On Mon, 16 Feb 2026 at 17:59, Avi Kivity <[email protected]> wrote:
>
> std::uninitialized_move{,_n} delegates to the corresponding
> std::uninitialized_copy() variant after wrapping with a move
> iterator, but the std::uninitialized_copy() doesn't unwrap the
> move iterator, therefore losing the memcpy optimization if the
> iterators were just pointers.
>
> Fix this by unwrapping the move iterator using __miter_base(). To
> prevent some greedy_ops tests starting to fail by consuming the
> pointer difference, we have to unwrap the move iterator before computing
> the iterator difference as well.
>
> libstdc++v3/Changelog:
>
> PR libstdc++/121789
> * include/bits/stl_uninitialized.h (uninitialized_copy):
> Unwrap move iterators
> *
> testsuite/20_util/specialized_algorithms/uninitialized_move/121789.cc:
> New test.
> ---
>
> Disclosure: I was assisted by AI.
>
> libstdc++-v3/include/bits/stl_uninitialized.h | 8 ++--
> .../uninitialized_move/121789.cc | 37 +++++++++++++++++++
> 2 files changed, 42 insertions(+), 3 deletions(-)
> create mode 100644
> libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_move/121789.cc
>
> diff --git a/libstdc++-v3/include/bits/stl_uninitialized.h
> b/libstdc++-v3/include/bits/stl_uninitialized.h
> index 82e4ba2ff7b..fa9d896fefc 100644
> --- a/libstdc++-v3/include/bits/stl_uninitialized.h
> +++ b/libstdc++-v3/include/bits/stl_uninitialized.h
> @@ -273,11 +273,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> // 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 _Src = decltype(std::__miter_base(std::__niter_base(__first)));
> using _ValT = typename iterator_traits<_ForwardIterator>::value_type;
>
> #if __glibcxx_raw_memory_algorithms >= 202411L // >= C++26
> if consteval {
> return std::__do_uninit_copy(__first, __last, __result);
> @@ -285,16 +285,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> #endif
> 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;
> + ptrdiff_t __n
> + = std::__miter_base(std::__niter_base(__last))
> + - std::__miter_base(std::__niter_base(__first));
This change seems completely unnecessary (I blame the AI assistance).
> if (__n > 0) [[__likely__]]
> {
> using _ValT = typename remove_pointer<_Src>::type;
> __builtin_memcpy(std::__niter_base(__result),
> - std::__niter_base(__first),
> + std::__miter_base(std::__niter_base(__first)),
> __n * sizeof(_ValT));
> __result += __n;
> }
> return __result;
> }
> diff --git
> a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_move/121789.cc
>
> b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_move/121789.cc
> new file mode 100644
> index 00000000000..7ac8cf6496f
> --- /dev/null
> +++
> b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_move/121789.cc
> @@ -0,0 +1,37 @@
> +// Copyright (C) 2025-2026 Free Software Foundation, Inc.
There should not be any copyright/license header here, see
https://gcc.gnu.org/onlinedocs/libstdc++/manual/test.html#test.new_tests
> +//
> +// This file is part of the GNU ISO C++ Library. This library is free
> +// software; you can redistribute it and/or modify it under the
> +// terms of the GNU General Public License as published by the
> +// Free Software Foundation; either version 3, or (at your option)
> +// any later version.
> +
> +// This library is distributed in the hope that it will be useful,
> +// but WITHOUT ANY WARRANTY; without even the implied warranty of
> +// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> +// GNU General Public License for more details.
> +
> +// You should have received a copy of the GNU General Public License along
> +// with this library; see the file COPYING3. If not see
> +// <http://www.gnu.org/licenses/>.
> +
> +// { dg-options "-O1 -fdump-tree-optimized" }
> +// { dg-do compile { target c++17 } }
> +// { dg-final { scan-tree-dump "memcpy" "optimized" } }
> +
> +// PR libstdc++/121789
> +// std::uninitialized_move_n() and friends don't optimize to memcpy
> +
> +#include <memory>
> +
> +struct T { int x; };
> +
> +void f(T* src, T* dst, unsigned n)
> +{
> + std::uninitialized_move(src, src + n, dst);
> +}
> +
> +void g(T* src, T* dst, unsigned n)
> +{
> + std::uninitialized_move_n(src, n, dst);
> +}
> --
> 2.53.0
>
