On Mon, 1 Apr 2024 at 23:16, Patrick Palka <ppa...@redhat.com> wrote:
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
This is a layout change for some specializations of slide_view, but
better to do that now than change it between gcc 14 and 15.
OK for trunk.
>
> -- >8 --
>
> Currently __maybe_present_t<false, T> maps to the same empty class
> type independent of T. This is suboptimal because it means adjacent
> __maybe_present_t<false, ...> members with the [[no_unique_address]]
> attribute can't overlap even if the conditionally present types are
> different.
>
> This patch fixes this by turning this empty class type into a template
> parameterized by the conditionally present type, so that
>
> [[no_unique_address]] __maybe_present_t<false, T> _M_a;
> [[no_unique_address]] __maybe_present_t<false, U> _M_b;
>
> now overlap if T and U are different.
>
> This patch goes a step further and also adds an optional integer
> discriminator parameter to allow for overlapping when T and U are
> the same.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/ranges (ranges::__detail::_Empty): Rename to ...
> (ranges::__detail::_Absent): ... this. Turn into a template
> parameterized by the absent type _Tp and discriminator _Disc.
> (ranges::__detail::__maybe_present_t): Add an optional
> discriminator parameter.
> (slide_view::_M_cached_begin): Pass a discriminator argument to
> __maybe_present_t.
> (slide_view::_M_cached_end): Likewise.
> * testsuite/std/ranges/adaptors/sizeof.cc: Verify the size of
> slide_view<V> is 3 instead 4 pointers.
> ---
> libstdc++-v3/include/std/ranges | 13 ++++++++-----
> .../testsuite/std/ranges/adaptors/sizeof.cc | 4 ++++
> 2 files changed, 12 insertions(+), 5 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> index 7d739852677..afce818376b 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -886,14 +886,17 @@ namespace views
>
> namespace __detail
> {
> - struct _Empty { };
> + template<typename _Tp, int _Disc>
> + struct _Absent { };
>
> // Alias for a type that is conditionally present
> // (and is an empty type otherwise).
> // Data members using this alias should use [[no_unique_address]] so that
> // they take no space when not needed.
> - template<bool _Present, typename _Tp>
> - using __maybe_present_t = __conditional_t<_Present, _Tp, _Empty>;
> + // The optional template parameter _Disc is for discriminating two
> otherwise
> + // equivalent absent types so that even they can overlap.
> + template<bool _Present, typename _Tp, int _Disc = 0>
> + using __maybe_present_t = __conditional_t<_Present, _Tp, _Absent<_Tp,
> _Disc>>;
>
> // Alias for a type that is conditionally const.
> template<bool _Const, typename _Tp>
> @@ -6553,10 +6556,10 @@ namespace views::__adaptor
> range_difference_t<_Vp> _M_n;
> [[no_unique_address]]
> __detail::__maybe_present_t<__detail::__slide_caches_first<_Vp>,
> - __detail::_CachedPosition<_Vp>>
> _M_cached_begin;
> + __detail::_CachedPosition<_Vp>, 0>
> _M_cached_begin;
> [[no_unique_address]]
> __detail::__maybe_present_t<__detail::__slide_caches_last<_Vp>,
> - __detail::_CachedPosition<_Vp>>
> _M_cached_end;
> + __detail::_CachedPosition<_Vp>, 1>
> _M_cached_end;
>
> template<bool> class _Iterator;
> class _Sentinel;
> diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/sizeof.cc
> b/libstdc++-v3/testsuite/std/ranges/adaptors/sizeof.cc
> index 12a9da3181d..08c01704d10 100644
> --- a/libstdc++-v3/testsuite/std/ranges/adaptors/sizeof.cc
> +++ b/libstdc++-v3/testsuite/std/ranges/adaptors/sizeof.cc
> @@ -49,3 +49,7 @@ static_assert(sizeof(ranges::lazy_split_view<V,
> std::string_view>) == 4*ptr);
>
> static_assert
> (sizeof(ranges::reverse_view<ranges::filter_view<V, decltype(pred_l)>>) ==
> 3*ptr);
> +
> +#if __cpp_lib_ranges_slide
> +static_assert(sizeof(ranges::slide_view<V>) == 3*ptr);
> +#endif
> --
> 2.44.0.448.gc2cbfbd2e2
>
