[PATCH] libstdc++: optimize ranges::for_each for segmented iterators

From 03ad1aa9f65f1b76226d3ea8a81f30d415675dfe Mon Sep 17 00:00:00 2001
From: Yuao Ma <[email protected]>
Date: Thu, 20 Aug 2026 00:10:23 +0800
Subject: [PATCH] libstdc++: optimize ranges::for_each for segmented iterators

Similar to r17-3419-g2d11dfb0e2edd9, we can optimize std::for_each for
segemented iterators.

libstdc++-v3/ChangeLog:

	* include/bits/ranges_algo.h (ranges::__for_each): Add
	segemented iterators logic. Split out naive for-loop from ...
	(__for_each_fn::operator()): ... here.
	* testsuite/performance/25_algorithms/for_each.cc: Add
	benchmark for ranges::for_each.
---
 libstdc++-v3/include/bits/ranges_algo.h       | 33 +++++++++++++++++--
 .../performance/25_algorithms/for_each.cc     | 23 ++++++++++++-
 2 files changed, 52 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
index 4330d3e70b8..f4c70dc7029 100644
--- a/libstdc++-v3/include/bits/ranges_algo.h
+++ b/libstdc++-v3/include/bits/ranges_algo.h
@@ -211,6 +211,33 @@ namespace ranges
   template<typename _Iter, typename _Fp>
     using for_each_result = in_fun_result<_Iter, _Fp>;
 
+  // Apply __f to the result of applying __proj to each element in
+  // [__first, __last).
+  // Dispatches to __for_each_segment for segmented iterators
+  // (e.g. deque::iterator).
+  // Returns an iterator equal to __last.
+  template<typename _InputIterator, typename _Sentinel, typename _Function,
+	   typename _Proj>
+    constexpr _InputIterator
+    __for_each(_InputIterator __first, _Sentinel __last, _Function&& __f,
+	       _Proj& __proj)
+    {
+      if constexpr (__segmented_iterator<_InputIterator>
+		    && same_as<_InputIterator, _Sentinel>)
+	{
+	  std::__for_each_segment(__first, __last,
+	    [&](auto __lfirst, auto __llast)
+	    { return ranges::__for_each(__lfirst, __llast, __f, __proj); });
+	  return __last;
+	}
+      else
+	{
+	  for (; __first != __last; ++__first)
+	    std::__invoke(__f, std::__invoke(__proj, *__first));
+	  return __first;
+	}
+    }
+
   struct __for_each_fn
   {
     template<input_iterator _Iter, sentinel_for<_Iter> _Sent,
@@ -219,9 +246,9 @@ namespace ranges
       constexpr for_each_result<_Iter, _Fun>
       operator()(_Iter __first, _Sent __last, _Fun __f, _Proj __proj = {}) const
       {
-	for (; __first != __last; ++__first)
-	  std::__invoke(__f, std::__invoke(__proj, *__first));
-	return { std::move(__first), std::move(__f) };
+	auto __end = ranges::__for_each(std::move(__first), std::move(__last), __f,
+					__proj);
+	return { std::move(__end), std::move(__f) };
       }
 
     template<input_range _Range, typename _Proj = identity,
diff --git a/libstdc++-v3/testsuite/performance/25_algorithms/for_each.cc b/libstdc++-v3/testsuite/performance/25_algorithms/for_each.cc
index 37af5ab5981..dd8b5ae83b5 100644
--- a/libstdc++-v3/testsuite/performance/25_algorithms/for_each.cc
+++ b/libstdc++-v3/testsuite/performance/25_algorithms/for_each.cc
@@ -1,3 +1,4 @@
+// STD=gnu++20
 #include <testsuite_performance.h>
 
 #include <algorithm>
@@ -15,7 +16,20 @@ void bench_seq(const char* label, __gnu_test::time_counter& time,
   start_counters(time, resource);
   for (int i = 0; i < 20000; ++i)
     std::for_each(c.begin(), c.end(),
-                  [](T& x) { x = std::min<T>(100, std::max<T>(x, 10)); });
+                  [](T& x) { x = std::clamp<T>(x, 10, 100); });
+  stop_counters(time, resource);
+  report_performance(__FILE__, label, time, resource);
+  clear_counters(time, resource);
+}
+
+template <typename Container>
+void bench_seq_ranges(const char* label, __gnu_test::time_counter& time,
+                      __gnu_test::resource_counter& resource) {
+  using T = typename Container::value_type;
+  Container c(size, 1);
+  start_counters(time, resource);
+  for (int i = 0; i < 20000; ++i)
+    std::ranges::for_each(c, [](T& x) { x = std::clamp<T>(x, 10, 100); });
   stop_counters(time, resource);
   report_performance(__FILE__, label, time, resource);
   clear_counters(time, resource);
@@ -29,4 +43,11 @@ int main() {
   bench_seq<std::vector<int>>("std::for_each vector<int>", time, resource);
   bench_seq<std::deque<int>>("std::for_each deque<int>", time, resource);
   bench_seq<std::list<int>>("std::for_each list<int>", time, resource);
+
+  bench_seq_ranges<std::vector<int>>("std::ranges::for_each vector<int>", time,
+                                     resource);
+  bench_seq_ranges<std::deque<int>>("std::ranges::for_each deque<int>", time,
+                                    resource);
+  bench_seq_ranges<std::list<int>>("std::ranges::for_each list<int>", time,
+                                   resource);
 }
-- 
2.54.0