https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85828
--- Comment #3 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:c2fb0a1a2e7a0fb15cf3cf876f621902ccd273f0 commit r11-2677-gc2fb0a1a2e7a0fb15cf3cf876f621902ccd273f0 Author: Jonathan Wakely <jwak...@redhat.com> Date: Wed Aug 12 20:36:00 2020 +0100 libstdc++: Make self-move well-defined for containers [PR 85828] The C++ LWG recently confirmed that self-move assignment should not have undefined behaviour for standard containers (see the proposed resolution of LWG 2839). The result should be a valid but unspecified value, just like other times when a container is moved from. Our std::list, std::__cxx11::basic_string and unordered containers all have bugs which result in undefined behaviour. For std::list the problem is that we clear the previous contents using _M_clear() instead of clear(). This means the _M_next, _M_prev and _M_size members are not zeroed, and so after we "update" them (with their existing values), we are left with dangling pointers and a non-zero size, but no elements. For the unordered containers the problem is similar. _Hashtable first deallocates the existing contents, then takes ownership of the pointers from the RHS object (which has just had its contents deallocated so the pointers are dangling). For std::basic_string it's a little more subtle. When the string is local (i.e. fits in the SSO buffer) we use char_traits::copy to copy the contents from this->data() to __rhs.data(). When &__rhs == this that copy violates the precondition that the ranges don't overlap. We only need to check for self-move for this case where it's local, because the only other case that can be true for self-move is that it's non-local but the allocators compare equal. In that case the data pointer is neither deallocated nor leaked, so the result is well-defined. This patch also makes a small optimization for std::deque move assignment, to use the efficient move when is_always_equal is false, but the allocators compare equal at runtime. Finally, we need to remove all the Debug Mode checks which abort the program when a self-move is detected, because it's not undefined to do that. Before PR 85828 can be closed we should also look into fixing std::shuffle so it doesn't do any redundant self-swaps. libstdc++-v3/ChangeLog: PR libstdc++/85828 * include/bits/basic_string.h (operator=(basic_string&&)): Check for self-move before copying with char_traits::copy. * include/bits/hashtable.h (operator=(_Hashtable&&)): Check for self-move. * include/bits/stl_deque.h (_M_move_assign1(deque&&, false_type)): Check for equal allocators. * include/bits/stl_list.h (_M_move_assign(list&&, true_type)): Call clear() instead of _M_clear(). * include/debug/formatter.h (__msg_self_move_assign): Change comment. * include/debug/macros.h (__glibcxx_check_self_move_assign): (_GLIBCXX_DEBUG_VERIFY): Remove. * include/debug/safe_container.h (operator=(_Safe_container&&)): Remove assertion check for safe move and make it well-defined. * include/debug/safe_iterator.h (operator=(_Safe_iterator&&)): Remove assertion check for self-move. * include/debug/safe_local_iterator.h (operator=(_Safe_local_iterator&&)): Likewise. * testsuite/21_strings/basic_string/cons/char/self_move.cc: New test. * testsuite/23_containers/deque/cons/self_move.cc: New test. * testsuite/23_containers/forward_list/cons/self_move.cc: New test. * testsuite/23_containers/list/cons/self_move.cc: New test. * testsuite/23_containers/set/cons/self_move.cc: New test. * testsuite/23_containers/unordered_set/cons/self_move.cc: New test. * testsuite/23_containers/vector/cons/self_move.cc: New test.
