[committed] libstdc++: Implement std::atomic<T*>::compare_exchange_weak

[committed] libstdc++: Implement std::atomic<T*>::compare_exchange_weak

[Jonathan Wakely]

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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 <jwakely@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.


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commit 89cf858571c58a58ca51dbbf3975582ebab41e2d
Author: Jonathan Wakely <jwakely@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 <jwakely@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,