libstdc++: Refactor std::uninitialized_{copy, fill, fill_n} algos [PR68350]

Tested x86_64-linux.

-- >8 --

This refactors the std::uninitialized_copy, std::uninitialized_fill and
std::uninitialized_fill_n algorithms to directly perform memcpy/memset
optimizations instead of dispatching to std::copy/std::fill/std::fill_n.

The reasons for this are:

- Use 'if constexpr' to simplify and optimize compilation throughput, so
  dispatching to specialized class templates is only needed for C++98
  mode.
- Relax the conditions for using memcpy/memset, because the C++20 rules
  on implicit-lifetime types mean that we can rely on memcpy to begin
  lifetimes of trivially copyable types.  We don't need to require
  trivially default constructible, so don't need to limit the
  optimization to trivial types. See PR 68350 for more details.
- The conditions on non-overlapping ranges are stronger for
  std::uninitialized_copy than for std::copy so we can use memcpy instead
  of memmove, which might be a minor optimization.
- Avoid including <bits/stl_algobase.h> in <bits/stl_uninitialized.h>.
  It only needs some iterator utilities from that file now, which belong
  in <bits/stl_iterator.h> anyway, so this moves them there.

Several tests need changes to the diagnostics matched by dg-error
because we no longer use the __constructible() function that had a
static assert in. Now we just get straightforward errors for attempting
to use a deleted constructor.

Two tests needed more signficant changes to the actual expected results
of executing the tests, because they were checking for old behaviour
which was incorrect according to the standard.
20_util/specialized_algorithms/uninitialized_copy/64476.cc was expecting
std::copy to be used for a call to std::uninitialized_copy involving two
trivially copyable types. That was incorrect behaviour, because a
non-trivial constructor should have been used, but using std::copy used
trivial default initialization followed by assignment.
20_util/specialized_algorithms/uninitialized_fill_n/sizes.cc was testing
the behaviour with a non-integral Size passed to uninitialized_fill_n,
but I wrote the test looking at the requirements of uninitialized_copy_n
which are not the same as uninitialized_fill_n. The former uses --n and
tests n > 0, but the latter just tests n-- (which will never be false
for a floating-point value with a fractional part).

libstdc++-v3/ChangeLog:

	PR libstdc++/68350
	PR libstdc++/93059
	* include/bits/stl_algobase.h (__niter_base, __niter_wrap): Move
	to ...
	* include/bits/stl_iterator.h: ... here.
	* include/bits/stl_uninitialized.h (__check_constructible)
	(_GLIBCXX_USE_ASSIGN_FOR_INIT): Remove.
	[C++98] (__unwrappable_niter): New trait.
	(__uninitialized_copy<true>): Replace use of std::copy.
	(uninitialized_copy): Fix Doxygen comments. Open-code memcpy
	optimization for C++11 and later.
	(__uninitialized_fill<true>): Replace use of std::fill.
	(uninitialized_fill): Fix Doxygen comments. Open-code memset
	optimization for C++11 and later.
	(__uninitialized_fill_n<true>): Replace use of std::fill_n.
	(uninitialized_fill_n): Fix Doxygen comments. Open-code memset
	optimization for C++11 and later.
	* testsuite/20_util/specialized_algorithms/uninitialized_copy/64476.cc:
	Adjust expected behaviour to match what the standard specifies.
	* testsuite/20_util/specialized_algorithms/uninitialized_fill_n/sizes.cc:
	Likewise.
	* testsuite/20_util/specialized_algorithms/uninitialized_copy/1.cc:
	Adjust dg-error directives.
	* testsuite/20_util/specialized_algorithms/uninitialized_copy/89164.cc:
	Likewise.
	* testsuite/20_util/specialized_algorithms/uninitialized_copy_n/89164.cc:
	Likewise.
	* testsuite/20_util/specialized_algorithms/uninitialized_fill/89164.cc:
	Likewise.
	* testsuite/20_util/specialized_algorithms/uninitialized_fill_n/89164.cc:
	Likewise.
	* testsuite/23_containers/vector/cons/89164.cc: Likewise.
	* testsuite/23_containers/vector/cons/89164_c++17.cc: Likewise.
---
 libstdc++-v3/include/bits/stl_algobase.h      |  45 --
 libstdc++-v3/include/bits/stl_iterator.h      |  54 +++
 libstdc++-v3/include/bits/stl_uninitialized.h | 385 +++++++++++++-----
 .../uninitialized_copy/1.cc                   |   3 +-
 .../uninitialized_copy/64476.cc               |   6 +-
 .../uninitialized_copy/89164.cc               |   3 +-
 .../uninitialized_copy_n/89164.cc             |   3 +-
 .../uninitialized_fill/89164.cc               |   3 +-
 .../uninitialized_fill_n/89164.cc             |   3 +-
 .../uninitialized_fill_n/sizes.cc             |  22 +-
 .../23_containers/vector/cons/89164.cc        |   5 +-
 .../23_containers/vector/cons/89164_c++17.cc  |   3 +-
 12 files changed, 383 insertions(+), 152 deletions(-)

This causes some codegen regressions on (at least) aarch64 and power64:

FAIL: g++.dg/pr104547.C  -std=gnu++11  scan-tree-dump-not vrp2
"_M_default_append"
FAIL: g++.dg/pr104547.C  -std=gnu++17  scan-tree-dump-not vrp2
"_M_default_append"

For some reason the PR 104547 fix to stop the unused _M_default_append
function being emitted in the assembly code no longer works.

I also see new bogus stringop-overflow warnings for these tests:

FAIL: 27_io/basic_ios/imbue/14072.cc  -std=gnu++17 (test for excess errors)
FAIL: 27_io/basic_istream/extractors_character/char/9826.cc
-std=gnu++17 (test for excess errors)
FAIL: 27_io/basic_stringbuf/requirements/explicit_instantiation/2.cc
-std=gnu++17 (test for excess errors)
FAIL: 27_io/basic_stringstream/requirements/explicit_instantiation.cc
-std=gnu++17 (test for excess errors)





