commit dba1ab212292839572fda60df00965e094a11252
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Sep 2 15:29:22 2021

    libstdc++: Remove "no stronger" assertion in compare exchange [PR102177]
    
    P0418R2 removed some preconditions from std::atomic::compare_exchange_*
    but we still enforce them via __glibcxx_assert. This removes those
    assertions.
    
    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
    
    libstdc++-v3/ChangeLog:
    
            PR c++/102177
            * include/bits/atomic_base.h (__is_valid_cmpexch_failure_order):
            New function to check if a memory order is valid for the failure
            case of compare exchange operations.
            (__atomic_base<I>::compare_exchange_weak): Simplify assertions
            by using __is_valid_cmpexch_failure_order.
            (__atomic_base<I>::compare_exchange_strong): Likewise.
            (__atomic_base<P*>::compare_exchange_weak): Likewise.
            (__atomic_base<P*>::compare_exchange_strong): Likewise.
            (__atomic_impl::compare_exchange_weak): Add assertion.
            (__atomic_impl::compare_exchange_strong): Likewise.
            * include/std/atomic (atomic::compare_exchange_weak): Likewise.
            (atomic::compare_exchange_strong): Likewise.

diff --git a/libstdc++-v3/include/bits/atomic_base.h b/libstdc++-v3/include/bits/atomic_base.h
index 20cf1343c58..cbe1da6d125 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@ -121,6 +121,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       | __memory_order_modifier(__m & __memory_order_modifier_mask));
   }
 
+  constexpr bool
+  __is_valid_cmpexch_failure_order(memory_order __m) noexcept
+  {
+    return (__m & __memory_order_mask) != memory_order_release
+	&& (__m & __memory_order_mask) != memory_order_acq_rel;
+  }
+
   _GLIBCXX_ALWAYS_INLINE void
   atomic_thread_fence(memory_order __m) noexcept
   { __atomic_thread_fence(int(__m)); }
@@ -511,13 +518,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       compare_exchange_weak(__int_type& __i1, __int_type __i2,
 			    memory_order __m1, memory_order __m2) noexcept
       {
-	memory_order __b2 __attribute__ ((__unused__))
-	  = __m2 & __memory_order_mask;
-	memory_order __b1 __attribute__ ((__unused__))
-	  = __m1 & __memory_order_mask;
-	__glibcxx_assert(__b2 != memory_order_release);
-	__glibcxx_assert(__b2 != memory_order_acq_rel);
-	__glibcxx_assert(__b2 <= __b1);
+	__glibcxx_assert(__is_valid_cmpexch_failure_order(__m2));
 
 	return __atomic_compare_exchange_n(&_M_i, &__i1, __i2, 1,
 					   int(__m1), int(__m2));
@@ -528,13 +529,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 			    memory_order __m1,
 			    memory_order __m2) volatile noexcept
       {
-	memory_order __b2 __attribute__ ((__unused__))
-	  = __m2 & __memory_order_mask;
-	memory_order __b1 __attribute__ ((__unused__))
-	  = __m1 & __memory_order_mask;
-	__glibcxx_assert(__b2 != memory_order_release);
-	__glibcxx_assert(__b2 != memory_order_acq_rel);
-	__glibcxx_assert(__b2 <= __b1);
+	__glibcxx_assert(__is_valid_cmpexch_failure_order(__m2));
 
 	return __atomic_compare_exchange_n(&_M_i, &__i1, __i2, 1,
 					   int(__m1), int(__m2));
@@ -560,13 +555,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       compare_exchange_strong(__int_type& __i1, __int_type __i2,
 			      memory_order __m1, memory_order __m2) noexcept
       {
-	memory_order __b2 __attribute__ ((__unused__))
-	  = __m2 & __memory_order_mask;
-	memory_order __b1 __attribute__ ((__unused__))
-	  = __m1 & __memory_order_mask;
-	__glibcxx_assert(__b2 != memory_order_release);
-	__glibcxx_assert(__b2 != memory_order_acq_rel);
-	__glibcxx_assert(__b2 <= __b1);
+	__glibcxx_assert(__is_valid_cmpexch_failure_order(__m2));
 
