[libcxx] Optimize std::generate for segmented iterators (llvm#163006)

Part of llvm#102817.

This patch attempts to optimize the performance of `std::generate` for
segmented iterators. Below are the benchmark numbers from
`libcxx\test\benchmarks\algorithms\modifying\generate.bench.cpp`. Test
cases that use segmented iterators have also been added.

- before

```
std::generate(deque<int>)/32           194 ns          193 ns      3733333
std::generate(deque<int>)/50           276 ns          276 ns      2488889
std::generate(deque<int>)/1024        5096 ns         5022 ns       112000
std::generate(deque<int>)/8192       40806 ns        40806 ns        17231
```

- after

```
std::generate(deque<int>)/32           106 ns          105 ns      6400000
std::generate(deque<int>)/50           139 ns          138 ns      4977778
std::generate(deque<int>)/1024        2713 ns         2699 ns       248889
std::generate(deque<int>)/8192       18983 ns        19252 ns        37333
```

---------

Co-authored-by: A. Jiang <[email protected]>

Author: c8ef

libcxx/docs/ReleaseNotes/22.rst

@@ -75,6 +75,8 @@ Improvements and New Features
 - The ``std::{fill, fill_n}`` and ``std::ranges::{fill, fill_n}`` algorithms have been optimized for segmented iterators,
   resulting in a performance improvement of at least 10x for ``std::deque<int>`` iterators and
   ``std::join_view<std::vector<std::vector<int>>>`` iterators.
+- The ``std::generate`` algorithm has been optimized for segmented iterators, resulting in a performance improvement for
+  ``std::deque<short>`` and ``std::join_view<vector<vector<short>>>`` iterators.
 
 Deprecations and Removals
 -------------------------

libcxx/include/__algorithm/generate.h

@@ -9,7 +9,9 @@
 #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
@@ -20,8 +22,8 @@ _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();
+  using __iter_ref = decltype(*__first);
+  std::for_each(__first, __last, [&](__iter_ref __element) { std::forward<__iter_ref>(__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());