[patch] fix libstdc++/56267 - local iterator requirements

[Jonathan Wakely]

On 17 January 2014 15:11, Jonathan Wakely wrote:
> The issue in PR 56267 is that unordered_xxx::local_iterator sometimes
> inherits from the user-defined hash function (via _Hash_code_base,
> which inherits from the hash function to use the EBO), and
> local_iterator must be DefaultConstructible and Assignable, even when
> the hash function isn't.
>
> My solution is to remove the inheritance from _Hash_code_base, and
> instead construct/destroy the _Hash_code_base in a block of
> uninitialized memory (via __gnu_cxx::__aligned_buffer). This would
> mean we can't use the EBO and increase the size of local_iterator, and
> past measurements have shown that the unordered containers'
> performance is sensitive to such changes, so there's a partial
> specialization that doesn't have the __aligned_buffer member for the
> case where the _Hash_code_base is empty and needs no storage.
>
> François, do you have any comments on this? Can you see a better solution?
>
> While working on this I decided I didn't like everything in
> _Local_iterator_base being public, so I added some accessors to the
> only members that are needed by unrelated types.

Tested x86_64-linux and committed to trunk.

2014-01-20  Jonathan Wakely  <jwakely@redhat.com>

        PR libstdc++/56267
        * include/bits/hashtable_policy.h (_Hash_code_base<... false>): Grant
        friendship to _Local_iterator_base<..., false>.
        (_Local_iterator_base): Give protected access to all existing members.
        (_Local_iterator_base::_M_curr()): New public accessor.
        (_Local_iterator_base::_M_get_bucket()): New public accessor.
        (_Local_iterator_base<..., false>::_M_init()): New function to manage
        the lifetime of the _Hash_code_base explicitly.
        (_Local_iterator_base<..., false>::_M_destroy()): Likewise.
        (_Local_iterator_base<..., false>): Define copy constructor and copy
        assignment operator that use new functions to manage _Hash_code_base.
        (operator==(const _Local_iterator_base&, const _Local_iterator_base&),
        operator==(const _Local_iterator_base&, const _Local_iterator_base&)):
        Use public API for _Local_iterator_base.
        * include/debug/safe_local_iterator.h (_Safe_local_iterator): Likewise.
        * include/debug/unordered_map (__debug::unordered_map::erase(),
        __debug::unordered_multimap::erase()): Likewise.
        * include/debug/unordered_set (__debug::unordered_set::erase(),
        __debug::unordered_multiset::erase()): Likewise.
        * testsuite/23_containers/unordered_set/56267-2.cc: New test.