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From: "fdumont at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/95079] unorderd_map::insert_or_assign and try_emplace
 should only hash and mod once unless there is a rehash.
Date: Tue, 02 Mar 2021 16:49:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95079

--- Comment #6 from Fran=C3=A7ois Dumont <fdumont at gcc dot gnu.org> ---
Thanks for the feedback.

If this is still a problem for you after this enhancement you should perhaps
try the _Power2_rehash_policy provided as an extension. In
testsuite/23_containers/unordered_set/insert/hash_policy.cc you'll see an
example with a unordered_set like container defined as:

template<typename _Value, typename _Hash,
         typename _Pred, typename _Alloc>
  using unordered_set_power2_rehash =3D
  std::_Hashtable<_Value, _Value, _Alloc,
                  std::__detail::_Identity,
                  _Pred,
                  _Hash,
                  std::__detail::_Mask_range_hashing,
                  std::__detail::_Default_ranged_hash,
                  std::__detail::_Power2_rehash_policy,
                  std::__detail::_Hashtable_traits<false, true, true>>;

As stated by its name _Power2_rehash_policy will make sure that number buck=
ets
is a power of 2 and so make the modulo much trivial.=