[PATCH] PR libstdc++/71312 Increase alignment of pooled mutexes

[Jonathan Wakely]

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

Tested powerpc64le-linux, committed to trunk.

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commit 7bdfcdf0100b2e340339388e5cef610c9f6a251f
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Mar 11 21:18:05 2019 +0000

    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.

diff --git a/libstdc++-v3/src/c++11/shared_ptr.cc b/libstdc++-v3/src/c++11/shared_ptr.cc
index 1f1323ef89f..a4e833bf4c8 100644
--- 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 put each lock on a separate cache line
+    struct alignas(64) M : __gnu_cxx::__mutex { };
+    static M m[mask + 1];
     return m[i];
   }
 }