Hi!
Similar to ranges::distance, this patch optimizes std::for_each for
segmented iterators.
Fully tested on x86_64-linux with no regressions.
Using the newly added benchmark, it shows a 3x improvement when using
std::for_each with std::deque.
=== Sun Aug 2 09:30:52 AM UTC 2026 ===
for_each.cc std::for_each vector<int> 2r 1u 0s
0mem 0pf
for_each.cc std::for_each deque<int> 6r 6u 0s
0mem 0pf
for_each.cc std::for_each list<int> 13r 13u 0s
0mem 0pf
=== Sun Aug 2 09:31:08 AM UTC 2026 ===
for_each.cc std::for_each vector<int> 2r 1u 0s
0mem 0pf
for_each.cc std::for_each deque<int> 2r 2u 0s
0mem 0pf
for_each.cc std::for_each list<int> 13r 13u 0s
0mem 0pf
Please take a look when you are available, thanks!
Yuao
From c34403d39adaa034f30f54d0dcb7e5870b1d545c Mon Sep 17 00:00:00 2001
From: Yuao Ma <[email protected]>
Date: Sun, 2 Aug 2026 17:44:46 +0800
Subject: [PATCH] libstdc++: optimize std::for_each for segmented iterators
Similar to r17-2859-g15505d012872dd, we can optimize std::for_each for
segemented iterators.
libstdc++-v3/ChangeLog:
* include/bits/stl_algo.h (__for_each): Add segemented
iterators logic. Split out naive for-loop from ...
(for_each): ... here.
* testsuite/performance/25_algorithms/for_each.cc: New test.
---
libstdc++-v3/include/bits/stl_algo.h | 29 ++++++++++++++--
.../performance/25_algorithms/for_each.cc | 34 +++++++++++++++++++
2 files changed, 61 insertions(+), 2 deletions(-)
create mode 100644 libstdc++-v3/testsuite/performance/25_algorithms/for_each.cc
diff --git a/libstdc++-v3/include/bits/stl_algo.h
b/libstdc++-v3/include/bits/stl_algo.h
index 800c176cd5b..54d73ae8c62 100644
--- a/libstdc++-v3/include/bits/stl_algo.h
+++ b/libstdc++-v3/include/bits/stl_algo.h
@@ -132,6 +132,32 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
return __first;
}
+ /// Apply __f 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 _Function>
+ _GLIBCXX20_CONSTEXPR
+ _InputIterator
+ __for_each(_InputIterator __first, _InputIterator __last, _Function& __f)
+ {
+#if __cplusplus >= 201703L
+ if constexpr (__enable_for_each_segment<_InputIterator>)
+ {
+ std::__for_each_segment(__first, __last,
+ [&](auto __lfirst, auto __llast)
+ { return std::__for_each(__lfirst, __llast, __f); });
+ return __last;
+ }
+ else
+#endif // C++17
+ {
+ for (; __first != __last; ++__first)
+ __f(*__first);
+ return __first;
+ }
+ }
+
// set_difference
// set_intersection
// set_symmetric_difference
@@ -3813,8 +3839,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
// concept requirements
__glibcxx_function_requires(_InputIteratorConcept<_InputIterator>)
__glibcxx_requires_valid_range(__first, __last);
- for (; __first != __last; ++__first)
- __f(*__first);
+ std::__for_each(__first, __last, __f);
return __f; // N.B. [alg.foreach] says std::move(f) but it's redundant.
}
diff --git a/libstdc++-v3/testsuite/performance/25_algorithms/for_each.cc
b/libstdc++-v3/testsuite/performance/25_algorithms/for_each.cc
new file mode 100644
index 00000000000..22981eb9b64
--- /dev/null
+++ b/libstdc++-v3/testsuite/performance/25_algorithms/for_each.cc
@@ -0,0 +1,34 @@
+// STD=gnu++17
+
+#include <testsuite_performance.h>
+
+#include <algorithm>
+#include <deque>
+#include <list>
+#include <vector>
+
+const std::size_t size = 8192;
+
+template <typename Container>
+void bench_seq(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::for_each(c.begin(), c.end(),
+ [](T& x) { x = std::clamp<T>(x, 10, 100); });
+ stop_counters(time, resource);
+ report_performance(__FILE__, label, time, resource);
+ clear_counters(time, resource);
+}
+
+int main() {
+ using namespace __gnu_test;
+ time_counter time;
+ resource_counter resource;
+
+ 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);
+}
--
2.54.0
