On Wed, 26 Feb 2025 at 15:06, Patrick Palka <ppa...@redhat.com> wrote:
>
> On Tue, 25 Feb 2025, Jonathan Wakely wrote:
>
> > On Thu, 20 Feb 2025 at 16:23, Patrick Palka <ppa...@redhat.com> wrote:
> > >
> > > On Sun, 16 Feb 2025, Giuseppe D'Angelo wrote:
> > >
> > > > Hello,
> > > >
> > > > the attached patch implements the C++26 papers that add `constexpr` to
> > > > the
> > > > specialized memory algorithms (the uninitialized_* family). Tested on
> > > > x86-64
> > > > Linux.
> > > >
> > > > Thank you,
> > > > --
> > > > Giuseppe D'Angelo
> > > >
> > >
> > > > Subject: [PATCH] libstdc++: implement constexpr memory algorithms
> > > >
> > > > This commit adds support for C++26's constexpr specialized memory
> > > > algorithms, introduced by P2283R2, P3508R0, P3369R0.
> > > >
> > > > The uninitialized_default, value, copy, move and fill algorithms are
> > > > affected, in all of their variants (iterator-based, range-based and _n
> > > > versions.)
> > > >
> > > > The changes are mostly mechanical -- add `constexpr` to a number of
> > > > signatures. I've introduced a helper macro to conditionally expand to
> > > > `constexpr` only in C++26 and above modes. The internal helper guard
> > > > class for range algorithms instead can be marked unconditionally.
> > > >
> > > > uninitialized_fill is the only algorithm where I had to add a branch to
> > > > a constexpr-friendly version (already existing).
> > >
> > > Seems the patch also adds code to uninitialized_copy and
> > > uninitialized_fill_n?
> > >
> > > >
> > > > For each algorithm family I've added only one test to cover it and its
> > > > variants; the idea is to avoid too much repetition and simplify future
> > > > maintenance.
> > > >
> > > > libstdc++-v3/ChangeLog:
> > > >
> > > > * include/bits/ranges_uninitialized.h: Mark the specialized
> > > > memory algorithms as constexpr in C++26. Also mark the members
> > > > of the _DestroyGuard helper class.
> > > > * include/bits/stl_uninitialized.h: Ditto.
> > > > * include/bits/stl_construct.h: Mark _Construct_novalue (which
> > > > uses placement new to do default initialization) as constexpr
> > > > in C++26. This is possible due to P2747R2, which GCC already
> > > > implements; check P2747's feature-testing macro to avoid
> > > > issues with other compilers.
> > > > * include/bits/version.def: Bump the feature-testing macro.
> > > > * include/bits/version.h: Regenerate.
> > > > * testsuite/20_util/specialized_algorithms/feature_test_macro.cc:
> > > > New test.
> > > > *
> > > > testsuite/20_util/specialized_algorithms/uninitialized_copy/constexpr.cc:
> > > > New test.
> > > > *
> > > > testsuite/20_util/specialized_algorithms/uninitialized_default_construct/constexpr.cc:
> > > > New test.
> > > > *
> > > > testsuite/20_util/specialized_algorithms/uninitialized_fill/constexpr.cc:
> > > > New test.
> > > > *
> > > > testsuite/20_util/specialized_algorithms/uninitialized_move/constexpr.cc:
> > > > New test.
> > > > *
> > > > testsuite/20_util/specialized_algorithms/uninitialized_value_construct/constexpr.cc:
> > > > New test.
