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?
>
> > inline void
> > _Construct_novalue(_T1* __p)
> > { ::new(static_cast<void*>(__p)) _T1; }
> > diff --git a/libstdc++-v3/include/bits/stl_uninitialized.h
> > b/libstdc++-v3/include/bits/stl_uninitialized.h
> > index ed836663a44..04f9792d37e 100644
> > --- a/libstdc++-v3/include/bits/stl_uninitialized.h
> > +++ b/libstdc++-v3/include/bits/stl_uninitialized.h
> > @@ -68,6 +68,12 @@
> > #include <bits/stl_iterator.h> // __niter_base
> > #include <ext/alloc_traits.h> // __alloc_traits
> >
> > +#if __glibcxx_raw_memory_algorithms >= 202411L // >= C++26
> > +# define _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS constexpr
> > +#else
> > +# define _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > +#endif
>
> Similarly we could just use _GLIBCXX26_CONSTEXPR unconditionally
> throughout, instead of indirectly conditioning on __cpp_constexpr.
>
> > +
> > namespace std _GLIBCXX_VISIBILITY(default)
> > {
> > _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > @@ -226,6 +232,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > * Like std::copy, but does not require an initialized output range.
> > */
> > template<typename _InputIterator, typename _ForwardIterator>
> > + _GLIBCXX26_CONSTEXPR_RAW_MEMORY_ALGORITHMS
> > inline _ForwardIterator
> > uninitialized_copy(_InputIterator __first, _InputIterator __last,
> > _ForwardIterator __result)
> > @@ -256,6 +263,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > using _Src = decltype(std::__niter_base(__first));
> > using _ValT = typename iterator_traits<_ForwardIterator>::value_type;
> >
> > +
> > + if (__is_constant_evaluated())
>
> We typically call __is_constant_evaluated fully qualified (though I
> don't remember why since it's not eligible for ADL?)
I don't think we're consistent, and it's not necessary.
But we could simplify things a little by doing:
#if __glibcxx_raw_memory_algorithms >= 202411L // >= C++26
if consteval {
return std::__do_uninit_copy(__first, __last, __result);
}
#endif
We don't need to use the __is_constant_evaluated() wrapper, or even
the std::is_constant_evaluated() function, because this is C++26 code
so we know 'if consteval' works. Clang supports it since version 14,
which is too old to support any C++26 mode, so every Clang that
supports -std=c++2c also supports 'if consteval'.
