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| author | Patrick Palka <ppalka@redhat.com> | 2024-04-02 13:07:07 -0400 |
|---|---|---|
| committer | Patrick Palka <ppalka@redhat.com> | 2024-04-02 13:07:07 -0400 |
| commit | 0e64bbb8823f7b3757befc878ed177dfb59943d1 (patch) | |
| tree | 1b7be5167af2481afc7f445b388ca4c42d959235 | |
| parent | d5aa2ca06aa7a6a2f826c4da19204b6db1402995 (diff) | |
| download | gcc-0e64bbb8823f7b3757befc878ed177dfb59943d1.zip gcc-0e64bbb8823f7b3757befc878ed177dfb59943d1.tar.gz gcc-0e64bbb8823f7b3757befc878ed177dfb59943d1.tar.bz2 | |
libstdc++: Allow adjacent __maybe_present_t<false, ...> fields to overlap
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 turns 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.
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
| -rw-r--r-- | libstdc++-v3/include/std/ranges | 13 | ||||
| -rw-r--r-- | libstdc++-v3/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 7d73985..afce818 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 12a9da31..08c0170 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 |