[PATCH v1] libstdc++: Fix bug in default ctor of extents.

[Jonathan Wakely]

On Mon, 26 May 2025 at 08:49, Tomasz Kaminski <tkaminsk@redhat.com> wrote:
>
>
>
> On Sat, May 24, 2025 at 1:29 PM Luc Grosheintz <luc.grosheintz@gmail.com> wrote:
>>
>> The array that stores the dynamic extents used to be default
>> initialized. The standard requires value intialization. This
>> commit fixes the bug and adds a test.
>>
>> libstdc++-v3/ChangeLog:
>>
>>         * include/std/mdspan: Value initialize the array storing the
>>         dynamic extents.
>>         * testsuite/23_containers/mdspan/extents/ctor_default.cc: New
>>         test.
>>
>> Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
>> ---
>
> LGTM, thanks for noticing and fixing it.
>  We also need approval from the maintainer.

OK for trunk.

>>
>>  libstdc++-v3/include/std/mdspan               |  2 +-
>>  .../mdspan/extents/ctor_default.cc            | 41 +++++++++++++++++++
>>  2 files changed, 42 insertions(+), 1 deletion(-)
>>  create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/extents/ctor_default.cc
>>
>> diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
>> index 47cfa405e44..bcf2fa60fea 100644
>> --- a/libstdc++-v3/include/std/mdspan
>> +++ b/libstdc++-v3/include/std/mdspan
>> @@ -146,7 +146,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>
>>        private:
>>         using _S_storage = __array_traits<_IndexType, _S_rank_dynamic>::_Type;
>> -       [[no_unique_address]] _S_storage _M_dynamic_extents;
>> +       [[no_unique_address]] _S_storage _M_dynamic_extents{};
>
> We know that these are integral types, so we can use {}.
>>
>>        };
>>
>>      template<typename _OIndexType, typename _SIndexType>
>> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/extents/ctor_default.cc b/libstdc++-v3/testsuite/23_containers/mdspan/extents/ctor_default.cc
>> new file mode 100644
>> index 00000000000..eec300f6896
>> --- /dev/null
>> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/extents/ctor_default.cc
>> @@ -0,0 +1,41 @@
>> +// { dg-do run { target c++23 } }
>> +#include <mdspan>
>> +
>> +#include <cstdint>
>> +#include <testsuite_hooks.h>
>> +
>> +constexpr auto dyn = std::dynamic_extent;
>> +
>> +template<typename Extents>
>> +  constexpr void
>> +  test_default_ctor()
>> +  {
>> +    Extents exts;
>> +    for(size_t i = 0; i < Extents::rank(); ++i)
>> +      if(exts.static_extent(i) == std::dynamic_extent)
>> +       VERIFY(exts.extent(i) == 0);
>> +      else
>> +       VERIFY(exts.extent(i) == Extents::static_extent(i));
>> +  }
>> +
>> +constexpr bool
>> +test_default_ctor_all()
>> +{
>> +  test_default_ctor<std::extents<int, 1>>();
>> +  test_default_ctor<std::extents<int, dyn>>();
>> +  test_default_ctor<std::extents<int, 1, 2>>();
>> +  test_default_ctor<std::extents<int, dyn, 2>>();
>> +  test_default_ctor<std::extents<int, dyn, dyn>>();
>> +  test_default_ctor<std::extents<int, 1, 2, 3>>();
>> +  test_default_ctor<std::extents<int, dyn, 2, dyn>>();
>> +  test_default_ctor<std::extents<int, dyn, dyn, dyn>>();
>> +  return true;
>> +}
>> +
>> +int
>> +main()
>> +{
>> +  test_default_ctor_all();
>> +  static_assert(test_default_ctor_all());
>> +  return 0;
>> +}
>> --
>> 2.49.0
>>