[libcxx-commits] [libcxx] [libcxx] Optimize std::generate for segmented iterators (PR #163006)

[A. Jiang via libcxx-commits]

================
@@ -9,19 +9,36 @@
 #ifndef _LIBCPP___ALGORITHM_GENERATE_H
 #define _LIBCPP___ALGORITHM_GENERATE_H
 
+#include <__algorithm/for_each.h>
 #include <__config>
+#include <__utility/forward.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+namespace __generate {
+
+template <typename _Generator>
+struct __fn {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __fn(_Generator& __gen) : __gen_(__gen) {}
+
+  template <typename _Tp>
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void operator()(_Tp&& __element) const {
+    std::forward<_Tp>(__element) = __gen_();
+  }
+
+  _Generator& __gen_;
+};
+
+} // namespace __generate
+
 template <class _ForwardIterator, class _Generator>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
 generate(_ForwardIterator __first, _ForwardIterator __last, _Generator __gen) {
-  for (; __first != __last; ++__first)
-    *__first = __gen();
+  std::for_each(__first, __last, __generate::__fn<_Generator>(__gen));
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frederick-vs-ja wrote:

Given the wrapper is never used in a way involving ADL, I think it's clearer to avoid introducing a nested namespace. How about renaming it to `__generate_assigner`?

https://github.com/llvm/llvm-project/pull/163006