On Mon, 14 Oct 2024 at 00:53, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> Tested x86_64-linux.
>
> -- >8 --
>
> This refactors the std::uninitialized_copy, std::uninitialized_fill and
> std::uninitialized_fill_n algorithms to directly perform memcpy/memset
> optimizations instead of dispatching to std::copy/std::fill/std::fill_n.
>
> The reasons for this are:
>
> - Use 'if constexpr' to simplify and optimize compilation throughput, so
>   dispatching to specialized class templates is only needed for C++98
>   mode.
> - Relax the conditions for using memcpy/memset, because the C++20 rules
>   on implicit-lifetime types mean that we can rely on memcpy to begin
>   lifetimes of trivially copyable types.  We don't need to require
>   trivially default constructible, so don't need to limit the
>   optimization to trivial types. See PR 68350 for more details.
> - The conditions on non-overlapping ranges are stronger for
>   std::uninitialized_copy than for std::copy so we can use memcpy instead
>   of memmove, which might be a minor optimization.
> - Avoid including <bits/stl_algobase.h> in <bits/stl_uninitialized.h>.
>   It only needs some iterator utilities from that file now, which belong
>   in <bits/stl_iterator.h> anyway, so this moves them there.
>
> Several tests need changes to the diagnostics matched by dg-error
> because we no longer use the __constructible() function that had a
> static assert in. Now we just get straightforward errors for attempting
> to use a deleted constructor.
>
> Two tests needed more signficant changes to the actual expected results
> of executing the tests, because they were checking for old behaviour
> which was incorrect according to the standard.
> 20_util/specialized_algorithms/uninitialized_copy/64476.cc was expecting
> std::copy to be used for a call to std::uninitialized_copy involving two
> trivially copyable types. That was incorrect behaviour, because a
> non-trivial constructor should have been used, but using std::copy used
> trivial default initialization followed by assignment.
> 20_util/specialized_algorithms/uninitialized_fill_n/sizes.cc was testing
> the behaviour with a non-integral Size passed to uninitialized_fill_n,
> but I wrote the test looking at the requirements of uninitialized_copy_n
> which are not the same as uninitialized_fill_n. The former uses --n and
> tests n > 0, but the latter just tests n-- (which will never be false
> for a floating-point value with a fractional part).
>
> libstdc++-v3/ChangeLog:
>
>         PR libstdc++/68350
>         PR libstdc++/93059
>         * include/bits/stl_algobase.h (__niter_base, __niter_wrap): Move
>         to ...
>         * include/bits/stl_iterator.h: ... here.
>         * include/bits/stl_uninitialized.h (__check_constructible)
>         (_GLIBCXX_USE_ASSIGN_FOR_INIT): Remove.
>         [C++98] (__unwrappable_niter): New trait.
>         (__uninitialized_copy<true>): Replace use of std::copy.
>         (uninitialized_copy): Fix Doxygen comments. Open-code memcpy
>         optimization for C++11 and later.
>         (__uninitialized_fill<true>): Replace use of std::fill.
>         (uninitialized_fill): Fix Doxygen comments. Open-code memset
>         optimization for C++11 and later.
>         (__uninitialized_fill_n<true>): Replace use of std::fill_n.
>         (uninitialized_fill_n): Fix Doxygen comments. Open-code memset
>         optimization for C++11 and later.
>         * testsuite/20_util/specialized_algorithms/uninitialized_copy/64476.cc:
>         Adjust expected behaviour to match what the standard specifies.
>         * testsuite/20_util/specialized_algorithms/uninitialized_fill_n/sizes.cc:
>         Likewise.
>         * testsuite/20_util/specialized_algorithms/uninitialized_copy/1.cc:
>         Adjust dg-error directives.
>         * testsuite/20_util/specialized_algorithms/uninitialized_copy/89164.cc:
>         Likewise.
>         * testsuite/20_util/specialized_algorithms/uninitialized_copy_n/89164.cc:
>         Likewise.
>         * testsuite/20_util/specialized_algorithms/uninitialized_fill/89164.cc:
>         Likewise.
>         * testsuite/20_util/specialized_algorithms/uninitialized_fill_n/89164.cc:
>         Likewise.
>         * testsuite/23_containers/vector/cons/89164.cc: Likewise.
>         * testsuite/23_containers/vector/cons/89164_c++17.cc: Likewise.
> ---
>  libstdc++-v3/include/bits/stl_algobase.h      |  45 --
>  libstdc++-v3/include/bits/stl_iterator.h      |  54 +++
>  libstdc++-v3/include/bits/stl_uninitialized.h | 385 +++++++++++++-----
>  .../uninitialized_copy/1.cc                   |   3 +-
>  .../uninitialized_copy/64476.cc               |   6 +-
>  .../uninitialized_copy/89164.cc               |   3 +-
>  .../uninitialized_copy_n/89164.cc             |   3 +-
>  .../uninitialized_fill/89164.cc               |   3 +-
>  .../uninitialized_fill_n/89164.cc             |   3 +-
>  .../uninitialized_fill_n/sizes.cc             |  22 +-
>  .../23_containers/vector/cons/89164.cc        |   5 +-
>  .../23_containers/vector/cons/89164_c++17.cc  |   3 +-
>  12 files changed, 383 insertions(+), 152 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/stl_algobase.h b/libstdc++-v3/include/bits/stl_algobase.h
> index 384e5fdcdc9..751b7ad119b 100644
> --- a/libstdc++-v3/include/bits/stl_algobase.h
> +++ b/libstdc++-v3/include/bits/stl_algobase.h
> @@ -308,51 +308,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        return __a;
>      }
>
> -  // Fallback implementation of the function in bits/stl_iterator.h used to
> -  // remove the __normal_iterator wrapper. See copy, fill, ...
> -  template<typename _Iterator>
> -    _GLIBCXX20_CONSTEXPR
> -    inline _Iterator
> -    __niter_base(_Iterator __it)
> -    _GLIBCXX_NOEXCEPT_IF(std::is_nothrow_copy_constructible<_Iterator>::value)
> -    { return __it; }
> -
> -#if __cplusplus < 201103L
> -  template<typename _Ite, typename _Seq>
> -    _Ite
> -    __niter_base(const ::__gnu_debug::_Safe_iterator<_Ite, _Seq,
> -                std::random_access_iterator_tag>&);
> -
> - template<typename _Ite, typename _Cont, typename _Seq>
> -    _Ite
> -    __niter_base(const ::__gnu_debug::_Safe_iterator<
> -                ::__gnu_cxx::__normal_iterator<_Ite, _Cont>, _Seq,
> -                std::random_access_iterator_tag>&);
> -#else
> -  template<typename _Ite, typename _Seq>
> -    _GLIBCXX20_CONSTEXPR
> -    decltype(std::__niter_base(std::declval<_Ite>()))
> -    __niter_base(const ::__gnu_debug::_Safe_iterator<_Ite, _Seq,
> -                std::random_access_iterator_tag>&)
> -    noexcept(std::is_nothrow_copy_constructible<_Ite>::value);
> -#endif
> -
> -  // Reverse the __niter_base transformation to get a
> -  // __normal_iterator back again (this assumes that __normal_iterator
> -  // is only used to wrap random access iterators, like pointers).
> -  template<typename _From, typename _To>
> -    _GLIBCXX20_CONSTEXPR
> -    inline _From
> -    __niter_wrap(_From __from, _To __res)
> -    { return __from + (std::__niter_base(__res) - std::__niter_base(__from)); }
> -
> -  // No need to wrap, iterator already has the right type.
> -  template<typename _Iterator>
> -    _GLIBCXX20_CONSTEXPR
> -    inline _Iterator
> -    __niter_wrap(const _Iterator&, _Iterator __res)
> -    { return __res; }
> -
>    // All of these auxiliary structs serve two purposes.  (1) Replace
>    // calls to copy with memmove whenever possible.  (Memmove, not memcpy,
>    // because the input and output ranges are permitted to overlap.)
> diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
> index 28a600c81cb..85b98ffff61 100644
> --- a/libstdc++-v3/include/bits/stl_iterator.h
> +++ b/libstdc++-v3/include/bits/stl_iterator.h
> @@ -1338,10 +1338,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  _GLIBCXX_END_NAMESPACE_VERSION
>  } // namespace
>
> +namespace __gnu_debug
> +{
> +  template<typename _Iterator, typename _Sequence, typename _Category>
> +    class _Safe_iterator;
> +}
> +
>  namespace std _GLIBCXX_VISIBILITY(default)
>  {
>  _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> +  // Unwrap a __normal_iterator to get the underlying iterator
> +  // (usually a pointer)
>    template<typename _Iterator, typename _Container>
>      _GLIBCXX20_CONSTEXPR
>      _Iterator
> @@ -1349,6 +1357,52 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      _GLIBCXX_NOEXCEPT_IF(std::is_nothrow_copy_constructible<_Iterator>::value)
>      { return __it.base(); }
>
> +  // Fallback implementation of the function in bits/stl_iterator.h used to
> +  // remove the __normal_iterator wrapper. See std::copy, std::fill, etc.
> +  template<typename _Iterator>
> +    _GLIBCXX20_CONSTEXPR
> +    inline _Iterator
> +    __niter_base(_Iterator __it)
> +    _GLIBCXX_NOEXCEPT_IF(std::is_nothrow_copy_constructible<_Iterator>::value)
> +    { return __it; }
> +
> +  // Overload for _Safe_iterator needs to be declared before __niter_base uses.
> +#if __cplusplus < 201103L
> +  template<typename _Ite, typename _Seq>
> +    _Ite
> +    __niter_base(const ::__gnu_debug::_Safe_iterator<_Ite, _Seq,
> +                std::random_access_iterator_tag>&);
> +
> + template<typename _Ite, typename _Cont, typename _Seq>
> +    _Ite
> +    __niter_base(const ::__gnu_debug::_Safe_iterator<
> +                ::__gnu_cxx::__normal_iterator<_Ite, _Cont>, _Seq,
> +                std::random_access_iterator_tag>&);
> +#else
> +  template<typename _Ite, typename _Seq>
> +    _GLIBCXX20_CONSTEXPR
> +    decltype(std::__niter_base(std::declval<_Ite>()))
> +    __niter_base(const ::__gnu_debug::_Safe_iterator<_Ite, _Seq,
> +                std::random_access_iterator_tag>&)
> +    noexcept(std::is_nothrow_copy_constructible<_Ite>::value);
> +#endif
> +
> +  // Reverse the __niter_base transformation to get a
> +  // __normal_iterator back again (this assumes that __normal_iterator
> +  // is only used to wrap random access iterators, like pointers).
> +  template<typename _From, typename _To>
> +    _GLIBCXX20_CONSTEXPR
> +    inline _From
> +    __niter_wrap(_From __from, _To __res)
> +    { return __from + (std::__niter_base(__res) - std::__niter_base(__from)); }
> +
> +  // No need to wrap, iterator already has the right type.
> +  template<typename _Iterator>
> +    _GLIBCXX20_CONSTEXPR
> +    inline _Iterator
> +    __niter_wrap(const _Iterator&, _Iterator __res)
> +    { return __res; }
> +
>  #if __cplusplus >= 201103L && __cplusplus <= 201703L
>    // Need to overload __to_address because the pointer_traits primary template
>    // will deduce element_type of __normal_iterator<T*, C> as T* rather than T.
> diff --git a/libstdc++-v3/include/bits/stl_uninitialized.h b/libstdc++-v3/include/bits/stl_uninitialized.h
> index f663057b1a1..1bad82bf5bf 100644
> --- a/libstdc++-v3/include/bits/stl_uninitialized.h
> +++ b/libstdc++-v3/include/bits/stl_uninitialized.h
> @@ -57,16 +57,16 @@
>  #define _STL_UNINITIALIZED_H 1
>
>  #if __cplusplus >= 201103L
> -#include <type_traits>
> +# include <type_traits>
> +# include <bits/ptr_traits.h>      // __to_address
> +# include <bits/stl_pair.h>        // pair
>  #endif
>
> -#include <bits/stl_algobase.h>    // copy
> +#include <bits/cpp_type_traits.h> // __is_pointer
> +#include <bits/stl_iterator_base_funcs.h> // distance, advance
> +#include <bits/stl_iterator.h>    // __niter_base
>  #include <ext/alloc_traits.h>     // __alloc_traits
>
> -#if __cplusplus >= 201703L
> -#include <bits/stl_pair.h>
> -#endif
> -
>  namespace std _GLIBCXX_VISIBILITY(default)
>  {
>  _GLIBCXX_BEGIN_NAMESPACE_VERSION
> @@ -77,36 +77,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
>    /// @cond undocumented
>
> -#if __cplusplus >= 201103L
> -  template<typename _ValueType, typename _Tp>
> -    constexpr bool
> -    __check_constructible()
> -    {
> -      // Trivial types can have deleted constructors, but std::copy etc.
> -      // only use assignment (or memmove) not construction, so we need an
> -      // explicit check that construction from _Tp is actually valid,
> -      // otherwise some ill-formed uses of std::uninitialized_xxx would
> -      // compile without errors. This gives a nice clear error message.
> -      static_assert(is_constructible<_ValueType, _Tp>::value,
> -         "result type must be constructible from input type");
> -
> -      return true;
> -    }
> -
> -// If the type is trivial we don't need to construct it, just assign to it.
> -// But trivial types can still have deleted or inaccessible assignment,
> -// so don't try to use std::copy or std::fill etc. if we can't assign.
> -# define _GLIBCXX_USE_ASSIGN_FOR_INIT(T, U) \
> -    __is_trivial(T) && __is_assignable(T&, U) \
> -    && std::__check_constructible<T, U>()
> -#else
> -// No need to check if is_constructible<T, U> for C++98. Trivial types have
> -// no user-declared constructors, so if the assignment is valid, construction
> -// should be too.
> -# define _GLIBCXX_USE_ASSIGN_FOR_INIT(T, U) \
> -    __is_trivial(T) && __is_assignable(T&, U)
> -#endif
> -
>    template<typename _ForwardIterator, typename _Alloc = void>
>      struct _UninitDestroyGuard
>      {
> @@ -160,6 +130,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        _UninitDestroyGuard(const _UninitDestroyGuard&);
>      };
>
> +  // This is the default implementation of std::uninitialized_copy.
>    template<typename _InputIterator, typename _ForwardIterator>
>      _GLIBCXX20_CONSTEXPR
>      _ForwardIterator
> @@ -173,7 +144,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        return __result;
>      }
>
> -  template<bool _TrivialValueTypes>