> > > >
> > > > Signed-off-by: Giuseppe D'Angelo <giuseppe.dang...@kdab.com>
> > > > ---
> > > > .../include/bits/ranges_uninitialized.h | 29 ++++++++
> > > > libstdc++-v3/include/bits/stl_construct.h | 3 +
> > > > libstdc++-v3/include/bits/stl_uninitialized.h | 42 ++++++++++++
> > > > libstdc++-v3/include/bits/version.def | 5 ++
> > > > libstdc++-v3/include/bits/version.h | 7 +-
> > > > .../feature_test_macro.cc | 14 ++++
> > > > .../uninitialized_copy/constexpr.cc | 58 ++++++++++++++++
> > > > .../constexpr.cc | 67 ++++++++++++++++++
> > > > .../uninitialized_fill/constexpr.cc | 68 +++++++++++++++++++
> > > > .../uninitialized_move/constexpr.cc | 51 ++++++++++++++
> > > > .../constexpr.cc | 64 +++++++++++++++++
> > > > 11 files changed, 407 insertions(+), 1 deletion(-)
> > > > create mode 100644
> > > > libstdc++-v3/testsuite/20_util/specialized_algorithms/feature_test_macro.cc
> > > > create mode 100644
> > > > libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/constexpr.cc
> > > > create mode 100644
> > > > libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_default_construct/constexpr.cc
> > > > create mode 100644
> > > > libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill/constexpr.cc
> > > > create mode 100644
> > > > libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_move/constexpr.cc
> > > > create mode 100644
> > > > libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_value_construct/constexpr.cc
> > > >
> > > > diff --git a/libstdc++-v3/include/bits/ranges_uninitialized.h
> > > > b/libstdc++-v3/include/bits/ranges_uninitialized.h
> > > > index ced7bda5e37..337d321702d 100644
> > > > --- a/libstdc++-v3/include/bits/ranges_uninitialized.h
> > > > +++ b/libstdc++-v3/include/bits/ranges_uninitialized.h
> > > > @@ -35,6 +35,12 @@
> > > >
> > > > #include <bits/ranges_algobase.h>
> > > >
> > > > +#if __glibcxx_raw_memory_algorithms >= 202411L // >= C++26
> > > > +# define _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS constexpr
> > > > +#else
> > > > +# define _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > > > +#endif
> > > > +
> > > > namespace std _GLIBCXX_VISIBILITY(default)
> > > > {
> > > > _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > > > @@ -105,15 +111,18 @@ namespace ranges
> > > > const _Iter* _M_cur;
> > > >
> > > > public:
> > > > + constexpr
> > > > explicit
> > > > _DestroyGuard(const _Iter& __iter)
> > > > : _M_first(__iter), _M_cur(std::__addressof(__iter))
> > > > { }
> > > >
> > > > + constexpr
> > > > void
> > > > release() noexcept
> > > > { _M_cur = nullptr; }
> > > >
> > > > + constexpr
> > > > ~_DestroyGuard()
> > > > {
> > > > if (_M_cur != nullptr)
> > > > @@ -126,10 +135,12 @@ namespace ranges
> > > > && is_trivially_destructible_v<iter_value_t<_Iter>>
> > > > struct _DestroyGuard<_Iter>
> > > > {
> > > > + constexpr
> > > > explicit
> > > > _DestroyGuard(const _Iter&)
> > > > { }
> > > >
> > > > + constexpr
> > > > void
> > > > release() noexcept
> > > > { }
> > > > @@ -141,6 +152,7 @@ namespace ranges
> > > > template<__detail::__nothrow_forward_iterator _Iter,
> > > > __detail::__nothrow_sentinel<_Iter> _Sent>
> > > > requires default_initializable<iter_value_t<_Iter>>
> > > > + _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > > > _Iter
> > > > operator()(_Iter __first, _Sent __last) const
> > > > {
> > > > @@ -159,6 +171,7 @@ namespace ranges
> > > >
> > > > template<__detail::__nothrow_forward_range _Range>
> > > > requires default_initializable<range_value_t<_Range>>
> > > > + _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > > > borrowed_iterator_t<_Range>
