Pushed to trunk.
On Thu, 27 Jun 2024 at 10:03, Jonathan Wakely <jwak...@redhat.com> wrote:
>
> I'm planning to push this, although arguably the first change isn't
> worth doing if we can't use it everywhere. If we need to keep the old
> code for EDG, maybe we should just keep using that? The new version
> probably compiles faster though.
>
> Removing the dependency on std::aligned_storage and adding the test is
> surely useful though.
>
> Tested x86_64-linux.
>
> -- >8 --
>
> As noted in a comment, the __gnu_cxx::__aligned_membuf class template
> can be simplified, because alignof(T) and alignas(T) use the correct
> alignment for a data member. That's true since GCC 8 and Clang 8. The
> EDG front end (as used by Intel icc, aka "Intel C++ Compiler Classic")
> does not implement the PR c++/69560 change, so keep using the old
> implementation when __EDG__ is defined, to avoid an ABI change for icc.
>
> For __gnu_cxx::__aligned_buffer<T> all supported compilers agree on the
> value of __alignof__(T), but we can still simplify it by removing the
> dependency on std::aligned_storage<sizeof(T), __alignof__(T)>.
>
> Add a test that checks that the aligned buffer types have the expected
> alignment, so that we can tell if changes like this affect their ABI
> properties.
>
> libstdc++-v3/ChangeLog:
>
> * include/ext/aligned_buffer.h (__aligned_membuf): Use
> alignas(T) directly instead of defining a struct and using 9its
> alignment.
> (__aligned_buffer): Remove use of std::aligned_storage.
> * testsuite/abi/aligned_buffers.cc: New test.
> ---
> libstdc++-v3/include/ext/aligned_buffer.h | 20 ++++-----
> libstdc++-v3/testsuite/abi/aligned_buffers.cc | 42 +++++++++++++++++++
> 2 files changed, 52 insertions(+), 10 deletions(-)
> create mode 100644 libstdc++-v3/testsuite/abi/aligned_buffers.cc
>
> diff --git a/libstdc++-v3/include/ext/aligned_buffer.h
> b/libstdc++-v3/include/ext/aligned_buffer.h
> index 26b36609fa5..9c2c628e54a 100644
> --- a/libstdc++-v3/include/ext/aligned_buffer.h
> +++ b/libstdc++-v3/include/ext/aligned_buffer.h
> @@ -49,11 +49,15 @@ namespace __gnu_cxx
> // Target macro ADJUST_FIELD_ALIGN can produce different alignment for
> // types when used as class members. __aligned_membuf is intended
> // for use as a class member, so align the buffer as for a class
> member.
> - // Since GCC 8 we could just use alignof(_Tp) instead, but older
> - // versions of non-GNU compilers might still need this trick.
> + // Since GCC 8 we can just use alignas(_Tp) to get the right alignment.
> +#ifdef __EDG__
> + // The EDG front end does not implement the PR c++/69560 alignof
> change.
> struct _Tp2 { _Tp _M_t; };
> -
> - alignas(__alignof__(_Tp2::_M_t)) unsigned char _M_storage[sizeof(_Tp)];
> + alignas(__alignof__(_Tp2::_M_t))
> +#else
> + alignas(_Tp)
> +#endif
> + unsigned char _M_storage[sizeof(_Tp)];
>
> __aligned_membuf() = default;
>
> @@ -81,8 +85,6 @@ namespace __gnu_cxx
> template<typename _Tp>
> using __aligned_buffer = __aligned_membuf<_Tp>;
> #else
> -#pragma GCC diagnostic push
> -#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
> // Similar to __aligned_membuf but aligned for complete objects, not
> members.
> // This type is used in <forward_list>, <future>, <bits/shared_ptr_base.h>
> // and <bits/hashtable_policy.h>, but ideally they would use
> __aligned_membuf
> @@ -90,10 +92,9 @@ namespace __gnu_cxx
> // This type is still used to avoid an ABI change.
> template<typename _Tp>
> struct __aligned_buffer
> - : std::aligned_storage<sizeof(_Tp), __alignof__(_Tp)>
> {
> - typename
> - std::aligned_storage<sizeof(_Tp), __alignof__(_Tp)>::type _M_storage;
> + // Using __alignof__ gives the alignment for a complete object.
> + alignas(__alignof__(_Tp)) unsigned char _M_storage[sizeof(_Tp)];
>
> __aligned_buffer() = default;
>
> @@ -120,7 +121,6 @@ namespace __gnu_cxx
> _M_ptr() const noexcept
> { return static_cast<const _Tp*>(_M_addr()); }
> };
> -#pragma GCC diagnostic pop
> #endif
>
> } // namespace
> diff --git a/libstdc++-v3/testsuite/abi/aligned_buffers.cc
> b/libstdc++-v3/testsuite/abi/aligned_buffers.cc
> new file mode 100644
> index 00000000000..b4b8ea13970
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/abi/aligned_buffers.cc
> @@ -0,0 +1,42 @@
> +// { dg-do compile { target c++11 } }
> +
> +// Check alignment of the buffer types used for uninitialized storage.
> +
> +#include <ext/aligned_buffer.h>
> +
> +template<typename T> using membuf = __gnu_cxx::__aligned_membuf<T>;
> +template<typename T> using objbuf = __gnu_cxx::__aligned_buffer<T>;
> +
> +template<typename T>
> +constexpr bool
> +check_alignof_membuf()
> +{
> + return alignof(membuf<T>) == alignof(T)
> + && __alignof__(membuf<T>) == alignof(T);
> +}
> +
> +template<typename T>
> +constexpr bool
> +check_alignof_objbuf()
> +{
> +#if _GLIBCXX_INLINE_VERSION
> + // For the gnu-versioned-namespace ABI __aligned_buffer ==
> __aligned_membuf.
> + return check_alignof_membuf<T>();
> +#else
> + return alignof(objbuf<T>) == __alignof__(T)
> + && __alignof__(objbuf<T>) == __alignof__(T);
> +#endif
> +}
> +
> +struct S { long long l; };
> +struct alignas(128) X { char x; };
> +static_assert( check_alignof_membuf<int>(), "membuf<int>" );
> +static_assert( check_alignof_membuf<long long>(), "membuf<long long>" );
> +static_assert( check_alignof_membuf<void*>(), "membuf<void*>" );
> +static_assert( check_alignof_membuf<S>(), "membuf<S>" );
> +static_assert( check_alignof_membuf<X>(), "membuf<X>" );
> +static_assert( check_alignof_objbuf<int>(), "objbuf<int>" );
> +static_assert( check_alignof_objbuf<long long>(), "objbuf<long long>" );
> +static_assert( check_alignof_objbuf<void*>(), "objbuf<void*>" );
> +static_assert( check_alignof_objbuf<S>(), "objbuf<S>" );
> +static_assert( check_alignof_objbuf<X>(), "objbuf<X>" );
> --
> 2.45.2
>
