diff --git a/include/boost/smart_ptr/detail/sp_counted_base.hpp b/include/boost/smart_ptr/detail/sp_counted_base.hpp
index f9a5c14..536926a 100644
--- a/include/boost/smart_ptr/detail/sp_counted_base.hpp
+++ b/include/boost/smart_ptr/detail/sp_counted_base.hpp
@@ -17,14 +17,9 @@
 // http://www.boost.org/LICENSE_1_0.txt)
 //
 
-#include <boost/config.hpp>
+#include <boost/smart_ptr/detail/sp_has_gcc_intrinsics.hpp>
 #include <boost/smart_ptr/detail/sp_has_sync_intrinsics.hpp>
-
-#if !defined( __c2__ ) && defined( __clang__ ) && defined( __has_extension )
-# if __has_extension( __c_atomic__ )
-#   define BOOST_SP_HAS_CLANG_C11_ATOMICS
-# endif
-#endif
+#include <boost/config.hpp>
 
 #if defined( BOOST_SP_DISABLE_THREADS )
 # include <boost/smart_ptr/detail/sp_counted_base_nt.hpp>
@@ -41,18 +36,24 @@
 #elif defined( BOOST_DISABLE_THREADS ) && !defined( BOOST_SP_ENABLE_THREADS ) && !defined( BOOST_DISABLE_WIN32 )
 # include <boost/smart_ptr/detail/sp_counted_base_nt.hpp>
 
-#elif defined( BOOST_SP_HAS_CLANG_C11_ATOMICS )
-# include <boost/smart_ptr/detail/sp_counted_base_clang.hpp>
+#elif defined( BOOST_SP_HAS_GCC_INTRINSICS )
+# include <boost/smart_ptr/detail/sp_counted_base_gcc_atomic.hpp>
 
 #elif !defined( BOOST_NO_CXX11_HDR_ATOMIC )
 # include <boost/smart_ptr/detail/sp_counted_base_std_atomic.hpp>
 
-#elif defined( __SNC__ )
-# include <boost/smart_ptr/detail/sp_counted_base_snc_ps3.hpp>
+#elif defined( __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 )
+# include <boost/smart_ptr/detail/sp_counted_base_sync.hpp>
 
 #elif defined( __GNUC__ ) && ( defined( __i386__ ) || defined( __x86_64__ ) ) && !defined(__PATHSCALE__)
 # include <boost/smart_ptr/detail/sp_counted_base_gcc_x86.hpp>
 
+#elif defined( BOOST_SP_HAS_SYNC_INTRINSICS )
+# include <boost/smart_ptr/detail/sp_counted_base_sync.hpp>
+
+#elif defined( __SNC__ )
+# include <boost/smart_ptr/detail/sp_counted_base_snc_ps3.hpp>
+
 #elif defined(__HP_aCC) && defined(__ia64)
 # include <boost/smart_ptr/detail/sp_counted_base_acc_ia64.hpp>
 
@@ -71,9 +72,6 @@
 #elif defined( __GNUC__ ) && ( defined( __mips__ ) || defined( _mips ) ) && !defined(__PATHSCALE__) && !defined( __mips16 )
 # include <boost/smart_ptr/detail/sp_counted_base_gcc_mips.hpp>
 
-#elif defined( BOOST_SP_HAS_SYNC_INTRINSICS )
-# include <boost/smart_ptr/detail/sp_counted_base_sync.hpp>
-
 #elif defined(__GNUC__) && ( defined( __sparcv9 ) || ( defined( __sparcv8 ) && ( __GNUC__ * 100 + __GNUC_MINOR__ >= 402 ) ) )
 # include <boost/smart_ptr/detail/sp_counted_base_gcc_sparc.hpp>
 