> +#if __cplusplus < 201103L
> +
> +  // True if we can unwrap _Iter to get a pointer by using std::__niter_base.
> +  template<typename _Iter>
> +    struct __unwrappable_niter
> +    {
> +      template<typename> struct __is_ptr { enum { __value = 0 }; };
> +      template<typename _Tp> struct __is_ptr<_Tp*> { enum { __value = 1 }; };
> +
> +      typedef __decltype(std::__niter_base(*(_Iter*)0)) _Base;
> +
> +      enum { __value = __is_ptr<_Base>::__value };
> +    };
> +
> +  // Use template specialization for C++98 when 'if constexpr' can't be used.
> +  template<bool _CanMemcpy>
>      struct __uninitialized_copy
>      {
>        template<typename _InputIterator, typename _ForwardIterator>
> @@ -186,53 +172,150 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>    template<>
>      struct __uninitialized_copy<true>
>      {
> +      // Overload for generic iterators.
>        template<typename _InputIterator, typename _ForwardIterator>
>          static _ForwardIterator
>          __uninit_copy(_InputIterator __first, _InputIterator __last,
>                       _ForwardIterator __result)
> -        { return std::copy(__first, __last, __result); }
> -    };
> +       {
> +         if (__unwrappable_niter<_InputIterator>::__value
> +               && __unwrappable_niter<_ForwardIterator>::__value)
> +           {
> +             __uninit_copy(std::__niter_base(__first),
> +                           std::__niter_base(__last),
> +                           std::__niter_base(__result));
> +             std::advance(__result, std::distance(__first, __last));
> +             return __result;
> +           }
> +         else
> +           return std::__do_uninit_copy(__first, __last, __result);
> +       }
>
> +      // Overload for pointers.
> +      template<typename _Tp, typename _Up>
> +       static _Up*
> +       __uninit_copy(_Tp* __first, _Tp* __last, _Up* __result)
> +       {
> +         // Ensure that we don't successfully memcpy in cases that should be
> +         // ill-formed because is_constructible<_Up, _Tp&> is false.
> +         typedef __typeof__(static_cast<_Up>(*__first)) __check
> +           __attribute__((__unused__));
> +
> +         if (const ptrdiff_t __n = __last - __first)
> +           {
> +             __builtin_memcpy(__result, __first, __n * sizeof(_Tp));
> +             __result += __n;
> +           }
> +         return __result;
> +       }
> +    };
> +#endif
>    /// @endcond
>
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wc++17-extensions"
>    /**
>     *  @brief Copies the range [first,last) into result.
>     *  @param  __first  An input iterator.
>     *  @param  __last   An input iterator.
> -   *  @param  __result An output iterator.
> -   *  @return   __result + (__first - __last)
> +   *  @param  __result A forward iterator.
> +   *  @return   __result + (__last - __first)
>     *
> -   *  Like copy(), but does not require an initialized output range.
> +   *  Like std::copy, but does not require an initialized output range.
>    */
>    template<typename _InputIterator, typename _ForwardIterator>
>      inline _ForwardIterator
>      uninitialized_copy(_InputIterator __first, _InputIterator __last,
>                        _ForwardIterator __result)
>      {
> +      // We can use memcpy to copy the ranges under these conditions:
> +      //
> +      // _ForwardIterator and _InputIterator are both contiguous iterators,
> +      // so that we can turn them into pointers to pass to memcpy.
> +      // Before C++20 we can't detect all contiguous iterators, so we only
> +      // handle built-in pointers and __normal_iterator<T*, C> types.
> +      //
> +      // The value types of both iterators are trivially-copyable types,
> +      // so that memcpy is not undefined and can begin the lifetime of
> +      // new objects in the output range.
> +      //
> +      // Finally, memcpy from the source type, S, to the destination type, D,
> +      // must give the same value as initialization of D from S would give.
> +      // We require is_trivially_constructible<D, S> to be true, but that is
> +      // not sufficient. Some cases of trivial initialization are not just a
> +      // bitwise copy, even when sizeof(D) == sizeof(S),
> +      // e.g. bit_cast<unsigned>(1.0f) != 1u because the corresponding bits
> +      // of the value representations do not have the same meaning.
> +      // We cannot tell when this condition is true in general,
> +      // so we rely on the __memcpyable trait.
> +
> +#if __cplusplus >= 201103L
> +      using _Dest = decltype(std::__niter_base(__result));
> +      using _Src = decltype(std::__niter_base(__first));
> +      using _ValT = typename iterator_traits<_ForwardIterator>::value_type;
> +
> +      if constexpr (!__is_trivially_constructible(_ValT, decltype(*__first)))
> +       return std::__do_uninit_copy(__first, __last, __result);
> +      else if constexpr (__memcpyable<_Dest, _Src>::__value)
> +       {
> +         ptrdiff_t __n = __last - __first;
> +         if (__builtin_expect(__n > 1, true))
> +           {
> +             using _ValT = typename remove_pointer<_Src>::type;
> +             __builtin_memcpy(std::__niter_base(__result),
> +                              std::__niter_base(__first),
> +                              __n * sizeof(_ValT));
> +             __result += __n;
> +           }
> +         else if (__n == 1) // memcpy could overwrite tail padding
> +           std::_Construct(std::__addressof(*__result++), *__first);
> +         return __result;
> +       }
> +#if __cpp_lib_concepts
> +      else if constexpr (contiguous_iterator<_ForwardIterator>
> +                          && contiguous_iterator<_InputIterator>)
> +       {
> +         using _DestPtr = decltype(std::to_address(__result));
> +         using _SrcPtr = decltype(std::to_address(__first));
> +         if constexpr (__memcpyable<_DestPtr, _SrcPtr>::__value)
> +           {
> +             if (auto __n = __last - __first; __n > 1) [[likely]]
> +               {
> +                 void* __dest = std::to_address(__result);
> +                 const void* __src = std::to_address(__first);
> +                 size_t __nbytes = __n * sizeof(remove_pointer_t<_DestPtr>);
> +                 __builtin_memmove(__dest, __src, __nbytes);
> +                 __result += __n;
> +               }
> +             else if (__n == 1) // memcpy could overwrite tail padding
> +               std::construct_at(std::__addressof(*__result++), *__first);
> +             return __result;
> +           }
> +         else
> +           return std::__do_uninit_copy(__first, __last, __result);
> +       }
> +#endif
> +      else
> +       return std::__do_uninit_copy(__first, __last, __result);
> +#else // C++98
>        typedef typename iterator_traits<_InputIterator>::value_type
>         _ValueType1;
>        typedef typename iterator_traits<_ForwardIterator>::value_type
>         _ValueType2;
>
> -      // _ValueType1 must be trivially-copyable to use memmove, so don't
> -      // bother optimizing to std::copy if it isn't.
> -      // XXX Unnecessary because std::copy would check it anyway?
> -      const bool __can_memmove = __is_trivial(_ValueType1);
> +      const bool __can_memcpy
> +       = __memcpyable<_ValueType1*, _ValueType2*>::__value
> +           && __is_trivially_constructible(_ValueType2, __decltype(*__first));
>
> -#if __cplusplus < 201103L
> -      typedef typename iterator_traits<_InputIterator>::reference _From;
> -#else
> -      using _From = decltype(*__first);
> +      return __uninitialized_copy<__can_memcpy>::
> +              __uninit_copy(__first, __last, __result);
>  #endif
> -      const bool __assignable
> -       = _GLIBCXX_USE_ASSIGN_FOR_INIT(_ValueType2, _From);
> -
> -      return std::__uninitialized_copy<__can_memmove && __assignable>::
> -       __uninit_copy(__first, __last, __result);
>      }
> +#pragma GCC diagnostic pop
>
>    /// @cond undocumented
>
> +  // This is the default implementation of std::uninitialized_fill.
>    template<typename _ForwardIterator, typename _Tp>
>      _GLIBCXX20_CONSTEXPR void
>      __do_uninit_fill(_ForwardIterator __first, _ForwardIterator __last,
> @@ -244,12 +327,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        __guard.release();
>      }
>
> -  template<bool _TrivialValueType>
> +#if __cplusplus < 201103L
> +  // Use template specialization for C++98 when 'if constexpr' can't be used.
> +  template<bool _CanMemset>
>      struct __uninitialized_fill
>      {
>        template<typename _ForwardIterator, typename _Tp>
> -        static void
> -        __uninit_fill(_ForwardIterator __first, _ForwardIterator __last,
> +       static void
> +       __uninit_fill(_ForwardIterator __first, _ForwardIterator __last,
>                       const _Tp& __x)
>         { std::__do_uninit_fill(__first, __last, __x); }
>      };
> @@ -257,56 +342,129 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>    template<>
>      struct __uninitialized_fill<true>
>      {
> +      // Overload for generic iterators.
>        template<typename _ForwardIterator, typename _Tp>
> -        static void
> -        __uninit_fill(_ForwardIterator __first, _ForwardIterator __last,
> +       static void
> +       __uninit_fill(_ForwardIterator __first, _ForwardIterator __last,
>                       const _Tp& __x)
> -        { std::fill(__first, __last, __x); }
> -    };
> +       {
> +         if (__unwrappable_niter<_ForwardIterator>::__value)
> +           __uninit_fill(std::__niter_base(__first),
> +                         std::__niter_base(__last),
> +                         __x);
> +         else
> +           std::__do_uninit_copy(__first, __last, __x);
> +       }
>
> +      // Overload for pointers.
> +      template<typename _Up, typename _Tp>
> +       static void
> +       __uninit_fill(_Up* __first, _Up* __last, const _Tp& __x)
> +       {
> +         // Ensure that we don't successfully memset in cases that should be
> +         // ill-formed because is_constructible<_Up, const _Tp&> is false.
> +         typedef __typeof__(static_cast<_Up>(__x)) __check
> +           __attribute__((__unused__));
> +
> +         if (__first != __last)
> +           __builtin_memset(__first, (int)__x, __last - __first);
> +       }
> +    };
> +#endif
>    /// @endcond
>
>    /**
>     *  @brief Copies the value x into the range [first,last).
> -   *  @param  __first  An input iterator.
> -   *  @param  __last   An input iterator.
> +   *  @param  __first  A forward iterator.
> +   *  @param  __last   A forward iterator.
>     *  @param  __x      The source value.
>     *  @return   Nothing.
>     *
> -   *  Like fill(), but does not require an initialized output range.
> +   *  Like std::fill, but does not require an initialized output range.
>    */
>    template<typename _ForwardIterator, typename _Tp>
>      inline void
>      uninitialized_fill(_ForwardIterator __first, _ForwardIterator __last,
>                        const _Tp& __x)
>      {
> +      // We would like to use memset to optimize this loop when possible.
> +      // As for std::uninitialized_copy, the optimization requires
> +      // contiguous iterators and trivially copyable value types,
> +      // with the additional requirement that sizeof(_Tp) == 1 because
> +      // memset only writes single bytes.
> +
> +      // FIXME: We could additionally enable this for 1-byte enums.
> +      // Maybe any 1-byte Val if is_trivially_constructible<Val, const T&>?
> +
>        typedef typename iterator_traits<_ForwardIterator>::value_type
>         _ValueType;
>
> -      // Trivial types do not need a constructor to begin their lifetime,
> -      // so try to use std::fill to benefit from its memset optimization.
> -      const bool __can_fill
> -       = _GLIBCXX_USE_ASSIGN_FOR_INIT(_ValueType, const _Tp&);
> +#if __cplusplus >= 201103L
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wc++17-extensions"
> +      if constexpr (__is_byte<_ValueType>::__value)
> +       if constexpr (is_same<_ValueType, _Tp>::value
> +                       || is_integral<_Tp>::value)
> +         {
> +           using _BasePtr = decltype(std::__niter_base(__first));
> +           if constexpr (is_pointer<_BasePtr>::value)
> +             {
> +               void* __dest = std::__niter_base(__first);
> +               ptrdiff_t __n = __last - __first;
> +               if (__builtin_expect(__n > 1, true))
> +                 __builtin_memset(__dest, (unsigned char)__x, __n);
> +               return;
> +             }
> +#if __cpp_lib_concepts
> +           else if constexpr (contiguous_iterator<_ForwardIterator>)
> +             {
> +               auto __dest = std::__to_address(__first);
> +               auto __n = __last - __first;
> +               if (__builtin_expect(__n > 1, true))
> +                 __builtin_memset(__dest, (unsigned char)__x, __n);
> +               return;
> +             }
> +#endif
> +         }
> +      std::__do_uninit_fill(__first, __last, __x);
> +#pragma GCC diagnostic pop
> +#else // C++98
> +      const bool __can_memset = __is_byte<_ValueType>::__value
> +                                 && __is_integer<_Tp>::__value;
>
> -      std::__uninitialized_fill<__can_fill>::
> -       __uninit_fill(__first, __last, __x);
> +      __uninitialized_fill<__can_memset>::__uninit_fill(__first, __last, __x);
> +#endif
>      }
>
>    /// @cond undocumented
>
> +  // This is the default implementation of std::uninitialized_fill_n.
>    template<typename _ForwardIterator, typename _Size, typename _Tp>
