[gcc r12-5422] libstdc++: Use __is_single_threaded in locale initialization

[Jonathan Wakely]

https://gcc.gnu.org/g:5faf1c8c7ab64340945972ee5acb9eb0a655567a

commit r12-5422-g5faf1c8c7ab64340945972ee5acb9eb0a655567a
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Nov 19 12:26:49 2021 +0000

    libstdc++: Use __is_single_threaded in locale initialization
    
    This replaces a __gthread_active_p() check with __is_single_threaded()
    so that std::locale initialization doesn't use __gthread_once if it
    happens before the first thread is created.
    
    This means that _S_initialize_once() might now be called twice instead
    of only once, because if __is_single_threaded() changes to false then we
    will do the __gthread_once call even if _S_initialize_once() was already
    called. Add a check to _S_initialize_once() and return immediately if
    it is the second call.
    
    Also use __builtin_expect to _S_initialize, as the branch will be taken
    at most once in the lifetime of the program.
    
    libstdc++-v3/ChangeLog:
    
            * src/c++98/locale_init.cc (_S_initialize_once): Check if
            initialization has already been done.
            (_S_initialize): Replace __gthread_active_p with
            __is_single_threaded. Use __builtin_expect.

Diff:
---
 libstdc++-v3/src/c++98/locale_init.cc | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/src/c++98/locale_init.cc b/libstdc++-v3/src/c++98/locale_init.cc
index e96b1a336aa..ff563b80465 100644
--- a/libstdc++-v3/src/c++98/locale_init.cc
+++ b/libstdc++-v3/src/c++98/locale_init.cc
@@ -31,6 +31,7 @@
 #include <cctype>
 #include <cwctype>     // For towupper, etc.
 #include <locale>
+#include <ext/atomicity.h>
 #include <ext/concurrence.h>
 
 #if _GLIBCXX_USE_DUAL_ABI
@@ -321,6 +322,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   void
   locale::_S_initialize_once() throw()
   {
+    // Need to check this because we could get called once from _S_initialize()
+    // when the program is single-threaded, and then again (via __gthread_once)
+    // when it's multi-threaded.
+    if (_S_classic)
+      return;
+
     // 2 references.
     // One reference for _S_classic, one for _S_global
     _S_classic = new (&c_locale_impl) _Impl(2);
@@ -332,10 +339,10 @@ _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)
+    if (__builtin_expect(!_S_classic, 0))
       _S_initialize_once();
   }