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)
+ {
+ 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]);
+ 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
