Re: [PATCH] libstdc++: Ensure active union member is correctly set

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On Sat, 23 Sept 2023, 08:30 Nathaniel Shead, <nathanieloshead@gmail.com>
wrote:

> On Sat, Sep 23, 2023 at 07:40:48AM +0100, Jonathan Wakely wrote:
> > On Sat, 23 Sept 2023, 01:39 Nathaniel Shead via Libstdc++, <
> > libstdc++@gcc.gnu.org> wrote:
> >
> > > Now that bootstrap has finished, I have gotten regressions in the
> > > following libstdc++ tests:
> > >
> > > Running libstdc++:libstdc++-dg/conformance.exp ...
> > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++23 (test for excess
> > > errors)
> > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++26 (test for excess
> > > errors)
> > > FAIL: 20_util/variant/constexpr.cc -std=gnu++20 (test for excess
> errors)
> > > FAIL: 20_util/variant/constexpr.cc -std=gnu++26 (test for excess
> errors)
> > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++20 (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++26 (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++20
> (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++26
> (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > > -std=gnu++20 (test for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > > -std=gnu++26 (test for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++20
> > > (test for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++26
> > > (test for excess errors)
> > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++23 (test for excess
> > > errors)
> > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++23 compilation
> > > failed to produce executable
> > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++26 (test for excess
> > > errors)
> > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++26 compilation
> > > failed to produce executable
> > >
> > > On investigation though it looks like the issue might be with libstdc++
> > > rather than the patch itself; running the failing tests using clang
> with
> > > libstdc++ also produces similar errors, and my reading of the code
> > > suggests that this is correct.
> > >
> > > What's the way forward here? Should I look at creating a patch to fix
> > > the libstdc++ issues before resubmitting this patch for the C++
> > > frontend? Or should I submit a version of this patch without the
> > > `std::construct_at` changes and wait till libstdc++ gets fixed for
> that?
> > >
> >
> > I think we should fix libstdc++. There are probably only a few places
> that
> > need a fix, which cause all those failures.
> >
> > I can help with those fixes. I'll look into it after the weekend.
> >
>
> Thanks. I did end up getting a chance to look at it earlier today, and
> with the following patch I had no regressions when applying the frontend
> changes. Bootstrapped and regtested on x86_64-pc-linux-gnu.
>

Nice, thanks! This looks good at first glance, I'll review it properly ASAP.



