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From: "fdumont at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/96088] Range insertion into unordered_map is less
 effective than a loop with insertion
Date: Tue, 25 May 2021 19:27:47 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96088

Fran=C3=A7ois Dumont <fdumont at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.0
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #6 from Fran=C3=A7ois Dumont <fdumont at gcc dot gnu.org> ---
Fix with this commit:
https://gcc.gnu.org/git/?p=3Dgcc.git;a=3Dcommit;h=3D2c43f5ec9db163696de8691=
eb529df06c4999bcc

libstdc++: Limit allocation on iterator insertion in Hashtable [PR 96088]

When inserting into unordered_multiset or unordered_multimap first instanti=
ate
the node to store and compute the hash code from it to avoid a potential
intermediate key_type instantiation.

When inserting into unordered_set or unordered_map check if invoking the ha=
sh
functor with container key_type is noexcept and invoking the same hash func=
tor
with key part of the iterator value_type can throw. In this case create a
temporary key_type instance at Hashtable level and use it to compute the ha=
sh
code. This temporary instance is moved to the final storage location if nee=
ded.

libstdc++-v3/ChangeLog:

PR libstdc++/96088
* include/bits/hashtable_policy.h (_Select2nd): New.
(_NodeBuilder<>): New.
(_ReuseOrAllocNode<>::operator()): Use variadic template args.
(_AllocNode<>::operator()): Likewise.
* include/bits/hashtable.h
(_Hashtable<>::__node_builder_t): New.
(_Hashtable<>::_M_insert_unique<>(_Kt&&, _Arg&&, const _NodeGenerator&)):
 New.
(_Hashtable<>::_S_forward_key): New.
(_Hashtable<>::_M_insert): Use latter.
(_Hashtable<>::_M_insert(const_iterator, _Arg&&, const _NodeGenerator&,
false_type)):
Instantiate node first, compute hash code second.
* testsuite/23_containers/unordered_map/96088.cc: New test.
* testsuite/23_containers/unordered_multimap/96088.cc: New test.
* testsuite/23_containers/unordered_multiset/96088.cc: New test.
* testsuite/23_containers/unordered_set/96088.cc: New test.
* testsuite/util/replacement_memory_operators.h
(counter::_M_increment): New.
(counter::_M_decrement): New.
(counter::reset()): New.=