[libcxx] Optimize std::generate for segmented iterators

libcxx/include/__algorithm/generate.h

@@ -9,21 +9,44 @@
 #ifndef _LIBCPP___ALGORITHM_GENERATE_H
 #define _LIBCPP___ALGORITHM_GENERATE_H
 
+#include <__algorithm/for_each_segment.h>
 #include <__config>
+#include <__iterator/segmented_iterator.h>
+#include <__type_traits/enable_if.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class _ForwardIterator, class _Generator>
+template <class _ForwardIterator, class _Sent, class _Generator>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
-generate(_ForwardIterator __first, _ForwardIterator __last, _Generator __gen) {
+__generate(_ForwardIterator __first, _Sent __last, _Generator& __gen) {
   for (; __first != __last; ++__first)
     *__first = __gen();
 }
 
+#ifndef _LIBCPP_CXX03_LANG
+template <class _SegmentedIterator,
+          class _Generator,
+          __enable_if_t<__is_segmented_iterator_v<_SegmentedIterator>, int> = 0>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
+_SegmentedIterator __generate(_SegmentedIterator __first, _SegmentedIterator __last, _Generator& __gen) {
+  using __local_iterator_t = typename __segmented_iterator_traits<_SegmentedIterator>::__local_iterator;
+  std::__for_each_segment(__first, __last, [&](__local_iterator_t __lfirst, __local_iterator_t __llast) {
+    std::__generate(__lfirst, __llast, __gen);
+  });
+  return __last;
+}
+#endif // !_LIBCPP_CXX03_LANG
+
+template <class _ForwardIterator, class _Generator>
+inline _LIBCPP_HIDE_FROM_ABI
+_LIBCPP_CONSTEXPR_SINCE_CXX20 void generate(_ForwardIterator __first, _ForwardIterator __last, _Generator __gen) {
+  std::__generate(__first, __last, __gen);
+}
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___ALGORITHM_GENERATE_H

libcxx/test/std/algorithms/alg.modifying.operations/alg.generate/generate.pass.cpp

@@ -16,6 +16,7 @@
 
 #include <algorithm>
 #include <cassert>
+#include <deque>
 
 #include "test_macros.h"
 #include "test_iterators.h"
@@ -51,12 +52,22 @@ test()
     assert(ia[3] == 1);
 }
 
+void deque_test() {
+  int sizes[] = {0, 1, 2, 1023, 1024, 1025, 2047, 2048, 2049};
+  for (const int size : sizes) {
+    std::deque<int> d(size);
+    std::generate(d.begin(), d.end(), gen_test());
+    assert(std::all_of(d.begin(), d.end(), [](int x) { return x == 1; }));
+  }
+}
+
 int main(int, char**)
 {
     test<forward_iterator<int*> >();
     test<bidirectional_iterator<int*> >();
     test<random_access_iterator<int*> >();
     test<int*>();
+    deque_test();
 
 #if TEST_STD_VER > 17
     static_assert(test_constexpr());