| Message ID | 20240401221539.842841-1-ppalka@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | libstdc++: Allow adjacent __maybe_present_t<false, ...> to overlap | expand |
On Mon, 1 Apr 2024 at 23:16, Patrick Palka <ppalka@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 >
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