[PATCH] Avoid copies in std::scoped_allocator_adaptor piecewise construction

Jonathan Wakely jwakely@redhat.com
Fri Oct 14 16:11:00 GMT 2016


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.


-------------- next part --------------
commit 2af91ec5027f51a60cb548379cccf4fb69e34897
Author: Jonathan Wakely <jwakely@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();
+}


More information about the Gcc-patches mailing list