https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112480
--- Comment #9 from CVS 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:2c492f99fc1fcb5f598286c3f3a21a05bca69d9e commit r14-5421-g2c492f99fc1fcb5f598286c3f3a21a05bca69d9e Author: Jonathan Wakely <jwak...@redhat.com> Date: Sat Nov 11 00:35:18 2023 +0000 libstdc++: Micro-optimization for std::optional [PR112480] This small change removes a branch when clearing a std::optional<T> for types with no-op destructors. For types where the destructor can be optimized away (e.g. because it's trivial, or empty and can be inlined) the _M_destroy() function does nothing but set _M_engaged to false. Setting _M_engaged=false unconditionally is cheaper than only doing it when initially true, because it allows the compiler to remove a branch. The compiler thinks it would be incorrect to unconditionally introduce a store there, because it could conflict with reads in other threads, so it won't do that optimization itself. We know it's safe to do because we're in a non-const member function, so the standard forbids any potentially concurrent calls to other member functions of the same object. Making the store unconditional can't create a data race that isn't already present in the program. libstdc++-v3/ChangeLog: PR libstdc++/112480 * include/std/optional (_Optional_payload_base::_M_reset): Set _M_engaged to false unconditionally.