>      _GLIBCXX20_CONSTEXPR
>      _ForwardIterator
>      __do_uninit_fill_n(_ForwardIterator __first, _Size __n, const _Tp& __x)
>      {
>        _UninitDestroyGuard<_ForwardIterator> __guard(__first);
> -      for (; __n > 0; --__n, (void) ++__first)
> +#if __cplusplus >= 201103L
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wc++17-extensions"
> +      if constexpr (is_integral<_Size>::value)
> +       // Loop will never terminate if __n is negative.
> +       __glibcxx_assert(__n >= 0);
> +      else if constexpr (is_floating_point<_Size>::value)
> +       // Loop will never terminate if __n is not an integer.
> +       __glibcxx_assert(__n >= 0 && static_cast<size_t>(__n) == __n);
> +#pragma GCC diagnostic pop
> +#endif
> +      for (; __n--; ++__first)
>         std::_Construct(std::__addressof(*__first), __x);
>        __guard.release();
>        return __first;
>      }
>
> -  template<bool _TrivialValueType>
> +#if __cplusplus < 201103L
> +  // Use template specialization for C++98 when 'if constexpr' can't be used.
> +  template<bool _CanMemset>
>      struct __uninitialized_fill_n
>      {
>        template<typename _ForwardIterator, typename _Size, typename _Tp>
> @@ -319,47 +477,92 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>    template<>
>      struct __uninitialized_fill_n<true>
>      {
> +      // Overload for generic iterators.
>        template<typename _ForwardIterator, typename _Size, typename _Tp>
>         static _ForwardIterator
>          __uninit_fill_n(_ForwardIterator __first, _Size __n,
>                         const _Tp& __x)
> -        { return std::fill_n(__first, __n, __x); }
> +       {
> +         if (__unwrappable_niter<_ForwardIterator>::__value)
> +           {
> +             _ForwardIterator __last = __first;
> +             std::advance(__last, __n);
> +             __uninitialized_fill<true>::__uninit_fill(__first, __last, __x);
> +             return __last;
> +           }
> +         else
> +           return std::__do_uninit_fill_n(__first, __n, __x);
> +       }
>      };
> -
> +#endif
>    /// @endcond
>
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wc++17-extensions"
>     // _GLIBCXX_RESOLVE_LIB_DEFECTS
>     // DR 1339. uninitialized_fill_n should return the end of its range
>    /**
>     *  @brief Copies the value x into the range [first,first+n).
> -   *  @param  __first  An input iterator.
> +   *  @param  __first  A forward iterator.
>     *  @param  __n      The number of copies to make.
>     *  @param  __x      The source value.
> -   *  @return   Nothing.
> +   *  @return   __first + __n.
>     *
> -   *  Like fill_n(), but does not require an initialized output range.
> +   *  Like std::fill_n, but does not require an initialized output range.
>    */
>    template<typename _ForwardIterator, typename _Size, typename _Tp>
>      inline _ForwardIterator
>      uninitialized_fill_n(_ForwardIterator __first, _Size __n, const _Tp& __x)
>      {
> +      // See uninitialized_fill conditions. We also require _Size to be
> +      // an integer. The standard only requires _Size to be decrementable
> +      // and contextually convertible to bool, so don't assume first+n works.
> +
> +      // FIXME: We could additionally enable this for 1-byte enums.
> +
>        typedef typename iterator_traits<_ForwardIterator>::value_type
>         _ValueType;
>
> -      // Trivial types do not need a constructor to begin their lifetime,
> -      // so try to use std::fill_n to benefit from its optimizations.
> -      const bool __can_fill
> -       = _GLIBCXX_USE_ASSIGN_FOR_INIT(_ValueType, const _Tp&)
> -      // For arbitrary class types and floating point types we can't assume
> -      // that __n > 0 and std::__size_to_integer(__n) > 0 are equivalent,
> -      // so only use std::fill_n when _Size is already an integral type.
> -       && __is_integer<_Size>::__value;
> +#if __cplusplus >= 201103L
> +      if constexpr (__is_byte<_ValueType>::__value)
> +       if constexpr (is_integral<_Tp>::value)
> +         if constexpr (is_integral<_Size>::value)
> +           {
> +             using _BasePtr = decltype(std::__niter_base(__first));
> +             if constexpr (is_pointer<_BasePtr>::value)
> +               {
> +                 void* __dest = std::__niter_base(__first);
> +                 if (__builtin_expect(__n > 1, true))
> +                   {
> +                     __builtin_memset(__dest, (unsigned char)__x, __n);
> +                     __first += __n;
> +                   }
> +                 return __first;
> +               }
> +#if __cpp_lib_concepts
> +             else if constexpr (contiguous_iterator<_ForwardIterator>)
> +               {
> +                 auto __dest = std::__to_address(__first);
> +                 if (__builtin_expect(__n > 1, true))
> +                   {
> +                     __builtin_memset(__dest, (unsigned char)__x, __n);
> +                     __first += __n;
> +                   }
> +                 return __first;
> +               }
> +#endif
> +           }
> +      return std::__do_uninit_fill_n(__first, __n, __x);
> +#else // C++98
> +      const bool __can_memset = __is_byte<_ValueType>::__value
> +                                 && __is_integer<_Tp>::__value
> +                                 && __is_integer<_Size>::__value;
>
> -      return __uninitialized_fill_n<__can_fill>::
> +      return __uninitialized_fill_n<__can_memset>::
>         __uninit_fill_n(__first, __n, __x);
> +#endif
>      }
> -
> -#undef _GLIBCXX_USE_ASSIGN_FOR_INIT
> +#pragma GCC diagnostic pop
>
>    /// @cond undocumented
>
> @@ -619,7 +822,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>             = std::__addressof(*__first);
>           std::_Construct(__val);
>           if (++__first != __last)
> -           std::fill(__first, __last, *__val);
> +           std::uninitialized_fill(__first, __last, *__val);
>         }
>      };
>
> @@ -653,7 +856,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>                 = std::__addressof(*__first);
>               std::_Construct(__val);
>               ++__first;
> -             __first = std::fill_n(__first, __n - 1, *__val);
> +             __first = std::uninitialized_fill_n(__first, __n - 1, *__val);
>             }
>           return __first;
>         }
> diff --git a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/1.cc b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/1.cc
> index 398d8690b56..27b3100d362 100644
> --- a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/1.cc
> +++ b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/1.cc
> @@ -34,4 +34,5 @@ test01(T* result)
>    T t[1];
>    std::uninitialized_copy(t, t+1, result); // { dg-error "here" }
>  }
> -// { dg-error "must be constructible from input type" "" { target *-*-* } 0 }
> +// { dg-error "no matching function" "construct_at" { target c++20 } 0 }
> +// { dg-error "use of deleted function" "T::T(const T&)" { target *-*-* } 0 }
> diff --git a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/64476.cc b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/64476.cc
> index 2f7dda3417d..e99338dff39 100644
> --- a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/64476.cc
> +++ b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/64476.cc
> @@ -54,8 +54,10 @@ test01()
>
>    std::uninitialized_copy(a, a+10, b);
>
> -  VERIFY(constructed == 0);
> -  VERIFY(assigned == 10);
> +  // In GCC 14 and older std::uninitialized_copy was optimized to std::copy
> +  // and so used assignments not construction, but that was non-conforming.
> +  VERIFY(constructed == 10);
> +  VERIFY(assigned == 0);
>  }
>
>  int
> diff --git a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/89164.cc b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/89164.cc
> index 48c16da4d32..6e978a7e36c 100644
> --- a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/89164.cc
> +++ b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/89164.cc
> @@ -35,4 +35,5 @@ void test01()
>
>    std::uninitialized_copy(x, x+1, p); // { dg-error "here" }
>  }
> -// { dg-error "must be constructible" "" { target *-*-* } 0 }
> +// { dg-error "no matching function" "construct_at" { target c++20 } 0 }
> +// { dg-error "use of deleted function" "X(const X&)" { target *-*-* } 0 }
> diff --git a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy_n/89164.cc b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy_n/89164.cc
> index 4e8fb0f4af2..96156208372 100644
> --- a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy_n/89164.cc
> +++ b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy_n/89164.cc
> @@ -32,4 +32,5 @@ void test01()
>
>    std::uninitialized_copy_n(x, 1, p); // { dg-error "here" }
>  }
> -// { dg-error "must be constructible" "" { target *-*-* } 0 }
> +// { dg-error "no matching function" "construct_at" { target c++20 } 0 }
> +// { dg-error "use of deleted function" "X(const X&)" { target *-*-* } 0 }
> diff --git a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill/89164.cc b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill/89164.cc
> index 8353b5882f0..0dcaa1aa9c3 100644
> --- a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill/89164.cc
> +++ b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill/89164.cc
> @@ -32,4 +32,5 @@ void f()
>
>    std::uninitialized_fill(p, p+1, x); // { dg-error "here" }
>  }
> -// { dg-error "must be constructible" "" { target *-*-* } 0 }
> +// { dg-error "no matching function" "construct_at" { target c++20 } 0 }
> +// { dg-error "use of deleted function" "X(const X&)" { target *-*-* } 0 }
> diff --git a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill_n/89164.cc b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill_n/89164.cc
> index 4b38c673d32..9b61157b934 100644
> --- a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill_n/89164.cc
> +++ b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill_n/89164.cc
> @@ -32,4 +32,5 @@ void test01()
>
>    std::uninitialized_fill_n(p, 1, x); // { dg-error "here" }
>  }
> -// { dg-error "must be constructible" "" { target *-*-* } 0 }
> +// { dg-error "no matching function" "construct_at" { target c++20 } 0 }
> +// { dg-error "use of deleted function" "X(const X&)" { target *-*-* } 0 }
> diff --git a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill_n/sizes.cc b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill_n/sizes.cc
> index e2ba9355c56..876ec5443fb 100644
> --- a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill_n/sizes.cc
> +++ b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill_n/sizes.cc
> @@ -24,21 +24,35 @@ void
>  test01()
>  {
>    int i[4] = { };
> -  std::uninitialized_fill_n(i, 2.0001, 0xabcd);
> +  // Floating-point n should work, but only if it's an integer value.
> +  std::uninitialized_fill_n(i, 3.0, 0xabcd);
>    VERIFY( i[0] == 0xabcd );
>    VERIFY( i[1] == 0xabcd );
>    VERIFY( i[2] == 0xabcd );
>    VERIFY( i[3] == 0 );
>  }
>
> -// The standard only requires that n>0 and --n are valid expressions.
> +// The standard only requires that `if (n--)` is a valid expression.
>  struct Size
>  {
>    int value;
>
> -  void operator--() { --value; }
> +  struct testable
> +  {
> +#if __cplusplus >= 201103L
> +    explicit
> +#endif
> +    operator bool() const { return nonzero; }
>
> -  int operator>(void*) { return value != 0; }
> +    bool nonzero;
> +  };
> +
> +  testable operator--(int)
> +  {
> +    testable t = { value != 0 };
> +    --value;
> +    return t;
> +  }
>  };
>
>  void
> diff --git a/libstdc++-v3/testsuite/23_containers/vector/cons/89164.cc b/libstdc++-v3/testsuite/23_containers/vector/cons/89164.cc
> index 106963ecbb9..36907dc508e 100644
> --- a/libstdc++-v3/testsuite/23_containers/vector/cons/89164.cc
> +++ b/libstdc++-v3/testsuite/23_containers/vector/cons/89164.cc
> @@ -32,7 +32,7 @@ void test01()
>    X x[1];
>    // Should not be able to create vector using uninitialized_copy:
>    std::vector<X> v1{x, x+1};   // { dg-error "here" "" { target c++17_down } }
> -  // { dg-error "deleted function 'X::X" "" { target c++20 } 0 }
> +  // { dg-error "deleted function 'X::X" "" { target *-*-* } 0 }
>  }
>
>  void test02()
> @@ -41,8 +41,7 @@ void test02()
>
>    // Should not be able to create vector using uninitialized_fill_n:
>    std::vector<Y> v2{2u, Y{}};  // { dg-error "here" "" { target c++17_down } }
> -  // { dg-error "deleted function .*Y::Y" "" { target c++20 } 0 }
> +  // { dg-error "deleted function .*Y::Y" "" { target *-*-* } 0 }
>  }
>
> -// { dg-error "must be constructible from input type" "" { target *-*-* } 0 }
>  // { dg-prune-output "construct_at" }
> diff --git a/libstdc++-v3/testsuite/23_containers/vector/cons/89164_c++17.cc b/libstdc++-v3/testsuite/23_containers/vector/cons/89164_c++17.cc
> index 09d3dc6f93d..07d4bab9117 100644
> --- a/libstdc++-v3/testsuite/23_containers/vector/cons/89164_c++17.cc
> +++ b/libstdc++-v3/testsuite/23_containers/vector/cons/89164_c++17.cc
> @@ -32,8 +32,7 @@ void test03()
>    // Can create initializer_list<Y> with C++17 guaranteed copy elision,
>    // but shouldn't be able to copy from it with uninitialized_copy:
>    std::vector<X> v3{X{}, X{}, X{}};   // { dg-error "here" "" { target c++17_only } }
> -  // { dg-error "deleted function .*X::X" "" { target c++20 } 0 }
> +  // { dg-error "deleted function .*X::X" "" { target *-*-* } 0 }
>  }
>
> -// { dg-error "must be constructible from input type" "" { target *-*-* } 0 }
>  // { dg-prune-output "construct_at" }
> --
> 2.46.2
>

