Tested on x86_64-pc-linux-gnu, does this look 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
