While looking at LWG 2511 I realised that we can prevent
scoped_allocator_adaptor::construct(pair<T, U>*, ...) from making any
copies internally. The transformed tuples that get passed to the
std::pair constructor (with additional allocator arguments) can be
tuples of references even if the incoming tuples are not.
See P0475R0 (which will be in the pre-Issaquah mailing) for a proposal
to resolve LWG 2511 by requiring this behaviour.
* include/std/scoped_allocator (scoped_allocator_adaptor): Forward
piecewise construction arguments as tuples of references, to avoid
copies (related to LWG 2511).
* testsuite/20_util/scoped_allocator/construct_pair.cc: New test.
Tested powerpc64le-linux, committed to trunk.
commit 2af91ec5027f51a60cb548379cccf4fb69e34897
Author: Jonathan Wakely <jwak...@redhat.com>
Date: Tue Jul 21 13:14:33 2015 +0100
Avoid copies in std::scoped_allocator_adaptor piecewise construction
* include/std/scoped_allocator (scoped_allocator_adaptor): Forward
piecewise construction arguments as tuples of references, to avoid
copies (related to LWG 2511).
* testsuite/20_util/scoped_allocator/construct_pair.cc: New test.
diff --git a/libstdc++-v3/include/std/scoped_allocator
b/libstdc++-v3/include/std/scoped_allocator
index 39762fe..dcb97df 100644
--- a/libstdc++-v3/include/std/scoped_allocator
+++ b/libstdc++-v3/include/std/scoped_allocator
@@ -369,10 +369,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
= __use_alloc<_T1, inner_allocator_type, _Args1...>(__inner);
auto __y_use_tag
= __use_alloc<_T2, inner_allocator_type, _Args2...>(__inner);
+ typename _Build_index_tuple<sizeof...(_Args1)>::__type __x_indices;
+ typename _Build_index_tuple<sizeof...(_Args2)>::__type __y_indices;
typedef __outermost_alloc_traits<scoped_allocator_adaptor> _O_traits;
_O_traits::construct(__outermost(*this), __p, piecewise_construct,
- _M_construct_p(__x_use_tag, __x),
- _M_construct_p(__y_use_tag, __y));
+ _M_construct_p(__x_use_tag, __x_indices, __x),
+ _M_construct_p(__y_use_tag, __y_indices, __y));
}
template<typename _T1, typename _T2>
@@ -428,26 +430,27 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
const scoped_allocator_adaptor<_OutA2, _InA...>& __b)
noexcept;
private:
- template<typename _Tuple>
- _Tuple&&
- _M_construct_p(__uses_alloc0, _Tuple& __t)
+ template<typename _Ind, typename... _Args>
+ tuple<_Args&&...>
+ _M_construct_p(__uses_alloc0, _Ind, tuple<_Args...>& __t)
{ return std::move(__t); }
- template<typename... _Args>
- std::tuple<allocator_arg_t, inner_allocator_type&, _Args...>
- _M_construct_p(__uses_alloc1_, std::tuple<_Args...>& __t)
+ template<size_t... _Ind, typename... _Args>
+ tuple<allocator_arg_t, inner_allocator_type&, _Args&&...>
+ _M_construct_p(__uses_alloc1_, _Index_tuple<_Ind...>,
+ tuple<_Args...>& __t)
{
- typedef std::tuple<allocator_arg_t, inner_allocator_type&> _Tuple;
- return std::tuple_cat(_Tuple(allocator_arg, inner_allocator()),
- std::move(__t));
+ return { allocator_arg, inner_allocator(),
+ std::get<_Ind>(std::move(__t))...
+ };
}
- template<typename... _Args>
- std::tuple<_Args..., inner_allocator_type&>
- _M_construct_p(__uses_alloc2_, std::tuple<_Args...>& __t)
+ template<size_t... _Ind, typename... _Args>
+ tuple<_Args&&..., inner_allocator_type&>
+ _M_construct_p(__uses_alloc2_, _Index_tuple<_Ind...>,
+ tuple<_Args...>& __t)
{
- typedef std::tuple<inner_allocator_type&> _Tuple;
- return std::tuple_cat(std::move(__t), _Tuple(inner_allocator()));
+ return { std::get<_Ind>(std::move(__t))..., inner_allocator() };
}
};
diff --git a/libstdc++-v3/testsuite/20_util/scoped_allocator/construct_pair.cc
b/libstdc++-v3/testsuite/20_util/scoped_allocator/construct_pair.cc
new file mode 100644
index 0000000..2996412
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/scoped_allocator/construct_pair.cc
@@ -0,0 +1,81 @@
+// Copyright (C) 2016 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3. If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-do run { target c++11 } }
+
+#include <utility>
+#include <tuple>
+#include <scoped_allocator>
+
+struct do_not_copy {
+ do_not_copy() = default;
+ do_not_copy(const do_not_copy&) { throw 1; }
+};
+
+void
+test01()
+{
+ struct X {
+ X(do_not_copy&&) { }
+ };
+
+ using pair = std::pair<X, int>;
+ std::scoped_allocator_adaptor<std::allocator<pair>> a;
+ auto ptr = a.allocate(1);
+ a.construct(ptr, std::piecewise_construct,
+ std::tuple<do_not_copy>{}, std::make_tuple(1));
+ a.deallocate(ptr, 1);
+}
+
+void
+test02()
+{
+ struct X {
+ using allocator_type = std::allocator<int>;
+ X(do_not_copy&&, const allocator_type&) { }
+ };
+
+ using pair = std::pair<X, int>;
+ std::scoped_allocator_adaptor<std::allocator<pair>> a;
+ auto ptr = a.allocate(1);
+ a.construct(ptr, std::piecewise_construct,
+ std::tuple<do_not_copy>{}, std::make_tuple(1));
+ a.deallocate(ptr, 1);
+}
+
+void
+test03()
+{
+ struct X {
+ using allocator_type = std::allocator<int>;
+ X(std::allocator_arg_t, const allocator_type&, do_not_copy&&) { }
+ };
+
+ using pair = std::pair<X, int>;
+ std::scoped_allocator_adaptor<std::allocator<pair>> a;
+ auto ptr = a.allocate(1);
+ a.construct(ptr, std::piecewise_construct,
+ std::tuple<do_not_copy>{}, std::make_tuple(1));
+ a.deallocate(ptr, 1);
+}
+
+int main()
+{
+ test01();
+ test02();
+ test03();
+}
