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diff --git a/boost/smart_ptr/detail/sp_counted_base_clang.hpp b/boost/smart_ptr/detail/sp_counted_base_clang.hpp
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+++ b/boost/smart_ptr/detail/sp_counted_base_clang.hpp
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+#ifndef BOOST_SMART_PTR_DETAIL_SP_COUNTED_BASE_CLANG_HPP_INCLUDED
+#define BOOST_SMART_PTR_DETAIL_SP_COUNTED_BASE_CLANG_HPP_INCLUDED
+
+// MS compatible compilers support #pragma once
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1020)
+# pragma once
+#endif
+
+//  detail/sp_counted_base_clang.hpp - __c11 clang intrinsics
+//
+//  Copyright (c) 2007, 2013, 2015 Peter Dimov
+//
+//  Distributed under the Boost Software License, Version 1.0.
+//  See accompanying file LICENSE_1_0.txt or copy at
+//  http://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/detail/sp_typeinfo.hpp>
+#include <boost/cstdint.hpp>
+
+namespace boost
+{
+
+namespace detail
+{
+
+typedef _Atomic( boost::int_least32_t ) atomic_int_least32_t;
+
+inline void atomic_increment( atomic_int_least32_t * pw )
+{
+    __c11_atomic_fetch_add( pw, 1, __ATOMIC_RELAXED );
+}
+
+inline boost::int_least32_t atomic_decrement( atomic_int_least32_t * pw )
+{
+    return __c11_atomic_fetch_sub( pw, 1, __ATOMIC_ACQ_REL );
+}
+
+inline boost::int_least32_t atomic_conditional_increment( atomic_int_least32_t * pw )
+{
+    // long r = *pw;
+    // if( r != 0 ) ++*pw;
+    // return r;
+
+    boost::int_least32_t r = __c11_atomic_load( pw, __ATOMIC_RELAXED );
+
+    for( ;; )
+    {
+        if( r == 0 )
+        {
+            return r;
+        }
+
+        if( __c11_atomic_compare_exchange_weak( pw, &r, r + 1, __ATOMIC_RELAXED, __ATOMIC_RELAXED ) )
+        {
+            return r;
+        }
+    }    
+}
+
+class sp_counted_base
+{
+private:
+
+    sp_counted_base( sp_counted_base const & );
+    sp_counted_base & operator= ( sp_counted_base const & );
+
+    atomic_int_least32_t use_count_;	// #shared
+    atomic_int_least32_t weak_count_;	// #weak + (#shared != 0)
+
+public:
+
+    sp_counted_base()
+    {
+        __c11_atomic_init( &use_count_, 1 );
+        __c11_atomic_init( &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_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 __c11_atomic_load( const_cast< atomic_int_least32_t* >( &use_count_ ), __ATOMIC_ACQUIRE );
+    }
+};
+
+} // namespace detail
+
+} // namespace boost
+
+#endif  // #ifndef BOOST_SMART_PTR_DETAIL_SP_COUNTED_BASE_CLANG_HPP_INCLUDED