On Mon, 14 Oct 2024 at 14:59, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> This causes some codegen regressions on (at least) aarch64 and power64:
>
> FAIL: g++.dg/pr104547.C  -std=gnu++11  scan-tree-dump-not vrp2
> "_M_default_append"
> FAIL: g++.dg/pr104547.C  -std=gnu++17  scan-tree-dump-not vrp2
> "_M_default_append"
>
> For some reason the PR 104547 fix to stop the unused _M_default_append
> function being emitted in the assembly code no longer works.
>
> I also see new bogus stringop-overflow warnings for these tests:
>
> FAIL: 27_io/basic_ios/imbue/14072.cc  -std=gnu++17 (test for excess errors)
> FAIL: 27_io/basic_istream/extractors_character/char/9826.cc
> -std=gnu++17 (test for excess errors)
> FAIL: 27_io/basic_stringbuf/requirements/explicit_instantiation/2.cc
> -std=gnu++17 (test for excess errors)
> FAIL: 27_io/basic_stringstream/requirements/explicit_instantiation.cc
> -std=gnu++17 (test for excess errors)

Actually the bogus -Wstringop-overflow warnings are due to a different
patch I'm testing locally.

The FAIL: g++.dg/pr104547.C regression is caused by this one though.


>
>
>
>
> On Mon, 14 Oct 2024 at 00:53, Jonathan Wakely <jwakely@redhat.com> wrote:
> >
> > Tested x86_64-linux.
> >
> > -- >8 --
> >
> > This refactors the std::uninitialized_copy, std::uninitialized_fill and
> > std::uninitialized_fill_n algorithms to directly perform memcpy/memset
> > optimizations instead of dispatching to std::copy/std::fill/std::fill_n.
> >
> > The reasons for this are:
> >
> > - Use 'if constexpr' to simplify and optimize compilation throughput, so
> >   dispatching to specialized class templates is only needed for C++98
> >   mode.
> > - Relax the conditions for using memcpy/memset, because the C++20 rules
> >   on implicit-lifetime types mean that we can rely on memcpy to begin
> >   lifetimes of trivially copyable types.  We don't need to require
> >   trivially default constructible, so don't need to limit the
> >   optimization to trivial types. See PR 68350 for more details.
> > - The conditions on non-overlapping ranges are stronger for
> >   std::uninitialized_copy than for std::copy so we can use memcpy instead
> >   of memmove, which might be a minor optimization.
> > - Avoid including <bits/stl_algobase.h> in <bits/stl_uninitialized.h>.
> >   It only needs some iterator utilities from that file now, which belong
> >   in <bits/stl_iterator.h> anyway, so this moves them there.
> >
> > Several tests need changes to the diagnostics matched by dg-error
> > because we no longer use the __constructible() function that had a
> > static assert in. Now we just get straightforward errors for attempting
> > to use a deleted constructor.
> >
> > Two tests needed more signficant changes to the actual expected results
> > of executing the tests, because they were checking for old behaviour
> > which was incorrect according to the standard.
> > 20_util/specialized_algorithms/uninitialized_copy/64476.cc was expecting
> > std::copy to be used for a call to std::uninitialized_copy involving two
> > trivially copyable types. That was incorrect behaviour, because a
> > non-trivial constructor should have been used, but using std::copy used
> > trivial default initialization followed by assignment.
> > 20_util/specialized_algorithms/uninitialized_fill_n/sizes.cc was testing
> > the behaviour with a non-integral Size passed to uninitialized_fill_n,
> > but I wrote the test looking at the requirements of uninitialized_copy_n
> > which are not the same as uninitialized_fill_n. The former uses --n and
> > tests n > 0, but the latter just tests n-- (which will never be false
> > for a floating-point value with a fractional part).
> >
> > libstdc++-v3/ChangeLog:
> >
> >         PR libstdc++/68350
> >         PR libstdc++/93059
> >         * include/bits/stl_algobase.h (__niter_base, __niter_wrap): Move
> >         to ...
> >         * include/bits/stl_iterator.h: ... here.
> >         * include/bits/stl_uninitialized.h (__check_constructible)
> >         (_GLIBCXX_USE_ASSIGN_FOR_INIT): Remove.
> >         [C++98] (__unwrappable_niter): New trait.
> >         (__uninitialized_copy<true>): Replace use of std::copy.
> >         (uninitialized_copy): Fix Doxygen comments. Open-code memcpy
> >         optimization for C++11 and later.
> >         (__uninitialized_fill<true>): Replace use of std::fill.
> >         (uninitialized_fill): Fix Doxygen comments. Open-code memset
> >         optimization for C++11 and later.
> >         (__uninitialized_fill_n<true>): Replace use of std::fill_n.
> >         (uninitialized_fill_n): Fix Doxygen comments. Open-code memset
> >         optimization for C++11 and later.
> >         * testsuite/20_util/specialized_algorithms/uninitialized_copy/64476.cc:
> >         Adjust expected behaviour to match what the standard specifies.
> >         * testsuite/20_util/specialized_algorithms/uninitialized_fill_n/sizes.cc:
> >         Likewise.
> >         * testsuite/20_util/specialized_algorithms/uninitialized_copy/1.cc:
> >         Adjust dg-error directives.
> >         * testsuite/20_util/specialized_algorithms/uninitialized_copy/89164.cc:
> >         Likewise.
> >         * testsuite/20_util/specialized_algorithms/uninitialized_copy_n/89164.cc:
> >         Likewise.
> >         * testsuite/20_util/specialized_algorithms/uninitialized_fill/89164.cc:
> >         Likewise.
> >         * testsuite/20_util/specialized_algorithms/uninitialized_fill_n/89164.cc:
> >         Likewise.
> >         * testsuite/23_containers/vector/cons/89164.cc: Likewise.
> >         * testsuite/23_containers/vector/cons/89164_c++17.cc: Likewise.
> > ---
> >  libstdc++-v3/include/bits/stl_algobase.h      |  45 --
> >  libstdc++-v3/include/bits/stl_iterator.h      |  54 +++
> >  libstdc++-v3/include/bits/stl_uninitialized.h | 385 +++++++++++++-----
> >  .../uninitialized_copy/1.cc                   |   3 +-
> >  .../uninitialized_copy/64476.cc               |   6 +-
> >  .../uninitialized_copy/89164.cc               |   3 +-
> >  .../uninitialized_copy_n/89164.cc             |   3 +-
> >  .../uninitialized_fill/89164.cc               |   3 +-
> >  .../uninitialized_fill_n/89164.cc             |   3 +-
> >  .../uninitialized_fill_n/sizes.cc             |  22 +-
> >  .../23_containers/vector/cons/89164.cc        |   5 +-
> >  .../23_containers/vector/cons/89164_c++17.cc  |   3 +-
> >  12 files changed, 383 insertions(+), 152 deletions(-)
> >
> > diff --git a/libstdc++-v3/include/bits/stl_algobase.h b/libstdc++-v3/include/bits/stl_algobase.h
> > index 384e5fdcdc9..751b7ad119b 100644
> > --- a/libstdc++-v3/include/bits/stl_algobase.h
> > +++ b/libstdc++-v3/include/bits/stl_algobase.h
> > @@ -308,51 +308,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >        return __a;
> >      }
> >
> > -  // Fallback implementation of the function in bits/stl_iterator.h used to
> > -  // remove the __normal_iterator wrapper. See copy, fill, ...
> > -  template<typename _Iterator>
> > -    _GLIBCXX20_CONSTEXPR
> > -    inline _Iterator
> > -    __niter_base(_Iterator __it)
> > -    _GLIBCXX_NOEXCEPT_IF(std::is_nothrow_copy_constructible<_Iterator>::value)
> > -    { return __it; }
> > -
> > -#if __cplusplus < 201103L
> > -  template<typename _Ite, typename _Seq>
> > -    _Ite
> > -    __niter_base(const ::__gnu_debug::_Safe_iterator<_Ite, _Seq,
> > -                std::random_access_iterator_tag>&);
> > -
> > - template<typename _Ite, typename _Cont, typename _Seq>
> > -    _Ite
> > -    __niter_base(const ::__gnu_debug::_Safe_iterator<
> > -                ::__gnu_cxx::__normal_iterator<_Ite, _Cont>, _Seq,
> > -                std::random_access_iterator_tag>&);
> > -#else
> > -  template<typename _Ite, typename _Seq>
> > -    _GLIBCXX20_CONSTEXPR
> > -    decltype(std::__niter_base(std::declval<_Ite>()))
> > -    __niter_base(const ::__gnu_debug::_Safe_iterator<_Ite, _Seq,
> > -                std::random_access_iterator_tag>&)
> > -    noexcept(std::is_nothrow_copy_constructible<_Ite>::value);
> > -#endif
> > -
> > -  // Reverse the __niter_base transformation to get a
> > -  // __normal_iterator back again (this assumes that __normal_iterator
> > -  // is only used to wrap random access iterators, like pointers).
> > -  template<typename _From, typename _To>
> > -    _GLIBCXX20_CONSTEXPR
> > -    inline _From
> > -    __niter_wrap(_From __from, _To __res)
> > -    { return __from + (std::__niter_base(__res) - std::__niter_base(__from)); }
> > -
> > -  // No need to wrap, iterator already has the right type.
> > -  template<typename _Iterator>
> > -    _GLIBCXX20_CONSTEXPR
> > -    inline _Iterator
> > -    __niter_wrap(const _Iterator&, _Iterator __res)
> > -    { return __res; }
> > -
> >    // All of these auxiliary structs serve two purposes.  (1) Replace
> >    // calls to copy with memmove whenever possible.  (Memmove, not memcpy,
> >    // because the input and output ranges are permitted to overlap.)
> > diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
> > index 28a600c81cb..85b98ffff61 100644
> > --- a/libstdc++-v3/include/bits/stl_iterator.h
> > +++ b/libstdc++-v3/include/bits/stl_iterator.h
> > @@ -1338,10 +1338,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >  _GLIBCXX_END_NAMESPACE_VERSION
> >  } // namespace
> >
> > +namespace __gnu_debug
> > +{
> > +  template<typename _Iterator, typename _Sequence, typename _Category>
> > +    class _Safe_iterator;
> > +}
> > +
> >  namespace std _GLIBCXX_VISIBILITY(default)
> >  {
> >  _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >
> > +  // Unwrap a __normal_iterator to get the underlying iterator
> > +  // (usually a pointer)
> >    template<typename _Iterator, typename _Container>
> >      _GLIBCXX20_CONSTEXPR
> >      _Iterator
> > @@ -1349,6 +1357,52 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >      _GLIBCXX_NOEXCEPT_IF(std::is_nothrow_copy_constructible<_Iterator>::value)
> >      { return __it.base(); }
> >
> > +  // Fallback implementation of the function in bits/stl_iterator.h used to
> > +  // remove the __normal_iterator wrapper. See std::copy, std::fill, etc.
> > +  template<typename _Iterator>
> > +    _GLIBCXX20_CONSTEXPR
> > +    inline _Iterator
> > +    __niter_base(_Iterator __it)
> > +    _GLIBCXX_NOEXCEPT_IF(std::is_nothrow_copy_constructible<_Iterator>::value)
> > +    { return __it; }
> > +
> > +  // Overload for _Safe_iterator needs to be declared before __niter_base uses.
> > +#if __cplusplus < 201103L
> > +  template<typename _Ite, typename _Seq>
> > +    _Ite
> > +    __niter_base(const ::__gnu_debug::_Safe_iterator<_Ite, _Seq,
> > +                std::random_access_iterator_tag>&);
> > +
> > + template<typename _Ite, typename _Cont, typename _Seq>
> > +    _Ite
> > +    __niter_base(const ::__gnu_debug::_Safe_iterator<
> > +                ::__gnu_cxx::__normal_iterator<_Ite, _Cont>, _Seq,
> > +                std::random_access_iterator_tag>&);
> > +#else
> > +  template<typename _Ite, typename _Seq>
