[libcxx-commits] [libcxx] [libcxx] Optimize `ranges::fold_left_with_iter` for segmented iterators (PR #177853)
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Thu Feb 5 03:01:19 PST 2026
https://github.com/c8ef updated https://github.com/llvm/llvm-project/pull/177853
>From 50f1bd8d398bd8e24ff2f346ba538d38afde59cc Mon Sep 17 00:00:00 2001
From: c8ef <c8ef at outlook.com>
Date: Sun, 25 Jan 2026 20:37:09 +0800
Subject: [PATCH 1/5] optimize fold_left_with_iter
---
libcxx/docs/ReleaseNotes/23.rst | 4 ++++
libcxx/include/__algorithm/ranges_fold.h | 18 ++++++++++++------
2 files changed, 16 insertions(+), 6 deletions(-)
diff --git a/libcxx/docs/ReleaseNotes/23.rst b/libcxx/docs/ReleaseNotes/23.rst
index 73f5984768592..046ce4ad22b51 100644
--- a/libcxx/docs/ReleaseNotes/23.rst
+++ b/libcxx/docs/ReleaseNotes/23.rst
@@ -41,6 +41,10 @@ Implemented Papers
Improvements and New Features
-----------------------------
+- The ``std::ranges::fold_left_with_iter`` algorithm has been optimized for
+ segmented iterators, resulting in a performance improvement for
+ ``std::deque<int>`` iterators.
+
Deprecations and Removals
-------------------------
diff --git a/libcxx/include/__algorithm/ranges_fold.h b/libcxx/include/__algorithm/ranges_fold.h
index d2c3921398504..82614ec311164 100644
--- a/libcxx/include/__algorithm/ranges_fold.h
+++ b/libcxx/include/__algorithm/ranges_fold.h
@@ -10,12 +10,14 @@
#ifndef _LIBCPP___ALGORITHM_RANGES_FOLD_H
#define _LIBCPP___ALGORITHM_RANGES_FOLD_H
+#include <__algorithm/for_each.h>
#include <__concepts/assignable.h>
#include <__concepts/constructible.h>
#include <__concepts/convertible_to.h>
#include <__concepts/invocable.h>
#include <__concepts/movable.h>
#include <__config>
+#include <__functional/identity.h>
#include <__functional/invoke.h>
#include <__functional/reference_wrapper.h>
#include <__iterator/concepts.h>
@@ -80,18 +82,22 @@ concept __indirectly_binary_left_foldable =
struct __fold_left_with_iter {
template <input_iterator _Ip, sentinel_for<_Ip> _Sp, class _Tp, __indirectly_binary_left_foldable<_Tp, _Ip> _Fp>
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr auto operator()(_Ip __first, _Sp __last, _Tp __init, _Fp __f) {
- using _Up = decay_t<invoke_result_t<_Fp&, _Tp, iter_reference_t<_Ip>>>;
+ using _Up = decay_t<invoke_result_t<_Fp&, _Tp, iter_reference_t<_Ip>>>;
+ using __iter_ref = decltype(*__first);
if (__first == __last) {
return fold_left_with_iter_result<_Ip, _Up>{std::move(__first), _Up(std::move(__init))};
}
_Up __result = std::invoke(__f, std::move(__init), *__first);
- for (++__first; __first != __last; ++__first) {
- __result = std::invoke(__f, std::move(__result), *__first);
- }
-
- return fold_left_with_iter_result<_Ip, _Up>{std::move(__first), std::move(__result)};
+ ++__first;
+ auto __for_each_f = [&](__iter_ref __element) {
+ __result = std::invoke(__f, std::move(__result), std::forward<__iter_ref>(__element));
+ };
+ __identity __proj;
+ auto __end = std::__for_each(std::move(__first), std::move(__last), __for_each_f, __proj);
+
+ return fold_left_with_iter_result<_Ip, _Up>{std::move(__end), std::move(__result)};
}
template <input_range _Rp, class _Tp, __indirectly_binary_left_foldable<_Tp, iterator_t<_Rp>> _Fp>
>From 8fb997795da2e9e00ae940f57b7851e5977c21e7 Mon Sep 17 00:00:00 2001
From: c8ef <c8ef at outlook.com>
Date: Mon, 2 Feb 2026 23:20:55 +0800
Subject: [PATCH 2/5] mention improvement in release notes
---
libcxx/docs/ReleaseNotes/23.rst | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/libcxx/docs/ReleaseNotes/23.rst b/libcxx/docs/ReleaseNotes/23.rst
index 046ce4ad22b51..4a23ecf975aea 100644
--- a/libcxx/docs/ReleaseNotes/23.rst
+++ b/libcxx/docs/ReleaseNotes/23.rst
@@ -42,8 +42,8 @@ Improvements and New Features
-----------------------------
- The ``std::ranges::fold_left_with_iter`` algorithm has been optimized for
- segmented iterators, resulting in a performance improvement for
- ``std::deque<int>`` iterators.