> -- >8 --
>
> This patch ensures that the union members for std::string and
> std::variant are always properly set when a change occurs.
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/basic_string.h: (basic_string(basic_string&&)):
>         Activate _M_local_buf when needed.
>         (basic_string(basic_string&&, const _Alloc&)): Likewise.
>         * include/bits/basic_string.tcc: (basic_string::swap): Likewise.
>         * include/std/variant: (__detail::__variant::__construct_n): New.
>         (__detail::_variant::__emplace): Use __construct_n.
>
> Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> ---
>  libstdc++-v3/include/bits/basic_string.h   |  7 +++--
>  libstdc++-v3/include/bits/basic_string.tcc |  8 +++---
>  libstdc++-v3/include/std/variant           | 32 ++++++++++++++++++++--
>  3 files changed, 38 insertions(+), 9 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/basic_string.h
> b/libstdc++-v3/include/bits/basic_string.h
> index 09fd62afa66..7c342879827 100644
> --- a/libstdc++-v3/include/bits/basic_string.h
> +++ b/libstdc++-v3/include/bits/basic_string.h
> @@ -678,7 +678,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>        {
>         if (__str._M_is_local())
>           {
> -           traits_type::copy(_M_local_buf, __str._M_local_buf,
> +           traits_type::copy(_M_use_local_data(), __str._M_local_buf,
>                               __str.length() + 1);
>           }
>         else
> @@ -691,7 +691,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>         // basic_stringbuf relies on writing into unallocated capacity so
>         // we mess up the contents if we put a '\0' in the string.
>         _M_length(__str.length());
> -       __str._M_data(__str._M_local_data());
> +       __str._M_data(__str._M_use_local_data());
>         __str._M_set_length(0);
>        }
>
> @@ -717,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>        {
>         if (__str._M_is_local())
>           {
> +           _M_use_local_data();
>             traits_type::copy(_M_local_buf, __str._M_local_buf,
>                               __str.length() + 1);
>             _M_length(__str.length());
> @@ -728,7 +729,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>             _M_data(__str._M_data());
>             _M_length(__str.length());
>             _M_capacity(__str._M_allocated_capacity);
> -           __str._M_data(__str._M_local_buf);
> +           __str._M_data(__str._M_use_local_data());
>             __str._M_set_length(0);
>           }
>         else
> diff --git a/libstdc++-v3/include/bits/basic_string.tcc
> b/libstdc++-v3/include/bits/basic_string.tcc
> index 104a517f794..ee6e57da555 100644
> --- a/libstdc++-v3/include/bits/basic_string.tcc
> +++ b/libstdc++-v3/include/bits/basic_string.tcc
> @@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>               }
>             else if (__s.length())
>               {
> -               traits_type::copy(_M_local_buf, __s._M_local_buf,
> +               traits_type::copy(_M_use_local_data(), __s._M_local_buf,
>                                   __s.length() + 1);
>                 _M_length(__s.length());
>                 __s._M_set_length(0);
> @@ -87,7 +87,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>               }
>             else if (length())
>               {
> -               traits_type::copy(__s._M_local_buf, _M_local_buf,
> +               traits_type::copy(__s._M_use_local_data(), _M_local_buf,
>                                   length() + 1);
>                 __s._M_length(length());
>                 _M_set_length(0);
> @@ -97,7 +97,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>         else
>           {
>             const size_type __tmp_capacity = __s._M_allocated_capacity;
> -           traits_type::copy(__s._M_local_buf, _M_local_buf,
> +           traits_type::copy(__s._M_use_local_data(), _M_local_buf,
>                               length() + 1);
>             _M_data(__s._M_data());
>             __s._M_data(__s._M_local_buf);
> @@ -108,7 +108,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>           const size_type __tmp_capacity = _M_allocated_capacity;
>           if (__s._M_is_local())
>             {
> -             traits_type::copy(_M_local_buf, __s._M_local_buf,
> +             traits_type::copy(_M_use_local_data(), __s._M_local_buf,
>                                 __s.length() + 1);
>               __s._M_data(_M_data());
>               _M_data(_M_local_buf);
> diff --git a/libstdc++-v3/include/std/variant
> b/libstdc++-v3/include/std/variant
> index c0e41740dcf..7f24e760bb1 100644
> --- a/libstdc++-v3/include/std/variant
> +++ b/libstdc++-v3/include/std/variant
> @@ -320,6 +320,33 @@ namespace __variant
>      __get(_Variant&& __v) noexcept
>      { return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
>
> +  // Gets the _Uninitialized to construct into for __u.
> +  template<size_t _Np, typename _Union>
> +    constexpr decltype(auto)
> +    __construct_n(_Union& __u) noexcept
> +    {
> +      if constexpr (_Np == 0)
> +       return &__u._M_first;
> +      else if constexpr (_Np == 1)
> +       {
> +         std::_Construct(&__u._M_rest);
> +         return &__u._M_rest._M_first;
> +       }
> +      else if constexpr (_Np == 2)
> +       {
> +         std::_Construct(&__u._M_rest);
> +         std::_Construct(&__u._M_rest._M_rest);
> +         return &__u._M_rest._M_rest._M_first;
> +       }
> +      else
> +       {
> +         std::_Construct(&__u._M_rest);
> +         std::_Construct(&__u._M_rest._M_rest);
> +         std::_Construct(&__u._M_rest._M_rest._M_rest);
> +         return __variant::__construct_n<_Np -
> 3>(__u._M_rest._M_rest._M_rest);
> +       }
> +    }
> +
>    template<typename... _Types>
>      struct _Traits
>      {
> @@ -536,8 +563,9 @@ namespace __variant
>      __emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
>      {
>        __v._M_reset();
> -      auto* __addr = std::__addressof(__variant::__get_n<_Np>(__v._M_u));
> -      std::_Construct(__addr, std::forward<_Args>(__args)...);
> +      auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
> +      std::_Construct(__addr, in_place_index<0>,
> +                     std::forward<_Args>(__args)...);
>        // Construction didn't throw, so can set the new index now:
>        __v._M_index = _Np;
>      }
> --
> 2.41.0
>
>