> > +    _GLIBCXX20_CONSTEXPR
> > +    decltype(std::__niter_base(std::declval<_Ite>()))
> > +    __niter_base(const ::__gnu_debug::_Safe_iterator<_Ite, _Seq,
> > +                std::random_access_iterator_tag>&)
> > +    noexcept(std::is_nothrow_copy_constructible<_Ite>::value);
> > +#endif
> > +
> > +  // Reverse the __niter_base transformation to get a
> > +  // __normal_iterator back again (this assumes that __normal_iterator
> > +  // is only used to wrap random access iterators, like pointers).
> > +  template<typename _From, typename _To>
> > +    _GLIBCXX20_CONSTEXPR
> > +    inline _From
> > +    __niter_wrap(_From __from, _To __res)
> > +    { return __from + (std::__niter_base(__res) - std::__niter_base(__from)); }
> > +
> > +  // No need to wrap, iterator already has the right type.
> > +  template<typename _Iterator>
> > +    _GLIBCXX20_CONSTEXPR
> > +    inline _Iterator
> > +    __niter_wrap(const _Iterator&, _Iterator __res)
> > +    { return __res; }
> > +
> >  #if __cplusplus >= 201103L && __cplusplus <= 201703L
> >    // Need to overload __to_address because the pointer_traits primary template
> >    // will deduce element_type of __normal_iterator<T*, C> as T* rather than T.
> > diff --git a/libstdc++-v3/include/bits/stl_uninitialized.h b/libstdc++-v3/include/bits/stl_uninitialized.h
> > index f663057b1a1..1bad82bf5bf 100644
> > --- a/libstdc++-v3/include/bits/stl_uninitialized.h
> > +++ b/libstdc++-v3/include/bits/stl_uninitialized.h
> > @@ -57,16 +57,16 @@
> >  #define _STL_UNINITIALIZED_H 1
> >
> >  #if __cplusplus >= 201103L
> > -#include <type_traits>
> > +# include <type_traits>
> > +# include <bits/ptr_traits.h>      // __to_address
> > +# include <bits/stl_pair.h>        // pair
> >  #endif
> >
> > -#include <bits/stl_algobase.h>    // copy
> > +#include <bits/cpp_type_traits.h> // __is_pointer
> > +#include <bits/stl_iterator_base_funcs.h> // distance, advance
> > +#include <bits/stl_iterator.h>    // __niter_base
> >  #include <ext/alloc_traits.h>     // __alloc_traits
> >
> > -#if __cplusplus >= 201703L
> > -#include <bits/stl_pair.h>
> > -#endif
> > -
> >  namespace std _GLIBCXX_VISIBILITY(default)
> >  {
> >  _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > @@ -77,36 +77,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >
> >    /// @cond undocumented
> >
> > -#if __cplusplus >= 201103L
> > -  template<typename _ValueType, typename _Tp>
> > -    constexpr bool
> > -    __check_constructible()
> > -    {
> > -      // Trivial types can have deleted constructors, but std::copy etc.
> > -      // only use assignment (or memmove) not construction, so we need an
> > -      // explicit check that construction from _Tp is actually valid,
> > -      // otherwise some ill-formed uses of std::uninitialized_xxx would
> > -      // compile without errors. This gives a nice clear error message.
> > -      static_assert(is_constructible<_ValueType, _Tp>::value,
> > -         "result type must be constructible from input type");
> > -
> > -      return true;
> > -    }
> > -
> > -// If the type is trivial we don't need to construct it, just assign to it.
> > -// But trivial types can still have deleted or inaccessible assignment,
> > -// so don't try to use std::copy or std::fill etc. if we can't assign.
> > -# define _GLIBCXX_USE_ASSIGN_FOR_INIT(T, U) \
> > -    __is_trivial(T) && __is_assignable(T&, U) \
> > -    && std::__check_constructible<T, U>()
> > -#else
> > -// No need to check if is_constructible<T, U> for C++98. Trivial types have
> > -// no user-declared constructors, so if the assignment is valid, construction
> > -// should be too.
> > -# define _GLIBCXX_USE_ASSIGN_FOR_INIT(T, U) \
> > -    __is_trivial(T) && __is_assignable(T&, U)
> > -#endif
> > -
> >    template<typename _ForwardIterator, typename _Alloc = void>
> >      struct _UninitDestroyGuard
> >      {
> > @@ -160,6 +130,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >        _UninitDestroyGuard(const _UninitDestroyGuard&);
> >      };
> >
> > +  // This is the default implementation of std::uninitialized_copy.
> >    template<typename _InputIterator, typename _ForwardIterator>
> >      _GLIBCXX20_CONSTEXPR
> >      _ForwardIterator
> > @@ -173,7 +144,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >        return __result;
> >      }
> >
> > -  template<bool _TrivialValueTypes>
> > +#if __cplusplus < 201103L
> > +
> > +  // True if we can unwrap _Iter to get a pointer by using std::__niter_base.
> > +  template<typename _Iter>
> > +    struct __unwrappable_niter
> > +    {
> > +      template<typename> struct __is_ptr { enum { __value = 0 }; };
> > +      template<typename _Tp> struct __is_ptr<_Tp*> { enum { __value = 1 }; };
> > +
> > +      typedef __decltype(std::__niter_base(*(_Iter*)0)) _Base;
> > +
> > +      enum { __value = __is_ptr<_Base>::__value };
> > +    };
> > +
> > +  // Use template specialization for C++98 when 'if constexpr' can't be used.
> > +  template<bool _CanMemcpy>
> >      struct __uninitialized_copy
> >      {
> >        template<typename _InputIterator, typename _ForwardIterator>
> > @@ -186,53 +172,150 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >    template<>
> >      struct __uninitialized_copy<true>
> >      {
> > +      // Overload for generic iterators.
> >        template<typename _InputIterator, typename _ForwardIterator>
> >          static _ForwardIterator
> >          __uninit_copy(_InputIterator __first, _InputIterator __last,
> >                       _ForwardIterator __result)
> > -        { return std::copy(__first, __last, __result); }
> > -    };
> > +       {
> > +         if (__unwrappable_niter<_InputIterator>::__value
> > +               && __unwrappable_niter<_ForwardIterator>::__value)
> > +           {
> > +             __uninit_copy(std::__niter_base(__first),
> > +                           std::__niter_base(__last),
> > +                           std::__niter_base(__result));
> > +             std::advance(__result, std::distance(__first, __last));
> > +             return __result;
> > +           }
> > +         else
> > +           return std::__do_uninit_copy(__first, __last, __result);
> > +       }
> >
> > +      // Overload for pointers.
> > +      template<typename _Tp, typename _Up>
> > +       static _Up*
> > +       __uninit_copy(_Tp* __first, _Tp* __last, _Up* __result)
> > +       {
> > +         // Ensure that we don't successfully memcpy in cases that should be
> > +         // ill-formed because is_constructible<_Up, _Tp&> is false.
> > +         typedef __typeof__(static_cast<_Up>(*__first)) __check
> > +           __attribute__((__unused__));
> > +
> > +         if (const ptrdiff_t __n = __last - __first)
> > +           {
> > +             __builtin_memcpy(__result, __first, __n * sizeof(_Tp));
> > +             __result += __n;
> > +           }
> > +         return __result;
> > +       }
> > +    };
> > +#endif
> >    /// @endcond
> >
> > +#pragma GCC diagnostic push
> > +#pragma GCC diagnostic ignored "-Wc++17-extensions"
> >    /**
> >     *  @brief Copies the range [first,last) into result.
> >     *  @param  __first  An input iterator.
> >     *  @param  __last   An input iterator.
> > -   *  @param  __result An output iterator.
> > -   *  @return   __result + (__first - __last)
> > +   *  @param  __result A forward iterator.
> > +   *  @return   __result + (__last - __first)
> >     *
> > -   *  Like copy(), but does not require an initialized output range.
> > +   *  Like std::copy, but does not require an initialized output range.
> >    */
> >    template<typename _InputIterator, typename _ForwardIterator>
> >      inline _ForwardIterator
> >      uninitialized_copy(_InputIterator __first, _InputIterator __last,
> >                        _ForwardIterator __result)
> >      {
> > +      // We can use memcpy to copy the ranges under these conditions:
> > +      //
> > +      // _ForwardIterator and _InputIterator are both contiguous iterators,
> > +      // so that we can turn them into pointers to pass to memcpy.
> > +      // Before C++20 we can't detect all contiguous iterators, so we only
> > +      // handle built-in pointers and __normal_iterator<T*, C> types.
> > +      //
> > +      // The value types of both iterators are trivially-copyable types,
> > +      // so that memcpy is not undefined and can begin the lifetime of
> > +      // new objects in the output range.
> > +      //
> > +      // Finally, memcpy from the source type, S, to the destination type, D,
> > +      // must give the same value as initialization of D from S would give.
> > +      // We require is_trivially_constructible<D, S> to be true, but that is
> > +      // not sufficient. Some cases of trivial initialization are not just a
> > +      // bitwise copy, even when sizeof(D) == sizeof(S),
> > +      // e.g. bit_cast<unsigned>(1.0f) != 1u because the corresponding bits
> > +      // of the value representations do not have the same meaning.
> > +      // We cannot tell when this condition is true in general,
> > +      // so we rely on the __memcpyable trait.
> > +
> > +#if __cplusplus >= 201103L
> > +      using _Dest = decltype(std::__niter_base(__result));
> > +      using _Src = decltype(std::__niter_base(__first));
> > +      using _ValT = typename iterator_traits<_ForwardIterator>::value_type;
> > +
> > +      if constexpr (!__is_trivially_constructible(_ValT, decltype(*__first)))
> > +       return std::__do_uninit_copy(__first, __last, __result);
> > +      else if constexpr (__memcpyable<_Dest, _Src>::__value)
> > +       {
> > +         ptrdiff_t __n = __last - __first;
> > +         if (__builtin_expect(__n > 1, true))
> > +           {
> > +             using _ValT = typename remove_pointer<_Src>::type;
> > +             __builtin_memcpy(std::__niter_base(__result),
> > +                              std::__niter_base(__first),
> > +                              __n * sizeof(_ValT));
> > +             __result += __n;
> > +           }
> > +         else if (__n == 1) // memcpy could overwrite tail padding
> > +           std::_Construct(std::__addressof(*__result++), *__first);
> > +         return __result;
> > +       }
> > +#if __cpp_lib_concepts
> > +      else if constexpr (contiguous_iterator<_ForwardIterator>
> > +                          && contiguous_iterator<_InputIterator>)
> > +       {
> > +         using _DestPtr = decltype(std::to_address(__result));
> > +         using _SrcPtr = decltype(std::to_address(__first));
> > +         if constexpr (__memcpyable<_DestPtr, _SrcPtr>::__value)
> > +           {
> > +             if (auto __n = __last - __first; __n > 1) [[likely]]
> > +               {
> > +                 void* __dest = std::to_address(__result);
> > +                 const void* __src = std::to_address(__first);
> > +                 size_t __nbytes = __n * sizeof(remove_pointer_t<_DestPtr>);
> > +                 __builtin_memmove(__dest, __src, __nbytes);
