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libstdc++: Fix regression in std::move algorithm (PR 93872)

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The std::move and std::move_backward algorithms dispatch to the
std::__memmove helper when appropriate. That function uses a
pointer-to-const for the source values, preventing them from being
moved. The two callers of that function have the same problem.

Rather than altering __memmove and its callers to work with const or
non-const source pointers, this takes a more conservative approach of
casting away the const at the point where we want to do a move
assignment. This relies on the fact that we only use __memmove when the
type is trivially copyable, so we know the move assignment doesn't alter
the source anyway.

	PR libstdc++/93872
	* include/bits/stl_algobase.h (__memmove): Cast away const before
	doing move assignment.
	* testsuite/25_algorithms/move/93872.cc: New test.
	* testsuite/25_algorithms/move_backward/93872.cc: New test.
This commit is contained in:
Jonathan Wakely 2020-02-25 12:21:44 +00:00
parent 2877ad9ac2
commit 5b904f175f
3 changed files with 52 additions and 1 deletions
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8
libstdc++-v3/ChangeLog
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@ -1,3 +1,11 @@
2020-02-24 Jonathan Wakely <jwakely@redhat.com>
PR libstdc++/93872
* include/bits/stl_algobase.h (__memmove): Cast away const before
doing move assignment.
* testsuite/25_algorithms/move/93872.cc: New test.
* testsuite/25_algorithms/move_backward/93872.cc: New test.
2020-02-24 Patrick Palka <ppalka@redhat.com>
PR libstdc++/93884

6
libstdc++-v3/include/bits/stl_algobase.h
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@ -95,7 +95,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
for(; __num > 0; --__num)
{
if constexpr (_IsMove)
*__dst = std::move(*__src);
// This const_cast looks unsafe, but we only use this function
// for trivially-copyable types, which means this assignment
// is trivial and so doesn't alter the source anyway.
// See PR 93872 for why it's needed.
*__dst = std::move(*const_cast<_Tp*>(__src));
else
*__dst = *__src;
++__src;

39
libstdc++-v3/testsuite/25_algorithms/move/93872.cc Normal file
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@ -0,0 +1,39 @@
// Copyright (C) 2020 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-options "-std=gnu++2a" }
// { dg-do compile { target c++2a } }
#include <algorithm>
struct X
{
X() = default;
X(const X&) = delete;
X& operator=(const X&) = delete;
X(X&&) = default;
X& operator=(X&&) = default;
};
void
test01()
{
X a[2], b[2];
std::move(std::begin(a), std::end(a), std::begin(b));
}