PR libstdc++/71312 Increase alignment of pooled mutexes

	PR libstdc++/71312
	* src/c++11/shared_ptr.cc (get_mutex): Align pool mutexes to 64 bytes.

From-SVN: r270649

libstdc++-v3/ChangeLog

 2019-04-29  Jonathan Wakely  <jwakely@redhat.com>
 
+	PR libstdc++/71312
+	* src/c++11/shared_ptr.cc (get_mutex): Align pool mutexes to 64 bytes.
+
 	* include/bits/stl_bvector.h (vector<bool>::empty()): Add nodiscard
 	attribute.
 

libstdc++-v3/src/c++11/shared_ptr.cc

@@ -34,7 +34,9 @@ namespace __gnu_internal _GLIBCXX_VISIBILITY(hidden)
   __gnu_cxx::__mutex&
   get_mutex(unsigned char i)
   {
-    static __gnu_cxx::__mutex m[mask + 1];
+    // increase alignment to put each lock on a separate cache line
+    struct alignas(64) M : __gnu_cxx::__mutex { };
+    static M m[mask + 1];
     return m[i];
   }
 }