Re: [PATCH] libstdc++: fix memory clobbering in std::vector [PR110879]

[Jonathan Wakely <jwakely@redhat.com>]

On Thu, 17 Aug 2023 at 08:43, Vladimir Palevich <palevichva@gmail.com> wrote:
>
> On Thu, 17 Aug 2023 at 01:51, Jonathan Wakely <jwakely@redhat.com> wrote:
> >
> > On 09/08/23 01:34 +0300, Vladimir Palevich wrote:
> > >Because of the recent change in _M_realloc_insert and _M_default_append, call
> > >to deallocate was ordered after assignment to class members of std::vector
> > >(in the guard destructor), which is causing said members to be call-clobbered.
> > >This is preventing further optimization, the compiler is unable to move memory
> > >read out of a hot loop in this case.
> > >This patch reorders the call to before assignments by putting guard in its own
> > >block. Plus a new testsuite for this case.
> > >I'm not very happy with the new testsuite, but I don't know how to properly
> > >test this.
> > >
> > >Tested on x86_64-pc-linux-gnu.
> > >
> > >Maybe something could be done so that the compiler would be able to optimize
> > >such cases anyway. Reads could be moved just after the clobbering calls in
> > >unlikely branches, for example. This should be a fairly common case with
> > >destructors at the end of a function.
> > >
> > >Note: I don't have write access.
> > >
> > >-- >8 --
> > >
> > >Fix ordering to prevent clobbering of class members by a call to deallocate
> > >in _M_realloc_insert and _M_default_append.
> > >
> > >libstdc++-v3/ChangeLog:
> > >    PR libstdc++/110879
> > >    * include/bits/vector.tcc: End guard lifetime just before assignment to
> > >    class members.
> > >    * testsuite/libstdc++-dg/conformance.exp: Load scantree.exp.
> > >    * testsuite/23_containers/vector/110879.cc: New test.
> > >
> > >Signed-off-by: Vladimir Palevich  <palevichva@gmail.com>
> > >---
> > > libstdc++-v3/include/bits/vector.tcc          | 220 +++++++++---------
> > > .../testsuite/23_containers/vector/110879.cc  |  35 +++
> > > .../testsuite/libstdc++-dg/conformance.exp    |  13 ++
> > > 3 files changed, 163 insertions(+), 105 deletions(-)
> > > create mode 100644 libstdc++-v3/testsuite/23_containers/vector/110879.cc
> > >
> > >diff --git a/libstdc++-v3/include/bits/vector.tcc b/libstdc++-v3/include/bits/vector.tcc
> > >index ada396c9b30..80631d1e2a1 100644
> > >--- a/libstdc++-v3/include/bits/vector.tcc
> > >+++ b/libstdc++-v3/include/bits/vector.tcc
> > >@@ -488,78 +488,83 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
> > >       private:
> > >       _Guard(const _Guard&);
> > >       };
> > >-      _Guard __guard(__new_start, __len, _M_impl);
> > >
> > >-      // The order of the three operations is dictated by the C++11
> > >-      // case, where the moves could alter a new element belonging
> > >-      // to the existing vector.  This is an issue only for callers
> > >-      // taking the element by lvalue ref (see last bullet of C++11
> > >-      // [res.on.arguments]).
> > >+      {
> > >+      _Guard __guard(__new_start, __len, _M_impl);
> > >
> > >-      // If this throws, the existing elements are unchanged.
> > >+      // The order of the three operations is dictated by the C++11
> > >+      // case, where the moves could alter a new element belonging
> > >+      // to the existing vector.  This is an issue only for callers
> > >+      // taking the element by lvalue ref (see last bullet of C++11
> > >+      // [res.on.arguments]).
> > >+
> > >+      // If this throws, the existing elements are unchanged.
> > > #if __cplusplus >= 201103L
> > >-      _Alloc_traits::construct(this->_M_impl,
> > >-                             std::__to_address(__new_start + __elems_before),
> > >-                             std::forward<_Args>(__args)...);
> > >+      _Alloc_traits::construct(this->_M_impl,
> > >+                               std::__to_address(__new_start + __elems_before),
> > >+                               std::forward<_Args>(__args)...);
> > > #else
> > >-      _Alloc_traits::construct(this->_M_impl,
> > >-                             __new_start + __elems_before,
> > >-                             __x);
> > >+      _Alloc_traits::construct(this->_M_impl,
> > >+                               __new_start + __elems_before,
> > >+                               __x);
> > > #endif
> > >
> > > #if __cplusplus >= 201103L
> > >-      if _GLIBCXX17_CONSTEXPR (_S_use_relocate())
> > >-      {
> > >-        // Relocation cannot throw.