> > > > operator()(_Range&& __r) const
> > > > {
> > > > @@ -173,6 +186,7 @@ namespace ranges
> > > > {
> > > > template<__detail::__nothrow_forward_iterator _Iter>
> > > > requires default_initializable<iter_value_t<_Iter>>
> > > > + _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > > > _Iter
> > > > operator()(_Iter __first, iter_difference_t<_Iter> __n) const
> > > > {
> > > > @@ -198,6 +212,7 @@ namespace ranges
> > > > template<__detail::__nothrow_forward_iterator _Iter,
> > > > __detail::__nothrow_sentinel<_Iter> _Sent>
> > > > requires default_initializable<iter_value_t<_Iter>>
> > > > + _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > > > _Iter
> > > > operator()(_Iter __first, _Sent __last) const
> > > > {
> > > > @@ -217,6 +232,7 @@ namespace ranges
> > > >
> > > > template<__detail::__nothrow_forward_range _Range>
> > > > requires default_initializable<range_value_t<_Range>>
> > > > + _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > > > borrowed_iterator_t<_Range>
> > > > operator()(_Range&& __r) const
> > > > {
> > > > @@ -231,6 +247,7 @@ namespace ranges
> > > > {
> > > > template<__detail::__nothrow_forward_iterator _Iter>
> > > > requires default_initializable<iter_value_t<_Iter>>
> > > > + _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > > > _Iter
> > > > operator()(_Iter __first, iter_difference_t<_Iter> __n) const
> > > > {
> > > > @@ -261,6 +278,7 @@ namespace ranges
> > > > __detail::__nothrow_forward_iterator _Out,
> > > > __detail::__nothrow_sentinel<_Out> _OSent>
> > > > requires constructible_from<iter_value_t<_Out>,
> > > > iter_reference_t<_Iter>>
> > > > + _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > > > uninitialized_copy_result<_Iter, _Out>
> > > > operator()(_Iter __ifirst, _ISent __ilast,
> > > > _Out __ofirst, _OSent __olast) const
> > > > @@ -292,6 +310,7 @@ namespace ranges
> > > > template<input_range _IRange, __detail::__nothrow_forward_range
> > > > _ORange>
> > > > requires constructible_from<range_value_t<_ORange>,
> > > > range_reference_t<_IRange>>
> > > > + _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > > > uninitialized_copy_result<borrowed_iterator_t<_IRange>,
> > > > borrowed_iterator_t<_ORange>>
> > > > operator()(_IRange&& __inr, _ORange&& __outr) const
> > > > @@ -311,6 +330,7 @@ namespace ranges
> > > > template<input_iterator _Iter,
> > > > __detail::__nothrow_forward_iterator _Out,
> > > > __detail::__nothrow_sentinel<_Out> _Sent>
> > > > requires constructible_from<iter_value_t<_Out>,
> > > > iter_reference_t<_Iter>>
> > > > + _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > > > uninitialized_copy_n_result<_Iter, _Out>
> > > > operator()(_Iter __ifirst, iter_difference_t<_Iter> __n,
> > > > _Out __ofirst, _Sent __olast) const
> > > > @@ -350,6 +370,7 @@ namespace ranges
> > > > __detail::__nothrow_sentinel<_Out> _OSent>
> > > > requires constructible_from<iter_value_t<_Out>,
> > > > iter_rvalue_reference_t<_Iter>>
> > > > + _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > > > uninitialized_move_result<_Iter, _Out>
> > > > operator()(_Iter __ifirst, _ISent __ilast,
> > > > _Out __ofirst, _OSent __olast) const
> > > > @@ -384,6 +405,7 @@ namespace ranges
> > > > template<input_range _IRange, __detail::__nothrow_forward_range
> > > > _ORange>
> > > > requires constructible_from<range_value_t<_ORange>,
> > > > range_rvalue_reference_t<_IRange>>
> > > > + _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > > > uninitialized_move_result<borrowed_iterator_t<_IRange>,
> > > > borrowed_iterator_t<_ORange>>
