[gcc r12-7186] libstdc++: Strengthen memory order for atomic<T>::wait/notify

[Thomas Rodgers]

https://gcc.gnu.org/g:b25a7c05f120e46a60430dcefe529c72303b7022

commit r12-7186-gb25a7c05f120e46a60430dcefe529c72303b7022
Author: Thomas Rodgers <rodgert@appliantology.com>
Date:   Thu Feb 10 10:12:36 2022 -0800

    libstdc++: Strengthen memory order for atomic<T>::wait/notify
    
    This changes the memory order used in the spin wait code to match
    that of libc++.
    
    libstdc++-v3/ChangeLog:
            * include/bits/atomic_wait.h (__waiter_base::_S_do_spin,
            __waiter_base::_S_do_spin_v): Change memory order from relaxed
            to acquire.

Diff:
---
 libstdc++-v3/include/bits/atomic_wait.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h
index 6ce7f9343cf..125b1cad886 100644
--- a/libstdc++-v3/include/bits/atomic_wait.h
+++ b/libstdc++-v3/include/bits/atomic_wait.h
@@ -332,7 +332,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      }
 	    else
 	      {
-		__atomic_load(__addr, &__val, __ATOMIC_RELAXED);
+		__atomic_load(__addr, &__val, __ATOMIC_ACQUIRE);
 	      }
 	    return __atomic_spin(__pred, __spin);
 	  }
@@ -353,7 +353,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		     __platform_wait_t& __val,
 		     _Spin __spin = _Spin{ })
 	  {
-	    __atomic_load(__addr, &__val, __ATOMIC_RELAXED);
+	    __atomic_load(__addr, &__val, __ATOMIC_ACQUIRE);
 	    return __atomic_spin(__pred, __spin);
 	  }