> > >-        __new_finish = _S_relocate(__old_start, __position.base(),
> > >-                                   __new_start, _M_get_Tp_allocator());
> > >-        ++__new_finish;
> > >-        __new_finish = _S_relocate(__position.base(), __old_finish,
> > >-                                   __new_finish, _M_get_Tp_allocator());
> > >-      }
> > >-      else
> > >+      if _GLIBCXX17_CONSTEXPR (_S_use_relocate())
> > >+        {
> > >+          // Relocation cannot throw.
> > >+          __new_finish = _S_relocate(__old_start, __position.base(),
> > >+                                     __new_start, _M_get_Tp_allocator());
> > >+          ++__new_finish;
> > >+          __new_finish = _S_relocate(__position.base(), __old_finish,
> > >+                                     __new_finish, _M_get_Tp_allocator());
> > >+        }
> > >+      else
> > > #endif
> > >-      {
> > >-        // RAII type to destroy initialized elements.
> > >-        struct _Guard_elts
> > >         {
> > >-          pointer _M_first, _M_last;  // Elements to destroy
> > >-          _Tp_alloc_type& _M_alloc;
> > >-
> > >-          _GLIBCXX20_CONSTEXPR
> > >-          _Guard_elts(pointer __elt, _Tp_alloc_type& __a)
> > >-          : _M_first(__elt), _M_last(__elt + 1), _M_alloc(__a)
> > >-          { }
> > >-
> > >-          _GLIBCXX20_CONSTEXPR
> > >-          ~_Guard_elts()
> > >-          { std::_Destroy(_M_first, _M_last, _M_alloc); }
> > >-
> > >-        private:
> > >-          _Guard_elts(const _Guard_elts&);
> > >-        };
> > >-
> > >-        // Guard the new element so it will be destroyed if anything throws.
> > >-        _Guard_elts __guard_elts(__new_start + __elems_before, _M_impl);
> > >-
> > >-        __new_finish = std::__uninitialized_move_if_noexcept_a(
> > >-                         __old_start, __position.base(),
> > >-                         __new_start, _M_get_Tp_allocator());
> > >-
> > >-        ++__new_finish;
> > >-        // Guard everything before the new element too.
> > >-        __guard_elts._M_first = __new_start;
> > >-
> > >-        __new_finish = std::__uninitialized_move_if_noexcept_a(
> > >-                          __position.base(), __old_finish,
> > >-                          __new_finish, _M_get_Tp_allocator());
> > >-
> > >-        // New storage has been fully initialized, destroy the old elements.
> > >-        __guard_elts._M_first = __old_start;
> > >-        __guard_elts._M_last = __old_finish;
> > >-      }
> > >-      __guard._M_storage = __old_start;
> > >-      __guard._M_len = this->_M_impl._M_end_of_storage - __old_start;
> > >+          // RAII type to destroy initialized elements.
> > >+          struct _Guard_elts
> > >+          {
> > >+            pointer _M_first, _M_last;  // Elements to destroy
> > >+            _Tp_alloc_type& _M_alloc;
> > >+
> > >+            _GLIBCXX20_CONSTEXPR
> > >+            _Guard_elts(pointer __elt, _Tp_alloc_type& __a)
> > >+            : _M_first(__elt), _M_last(__elt + 1), _M_alloc(__a)
> > >+            { }
> > >+
> > >+            _GLIBCXX20_CONSTEXPR
> > >+            ~_Guard_elts()
> > >+            { std::_Destroy(_M_first, _M_last, _M_alloc); }
> > >+
> > >+          private:
> > >+            _Guard_elts(const _Guard_elts&);
> > >+          };
> > >+
> > >+          // Guard the new element so it will be destroyed if anything throws.
> > >+          _Guard_elts __guard_elts(__new_start + __elems_before, _M_impl);
> > >+
> > >+          __new_finish = std::__uninitialized_move_if_noexcept_a(
> > >+                           __old_start, __position.base(),
> > >+                           __new_start, _M_get_Tp_allocator());
> > >+
> > >+          ++__new_finish;
> > >+          // Guard everything before the new element too.
