PR libstdc++/86846 Alternative to pointer-width atomics

Define a class using std::mutex for when std::atomic<memory_resource*>
cannot be used to implement the default memory resource.

When std::mutex constructor is not constexpr the constant_init trick
won't work, so just define a global and use init_priority for it. The
compiler warns about using reserved priority, so put the definition in a
header file using #pragma GCC system_header to suppress the warning.

	PR libstdc++/86846
	* src/c++17/default_resource.h: New file, defining default_res.
	* src/c++17/memory_resource.cc [ATOMIC_POINTER_LOCK_FREE != 2]
	(atomic_mem_res): Define alternative for atomic<memory_resource*>
	using a mutex instead of atomics.

From-SVN: r263536

libstdc++-v3/ChangeLog

@@ -1,5 +1,11 @@
 2018-08-14  Jonathan Wakely  <jwakely@redhat.com>
 
+	PR libstdc++/86846
+	* src/c++17/default_resource.h: New file, defining default_res.
+	* src/c++17/memory_resource.cc [ATOMIC_POINTER_LOCK_FREE != 2]
+	(atomic_mem_res): Define alternative for atomic<memory_resource*>
+	using a mutex instead of atomics.
+
 	PR libstdc++/85343
 	* config/abi/pre/gnu.ver: Export new symbol.
 	* doc/xml/manual/abi.xml: Document new versions.

libstdc++-v3/src/c++17/default_resource.h

@@ -0,0 +1,11 @@
+// This is only in a header so we can use the system_header pragma,
+// to suppress the warning caused by using a reserved init_priority.
+#pragma GCC system_header
+
+#if ATOMIC_POINTER_LOCK_FREE == 2 || defined(__GTHREAD_MUTEX_INIT)
+# error "This file should not be included for this build"
+#endif
+
+struct {
+  atomic_mem_res obj = &newdel_res.obj;
+} default_res __attribute__ ((init_priority (100)));

libstdc++-v3/src/c++17/memory_resource.cc

@@ -25,6 +25,10 @@
 #include <memory_resource>
 #include <atomic>
 #include <new>
+#if ATOMIC_POINTER_LOCK_FREE != 2
+# include <bits/std_mutex.h>	// std::mutex, std::lock_guard
+# include <bits/move.h>		// std::exchange
+#endif
 
 namespace std _GLIBCXX_VISIBILITY(default)
 {
@@ -81,7 +85,42 @@ namespace pmr
 
     constant_init<newdel_res_t> newdel_res{};
     constant_init<null_res_t> null_res{};
-    constant_init<atomic<memory_resource*>> default_res{&newdel_res.obj};
+#if ATOMIC_POINTER_LOCK_FREE == 2
+    using atomic_mem_res = atomic<memory_resource*>;
+# define _GLIBCXX_ATOMIC_MEM_RES_CAN_BE_CONSTANT_INITIALIZED
+#else
+    // Can't use pointer-width atomics, define a type using a mutex instead:
+    struct atomic_mem_res
+    {
+# ifdef __GTHREAD_MUTEX_INIT
+#  define _GLIBCXX_ATOMIC_MEM_RES_CAN_BE_CONSTANT_INITIALIZED
+      // std::mutex has constexpr constructor
+      constexpr
+# endif
+      atomic_mem_res(memory_resource* r) : val(r) { }
+
+      mutex mx;
+      memory_resource* val;
+
+      memory_resource* load()
+      {
+	lock_guard<mutex> lock(mx);
+	return val;
+      }
+
+      memory_resource* exchange(memory_resource* r)
+      {
+	lock_guard<mutex> lock(mx);
+	return std::exchange(val, r);
+      }
+    };
+#endif // ATOMIC_POINTER_LOCK_FREE == 2
+
+#ifdef _GLIBCXX_ATOMIC_MEM_RES_CAN_BE_CONSTANT_INITIALIZED
+    constant_init<atomic_mem_res> default_res{&newdel_res.obj};
+#else
+# include "default_resource.h"
+#endif
   } // namespace
 
   memory_resource*