diff --git a/libstdc++-v3/include/bits/c++config b/libstdc++-v3/include/bits/c++config
index 32b8957f814..07465f7ecd5 100644
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -143,6 +143,15 @@
 # define _GLIBCXX_NODISCARD
 #endif
 
+// Macro for TSAN.
+#if __SANITIZE_THREAD__
+#  define _GLIBCXX_TSAN 1
+#elif defined __has_feature
+# if __has_feature(thread_sanitizer)
+#  define _GLIBCXX_TSAN 1
+# endif
+#endif
+
 
 
 #if __cplusplus
diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h b/libstdc++-v3/include/bits/shared_ptr_base.h
index 5be935d174d..b2397c8fddb 100644
--- a/libstdc++-v3/include/bits/shared_ptr_base.h
+++ b/libstdc++-v3/include/bits/shared_ptr_base.h
@@ -143,10 +143,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       virtual void*
       _M_get_deleter(const std::type_info&) noexcept = 0;
 
+      // Increment the use count (used when the count is greater than zero).
       void
       _M_add_ref_copy()
       { __gnu_cxx::__atomic_add_dispatch(&_M_use_count, 1); }
 
+      // Increment the use count if it is non-zero, throw otherwise.
       void
       _M_add_ref_lock()
       {
@@ -154,15 +156,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  __throw_bad_weak_ptr();
       }
 
+      // Increment the use count if it is non-zero.
       bool
       _M_add_ref_lock_nothrow() noexcept;
 
+      // Decrement the use count.
       void
-      _M_release() noexcept
-      {
-        // Be race-detector-friendly.  For more info see bits/c++config.
-        _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_use_count);
-	if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, -1) == 1)
+      _M_release() noexcept;
+
+      // Called by _M_release() when the use count reaches zero.
+      __attribute__((__noinline__))
+      void
+      _M_release_last_use() noexcept
       {
 	_GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count);
 	_M_dispose();
@@ -184,12 +189,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    _M_destroy();
 	  }
       }
-      }
 
+      // Increment the weak count.
       void
       _M_weak_add_ref() noexcept
       { __gnu_cxx::__atomic_add_dispatch(&_M_weak_count, 1); }
 
+      // Decrement the weak count.
       void
       _M_weak_release() noexcept
       {
@@ -286,6 +292,61 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
         }
     }
 
+  template<>
+    inline void
+    _Sp_counted_base<_S_mutex>::_M_release() noexcept
+    {
+      // Be race-detector-friendly.  For more info see bits/c++config.
+      _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_use_count);
+      if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, -1) == 1)
+	{
+	  _M_release_last_use();
+	}
+    }
+
+  template<>
+    inline void
+    _Sp_counted_base<_S_atomic>::_M_release() noexcept
+    {
+      _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_use_count);
+#if ! _GLIBCXX_TSAN
+      constexpr bool __lock_free
+	= __atomic_always_lock_free(sizeof(long long), 0)
+	&& __atomic_always_lock_free(sizeof(_Atomic_word), 0);
+      constexpr bool __double_word
+	= sizeof(long long) == 2 * sizeof(_Atomic_word);
+      // The ref-count members follow the vptr, so are aligned to
+      // alignof(void*).
+      constexpr bool __aligned = __alignof(long long) <= alignof(void*);
+      if _GLIBCXX17_CONSTEXPR (__lock_free && __double_word && __aligned)
+	{
+	  constexpr long long __unique_ref
+	    = 1LL + (1LL << (__CHAR_BIT__ * sizeof(_Atomic_word)));
+	  auto __both_counts = reinterpret_cast<long long*>(&_M_use_count);
+
+	  _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);
+	  if (__atomic_load_n(__both_counts, __ATOMIC_ACQUIRE) == __unique_ref)
+	    {
+	      // Both counts are 1, so there are no weak references and
+	      // we are releasing the last strong reference. No other
+	      // threads can observe the effects of this _M_release()
+	      // call (e.g. calling use_count()) without a data race.
+	      *(long long*)(&_M_use_count) = 0;
+	      _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count);
+	      _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
+	      _M_dispose();
+	      _M_destroy();
+	      __builtin_puts("double double bye bye");
+	      return;
+	    }
+	}
+#endif
+      if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, -1) == 1)
+	{
+	  _M_release_last_use();
+	}
+    }
+
   template<>
     inline void
     _Sp_counted_base<_S_single>::_M_weak_add_ref() noexcept