On Fri, 9 Jun 2023, Jonathan Wakely via Libstdc++ wrote:
> Tested powerpc64le-linux. Pushed to trunk.
>
> This makes sense to backport after some soak time on trunk.
>
> -- >8 --
>
> As reported in PR libstdc++/110167, std::to_array compiles extremely
> slowly for very large arrays. It needs to instantiate a very large
> specialization of std::index_sequence<N...> and then create a very large
> aggregate initializer from the pack expansion. For trivial types we can
> simply default-initialize the std::array and then use memcpy to copy the
> values. For non-trivial types we need to use the existing
> implementation, despite the compilation cost.
>
> As also noted in the PR, using a generic lambda instead of the
> __to_array helper compiles faster since gcc-13. It also produces
> slightly smaller code at -O1, due to additional inlining. The code at
> -Os, -O2 and -O3 seems to be the same. This new implementation requires
> __cpp_generic_lambdas >= 201707L (i.e. P0428R2) but that is supported
> since Clang 10 and since Intel icc 2021.5.0 (and since GCC 10.1).
>
> libstdc++-v3/ChangeLog:
>
> PR libstdc++/110167
> * include/std/array (to_array): Initialize arrays of trivial
> types using memcpy. For non-trivial types, use lambda
> expressions instead of a separate helper function.
> (__to_array): Remove.
> * testsuite/23_containers/array/creation/110167.cc: New test.
> ---
> libstdc++-v3/include/std/array | 53 +++++++++++++------
> .../23_containers/array/creation/110167.cc | 14 +++++
> 2 files changed, 51 insertions(+), 16 deletions(-)
> create mode 100644
> libstdc++-v3/testsuite/23_containers/array/creation/110167.cc
>
> diff --git a/libstdc++-v3/include/std/array b/libstdc++-v3/include/std/array
> index 70280c1beeb..b791d86ddb2 100644
> --- a/libstdc++-v3/include/std/array
> +++ b/libstdc++-v3/include/std/array
> @@ -414,19 +414,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> return std::move(std::get<_Int>(__arr));
> }
>
> -#if __cplusplus > 201703L
> +#if __cplusplus >= 202002L && __cpp_generic_lambdas >= 201707L
> #define __cpp_lib_to_array 201907L
> -
> - template<bool _Move = false, typename _Tp, size_t... _Idx>
> - constexpr array<remove_cv_t<_Tp>, sizeof...(_Idx)>
> - __to_array(_Tp (&__a)[sizeof...(_Idx)], index_sequence<_Idx...>)
> - {
> - if constexpr (_Move)
> - return {{std::move(__a[_Idx])...}};
> - else
> - return {{__a[_Idx]...}};
> - }
> -
> template<typename _Tp, size_t _Nm>
> [[nodiscard]]
> constexpr array<remove_cv_t<_Tp>, _Nm>
> @@ -436,8 +425,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> static_assert(!is_array_v<_Tp>);
> static_assert(is_constructible_v<_Tp, _Tp&>);
> if constexpr (is_constructible_v<_Tp, _Tp&>)
> - return __to_array(__a, make_index_sequence<_Nm>{});
> - __builtin_unreachable(); // FIXME: see PR c++/91388
> + {
> + if constexpr (is_trivial_v<_Tp> && _Nm != 0)
redundant _Nm != 0 test?
> + {
> + array<remove_cv_t<_Tp>, _Nm> __arr;
> + if (!__is_constant_evaluated() && _Nm != 0)
> + __builtin_memcpy(__arr.data(), __a, sizeof(__a));
> + else
> + for (size_t __i = 0; __i < _Nm; ++__i)
> + __arr._M_elems[__i] = __a[__i];
> + return __arr;
> + }
> + else
> + return [&__a]<size_t... _Idx>(index_sequence<_Idx...>) {
> + return array<remove_cv_t<_Tp>, _Nm>{{ __a[_Idx]... }};
> + }(make_index_sequence<_Nm>{});
> + }
> + else
> + __builtin_unreachable(); // FIXME: see PR c++/91388
> }
>
> template<typename _Tp, size_t _Nm>
> @@ -449,8 +454,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> static_assert(!is_array_v<_Tp>);
> static_assert(is_move_constructible_v<_Tp>);
> if constexpr (is_move_constructible_v<_Tp>)
> - return __to_array<1>(__a, make_index_sequence<_Nm>{});
> - __builtin_unreachable(); // FIXME: see PR c++/91388
> + {
> + if constexpr (is_trivial_v<_Tp>)
> + {
> + array<remove_cv_t<_Tp>, _Nm> __arr;
> + if (!__is_constant_evaluated() && _Nm != 0)
> + __builtin_memcpy(__arr.data(), __a, sizeof(__a));
> + else
> + for (size_t __i = 0; __i < _Nm; ++__i)
> + __arr._M_elems[__i] = std::move(__a[__i]);
IIUC this std::move is unnecessary for trivial arrays?
> + return __arr;
> + }
> + else
> + return [&__a]<size_t... _Idx>(index_sequence<_Idx...>) {
> + return array<remove_cv_t<_Tp>, _Nm>{{ std::move(__a[_Idx])... }};
> + }(make_index_sequence<_Nm>{});
> + }
> + else
> + __builtin_unreachable(); // FIXME: see PR c++/91388
> }
> #endif // C++20
>
> diff --git a/libstdc++-v3/testsuite/23_containers/array/creation/110167.cc
> b/libstdc++-v3/testsuite/23_containers/array/creation/110167.cc
> new file mode 100644
> index 00000000000..c2aecc911bd
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/23_containers/array/creation/110167.cc
> @@ -0,0 +1,14 @@
> +// { dg-options "-std=gnu++20" }
> +// { dg-do compile { target c++20 } }
> +
> +// PR libstdc++/110167 - excessive compile time when optimizing std::to_array
> +
> +#include <array>
> +
> +constexpr int N = 512 * 512;
> +
> +std::array<int, N>
> +make_std_array(int (&a)[N])
> +{
> + return std::to_array(a);
> +}
> --
> 2.40.1
>
>
