[gcc r16-3820] libstdc++: Fix bootstrap failure in atomicity.cc

[Jonathan Wakely]

https://gcc.gnu.org/g:4fe3b8b8db5c1915c0aa90965c875e4e3e4d1724

commit r16-3820-g4fe3b8b8db5c1915c0aa90965c875e4e3e4d1724
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Sep 12 11:33:20 2025 +0100

    libstdc++: Fix bootstrap failure in atomicity.cc
    
    My r16-3810-g6456da6bab8a2c changes broke bootstrap for targets that use
    the mutex-based atomic helpers. This fixes it by casting away the
    unnecessary volatile-qualification on the _Atomic_word* before passing
    it to __exchange_and_add_single.
    
    libstdc++-v3/ChangeLog:
    
            * config/cpu/generic/atomicity_mutex/atomicity.h
            (__exchange_and_add): Use const_cast to remove volatile.

Diff:
---
 libstdc++-v3/config/cpu/generic/atomicity_mutex/atomicity.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/config/cpu/generic/atomicity_mutex/atomicity.h b/libstdc++-v3/config/cpu/generic/atomicity_mutex/atomicity.h
index 7d5772d54840..d088f69fe9a4 100644
--- a/libstdc++-v3/config/cpu/generic/atomicity_mutex/atomicity.h
+++ b/libstdc++-v3/config/cpu/generic/atomicity_mutex/atomicity.h
@@ -44,7 +44,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   __exchange_and_add(volatile _Atomic_word* __mem, int __val) throw ()
   {
     __gnu_cxx::__scoped_lock sentry(get_atomic_mutex());
-    return __gnu_cxx::__exchange_and_add_single(__mem, __val);
+    // The volatile qualification is meaningless. All changes to the memory
+    // location happen while this mutex is locked so it's not volatile at all.
+    auto __mem2 = const_cast<_Atomic_word*>(__mem);
+    return __gnu_cxx::__exchange_and_add_single(__mem2, __val);
   }
 
   void