> > > > operator()(_IRange&& __inr, _ORange&& __outr) const
> > > > @@ -404,6 +426,7 @@ namespace ranges
> > > > __detail::__nothrow_sentinel<_Out> _Sent>
> > > > requires constructible_from<iter_value_t<_Out>,
> > > > iter_rvalue_reference_t<_Iter>>
> > > > + _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > > > uninitialized_move_n_result<_Iter, _Out>
> > > > operator()(_Iter __ifirst, iter_difference_t<_Iter> __n,
> > > > _Out __ofirst, _Sent __olast) const
> > > > @@ -441,6 +464,7 @@ namespace ranges
> > > > template<__detail::__nothrow_forward_iterator _Iter,
> > > > __detail::__nothrow_sentinel<_Iter> _Sent, typename _Tp>
> > > > requires constructible_from<iter_value_t<_Iter>, const _Tp&>
> > > > + _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > > > _Iter
> > > > operator()(_Iter __first, _Sent __last, const _Tp& __x) const
> > > > {
> > > > @@ -460,6 +484,7 @@ namespace ranges
> > > >
> > > > template<__detail::__nothrow_forward_range _Range, typename _Tp>
> > > > requires constructible_from<range_value_t<_Range>, const _Tp&>
> > > > + _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > > > borrowed_iterator_t<_Range>
> > > > operator()(_Range&& __r, const _Tp& __x) const
> > > > {
> > > > @@ -473,6 +498,7 @@ namespace ranges
> > > > {
> > > > template<__detail::__nothrow_forward_iterator _Iter, typename _Tp>
> > > > requires constructible_from<iter_value_t<_Iter>, const _Tp&>
> > > > + _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > > > _Iter
> > > > operator()(_Iter __first, iter_difference_t<_Iter> __n,
> > > > const _Tp& __x) const
> > > > @@ -573,6 +599,9 @@ namespace ranges
> > > > }
> > > > _GLIBCXX_END_NAMESPACE_VERSION
> > > > } // namespace std
> > > > +
> > > > +#undef _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > > > +
> > > > #endif // concepts
> > > > #endif // C++20
> > > > #endif // _RANGES_UNINITIALIZED_H
> > > > diff --git a/libstdc++-v3/include/bits/stl_construct.h
> > > > b/libstdc++-v3/include/bits/stl_construct.h
> > > > index bd8235e901b..6d34edf02da 100644
> > > > --- a/libstdc++-v3/include/bits/stl_construct.h
> > > > +++ b/libstdc++-v3/include/bits/stl_construct.h
> > > > @@ -144,6 +144,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > > > #endif
> > > >
> > > > template<typename _T1>
> > > > +#if __cpp_constexpr >= 202406L // >= C++26
> > > > + _GLIBCXX26_CONSTEXPR
> > > > +#endif
> > >
> > > Maybe we can get away with unconditionally declaring this
> > > _GLIBCXX26_CONSTEXPR? If the compiler doesn't support constexpr
> > > placement new then the 'constexpr' would be silently dropped at
> > > instantiation time. This would be in line with C++23 P2448R2 which
> > > made it no longer IFNDR to declare a constexpr function template
> > > for which no specialization is actually constexpr.
> >
> > Yeah, for internal functions that aren't ever compiled as C++98, we
> > can often just make them constexpr. It will never be called during
> > constant evaluation in C++20 or older, but that's usually fine.
> >
> > In this case though, would the placement new make it ill-formed in
> > Clang 18, which didn't support P2448R2?
>
> Clang 17/18 rejects 'constexpr' on non-template functions that use
> (non-constexpr) placement new but accepts it on templates (silently
> dropping constexpr at instantiation time): https://godbolt.org/z/Tqnvc1f1W
> So it seems Clang 17/18 behavior is consistent enough with P2448R2
> that there should be no compatibility issues.
Oh great, thanks for checking.
EDG is also OK with it, I checked back to v6.0 from 2020.
So making it unconditionally constexpr seems fine then.
