[Bug libstdc++/71312] mutexes for shared_ptr atomics should be padded to cacheline size

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71312

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
A slightly simpler fix:

--- a/libstdc++-v3/src/c++11/shared_ptr.cc
+++ b/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 ensure each lock is on a separate cache line
+    struct alignas(64) M : __gnu_cxx::__mutex { };
+    static M m[mask + 1];
     return m[i];
   }
 }