+ segmented iterators, resulting in a performance improvement of up to 1.38x
+ for ``std::deque<int>`` iterators.
Deprecations and Removals
-------------------------
>From ef6d421759ccda87ae87063ba04f188e29fc0eb2 Mon Sep 17 00:00:00 2001
From: c8ef <c8ef at outlook.com>
Date: Wed, 4 Feb 2026 23:29:02 +0800
Subject: [PATCH 3/5] remove for_each_f; add test coverage
---
libcxx/include/__algorithm/ranges_fold.h | 11 +++++++----
.../alg.nonmodifying/alg.fold/left_folds.pass.cpp | 13 +++++++++++++
2 files changed, 20 insertions(+), 4 deletions(-)
diff --git a/libcxx/include/__algorithm/ranges_fold.h b/libcxx/include/__algorithm/ranges_fold.h
index 82614ec311164..b99fe858efd5c 100644
--- a/libcxx/include/__algorithm/ranges_fold.h
+++ b/libcxx/include/__algorithm/ranges_fold.h
@@ -91,11 +91,14 @@ struct __fold_left_with_iter {
_Up __result = std::invoke(__f, std::move(__init), *__first);
++__first;
- auto __for_each_f = [&](__iter_ref __element) {
- __result = std::invoke(__f, std::move(__result), std::forward<__iter_ref>(__element));
- };
__identity __proj;
- auto __end = std::__for_each(std::move(__first), std::move(__last), __for_each_f, __proj);
+ auto __end = std::__for_each(
+ std::move(__first),
+ std::move(__last),
+ [&](__iter_ref __element) {
+ __result = std::invoke(__f, std::move(__result), std::forward<__iter_ref>(__element));
+ },
+ __proj);
return fold_left_with_iter_result<_Ip, _Up>{std::move(__end), std::move(__result)};
}
diff --git a/libcxx/test/std/algorithms/alg.nonmodifying/alg.fold/left_folds.pass.cpp b/libcxx/test/std/algorithms/alg.nonmodifying/alg.fold/left_folds.pass.cpp
index 4987ca9cac4ae..cd1db8d63b12c 100644
--- a/libcxx/test/std/algorithms/alg.nonmodifying/alg.fold/left_folds.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.nonmodifying/alg.fold/left_folds.pass.cpp
@@ -36,6 +36,7 @@
#include <iterator>
#include <list>
#include <ranges>
+#include <set>
#include <string_view>
#include <string>
#include <vector>
@@ -322,6 +323,18 @@ void runtime_only_test_case() {
// int(-15.5) + -6.6 = -15 + -6.6 = -21.6.
check(data, 0.0, plus, expected);
}
+
+ {
+ auto const data = std::set<double>{-1.1, -2.2, -3.3, -4.4, -5.5, -6.6};
+ auto plus = [](int const x, double const y) { return x + y; };
+ auto const expected = -21.1; // int( 0.0) + -6.6 = 0 + -6.6 = -6.6
+ // int(- 6.6) + -5.5 = - 6 + -5.5 = -11.5
+ // int(-11.5) + -4.4 = -11 + -4.4 = -15.4
+ // int(-15.4) + -3.3 = -15 + -3.3 = -18.3
+ // int(-18.3) + -2.2 = -18 + -2.2 = -20.2
+ // int(-20.2) + -1.1 = -20 + -1.1 = -21.1.
