[committed] libstdc++: Make self-move well-defined for containers [PR 85828]

Jonathan Wakely jwakely@redhat.com
Tue Aug 25 15:45:36 GMT 2020

On 12/08/20 20:37 +0100, Jonathan Wakely wrote:
>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
>Before PR 85828 can be closed we should also look into fixing
>std::shuffle so it doesn't do any redundant self-swaps.
>	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):
>	* 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.

This removes all the existing tests that expect a debug mode failure
that no longer happens.

Tested powerpc64le-linux. Committed to trunk.

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