[PATCH] PR libstdc++/86846 Alternative to pointer-width atomics

[Jonathan Wakely]

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.

Tested x86_64-linux, committed to trunk.


-------------- next part --------------
commit 81de951109506b684d079f0fbc7aebae2c228ba4
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Aug 13 12:34:56 2018 +0100

    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.

diff --git a/libstdc++-v3/src/c++17/default_resource.h b/libstdc++-v3/src/c++17/default_resource.h
new file mode 100644
index 00000000000..522cee13b90
--- /dev/null
+++ b/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)));
diff --git a/libstdc++-v3/src/c++17/memory_resource.cc b/libstdc++-v3/src/c++17/memory_resource.cc
index c3ae2b69f71..bd8f32d931e 100644
--- a/libstdc++-v3/src/c++17/memory_resource.cc
+++ b/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*