+ check(data, 0.0, plus, expected);
+ }
}
int main(int, char**) {
>From a9dbf70511c51915454a758b0c971a98aef310bd Mon Sep 17 00:00:00 2001
From: c8ef <c8ef at outlook.com>
Date: Thu, 5 Feb 2026 08:37:31 +0800
Subject: [PATCH 4/5] simpler test
---
.../alg.nonmodifying/alg.fold/left_folds.pass.cpp | 12 +++---------
1 file changed, 3 insertions(+), 9 deletions(-)
diff --git a/libcxx/test/std/algorithms/alg.nonmodifying/alg.fold/left_folds.pass.cpp b/libcxx/test/std/algorithms/alg.nonmodifying/alg.fold/left_folds.pass.cpp
index cd1db8d63b12c..107e09a63c96f 100644
--- a/libcxx/test/std/algorithms/alg.nonmodifying/alg.fold/left_folds.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.nonmodifying/alg.fold/left_folds.pass.cpp
@@ -325,15 +325,9 @@ void runtime_only_test_case() {
}
{
- auto const data = std::set<double>{-1.1, -2.2, -3.3, -4.4, -5.5, -6.6};
- auto plus = [](int const x, double const y) { return x + y; };
- auto const expected = -21.1; // int( 0.0) + -6.6 = 0 + -6.6 = -6.6
- // int(- 6.6) + -5.5 = - 6 + -5.5 = -11.5
- // int(-11.5) + -4.4 = -11 + -4.4 = -15.4
- // int(-15.4) + -3.3 = -15 + -3.3 = -18.3
- // int(-18.3) + -2.2 = -18 + -2.2 = -20.2
- // int(-20.2) + -1.1 = -20 + -1.1 = -21.1.
- check(data, 0.0, plus, expected);
+ auto const data = std::set<int>{2, 4, 6, 8, 10, 12};
+ auto const expected = triangular_sum(data);
+ check(data, 0, std::plus<long>(), static_cast<long>(expected));
}
}
>From d9a719fb3323bd7c70c712ffffd2e00db2bf5807 Mon Sep 17 00:00:00 2001
From: c8ef <c8ef at outlook.com>
Date: Thu, 5 Feb 2026 19:01:04 +0800
Subject: [PATCH 5/5] use auto&&
---
libcxx/include/__algorithm/ranges_fold.h | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)
diff --git a/libcxx/include/__algorithm/ranges_fold.h b/libcxx/include/__algorithm/ranges_fold.h
index b99fe858efd5c..c2d150a16d328 100644
--- a/libcxx/include/__algorithm/ranges_fold.h
+++ b/libcxx/include/__algorithm/ranges_fold.h
@@ -82,8 +82,7 @@ concept __indirectly_binary_left_foldable =
struct __fold_left_with_iter {
template <input_iterator _Ip, sentinel_for<_Ip> _Sp, class _Tp, __indirectly_binary_left_foldable<_Tp, _Ip> _Fp>
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr auto operator()(_Ip __first, _Sp __last, _Tp __init, _Fp __f) {
- using _Up = decay_t<invoke_result_t<_Fp&, _Tp, iter_reference_t<_Ip>>>;
- using __iter_ref = decltype(*__first);
+ using _Up = decay_t<invoke_result_t<_Fp&, _Tp, iter_reference_t<_Ip>>>;
if (__first == __last) {
return fold_left_with_iter_result<_Ip, _Up>{std::move(__first), _Up(std::move(__init))};
@@ -95,8 +94,8 @@ struct __fold_left_with_iter {
auto __end = std::__for_each(
std::move(__first),
std::move(__last),
- [&](__iter_ref __element) {
- __result = std::invoke(__f, std::move(__result), std::forward<__iter_ref>(__element));
+ [&](auto&& __element) {
+ __result = std::invoke(__f, std::move(__result), std::forward<decltype(__element)>(__element));
},
__proj);
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