https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121148
--- Comment #16 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jonathan Wakely <r...@gcc.gnu.org>: https://gcc.gnu.org/g:6456da6bab8a2c43e7899afda991589065d96595 commit r16-3810-g6456da6bab8a2c43e7899afda991589065d96595 Author: Jonathan Wakely <jwak...@redhat.com> Date: Thu Jul 17 22:02:45 2025 +0100 libstdc++: Make atomicity helpers use unsigned arithmetic [PR121148] The standard requires that std::atomic<integral-type>::fetch_add does not have undefined behaviour for signed overflow, instead it wraps like unsigned integers. The compiler ensures this is true for the atomic built-ins that std::atomic uses, but it's not currently true for the __gnu_cxx::__exchange_and_add and __gnu_cxx::__atomic_add functions defined in libstdc++, which operate on type _Atomic_word. For the inline __exchange_and_add_single function (used when there's only one thread in the process), we can copy the value to an unsigned long and do the addition on that, then assign it back to the _Atomic_word variable. The __exchange_and_add in config/cpu/generic/atomicity_mutex/atomicity.h locks a mutex and then performs exactly the same steps as __exchange_and_add_single. Calling __exchange_and_add_single instead of duplicating the code benefits from the fix just made to __exchange_and_add_single. For the remaining config/cpu/$arch/atomicity.h implementations, they either use inline assembly which uses wrapping instructions (so no changes needed), or we can fix them by compiling with -fwrapv. After ths change, UBsan no longer gives an error for: _Atomic_word i = INT_MAX; __gnu_cxx::__exchange_and_add_dispatch(&i, 1); /usr/include/c++/14/ext/atomicity.h:85:12: runtime error: signed integer overflow: 2147483647 + 1 cannot be represented in type 'int' libstdc++-v3/ChangeLog: PR libstdc++/121148 * config/cpu/generic/atomicity_mutex/atomicity.h (__exchange_and_add): Call __exchange_and_add_single. * include/ext/atomicity.h (__exchange_and_add_single): Use an unsigned type for the addition. * libsupc++/Makefile.am (atomicity.o): Compile with -fwrapv. * libsupc++/Makefile.in: Regenerate. Reviewed-by: Tomasz KamiÅski <tkami...@redhat.com>
