https://gcc.gnu.org/g:afc9351ebbac1ed55d7c472af817b172bdb662e6
commit r15-5212-gafc9351ebbac1ed55d7c472af817b172bdb662e6
Author: Jonathan Wakely <jwak...@redhat.com>
Date: Fri Nov 1 12:49:53 2024 +0000
libstdc++: Allow unordered_set assignment to assign to existing nodes
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 <fdum...@gcc.gnu.org>
Diff:
---
libstdc++-v3/include/bits/hashtable_policy.h | 37 +++++++++++++++-------
.../unordered_multiset/allocator/move_assign.cc | 5 +--
.../unordered_set/allocator/move_assign.cc | 10 +++---
3 files changed, 35 insertions(+), 17 deletions(-)
diff --git a/libstdc++-v3/include/bits/hashtable_policy.h
b/libstdc++-v3/include/bits/hashtable_policy.h
index b5f837e60619..7a3c66c37fd6 100644
--- a/libstdc++-v3/include/bits/hashtable_policy.h
+++ b/libstdc++-v3/include/bits/hashtable_policy.h
@@ -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;
diff --git
a/libstdc++-v3/testsuite/23_containers/unordered_multiset/allocator/move_assign.cc
b/libstdc++-v3/testsuite/23_containers/unordered_multiset/allocator/move_assign.cc
index 50608ec443ff..6d00354902ee 100644
---
a/libstdc++-v3/testsuite/23_containers/unordered_multiset/allocator/move_assign.cc
+++
b/libstdc++-v3/testsuite/23_containers/unordered_multiset/allocator/move_assign.cc
@@ -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()
diff --git
a/libstdc++-v3/testsuite/23_containers/unordered_set/allocator/move_assign.cc
b/libstdc++-v3/testsuite/23_containers/unordered_set/allocator/move_assign.cc
index 677ea67d0ea7..6be70022705a 100644
---
a/libstdc++-v3/testsuite/23_containers/unordered_set/allocator/move_assign.cc
+++
b/libstdc++-v3/testsuite/23_containers/unordered_set/allocator/move_assign.cc
@@ -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.
