Tested x86_64-linux. Pushed to trunk. -- >8 --
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. --- libstdc++-v3/include/std/optional | 2 ++ 1 file changed, 2 insertions(+) diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional index 53450c760d9..a8c97717b72 100644 --- a/libstdc++-v3/include/std/optional +++ b/libstdc++-v3/include/std/optional @@ -311,6 +311,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION { if (this->_M_engaged) _M_destroy(); + else // This seems redundant but improves codegen, see PR 112480. + this->_M_engaged = false; } }; -- 2.41.0