[libcxx] Optimize std::generate for segmented iterators

libcxx/include/__algorithm/generate.h

@@ -9,6 +9,7 @@
 #ifndef _LIBCPP___ALGORITHM_GENERATE_H
 #define _LIBCPP___ALGORITHM_GENERATE_H
 
+#include <__algorithm/for_each.h>
 #include <__config>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -20,8 +21,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 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, [&](auto& __element) { __element = __gen(); });
 }
 
 _LIBCPP_END_NAMESPACE_STD

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());