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| author | Connector Switch <c8ef@outlook.com> | 2025-10-21 18:01:36 +0800 |
|---|---|---|
| committer | GitHub <noreply@github.com> | 2025-10-21 12:01:36 +0200 |
| commit | c06ae43e26aa5cd472d0b25d5904c44d20e84067 (patch) | |
| tree | 18994c9c7f4454554c4692d59ec9e2da4ddeb409 /libcxx/include/__algorithm | |
| parent | 2eb9251481e1e749314694737600f40a07054be0 (diff) | |
[libcxx] Optimize `std::generate_n` for segmented iterators (#164266)
Part of #102817.
This is a natural follow-up to #163006. We are forwarding
`std::generate_n` to `std::__for_each_n` (`std::for_each_n` needs
c++17), resulting in improved performance for segmented iterators.
before:
```
std::generate_n(deque<int>)/32 17.5 ns 17.3 ns 40727273
std::generate_n(deque<int>)/50 25.7 ns 25.5 ns 26352941
std::generate_n(deque<int>)/1024 490 ns 487 ns 1445161
std::generate_n(deque<int>)/8192 3908 ns 3924 ns 179200
```
after:
```
std::generate_n(deque<int>)/32 11.1 ns 11.0 ns 64000000
std::generate_n(deque<int>)/50 16.1 ns 16.0 ns 44800000
std::generate_n(deque<int>)/1024 291 ns 292 ns 2357895
std::generate_n(deque<int>)/8192 2269 ns 2250 ns 298667
```
Diffstat (limited to 'libcxx/include/__algorithm')
| -rw-r--r-- | libcxx/include/__algorithm/generate_n.h | 13 |
1 files changed, 7 insertions, 6 deletions
diff --git a/libcxx/include/__algorithm/generate_n.h b/libcxx/include/__algorithm/generate_n.h index f36403fd0f94..e9da133f0570 100644 --- a/libcxx/include/__algorithm/generate_n.h +++ b/libcxx/include/__algorithm/generate_n.h @@ -9,8 +9,10 @@ #ifndef _LIBCPP___ALGORITHM_GENERATE_N_H #define _LIBCPP___ALGORITHM_GENERATE_N_H +#include <__algorithm/for_each_n.h> #include <__config> -#include <__utility/convert_to_integral.h> +#include <__functional/identity.h> +#include <__utility/forward.h> #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) # pragma GCC system_header @@ -21,11 +23,10 @@ _LIBCPP_BEGIN_NAMESPACE_STD template <class _OutputIterator, class _Size, class _Generator> inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutputIterator generate_n(_OutputIterator __first, _Size __orig_n, _Generator __gen) { - typedef decltype(std::__convert_to_integral(__orig_n)) _IntegralSize; - _IntegralSize __n = __orig_n; - for (; __n > 0; ++__first, (void)--__n) - *__first = __gen(); - return __first; + using __iter_ref = decltype(*__first); + __identity __proj; + auto __f = [&](__iter_ref __element) { std::forward<__iter_ref>(__element) = __gen(); }; + return std::__for_each_n(__first, __orig_n, __f, __proj); } _LIBCPP_END_NAMESPACE_STD |