@@ -91,6 +89,4 @@
 
 #endif
 
-#undef BOOST_SP_HAS_CLANG_C11_ATOMICS
-
 #endif  // #ifndef BOOST_SMART_PTR_DETAIL_SP_COUNTED_BASE_HPP_INCLUDED
diff --git a/include/boost/smart_ptr/detail/sp_counted_base_gcc_atomic.hpp b/include/boost/smart_ptr/detail/sp_counted_base_gcc_atomic.hpp
new file mode 100644
index 0000000..c2172e9
--- /dev/null
+++ b/include/boost/smart_ptr/detail/sp_counted_base_gcc_atomic.hpp
@@ -0,0 +1,148 @@
+#ifndef BOOST_SMART_PTR_DETAIL_SP_COUNTED_BASE_GCC_ATOMIC_HPP_INCLUDED
+#define BOOST_SMART_PTR_DETAIL_SP_COUNTED_BASE_GCC_ATOMIC_HPP_INCLUDED
+
+// MS compatible compilers support #pragma once
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1020)
+# pragma once
+#endif
+
+// detail/sp_counted_base_gc_atomic.hpp - g++ 4.7+ __atomic intrinsics
+//
+// Copyright 2007, 2020 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/smart_ptr/detail/sp_typeinfo_.hpp>
+#include <boost/config.hpp>
+#include <boost/cstdint.hpp>
+
+#if defined(BOOST_SP_REPORT_IMPLEMENTATION)
+
+#include <boost/config/pragma_message.hpp>
+BOOST_PRAGMA_MESSAGE("Using __atomic sp_counted_base")
+
+#endif
+
+namespace boost
+{
+
+namespace detail
+{
+
+inline void atomic_increment( boost::uint_least32_t * pw )
+{
+    __atomic_fetch_add( pw, 1, __ATOMIC_RELAXED );
+}
+
+inline boost::uint_least32_t atomic_decrement( boost::uint_least32_t * pw )
+{
+    return __atomic_fetch_sub( pw, 1, __ATOMIC_ACQ_REL );
+}
+
+inline boost::uint_least32_t atomic_conditional_increment( boost::uint_least32_t * pw )
+{
+    // long r = *pw;
+    // if( r != 0 ) ++*pw;
+    // return r;
+
+    boost::uint_least32_t r = __atomic_load_n( pw, __ATOMIC_RELAXED );
+
+    for( ;; )
+    {
+        if( r == 0 )
+        {
+            return r;
+        }
+
+        if( __atomic_compare_exchange_n( pw, &r, r + 1, true, __ATOMIC_RELAXED, __ATOMIC_RELAXED ) )
+        {
+            return r;
+        }
+    }    
+}
+
+inline boost::uint_least32_t atomic_load( boost::uint_least32_t const * pw )
+{
+    return __atomic_load_n( pw, __ATOMIC_ACQUIRE );
+}
+
+class BOOST_SYMBOL_VISIBLE sp_counted_base
+{
+private:
+
+    sp_counted_base( sp_counted_base const & );
+    sp_counted_base & operator= ( sp_counted_base const & );
+
+    boost::uint_least32_t use_count_;        // #shared
+    boost::uint_least32_t weak_count_;       // #weak + (#shared != 0)
+
+public:
+
+    sp_counted_base(): use_count_( 1 ), weak_count_( 1 )
+    {
+    }
+
+    virtual ~sp_counted_base() // nothrow
+    {
+    }
+
+    // dispose() is called when use_count_ drops to zero, to release
+    // the resources managed by *this.
+
+    virtual void dispose() = 0; // nothrow
+
+    // destroy() is called when weak_count_ drops to zero.
+
+    virtual void destroy() // nothrow
+    {
+        delete this;
+    }
+
+    virtual void * get_deleter( sp_typeinfo_ const & ti ) = 0;
+    virtual void * get_local_deleter( sp_typeinfo_ const & ti ) = 0;
+    virtual void * get_untyped_deleter() = 0;
+
+    void add_ref_copy()
+    {
+        atomic_increment( &use_count_ );
+    }
+
+    bool add_ref_lock() // true on success
+    {
+        return atomic_conditional_increment( &use_count_ ) != 0;
+    }
+
+    void release() // nothrow
+    {
+        if( atomic_decrement( &use_count_ ) == 1 )
+        {
+            dispose();
+            weak_release();
+        }
+    }
+
+    void weak_add_ref() // nothrow
+    {
+        atomic_increment( &weak_count_ );
+    }
+
+    void weak_release() // nothrow
+    {
+        if( atomic_decrement( &weak_count_ ) == 1 )
+        {
+            destroy();
+        }
+    }
+
+    long use_count() const // nothrow
+    {
+        return atomic_load( &use_count_ );
+    }
+};
+
+} // namespace detail
+
+} // namespace boost
+
+#endif  // #ifndef BOOST_SMART_PTR_DETAIL_SP_COUNTED_BASE_SYNC_HPP_INCLUDED
diff --git a/include/boost/smart_ptr/detail/sp_has_gcc_intrinsics.hpp b/include/boost/smart_ptr/detail/sp_has_gcc_intrinsics.hpp
new file mode 100644
index 0000000..2e15f79
--- /dev/null
+++ b/include/boost/smart_ptr/detail/sp_has_gcc_intrinsics.hpp
@@ -0,0 +1,27 @@
+#ifndef BOOST_SMART_PTR_DETAIL_SP_HAS_GCC_INTRINSICS_HPP_INCLUDED
+#define BOOST_SMART_PTR_DETAIL_SP_HAS_GCC_INTRINSICS_HPP_INCLUDED
+
+// MS compatible compilers support #pragma once
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1020)
+# pragma once
+#endif
+
+
+// boost/smart_ptr/detail/sp_has_gcc_intrinsics.hpp
+//
+// Copyright 2020 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+//
+// Defines the BOOST_SP_HAS_GCC_INTRINSICS macro if the __atomic_*
+// intrinsics are available.
+
+
+#if defined( __ATOMIC_RELAXED ) && defined( __ATOMIC_ACQUIRE ) && defined( __ATOMIC_RELEASE ) && defined( __ATOMIC_ACQ_REL )
+
+# define BOOST_SP_HAS_GCC_INTRINSICS
+
+#endif
+
+#endif // #ifndef BOOST_SMART_PTR_DETAIL_SP_HAS_GCC_INTRINSICS_HPP_INCLUDED