[libcxx-commits] [libcxx] [libc++] Merge the segmented iterator code for {copy, move}_backward (PR #165160)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 created https://github.com/llvm/llvm-project/pull/165160

This removes a bit of code duplication and might simplify future segmented iterator optimitations. 


>From ae7d2eb6038cb69e3f1a279c8f0a85d266e36dbe Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Sun, 26 Oct 2025 17:00:09 +0100
Subject: [PATCH] [libc++] Merge the segmented iterator code for
 {copy,move}_backward

---
 libcxx/include/__algorithm/copy_backward.h    | 25 +++---------------
 libcxx/include/__algorithm/for_each_segment.h | 26 +++++++++++++++++++
 libcxx/include/__algorithm/move_backward.h    | 26 ++++---------------
 3 files changed, 35 insertions(+), 42 deletions(-)

diff --git a/libcxx/include/__algorithm/copy_backward.h b/libcxx/include/__algorithm/copy_backward.h
index 6c9eba672e154..7ca60814c41ab 100644
--- a/libcxx/include/__algorithm/copy_backward.h
+++ b/libcxx/include/__algorithm/copy_backward.h
@@ -173,27 +173,10 @@ struct __copy_backward_impl {
   template <class _InIter, class _OutIter, __enable_if_t<__is_segmented_iterator_v<_InIter>, int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 pair<_InIter, _OutIter>
   operator()(_InIter __first, _InIter __last, _OutIter __result) const {
-    using _Traits = __segmented_iterator_traits<_InIter>;
-    auto __sfirst = _Traits::__segment(__first);
-    auto __slast  = _Traits::__segment(__last);
-    if (__sfirst == __slast) {
-      auto __iters =
-          std::__copy_backward<_AlgPolicy>(_Traits::__local(__first), _Traits::__local(__last), std::move(__result));
-      return std::make_pair(__last, __iters.second);
-    }
-
-    __result =
-        std::__copy_backward<_AlgPolicy>(_Traits::__begin(__slast), _Traits::__local(__last), std::move(__result))
-            .second;
-    --__slast;
-    while (__sfirst != __slast) {
-      __result =
-          std::__copy_backward<_AlgPolicy>(_Traits::__begin(__slast), _Traits::__end(__slast), std::move(__result))
-              .second;
-      --__slast;
-    }
-    __result = std::__copy_backward<_AlgPolicy>(_Traits::__local(__first), _Traits::__end(__slast), std::move(__result))
-                   .second;
+    using __local_iterator = typename __segmented_iterator_traits<_InIter>::__local_iterator;
+    std::__for_each_segment_backward(__first, __last, [&__result](__local_iterator __lfirst, __local_iterator __llast) {
+      __result = std::__copy_backward<_AlgPolicy>(std::move(__lfirst), std::move(__llast), std::move(__result));
+    });
     return std::make_pair(__last, std::move(__result));
   }
 
diff --git a/libcxx/include/__algorithm/for_each_segment.h b/libcxx/include/__algorithm/for_each_segment.h
index 93aa8259b2f7f..de0ed243d7acc 100644
--- a/libcxx/include/__algorithm/for_each_segment.h
+++ b/libcxx/include/__algorithm/for_each_segment.h
@@ -48,6 +48,32 @@ __for_each_segment(_SegmentedIterator __first, _SegmentedIterator __last, _Funct
   __func(_Traits::__begin(__sfirst), _Traits::__local(__last));
 }
 
+template <class _SegmentedIterator, class _Functor>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _SegmentedIterator
+__for_each_segment_backward(_SegmentedIterator __first, _SegmentedIterator __last, _Functor __func) {
+  using _Traits = __segmented_iterator_traits<_SegmentedIterator>;
+
+  auto __sfirst = _Traits::__segment(__first);
+  auto __slast = _Traits::__segment(__last);
+
+  // We are in a single segment, so we might not be at the beginning or end
+  if (__sfirst == __slast) {
+    __func(_Traits::__local(__first), _Traits::__local(__last));
+    return;
+  }
+
+  // We have more than one segment. Iterate over the last segment, since we might not start at the end
+  __func(_Traits::__begin(__slast), _Traits::__local(__last));
+  --__slast;
+  // iterate over the segments which are guaranteed to be completely in the range
+  while (__sfirst != __slast) {
+    __func(_Traits::__begin(__slast), _Traits::__end(__slast));
+    --__slast;
+  }
+  // iterate over the first segment
+  __func(_Traits::__local(__first), _Traits::__end(__slast));
+}
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___ALGORITHM_FOR_EACH_SEGMENT_H
diff --git a/libcxx/include/__algorithm/move_backward.h b/libcxx/include/__algorithm/move_backward.h
index a4698327b474d..d7bc88975ce93 100644
--- a/libcxx/include/__algorithm/move_backward.h
+++ b/libcxx/include/__algorithm/move_backward.h
@@ -11,6 +11,7 @@
 
 #include <__algorithm/copy_backward.h>
 #include <__algorithm/copy_move_common.h>
+#include <__algorithm/for_each_segment.h>
 #include <__algorithm/iterator_operations.h>
 #include <__algorithm/min.h>
 #include <__config>
@@ -54,27 +55,10 @@ struct __move_backward_impl {
   template <class _InIter, class _OutIter, __enable_if_t<__is_segmented_iterator_v<_InIter>, int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 pair<_InIter, _OutIter>
   operator()(_InIter __first, _InIter __last, _OutIter __result) const {
-    using _Traits = __segmented_iterator_traits<_InIter>;
-    auto __sfirst = _Traits::__segment(__first);
-    auto __slast  = _Traits::__segment(__last);
-    if (__sfirst == __slast) {
-      auto __iters =
-          std::__move_backward<_AlgPolicy>(_Traits::__local(__first), _Traits::__local(__last), std::move(__result));
-      return std::make_pair(__last, __iters.second);
-    }
-
-    __result =
-        std::__move_backward<_AlgPolicy>(_Traits::__begin(__slast), _Traits::__local(__last), std::move(__result))
-            .second;
-    --__slast;
-    while (__sfirst != __slast) {
-      __result =
-          std::__move_backward<_AlgPolicy>(_Traits::__begin(__slast), _Traits::__end(__slast), std::move(__result))
-              .second;
-      --__slast;
-    }
-    __result = std::__move_backward<_AlgPolicy>(_Traits::__local(__first), _Traits::__end(__slast), std::move(__result))
-                   .second;
+    using __local_iterator = typename __segmented_iterator_traits<_InIter>::__local_iterator;
+    std::__for_each_segment_backward(__first, __last, [&__result](__local_iterator __lfirst, __local_iterator __llast) {
+      __result = std::__move_backward<_AlgPolicy>(std::move(__lfirst), std::move(__llast), std::move(__result));
+    });
     return std::make_pair(__last, std::move(__result));
   }