[gcc r13-572] libstdc++: Relax memory ordering for default memory resource object

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:ddb1427defe95341ac2eb672e7bea7303f8c7db9

commit r13-572-gddb1427defe95341ac2eb672e7bea7303f8c7db9
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue May 17 16:17:21 2022 +0100

    libstdc++: Relax memory ordering for default memory resource object
    
    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  <trodgers@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.

Diff:
---
 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