       if (__gthread_active_p())
+       if (!__gnu_cxx::__is_single_threaded())
          {
            if (__atomic_always_lock_free(sizeof(_M_index), &_M_index))
              {
diff --git a/libstdc++-v3/src/c++98/locale_init.cc 
b/libstdc++-v3/src/c++98/locale_init.cc
index c3841ccbd3c9..fc8416ba01a6 100644
--- a/libstdc++-v3/src/c++98/locale_init.cc
+++ b/libstdc++-v3/src/c++98/locale_init.cc
@@ -320,7 +320,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   locale::_S_initialize()
   {
 #ifdef __GTHREADS
-    if (__gthread_active_p())
+    if (!__gnu_cxx::__is_single_threaded())
       __gthread_once(&_S_once, _S_initialize_once);
 #endif
     if (!_S_classic)