> > +                 __result += __n;
> > +               }
> > +             else if (__n == 1) // memcpy could overwrite tail padding
> > +               std::construct_at(std::__addressof(*__result++), *__first);
> > +             return __result;
> > +           }
> > +         else
> > +           return std::__do_uninit_copy(__first, __last, __result);
> > +       }
> > +#endif
> > +      else
> > +       return std::__do_uninit_copy(__first, __last, __result);
> > +#else // C++98
> >        typedef typename iterator_traits<_InputIterator>::value_type
> >         _ValueType1;
> >        typedef typename iterator_traits<_ForwardIterator>::value_type
> >         _ValueType2;
> >
> > -      // _ValueType1 must be trivially-copyable to use memmove, so don't
> > -      // bother optimizing to std::copy if it isn't.
> > -      // XXX Unnecessary because std::copy would check it anyway?
> > -      const bool __can_memmove = __is_trivial(_ValueType1);
> > +      const bool __can_memcpy
> > +       = __memcpyable<_ValueType1*, _ValueType2*>::__value
> > +           && __is_trivially_constructible(_ValueType2, __decltype(*__first));
> >
> > -#if __cplusplus < 201103L
> > -      typedef typename iterator_traits<_InputIterator>::reference _From;
> > -#else
> > -      using _From = decltype(*__first);
> > +      return __uninitialized_copy<__can_memcpy>::
> > +              __uninit_copy(__first, __last, __result);
> >  #endif
> > -      const bool __assignable
> > -       = _GLIBCXX_USE_ASSIGN_FOR_INIT(_ValueType2, _From);
> > -
> > -      return std::__uninitialized_copy<__can_memmove && __assignable>::
> > -       __uninit_copy(__first, __last, __result);
> >      }
> > +#pragma GCC diagnostic pop
> >
> >    /// @cond undocumented
> >
> > +  // This is the default implementation of std::uninitialized_fill.
> >    template<typename _ForwardIterator, typename _Tp>
> >      _GLIBCXX20_CONSTEXPR void
> >      __do_uninit_fill(_ForwardIterator __first, _ForwardIterator __last,
> > @@ -244,12 +327,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >        __guard.release();
> >      }
> >
> > -  template<bool _TrivialValueType>
> > +#if __cplusplus < 201103L
> > +  // Use template specialization for C++98 when 'if constexpr' can't be used.
> > +  template<bool _CanMemset>
> >      struct __uninitialized_fill
> >      {
> >        template<typename _ForwardIterator, typename _Tp>
> > -        static void
> > -        __uninit_fill(_ForwardIterator __first, _ForwardIterator __last,
> > +       static void
> > +       __uninit_fill(_ForwardIterator __first, _ForwardIterator __last,
> >                       const _Tp& __x)
> >         { std::__do_uninit_fill(__first, __last, __x); }
> >      };
> > @@ -257,56 +342,129 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >    template<>
> >      struct __uninitialized_fill<true>
> >      {
> > +      // Overload for generic iterators.
> >        template<typename _ForwardIterator, typename _Tp>
> > -        static void
> > -        __uninit_fill(_ForwardIterator __first, _ForwardIterator __last,
> > +       static void
> > +       __uninit_fill(_ForwardIterator __first, _ForwardIterator __last,
> >                       const _Tp& __x)
> > -        { std::fill(__first, __last, __x); }
> > -    };
> > +       {
> > +         if (__unwrappable_niter<_ForwardIterator>::__value)
> > +           __uninit_fill(std::__niter_base(__first),
> > +                         std::__niter_base(__last),
> > +                         __x);
> > +         else
> > +           std::__do_uninit_copy(__first, __last, __x);
> > +       }
> >
> > +      // Overload for pointers.
> > +      template<typename _Up, typename _Tp>
> > +       static void
> > +       __uninit_fill(_Up* __first, _Up* __last, const _Tp& __x)
> > +       {
> > +         // Ensure that we don't successfully memset in cases that should be
> > +         // ill-formed because is_constructible<_Up, const _Tp&> is false.
> > +         typedef __typeof__(static_cast<_Up>(__x)) __check
> > +           __attribute__((__unused__));
> > +
> > +         if (__first != __last)
> > +           __builtin_memset(__first, (int)__x, __last - __first);
> > +       }
> > +    };
> > +#endif
> >    /// @endcond
> >
> >    /**
> >     *  @brief Copies the value x into the range [first,last).
> > -   *  @param  __first  An input iterator.
> > -   *  @param  __last   An input iterator.
> > +   *  @param  __first  A forward iterator.
> > +   *  @param  __last   A forward iterator.
> >     *  @param  __x      The source value.
> >     *  @return   Nothing.
> >     *
> > -   *  Like fill(), but does not require an initialized output range.
> > +   *  Like std::fill, but does not require an initialized output range.
> >    */
> >    template<typename _ForwardIterator, typename _Tp>
> >      inline void
> >      uninitialized_fill(_ForwardIterator __first, _ForwardIterator __last,
> >                        const _Tp& __x)
> >      {
> > +      // We would like to use memset to optimize this loop when possible.
> > +      // As for std::uninitialized_copy, the optimization requires
> > +      // contiguous iterators and trivially copyable value types,
> > +      // with the additional requirement that sizeof(_Tp) == 1 because
> > +      // memset only writes single bytes.
> > +
> > +      // FIXME: We could additionally enable this for 1-byte enums.
> > +      // Maybe any 1-byte Val if is_trivially_constructible<Val, const T&>?
> > +
> >        typedef typename iterator_traits<_ForwardIterator>::value_type
> >         _ValueType;
> >
> > -      // Trivial types do not need a constructor to begin their lifetime,
> > -      // so try to use std::fill to benefit from its memset optimization.
> > -      const bool __can_fill
> > -       = _GLIBCXX_USE_ASSIGN_FOR_INIT(_ValueType, const _Tp&);
> > +#if __cplusplus >= 201103L
> > +#pragma GCC diagnostic push
> > +#pragma GCC diagnostic ignored "-Wc++17-extensions"
> > +      if constexpr (__is_byte<_ValueType>::__value)
> > +       if constexpr (is_same<_ValueType, _Tp>::value
> > +                       || is_integral<_Tp>::value)
> > +         {
> > +           using _BasePtr = decltype(std::__niter_base(__first));
> > +           if constexpr (is_pointer<_BasePtr>::value)
> > +             {
> > +               void* __dest = std::__niter_base(__first);
> > +               ptrdiff_t __n = __last - __first;
> > +               if (__builtin_expect(__n > 1, true))
> > +                 __builtin_memset(__dest, (unsigned char)__x, __n);
> > +               return;
> > +             }
> > +#if __cpp_lib_concepts
> > +           else if constexpr (contiguous_iterator<_ForwardIterator>)
> > +             {
> > +               auto __dest = std::__to_address(__first);
> > +               auto __n = __last - __first;
> > +               if (__builtin_expect(__n > 1, true))
> > +                 __builtin_memset(__dest, (unsigned char)__x, __n);
> > +               return;
> > +             }
> > +#endif
> > +         }
> > +      std::__do_uninit_fill(__first, __last, __x);
> > +#pragma GCC diagnostic pop
> > +#else // C++98
> > +      const bool __can_memset = __is_byte<_ValueType>::__value
> > +                                 && __is_integer<_Tp>::__value;
> >
> > -      std::__uninitialized_fill<__can_fill>::
> > -       __uninit_fill(__first, __last, __x);
> > +      __uninitialized_fill<__can_memset>::__uninit_fill(__first, __last, __x);
> > +#endif
> >      }
> >
> >    /// @cond undocumented
> >
> > +  // This is the default implementation of std::uninitialized_fill_n.
> >    template<typename _ForwardIterator, typename _Size, typename _Tp>
> >      _GLIBCXX20_CONSTEXPR
> >      _ForwardIterator
> >      __do_uninit_fill_n(_ForwardIterator __first, _Size __n, const _Tp& __x)
> >      {
> >        _UninitDestroyGuard<_ForwardIterator> __guard(__first);
> > -      for (; __n > 0; --__n, (void) ++__first)
> > +#if __cplusplus >= 201103L
> > +#pragma GCC diagnostic push
> > +#pragma GCC diagnostic ignored "-Wc++17-extensions"
> > +      if constexpr (is_integral<_Size>::value)
> > +       // Loop will never terminate if __n is negative.
> > +       __glibcxx_assert(__n >= 0);
> > +      else if constexpr (is_floating_point<_Size>::value)
> > +       // Loop will never terminate if __n is not an integer.
> > +       __glibcxx_assert(__n >= 0 && static_cast<size_t>(__n) == __n);
> > +#pragma GCC diagnostic pop
> > +#endif
> > +      for (; __n--; ++__first)
> >         std::_Construct(std::__addressof(*__first), __x);
> >        __guard.release();
> >        return __first;
> >      }
> >
> > -  template<bool _TrivialValueType>
> > +#if __cplusplus < 201103L
> > +  // Use template specialization for C++98 when 'if constexpr' can't be used.
> > +  template<bool _CanMemset>
> >      struct __uninitialized_fill_n
> >      {
> >        template<typename _ForwardIterator, typename _Size, typename _Tp>
> > @@ -319,47 +477,92 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >    template<>
> >      struct __uninitialized_fill_n<true>
> >      {
> > +      // Overload for generic iterators.
> >        template<typename _ForwardIterator, typename _Size, typename _Tp>
> >         static _ForwardIterator
> >          __uninit_fill_n(_ForwardIterator __first, _Size __n,
> >                         const _Tp& __x)
> > -        { return std::fill_n(__first, __n, __x); }
> > +       {
> > +         if (__unwrappable_niter<_ForwardIterator>::__value)
> > +           {
> > +             _ForwardIterator __last = __first;
> > +             std::advance(__last, __n);
> > +             __uninitialized_fill<true>::__uninit_fill(__first, __last, __x);
> > +             return __last;
> > +           }
> > +         else
> > +           return std::__do_uninit_fill_n(__first, __n, __x);
> > +       }
> >      };
> > -
> > +#endif
> >    /// @endcond
> >
> > +#pragma GCC diagnostic push
> > +#pragma GCC diagnostic ignored "-Wc++17-extensions"
> >     // _GLIBCXX_RESOLVE_LIB_DEFECTS
> >     // DR 1339. uninitialized_fill_n should return the end of its range
> >    /**
> >     *  @brief Copies the value x into the range [first,first+n).
> > -   *  @param  __first  An input iterator.
> > +   *  @param  __first  A forward iterator.
> >     *  @param  __n      The number of copies to make.
> >     *  @param  __x      The source value.
> > -   *  @return   Nothing.
> > +   *  @return   __first + __n.
> >     *
> > -   *  Like fill_n(), but does not require an initialized output range.
> > +   *  Like std::fill_n, but does not require an initialized output range.
> >    */
> >    template<typename _ForwardIterator, typename _Size, typename _Tp>
> >      inline _ForwardIterator
> >      uninitialized_fill_n(_ForwardIterator __first, _Size __n, const _Tp& __x)
> >      {
> > +      // See uninitialized_fill conditions. We also require _Size to be
