[committed] libstdc++: Relax memory ordering for default memory resource object

[Jonathan Wakely via Gcc-patches]

Tested powerpc64le-linux, pushed to trunk.

-- >8 --

Currently pmr::set_default_resource and pmr::get_default_resource both
use sequentially consistent memory ordering. This is overkill. The
standard only requires that a call to set_default_resource synchronizes
with subsequent calls to set_default_resource and get_default_resource.

Using acquire-release for the setter and acquire for the getter is
sufficient to meet the requirement.

Reviewed-by: Thomas Rodgers  <trodg...@redhat.com>

libstdc++-v3/ChangeLog:

        * src/c++17/memory_resource.cc (set_default_resource): Use
        memory_order_acq_rel.
        (get_default_resource): Use memory_order_acquire.
---
 libstdc++-v3/src/c++17/memory_resource.cc | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/libstdc++-v3/src/c++17/memory_resource.cc 
b/libstdc++-v3/src/c++17/memory_resource.cc
index bb6334c9694..8bc55a69f1f 100644
--- a/libstdc++-v3/src/c++17/memory_resource.cc
+++ b/libstdc++-v3/src/c++17/memory_resource.cc
@@ -112,13 +112,13 @@ namespace pmr
       mutex mx;
       memory_resource* val;
 
-      memory_resource* load()
+      memory_resource* load(std::memory_order)
       {
        lock_guard<mutex> lock(mx);
        return val;
       }
 
-      memory_resource* exchange(memory_resource* r)
+      memory_resource* exchange(memory_resource* r, std::memory_order)
       {
        lock_guard<mutex> lock(mx);
        return std::__exchange(val, r);
@@ -134,12 +134,12 @@ namespace pmr
 
       memory_resource* val;
 
-      memory_resource* load() const
+      memory_resource* load(std::memory_order) const
       {
        return val;
       }
 
-      memory_resource* exchange(memory_resource* r)
+      memory_resource* exchange(memory_resource* r, std::memory_order)
       {
        return std::__exchange(val, r);
       }
@@ -166,12 +166,12 @@ namespace pmr
   {
     if (r == nullptr)
       r = new_delete_resource();
-    return default_res.obj.exchange(r);
+    return default_res.obj.exchange(r, std::memory_order_acq_rel);
   }
 
   memory_resource*
   get_default_resource() noexcept
-  { return default_res.obj.load(); }
+  { return default_res.obj.load(std::memory_order_acquire); }
 
   // Member functions for std::pmr::monotonic_buffer_resource
 
-- 
2.34.3