[Bug libstdc++/99402] [10 Regression] std::copy creates _GLIBCXX_DEBUG false positive for attempt to subscript a dereferenceable (start-of-sequence) iterator

[François Dumont]

Hi

     Ok to backport this to gcc-10 branch ?

     Tested under Linux x86_64.

François


On 13/04/21 10:51 pm, redi at gcc dot gnu.org wrote:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99402
>
> Jonathan Wakely <redi at gcc dot gnu.org> changed:
>
>             What    |Removed                     |Added
> ----------------------------------------------------------------------------
>              Summary|[10/11 Regression]          |[10 Regression] std::copy
>                     |std::copy creates           |creates _GLIBCXX_DEBUG
>                     |_GLIBCXX_DEBUG false        |false positive for attempt
>                     |positive for attempt to     |to subscript a
>                     |subscript a dereferenceable |dereferenceable
>                     |(start-of-sequence)         |(start-of-sequence)
>                     |iterator                    |iterator
>
> --- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
> This was fixed on trunk by r11-8100:
>
> libstdc++: [_GLIBCXX_DEBUG] Fix management of __dp_sign_max_size [PR 99402]
>
> __dp_sign precision indicates that we found out what iterator comes first or
> last in the range. __dp_sign_max_size is the same plus it gives the information
> of the max size of the range that is to say the max_size value such that
> distance(lhs, rhs) < max_size.
> Thanks to this additional information we are able to tell when a copy of n
> elements
> to that range will fail even if we do not know exactly how large it is.
>
> This patch makes sure that we are properly using this information.
>
> libstdc++-v3/ChangeLog:
>
>          PR libstdc++/99402
>          * include/debug/helper_functions.h (__can_advance(_InputIterator,
>          const std::pair<_Diff, _Distance_precision>&, int)): New.
>          (__can_advance(const _Safe_iterator<>&,
>          const std::pair<_Diff, _Distance_precision>&, int)): New.
>          * include/debug/macros.h (__glibcxx_check_can_increment_dist): New,
>          use latter.
>          (__glibcxx_check_can_increment_range): Adapt to use latter.
>          (__glibcxx_check_can_decrement_range): Likewise.
>          * include/debug/safe_iterator.h
>          (_Safe_iterator<>::_M_can_advance(const std::pair<_Diff,
> _Distance_precision>&,
>          int)): New.
>          (__can_advance(const _Safe_iterator<>&,
>          const std::pair<_Diff, _Distance_precision>&, int)): New.
>          * include/debug/safe_iterator.tcc
>          (_Safe_iterator<>::_M_can_advance(const std::pair<_Diff,
> _Distance_precision>&,
>          int)): New.
>          (_Safe_iterator<>::_M_valid_range(const _Safe_iterator<>&,
>          std::pair<difference_type, _Distance_precision>&, bool)): Adapt for
>          __dp_sign_max_size.
>          (__copy_move_a): Adapt to use __glibcxx_check_can_increment_dist.
>          (__copy_move_backward_a): Likewise.
>          (__equal_aux): Likewise.
>          * include/debug/stl_iterator.h (__can_advance(const
> std::reverse_iterator<>&,
>          const std::pair<_Diff, _Distance_precision>&, int)): New.
>          (__can_advance(const std::move_iterator<>&,
>          const std::pair<_Diff, _Distance_precision>&, int)): New.
>          * testsuite/25_algorithms/copy/debug/99402.cc: New test.
>

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