> > +      // an integer. The standard only requires _Size to be decrementable
> > +      // and contextually convertible to bool, so don't assume first+n works.
> > +
> > +      // FIXME: We could additionally enable this for 1-byte enums.
> > +
> >        typedef typename iterator_traits<_ForwardIterator>::value_type
> >         _ValueType;
> >
> > -      // Trivial types do not need a constructor to begin their lifetime,
> > -      // so try to use std::fill_n to benefit from its optimizations.
> > -      const bool __can_fill
> > -       = _GLIBCXX_USE_ASSIGN_FOR_INIT(_ValueType, const _Tp&)
> > -      // For arbitrary class types and floating point types we can't assume
> > -      // that __n > 0 and std::__size_to_integer(__n) > 0 are equivalent,
> > -      // so only use std::fill_n when _Size is already an integral type.
> > -       && __is_integer<_Size>::__value;
> > +#if __cplusplus >= 201103L
> > +      if constexpr (__is_byte<_ValueType>::__value)
> > +       if constexpr (is_integral<_Tp>::value)
> > +         if constexpr (is_integral<_Size>::value)
> > +           {
> > +             using _BasePtr = decltype(std::__niter_base(__first));
> > +             if constexpr (is_pointer<_BasePtr>::value)
> > +               {
> > +                 void* __dest = std::__niter_base(__first);
> > +                 if (__builtin_expect(__n > 1, true))
> > +                   {
> > +                     __builtin_memset(__dest, (unsigned char)__x, __n);
> > +                     __first += __n;
> > +                   }
> > +                 return __first;
> > +               }
> > +#if __cpp_lib_concepts
> > +             else if constexpr (contiguous_iterator<_ForwardIterator>)
> > +               {
> > +                 auto __dest = std::__to_address(__first);
> > +                 if (__builtin_expect(__n > 1, true))
> > +                   {
> > +                     __builtin_memset(__dest, (unsigned char)__x, __n);
> > +                     __first += __n;
> > +                   }
> > +                 return __first;
> > +               }
> > +#endif
> > +           }
> > +      return std::__do_uninit_fill_n(__first, __n, __x);
> > +#else // C++98
> > +      const bool __can_memset = __is_byte<_ValueType>::__value
> > +                                 && __is_integer<_Tp>::__value
> > +                                 && __is_integer<_Size>::__value;
> >
> > -      return __uninitialized_fill_n<__can_fill>::
> > +      return __uninitialized_fill_n<__can_memset>::
> >         __uninit_fill_n(__first, __n, __x);
> > +#endif
> >      }
> > -
> > -#undef _GLIBCXX_USE_ASSIGN_FOR_INIT
> > +#pragma GCC diagnostic pop
> >
> >    /// @cond undocumented
> >
> > @@ -619,7 +822,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >             = std::__addressof(*__first);
> >           std::_Construct(__val);
> >           if (++__first != __last)
> > -           std::fill(__first, __last, *__val);
> > +           std::uninitialized_fill(__first, __last, *__val);
> >         }
> >      };
> >
> > @@ -653,7 +856,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >                 = std::__addressof(*__first);
> >               std::_Construct(__val);
> >               ++__first;
> > -             __first = std::fill_n(__first, __n - 1, *__val);
> > +             __first = std::uninitialized_fill_n(__first, __n - 1, *__val);
> >             }
> >           return __first;
> >         }
> > diff --git a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/1.cc b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/1.cc
> > index 398d8690b56..27b3100d362 100644
> > --- a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/1.cc
> > +++ b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/1.cc
> > @@ -34,4 +34,5 @@ test01(T* result)
> >    T t[1];
> >    std::uninitialized_copy(t, t+1, result); // { dg-error "here" }
> >  }
> > -// { dg-error "must be constructible from input type" "" { target *-*-* } 0 }
> > +// { dg-error "no matching function" "construct_at" { target c++20 } 0 }
> > +// { dg-error "use of deleted function" "T::T(const T&)" { target *-*-* } 0 }
> > diff --git a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/64476.cc b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/64476.cc
> > index 2f7dda3417d..e99338dff39 100644
> > --- a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/64476.cc
> > +++ b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/64476.cc
> > @@ -54,8 +54,10 @@ test01()
> >
> >    std::uninitialized_copy(a, a+10, b);
> >
> > -  VERIFY(constructed == 0);
> > -  VERIFY(assigned == 10);
> > +  // In GCC 14 and older std::uninitialized_copy was optimized to std::copy
> > +  // and so used assignments not construction, but that was non-conforming.
> > +  VERIFY(constructed == 10);
> > +  VERIFY(assigned == 0);
> >  }
> >
> >  int
> > diff --git a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/89164.cc b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/89164.cc
> > index 48c16da4d32..6e978a7e36c 100644
> > --- a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/89164.cc
> > +++ b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy/89164.cc
> > @@ -35,4 +35,5 @@ void test01()
> >
> >    std::uninitialized_copy(x, x+1, p); // { dg-error "here" }
> >  }
> > -// { dg-error "must be constructible" "" { target *-*-* } 0 }
> > +// { dg-error "no matching function" "construct_at" { target c++20 } 0 }
> > +// { dg-error "use of deleted function" "X(const X&)" { target *-*-* } 0 }
> > diff --git a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy_n/89164.cc b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy_n/89164.cc
> > index 4e8fb0f4af2..96156208372 100644
> > --- a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy_n/89164.cc
> > +++ b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_copy_n/89164.cc
> > @@ -32,4 +32,5 @@ void test01()
> >
> >    std::uninitialized_copy_n(x, 1, p); // { dg-error "here" }
> >  }
> > -// { dg-error "must be constructible" "" { target *-*-* } 0 }
> > +// { dg-error "no matching function" "construct_at" { target c++20 } 0 }
> > +// { dg-error "use of deleted function" "X(const X&)" { target *-*-* } 0 }
> > diff --git a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill/89164.cc b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill/89164.cc
> > index 8353b5882f0..0dcaa1aa9c3 100644
> > --- a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill/89164.cc
> > +++ b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill/89164.cc
> > @@ -32,4 +32,5 @@ void f()
> >
> >    std::uninitialized_fill(p, p+1, x); // { dg-error "here" }
> >  }
> > -// { dg-error "must be constructible" "" { target *-*-* } 0 }
> > +// { dg-error "no matching function" "construct_at" { target c++20 } 0 }
> > +// { dg-error "use of deleted function" "X(const X&)" { target *-*-* } 0 }
> > diff --git a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill_n/89164.cc b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill_n/89164.cc
> > index 4b38c673d32..9b61157b934 100644
> > --- a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill_n/89164.cc
> > +++ b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill_n/89164.cc
> > @@ -32,4 +32,5 @@ void test01()
> >
> >    std::uninitialized_fill_n(p, 1, x); // { dg-error "here" }
> >  }
> > -// { dg-error "must be constructible" "" { target *-*-* } 0 }
> > +// { dg-error "no matching function" "construct_at" { target c++20 } 0 }
> > +// { dg-error "use of deleted function" "X(const X&)" { target *-*-* } 0 }
> > diff --git a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill_n/sizes.cc b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill_n/sizes.cc
> > index e2ba9355c56..876ec5443fb 100644
> > --- a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill_n/sizes.cc
> > +++ b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_fill_n/sizes.cc
> > @@ -24,21 +24,35 @@ void
> >  test01()
> >  {
> >    int i[4] = { };
> > -  std::uninitialized_fill_n(i, 2.0001, 0xabcd);
> > +  // Floating-point n should work, but only if it's an integer value.
> > +  std::uninitialized_fill_n(i, 3.0, 0xabcd);
> >    VERIFY( i[0] == 0xabcd );
> >    VERIFY( i[1] == 0xabcd );
> >    VERIFY( i[2] == 0xabcd );
> >    VERIFY( i[3] == 0 );
> >  }
> >
> > -// The standard only requires that n>0 and --n are valid expressions.
> > +// The standard only requires that `if (n--)` is a valid expression.
> >  struct Size
> >  {
> >    int value;
> >
> > -  void operator--() { --value; }
> > +  struct testable
> > +  {
> > +#if __cplusplus >= 201103L
> > +    explicit
> > +#endif
> > +    operator bool() const { return nonzero; }
> >
> > -  int operator>(void*) { return value != 0; }
> > +    bool nonzero;
> > +  };
> > +
> > +  testable operator--(int)
> > +  {
> > +    testable t = { value != 0 };
> > +    --value;
> > +    return t;
> > +  }
> >  };
> >
> >  void
> > diff --git a/libstdc++-v3/testsuite/23_containers/vector/cons/89164.cc b/libstdc++-v3/testsuite/23_containers/vector/cons/89164.cc
> > index 106963ecbb9..36907dc508e 100644
> > --- a/libstdc++-v3/testsuite/23_containers/vector/cons/89164.cc
> > +++ b/libstdc++-v3/testsuite/23_containers/vector/cons/89164.cc
> > @@ -32,7 +32,7 @@ void test01()
> >    X x[1];
> >    // Should not be able to create vector using uninitialized_copy:
> >    std::vector<X> v1{x, x+1};   // { dg-error "here" "" { target c++17_down } }
> > -  // { dg-error "deleted function 'X::X" "" { target c++20 } 0 }
> > +  // { dg-error "deleted function 'X::X" "" { target *-*-* } 0 }
> >  }
> >
> >  void test02()
> > @@ -41,8 +41,7 @@ void test02()
> >
> >    // Should not be able to create vector using uninitialized_fill_n:
> >    std::vector<Y> v2{2u, Y{}};  // { dg-error "here" "" { target c++17_down } }
> > -  // { dg-error "deleted function .*Y::Y" "" { target c++20 } 0 }
> > +  // { dg-error "deleted function .*Y::Y" "" { target *-*-* } 0 }
> >  }
> >
> > -// { dg-error "must be constructible from input type" "" { target *-*-* } 0 }
> >  // { dg-prune-output "construct_at" }
> > diff --git a/libstdc++-v3/testsuite/23_containers/vector/cons/89164_c++17.cc b/libstdc++-v3/testsuite/23_containers/vector/cons/89164_c++17.cc
> > index 09d3dc6f93d..07d4bab9117 100644
> > --- a/libstdc++-v3/testsuite/23_containers/vector/cons/89164_c++17.cc
> > +++ b/libstdc++-v3/testsuite/23_containers/vector/cons/89164_c++17.cc
> > @@ -32,8 +32,7 @@ void test03()
> >    // Can create initializer_list<Y> with C++17 guaranteed copy elision,
> >    // but shouldn't be able to copy from it with uninitialized_copy:
> >    std::vector<X> v3{X{}, X{}, X{}};   // { dg-error "here" "" { target c++17_only } }
> > -  // { dg-error "deleted function .*X::X" "" { target c++20 } 0 }
> > +  // { dg-error "deleted function .*X::X" "" { target *-*-* } 0 }
> >  }
> >
> > -// { dg-error "must be constructible from input type" "" { target *-*-* } 0 }
> >  // { dg-prune-output "construct_at" }
> > --
> > 2.46.2
> >