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libstdc++: Allow unordered_set assignment to assign to existing nodes

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Currently the _ReuseOrAllocNode::operator(Args&&...) function always
destroys the value stored in recycled nodes and constructs a new value.

The _ReuseOrAllocNode type is only ever used for implementing
assignment, either from another unordered container of the same type, or
from std::initializer_list<value_type>. Consequently, the parameter pack
Args only ever consists of a single parameter or type const value_type&
or value_type.  We can replace the variadic parameter pack with a single
forwarding reference parameter, and when the value_type is assignable
from that type we can use assignment instead of destroying the existing
value and then constructing a new one.

Using assignment is typically only possible for sets, because for maps
the value_type is std::pair<const key_type, mapped_type> and in most
cases std::is_assignable_v<const key_type&, const key_type&> is false.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable_policy.h (_ReuseOrAllocNode::operator()):
	Replace parameter pack with a single parameter. Assign to
	existing value when possible.
	* testsuite/23_containers/unordered_multiset/allocator/move_assign.cc:
	Adjust expected count of operations.
	* testsuite/23_containers/unordered_set/allocator/move_assign.cc:
	Likewise.

Reviewed-by: François Dumont <fdumont@gcc.gnu.org>
This commit is contained in:
Jonathan Wakely 2024-11-01 12:49:53 +00:00 committed by Jonathan Wakely
parent 9fcbbb3d10
commit afc9351ebb
No known key found for this signature in database
3 changed files with 35 additions and 17 deletions
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37
libstdc++-v3/include/bits/hashtable_policy.h
View File

@ -172,24 +172,39 @@ namespace __detail
~_ReuseOrAllocNode()
{ _M_h._M_deallocate_nodes(_M_nodes); }
template<typename... _Args>
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
template<typename _Arg>
__node_ptr
operator()(_Args&&... __args)
operator()(_Arg&& __arg)
{
if (!_M_nodes)
return _M_h._M_allocate_node(std::forward<_Args>(__args)...);
return _M_h._M_allocate_node(std::forward<_Arg>(__arg));
using value_type = typename _NodeAlloc::value_type::value_type;
__node_ptr __node = _M_nodes;
_M_nodes = _M_nodes->_M_next();
__node->_M_nxt = nullptr;
auto& __a = _M_h._M_node_allocator();
__node_alloc_traits::destroy(__a, __node->_M_valptr());
_NodePtrGuard<__hashtable_alloc, __node_ptr> __guard { _M_h, __node };
__node_alloc_traits::construct(__a, __node->_M_valptr(),
std::forward<_Args>(__args)...);
__guard._M_ptr = nullptr;
if constexpr (is_assignable<value_type&, _Arg>::value)
{
__node->_M_v() = std::forward<_Arg>(__arg);
_M_nodes = _M_nodes->_M_next();
__node->_M_nxt = nullptr;
}
else
{
_M_nodes = _M_nodes->_M_next();
__node->_M_nxt = nullptr;
auto& __a = _M_h._M_node_allocator();
__node_alloc_traits::destroy(__a, __node->_M_valptr());
_NodePtrGuard<__hashtable_alloc, __node_ptr>
__guard{ _M_h, __node };
__node_alloc_traits::construct(__a, __node->_M_valptr(),
std::forward<_Arg>(__arg));
__guard._M_ptr = nullptr;
}
return __node;
}
#pragma GCC diagnostic pop
private:
__node_ptr _M_nodes;

5
libstdc++-v3/testsuite/23_containers/unordered_multiset/allocator/move_assign.cc
View File

@ -46,8 +46,9 @@ void test01()
VERIFY( 1 == v1.get_allocator().get_personality() );
VERIFY( 2 == v2.get_allocator().get_personality() );
VERIFY( counter_type::move_count == 1 );
VERIFY( counter_type::destructor_count == 2 );
VERIFY( counter_type::move_count == 0 );
// 1 element in v1 destroyed.
VERIFY( counter_type::destructor_count == 1 );
}
void test02()

10
libstdc++-v3/testsuite/23_containers/unordered_set/allocator/move_assign.cc
View File

@ -52,8 +52,9 @@ void test01()
VERIFY( 1 == v1.get_allocator().get_personality() );
VERIFY( 2 == v2.get_allocator().get_personality() );
VERIFY( counter_type::move_count == 1 );
VERIFY( counter_type::destructor_count == 2 );
VERIFY( counter_type::move_count == 0 );
// 1 element in v1 destroyed.
VERIFY( counter_type::destructor_count == 1 );
}
// Check there's nothing left allocated or constructed.
@ -130,8 +131,9 @@ void test03()
VERIFY( 1 == v1.get_allocator().get_personality() );
VERIFY( 2 == v2.get_allocator().get_personality() );
VERIFY( counter_type::move_count == 1 );
VERIFY( counter_type::destructor_count == i + 1 );
VERIFY( counter_type::move_count == 0 );
// (i - 1) elements in v2 destroyed, and 1 element in v1 destroyed.
VERIFY( counter_type::destructor_count == i );
}
// Check there's nothing left allocated or constructed.