> > >+          __guard_elts._M_first = __new_start;
> > >+
> > >+          __new_finish = std::__uninitialized_move_if_noexcept_a(
> > >+                            __position.base(), __old_finish,
> > >+                            __new_finish, _M_get_Tp_allocator());
> > >+
> > >+          // New storage has been fully initialized, destroy the old elements.
> > >+          __guard_elts._M_first = __old_start;
> > >+          __guard_elts._M_last = __old_finish;
> > >+        }
> > >+      __guard._M_storage = __old_start;
> > >+      __guard._M_len = this->_M_impl._M_end_of_storage - __old_start;
> > >+      }
> > >+      // deallocate should be called before assignments to _M_impl,
> > >+      // to avoid call-clobbering
> > >
> > >       this->_M_impl._M_start = __new_start;
> > >       this->_M_impl._M_finish = __new_finish;
> > >@@ -728,49 +733,54 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
> > >             private:
> > >               _Guard(const _Guard&);
> > >             };
> > >-            _Guard __guard(__new_start, __len, _M_impl);
> > >-
> > >-            std::__uninitialized_default_n_a(__new_start + __size, __n,
> > >-                                             _M_get_Tp_allocator());
> > >-
> > >-            if _GLIBCXX17_CONSTEXPR (_S_use_relocate())
> > >-              {
> > >-                _S_relocate(__old_start, __old_finish,
> > >-                            __new_start, _M_get_Tp_allocator());
> > >-              }
> > >-            else
> > >-              {
> > >-                // RAII type to destroy initialized elements.
> > >-                struct _Guard_elts
> > >+
> > >+            {
> > >+              _Guard __guard(__new_start, __len, _M_impl);
> > >+
> > >+              std::__uninitialized_default_n_a(__new_start + __size, __n,
> > >+                                               _M_get_Tp_allocator());
> > >+
> > >+              if _GLIBCXX17_CONSTEXPR (_S_use_relocate())
> > >                 {
> > >-                  pointer _M_first, _M_last;  // Elements to destroy
> > >-                  _Tp_alloc_type& _M_alloc;
> > >-
> > >-                  _GLIBCXX20_CONSTEXPR
> > >-                  _Guard_elts(pointer __first, size_type __n,
> > >-                              _Tp_alloc_type& __a)
> > >-                  : _M_first(__first), _M_last(__first + __n), _M_alloc(__a)
> > >-                  { }
> > >-
> > >-                  _GLIBCXX20_CONSTEXPR
> > >-                  ~_Guard_elts()
> > >-                  { std::_Destroy(_M_first, _M_last, _M_alloc); }
> > >-
> > >-                private:
> > >-                  _Guard_elts(const _Guard_elts&);
> > >-                };
> > >-                _Guard_elts __guard_elts(__new_start + __size, __n, _M_impl);
> > >-
> > >-                std::__uninitialized_move_if_noexcept_a(
> > >-                  __old_start, __old_finish, __new_start,
> > >-                  _M_get_Tp_allocator());
> > >-
> > >-                __guard_elts._M_first = __old_start;
> > >-                __guard_elts._M_last = __old_finish;
> > >-              }
> > >-            _GLIBCXX_ASAN_ANNOTATE_REINIT;
> > >-            __guard._M_storage = __old_start;
> > >-            __guard._M_len = this->_M_impl._M_end_of_storage - __old_start;
> > >+                  _S_relocate(__old_start, __old_finish,
> > >+                              __new_start, _M_get_Tp_allocator());
> > >+                }
> > >+              else
> > >+                {
> > >+                  // RAII type to destroy initialized elements.
