[libcxx-commits] [libcxx] [libc++] Backport segmented iterator optimization for std::for_each to C++03 (PR #134960)

[Nikolas Klauser via libcxx-commits]

================
@@ -26,28 +26,43 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class _InputIterator, class _Function>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Function
-for_each(_InputIterator __first, _InputIterator __last, _Function __f) {
+template <class _InputIterator, class _Sent, class _Func>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __for_each(_InputIterator __first, _Sent __last, _Func& __f) {
   for (; __first != __last; ++__first)
     __f(*__first);
-  return __f;
 }
 
-// __movable_box is available in C++20, but is actually a copyable-box, so optimization is only correct in C++23
-#if _LIBCPP_STD_VER >= 23
-template <class _SegmentedIterator, class _Function>
-  requires __is_segmented_iterator<_SegmentedIterator>::value
-_LIBCPP_HIDE_FROM_ABI constexpr _Function
-for_each(_SegmentedIterator __first, _SegmentedIterator __last, _Function __func) {
-  ranges::__movable_box<_Function> __wrapped_func(in_place, std::move(__func));
-  std::__for_each_segment(__first, __last, [&](auto __lfirst, auto __llast) {
-    __wrapped_func =
-        ranges::__movable_box<_Function>(in_place, std::for_each(__lfirst, __llast, std::move(*__wrapped_func)));
-  });
-  return std::move(*__wrapped_func);
+// __segment_processor handles the per-segment processing by applying the function object __func_ to each
+// element within the segment.
+template <class _SegmentedIterator, class _Func>
+struct __segment_processor {
+  using _Traits _LIBCPP_NODEBUG = __segmented_iterator_traits<_SegmentedIterator>;
+
+  _Func& __func_;
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR explicit __segment_processor(_Func& __f) : __func_(__f) {}
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
+  operator()(typename _Traits::__local_iterator __lfirst, typename _Traits::__local_iterator __llast) {
+    std::__for_each(__lfirst, __llast, __func_);
+  }
+};
+
+template <class _SegmentedIterator,
+          class _Function,
+          __enable_if_t<__is_segmented_iterator<_SegmentedIterator>::value, int> = 0>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Function
+__for_each(_SegmentedIterator __first, _SegmentedIterator __last, _Function __func) {
+  std::__for_each_segment(__first, __last, std::__segment_processor<_SegmentedIterator, _Function>(__func));
----------------
philnik777 wrote:

C++03 is a legacy platform. If you care about performance you don't use C++03, so I don't see the point of significantly complicating the code base for that. Re. "consistency" I don't see how that's relevant. They have the C++03 optimizations because people generally don't like it when we regress performance, even on a legacy platform. Why would you have to add a check for C++11 when forwarding? You can just forward the call no matter what - you just don't get the optimization.

https://github.com/llvm/llvm-project/pull/134960