 	return __atomic_compare_exchange_n(&_M_i, &__i1, __i2, 0,
 					   int(__m1), int(__m2));
@@ -577,14 +566,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 			      memory_order __m1,
 			      memory_order __m2) volatile noexcept
       {
-	memory_order __b2 __attribute__ ((__unused__))
-	  = __m2 & __memory_order_mask;
-	memory_order __b1 __attribute__ ((__unused__))
-	  = __m1 & __memory_order_mask;
-
-	__glibcxx_assert(__b2 != memory_order_release);
-	__glibcxx_assert(__b2 != memory_order_acq_rel);
-	__glibcxx_assert(__b2 <= __b1);
+	__glibcxx_assert(__is_valid_cmpexch_failure_order(__m2));
 
 	return __atomic_compare_exchange_n(&_M_i, &__i1, __i2, 0,
 					   int(__m1), int(__m2));
@@ -869,13 +851,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 			      memory_order __m1,
 			      memory_order __m2) noexcept
       {
-	memory_order __b2 __attribute__ ((__unused__))
-	  = __m2 & __memory_order_mask;
-	memory_order __b1 __attribute__ ((__unused__))
-	  = __m1 & __memory_order_mask;
-	__glibcxx_assert(__b2 != memory_order_release);
-	__glibcxx_assert(__b2 != memory_order_acq_rel);
-	__glibcxx_assert(__b2 <= __b1);
+	__glibcxx_assert(__is_valid_cmpexch_failure_order(__m2));
 
 	return __atomic_compare_exchange_n(&_M_p, &__p1, __p2, 0,
 					   int(__m1), int(__m2));
@@ -886,14 +862,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 			      memory_order __m1,
 			      memory_order __m2) volatile noexcept
       {
-	memory_order __b2 __attribute__ ((__unused__))
-	  = __m2 & __memory_order_mask;
-	memory_order __b1 __attribute__ ((__unused__))
-	  = __m1 & __memory_order_mask;
-
-	__glibcxx_assert(__b2 != memory_order_release);
-	__glibcxx_assert(__b2 != memory_order_acq_rel);
-	__glibcxx_assert(__b2 <= __b1);
+	__glibcxx_assert(__is_valid_cmpexch_failure_order(__m2));
 
 	return __atomic_compare_exchange_n(&_M_p, &__p1, __p2, 0,
 					   int(__m1), int(__m2));
@@ -996,6 +965,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 			    _Val<_Tp> __desired, memory_order __success,
 			    memory_order __failure) noexcept
       {
+	__glibcxx_assert(__is_valid_cmpexch_failure_order(__failure));
+
 	return __atomic_compare_exchange(__ptr, std::__addressof(__expected),
 					 std::__addressof(__desired), true,
 					 int(__success), int(__failure));
@@ -1007,6 +978,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 			      _Val<_Tp> __desired, memory_order __success,
 			      memory_order __failure) noexcept
       {
+	__glibcxx_assert(__is_valid_cmpexch_failure_order(__failure));
+
 	return __atomic_compare_exchange(__ptr, std::__addressof(__expected),
 					 std::__addressof(__desired), false,
 					 int(__success), int(__failure));
diff --git a/libstdc++-v3/include/std/atomic b/libstdc++-v3/include/std/atomic
index 9b1fb15ac41..b395c65d468 100644
--- a/libstdc++-v3/include/std/atomic
+++ b/libstdc++-v3/include/std/atomic
@@ -320,6 +320,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       compare_exchange_weak(_Tp& __e, _Tp __i, memory_order __s,
 			    memory_order __f) noexcept
       {
+	__glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
+
 	return __atomic_compare_exchange(std::__addressof(_M_i),
 					 std::__addressof(__e),
 					 std::__addressof(__i),
@@ -330,6 +332,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       compare_exchange_weak(_Tp& __e, _Tp __i, memory_order __s,
 			    memory_order __f) volatile noexcept
       {
+	__glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
+
 	return __atomic_compare_exchange(std::__addressof(_M_i),
 					 std::__addressof(__e),
 					 std::__addressof(__i),
@@ -352,6 +356,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       compare_exchange_strong(_Tp& __e, _Tp __i, memory_order __s,
 			      memory_order __f) noexcept
       {
+	__glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
+
 	return __atomic_compare_exchange(std::__addressof(_M_i),
 					 std::__addressof(__e),
 					 std::__addressof(__i),
@@ -362,6 +368,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       compare_exchange_strong(_Tp& __e, _Tp __i, memory_order __s,
 			      memory_order __f) volatile noexcept
       {
+	__glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
+
 	return __atomic_compare_exchange(std::__addressof(_M_i),
 					 std::__addressof(__e),
 					 std::__addressof(__i),