> > >+                  struct _Guard_elts
> > >+                  {
> > >+                    pointer _M_first, _M_last;  // Elements to destroy
> > >+                    _Tp_alloc_type& _M_alloc;
> > >+
> > >+                    _GLIBCXX20_CONSTEXPR
> > >+                    _Guard_elts(pointer __first, size_type __n,
> > >+                                _Tp_alloc_type& __a)
> > >+                    : _M_first(__first), _M_last(__first + __n), _M_alloc(__a)
> > >+                    { }
> > >+
> > >+                    _GLIBCXX20_CONSTEXPR
> > >+                    ~_Guard_elts()
> > >+                    { std::_Destroy(_M_first, _M_last, _M_alloc); }
> > >+
> > >+                  private:
> > >+                    _Guard_elts(const _Guard_elts&);
> > >+                  };
> > >+                  _Guard_elts __guard_elts(__new_start + __size, __n, _M_impl);
> > >+
> > >+                  std::__uninitialized_move_if_noexcept_a(
> > >+                    __old_start, __old_finish, __new_start,
> > >+                    _M_get_Tp_allocator());
> > >+
> > >+                  __guard_elts._M_first = __old_start;
> > >+                  __guard_elts._M_last = __old_finish;
> > >+                }
> > >+              _GLIBCXX_ASAN_ANNOTATE_REINIT;
> > >+              __guard._M_storage = __old_start;
> > >+              __guard._M_len = this->_M_impl._M_end_of_storage - __old_start;
> > >+            }
> > >+            // deallocate should be called before assignments to _M_impl,
> > >+            // to avoid call-clobbering
> > >
> > >             this->_M_impl._M_start = __new_start;
> > >             this->_M_impl._M_finish = __new_start + __size + __n;
> > >diff --git a/libstdc++-v3/testsuite/23_containers/vector/110879.cc b/libstdc++-v3/testsuite/23_containers/vector/110879.cc
> > >new file mode 100644
> > >index 00000000000..d38a5a0d1a3
> > >--- /dev/null
> > >+++ b/libstdc++-v3/testsuite/23_containers/vector/110879.cc
> > >@@ -0,0 +1,35 @@
> > >+// -*- C++ -*-
> > >+
> > >+// Copyright (C) 2023 Free Software Foundation, Inc.
> >
> > Do you actually have a FSF copyright assignment for GCC?
> >
> > If not, then this should not be here, as you can't assign it to the
> > FSF.
> >
> > You've already added a Signed-off-by tag, which I assume means you're
> > contributing under the terms of the DCO: https://gcc.gnu.org/dco.html
> >
> > In which case, this isn't copyright FSF.
> >
> > I've stopped putting the copyright and license header in tests, I
> > don't consider a 5 line function that does nothing interesting to be
> > copyrightable, or worth adding all this boilerplate to.
> >
> > If you don't mind, I'll just remove this boilerplate from the test.
>
> I've just copypasted this boilerplate from other test without thinking.
> You are correct, I don't have a FSF copyright assignment. I'm contributing
> under the terms of the DCO.
> You can just remove this boilerplate.
>
> >
> >
> > >+// 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 compile }
> > >+// { dg-options "-O3 -fdump-tree-optimized" }
> > >+
> > >+#include <vector>
> > >+
> > >+std::vector<int> f(std::size_t n) {
> > >+  std::vector<int> res;
> > >+  for (std::size_t i = 0; i < n; ++i) {
> > >+    res.push_back(i);
> > >+  }
> > >+  return res;
> > >+}
> > >+
> > >+// Reads of _M_finish should be optimized out.
> > >+// This regex matches all reads from res variable except for _M_end_of_storage field.
> > >+// { dg-final { scan-tree-dump-not "=\\s*\\S*res_(?!\\S*_M_end_of_storage;)" "optimized" } }
> >
> > If this was added to gcc/testsuite/g++.dg/ instead of
> > libstdc++-v3/testsuite then we wouldn't need scandump.exp and
> > scantree.exp in the library tests.
>
> Sure, you can move this test to a more appropriate place.

OK thanks - I've pushed it to trunk now.

Thanks again for the analysis of the problem and the patch to fix it!


>
> >
> > >diff --git a/libstdc++-v3/testsuite/libstdc++-dg/conformance.exp b/libstdc++-v3/testsuite/libstdc++-dg/conformance.exp
> > >index e8c6504a7f7..1d0588aeadc 100644
> > >--- a/libstdc++-v3/testsuite/libstdc++-dg/conformance.exp
> > >+++ b/libstdc++-v3/testsuite/libstdc++-dg/conformance.exp
> > >@@ -18,6 +18,19 @@
> > >
> > > # libstdc++-v3 testsuite that uses the 'dg.exp' driver.
> > >
> > >+# Damn dejagnu for not having proper library search paths for load_lib.
> > >+# We have to explicitly load everything that gcc-dg.exp wants to load.
> > >+
> > >+proc load_gcc_lib { filename } {
> > >+    global srcdir loaded_libs
> > >+
> > >+    load_file $srcdir/../../gcc/testsuite/lib/$filename
> > >+    set loaded_libs($filename) ""
> > >+}
> > >+
> > >+load_gcc_lib scandump.exp
> > >+load_gcc_lib scantree.exp
> > >+
> > > # Initialization.
> > > dg-init
> > >
> >
>