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libstdc++: Use __libc_single_threaded for locale initialization

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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:🆔:_M_id() const): Use __is_single_threaded.
	* src/c++98/locale_init.cc (locale::_S_initialize()):
	Likewise.
This commit is contained in:
Jonathan Wakely 2020-11-24 12:29:30 +00:00
parent 97981e13b7
commit e253d36214
2 changed files with 3 additions and 3 deletions
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4
libstdc++-v3/src/c++98/locale.cc
View File

@ -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))
{

2
libstdc++-v3/src/c++98/locale_init.cc
View File

@ -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)