Patchwork [v3] fix PR 53648

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Submitter Jonathan Wakely
Date June 14, 2012, 10:07 p.m.
Message ID <CAH6eHdQv28UAbzvw9G6ng_+eXfjm+2aG3f49CF7VDBiwuGrpJQ@mail.gmail.com>
Download mbox | patch
Permalink /patch/165022/
State New
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Comments

Jonathan Wakely - June 14, 2012, 10:07 p.m.
Using the empty base-class optimization in std::tuple can result in
ambiguous base classes when you have a tuple of tuples. This patch
disables the EBO for tuple elements that are themselves tuples, with
tuple_size > 1 (because for a zero-size tuple the base class will be
_Tuple_impl<1> and the tuple that contains it must have _Tuple_impl<N>
where N>1, because it contains at least one element, the zero-sized
tuple).

        PR libstdc++/53648
        * include/std/tuple (__empty_not_final): Do not use EBO for tuples.
        * testsuite/20_util/tuple/53648.cc: New.
        * testsuite/20_util/uses_allocator/cons_neg.cc: Adjust dg-error line
        number.

Tested x86_64-linux, committed to trunk.  Although I think it would be
safe I have no plans to apply it to release branches.
commit b069e0d58ce0c4d54d67aa5c85e88022a317817a
Author: Jonathan Wakely <jwakely.gcc@gmail.com>
Date:   Thu Jun 14 22:02:26 2012 +0100

    	PR libstdc++/53648
    	* include/std/tuple (__empty_not_final): Do not use EBO for tuples.
    	* testsuite/20_util/tuple/53648.cc: New.
    	* testsuite/20_util/uses_allocator/cons_neg.cc: Adjust dg-error line
    	number.

Patch

diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index 4d4691f..fb9e09f 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -1,6 +1,7 @@ 
 // <tuple> -*- C++ -*-
 
-// Copyright (C) 2007, 2008, 2009, 2010, 2011 Free Software Foundation, Inc.
+// Copyright (C) 2007, 2008, 2009, 2010, 2011, 2012
+// 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
@@ -201,10 +202,18 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       void _M_swap(_Tuple_impl&) noexcept { /* no-op */ }
     };
 
+  template<typename _Tp>
+    struct __is_empty_non_tuple : is_empty<_Tp> { };
+
+  // Using EBO for elements that are tuples causes ambiguous base errors.
+  template<typename _El0, typename... _El>
+    struct __is_empty_non_tuple<tuple<_El0, _El...>> : false_type { };
+
   // Use the Empty Base-class Optimization for empty, non-final types.
   template<typename _Tp>
     using __empty_not_final
-      = typename conditional<__is_final(_Tp), false_type, is_empty<_Tp>>::type;
+    = typename conditional<__is_final(_Tp), false_type,
+			   __is_empty_non_tuple<_Tp>>::type;
 
   /**
    * Recursive tuple implementation. Here we store the @c Head element
diff --git a/libstdc++-v3/testsuite/20_util/tuple/53648.cc b/libstdc++-v3/testsuite/20_util/tuple/53648.cc
new file mode 100644
index 0000000..5671e44
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/tuple/53648.cc
@@ -0,0 +1,44 @@ 
+// { dg-options "-std=gnu++0x" }
+// { dg-do compile }
+
+// Copyright (C) 2012 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/>.
+
+// libstdc++/53648
+
+#include <tuple>
+#include <type_traits>
+
+using std::tuple;
+
+struct A { };
+
+template class tuple<tuple<>>;
+template class tuple<tuple<tuple<>>>;
+template class tuple<A, tuple<A, tuple<A, tuple<A>>>>;
+template class tuple<tuple<tuple<A, A>, A>, A>;
+
+// Verify the following QoI properties are preserved
+
+static_assert( std::is_empty<tuple<>>::value, "tuple<> is empty" );
+
+static_assert( std::is_empty<tuple<tuple<>>>::value,
+               "tuple<tuple<>> is empty" );
+
+static_assert( sizeof(tuple<char, tuple<>>) == sizeof(char),
+               "tuple<> is eligible for EBO" );
+
diff --git a/libstdc++-v3/testsuite/20_util/uses_allocator/cons_neg.cc b/libstdc++-v3/testsuite/20_util/uses_allocator/cons_neg.cc
index 73a0d0f..d81dd35 100644
--- a/libstdc++-v3/testsuite/20_util/uses_allocator/cons_neg.cc
+++ b/libstdc++-v3/testsuite/20_util/uses_allocator/cons_neg.cc
@@ -44,4 +44,4 @@  void test01()
 
   tuple<Type> t(allocator_arg, a, 1);
 }
-// { dg-error "no matching function" "" { target *-*-* } 112 }
+// { dg-error "no matching function" "" { target *-*-* } 113 }