[committed] libstdc++: Implement std::atomic::compare_exchange_weak

[Jonathan Wakely via Gcc-patches]

For some reason r170217 didn't add compare_exchange_weak to the
__atomic_base<T*> partial specialization, and so weak compare exchange
operations on pointers use compare_exchange_strong instead.

This adds __atomic_base<T*>::compare_exchange_weak and then uses it in
std::atomic<T*>::compare_exchange_weak.

Signed-off-by: Jonathan Wakely <jwak...@redhat.com>

libstdc++-v3/ChangeLog:

        * include/bits/atomic_base.h (__atomic_base<P*>::compare_exchange_weak):
        Add new functions.
        * include/std/atomic (atomic<T*>::compare_exchange_weak): Use
        it.

Tested x86_64-linux. Committed to trunk.


commit 89cf858571c58a58ca51dbbf3975582ebab41e2d
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Thu Sep 2 16:47:31 2021

    libstdc++: Implement std::atomic<T*>::compare_exchange_weak
    
    For some reason r170217 didn't add compare_exchange_weak to the
    __atomic_base<T*> partial specialization, and so weak compare exchange
    operations on pointers use compare_exchange_strong instead.
    
    This adds __atomic_base<T*>::compare_exchange_weak and then uses it in
    std::atomic<T*>::compare_exchange_weak.
    
    Signed-off-by: Jonathan Wakely <jwak...@redhat.com>
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/atomic_base.h 
(__atomic_base<P*>::compare_exchange_weak):
            Add new functions.
            * include/std/atomic (atomic<T*>::compare_exchange_weak): Use
            it.

diff --git a/libstdc++-v3/include/bits/atomic_base.h 
b/libstdc++-v3/include/bits/atomic_base.h
index cbe1da6d125..71e1de078b5 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@ -846,6 +846,28 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        return __atomic_exchange_n(&_M_p, __p, int(__m));
       }
 
+      _GLIBCXX_ALWAYS_INLINE bool
+      compare_exchange_weak(__pointer_type& __p1, __pointer_type __p2,
+                           memory_order __m1,
+                           memory_order __m2) noexcept
+      {
+       __glibcxx_assert(__is_valid_cmpexch_failure_order(__m2));
+
+       return __atomic_compare_exchange_n(&_M_p, &__p1, __p2, 1,
+                                          int(__m1), int(__m2));
+      }
+
+      _GLIBCXX_ALWAYS_INLINE bool
+      compare_exchange_weak(__pointer_type& __p1, __pointer_type __p2,
+                           memory_order __m1,
+                           memory_order __m2) volatile noexcept
+      {
+       __glibcxx_assert(__is_valid_cmpexch_failure_order(__m2));
+
+       return __atomic_compare_exchange_n(&_M_p, &__p1, __p2, 1,
+                                          int(__m1), int(__m2));
+      }
+
       _GLIBCXX_ALWAYS_INLINE bool
       compare_exchange_strong(__pointer_type& __p1, __pointer_type __p2,
                              memory_order __m1,
diff --git a/libstdc++-v3/include/std/atomic b/libstdc++-v3/include/std/atomic
index bdbbfd5c8f8..936dd50ba1c 100644
--- a/libstdc++-v3/include/std/atomic
+++ b/libstdc++-v3/include/std/atomic
@@ -595,13 +595,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       bool
       compare_exchange_weak(__pointer_type& __p1, __pointer_type __p2,
                            memory_order __m1, memory_order __m2) noexcept
-      { return _M_b.compare_exchange_strong(__p1, __p2, __m1, __m2); }
+      { return _M_b.compare_exchange_weak(__p1, __p2, __m1, __m2); }
 
       bool
       compare_exchange_weak(__pointer_type& __p1, __pointer_type __p2,
                            memory_order __m1,
                            memory_order __m2) volatile noexcept
-      { return _M_b.compare_exchange_strong(__p1, __p2, __m1, __m2); }
+      { return _M_b.compare_exchange_weak(__p1, __p2, __m1, __m2); }
 
       bool
       compare_exchange_weak(__pointer_type& __p1, __pointer_type __p2,