[libcxx-commits] [libcxx] [libcxx] Optimize std::generate for segmented iterators (PR #163006)
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Wed Oct 15 08:02:21 PDT 2025
https://github.com/c8ef updated https://github.com/llvm/llvm-project/pull/163006
>From 962695a02114a3355b5a31727936008d866f53f8 Mon Sep 17 00:00:00 2001
From: c8ef <c8ef at outlook.com>
Date: Sat, 11 Oct 2025 23:22:15 +0800
Subject: [PATCH 1/6] [libcxx] Optimize std::generate for segmented iterators
---
libcxx/include/__algorithm/generate.h | 27 +++++++++++++++++--
.../alg.generate/generate.pass.cpp | 11 ++++++++
2 files changed, 36 insertions(+), 2 deletions(-)
diff --git a/libcxx/include/__algorithm/generate.h b/libcxx/include/__algorithm/generate.h
index c95b527402f5d..91e2ada7daf77 100644
--- a/libcxx/include/__algorithm/generate.h
+++ b/libcxx/include/__algorithm/generate.h
@@ -9,7 +9,10 @@
#ifndef _LIBCPP___ALGORITHM_GENERATE_H
#define _LIBCPP___ALGORITHM_GENERATE_H
+#include <__algorithm/for_each_segment.h>
#include <__config>
+#include <__iterator/segmented_iterator.h>
+#include <__type_traits/enable_if.h>
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
# pragma GCC system_header
@@ -17,13 +20,33 @@
_LIBCPP_BEGIN_NAMESPACE_STD
-template <class _ForwardIterator, class _Generator>
+template <class _ForwardIterator, class _Sent, class _Generator>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
-generate(_ForwardIterator __first, _ForwardIterator __last, _Generator __gen) {
+__generate(_ForwardIterator __first, _Sent __last, _Generator __gen) {
for (; __first != __last; ++__first)
*__first = __gen();
}
+#ifndef _LIBCPP_CXX03_LANG
+template <class _SegmentedIterator,
+ class _Generator,
+ __enable_if_t<__is_segmented_iterator_v<_SegmentedIterator>, int> = 0>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
+_SegmentedIterator __generate(_SegmentedIterator __first, _SegmentedIterator __last, _Generator& __gen) {
+ using __local_iterator_t = typename __segmented_iterator_traits<_SegmentedIterator>::__local_iterator;
+ std::__for_each_segment(__first, __last, [&](__local_iterator_t __lfirst, __local_iterator_t __llast) {
+ std::__generate(__lfirst, __llast, __gen);
+ });
+ return __last;
+}
+#endif // !_LIBCPP_CXX03_LANG
+
+template <class _ForwardIterator, class _Generator>
+inline _LIBCPP_HIDE_FROM_ABI
+_LIBCPP_CONSTEXPR_SINCE_CXX20 void generate(_ForwardIterator __first, _ForwardIterator __last, _Generator __gen) {
+ std::__generate(__first, __last, __gen);
+}
+
_LIBCPP_END_NAMESPACE_STD
#endif // _LIBCPP___ALGORITHM_GENERATE_H
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.generate/generate.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.generate/generate.pass.cpp
index 29d32d7156742..4591d7ece4645 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.generate/generate.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.generate/generate.pass.cpp
@@ -16,6 +16,7 @@
#include <algorithm>
#include <cassert>
+#include <deque>
#include "test_macros.h"
#include "test_iterators.h"
@@ -51,12 +52,22 @@ test()
assert(ia[3] == 1);
}
+void deque_test() {
+ int sizes[] = {0, 1, 2, 1023, 1024, 1025, 2047, 2048, 2049};
+ for (const int size : sizes) {
+ std::deque<int> d(size);
+ std::generate(d.begin(), d.end(), gen_test());
+ assert(std::all_of(d.begin(), d.end(), [](int x) { return x == 1; }));
+ }
+}
+
int main(int, char**)
{
test<forward_iterator<int*> >();
test<bidirectional_iterator<int*> >();
test<random_access_iterator<int*> >();
test<int*>();
+ deque_test();
#if TEST_STD_VER > 17
static_assert(test_constexpr());
>From 2f00429ee9d178e6a27cf94fbafb213a735b752b Mon Sep 17 00:00:00 2001
From: c8ef <c8ef at outlook.com>
Date: Sat, 11 Oct 2025 23:56:28 +0800
Subject: [PATCH 2/6] fix copy ci
---
.../algorithms/robust_against_copying_comparators.pass.cpp | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/libcxx/test/libcxx/algorithms/robust_against_copying_comparators.pass.cpp b/libcxx/test/libcxx/algorithms/robust_against_copying_comparators.pass.cpp
index 256251686bb3e..7a3b17b8a6246 100644
--- a/libcxx/test/libcxx/algorithms/robust_against_copying_comparators.pass.cpp
+++ b/libcxx/test/libcxx/algorithms/robust_against_copying_comparators.pass.cpp
@@ -149,7 +149,8 @@ TEST_CONSTEXPR_CXX20 bool all_the_algorithms() {
assert(copies == 0);
#endif
(void)std::generate(first, last, NullaryValue<T>(&copies));
- assert(copies == 0);
+ assert(copies == 1);
+ copies = 0;
(void)std::generate_n(first, count, NullaryValue<T>(&copies));
assert(copies == 0);
(void)std::includes(first, last, first2, last2, Less<T>(&copies));
>From 252611080c81a2e0d5489b80f48176e10b7ac62d Mon Sep 17 00:00:00 2001
From: c8ef <c8ef at outlook.com>
Date: Sun, 12 Oct 2025 00:00:44 +0800
Subject: [PATCH 3/6] fix missing &
---
libcxx/include/__algorithm/generate.h | 2 +-
.../algorithms/robust_against_copying_comparators.pass.cpp | 3 +--
2 files changed, 2 insertions(+), 3 deletions(-)
diff --git a/libcxx/include/__algorithm/generate.h b/libcxx/include/__algorithm/generate.h
index 91e2ada7daf77..188f10111d454 100644
--- a/libcxx/include/__algorithm/generate.h
+++ b/libcxx/include/__algorithm/generate.h
@@ -22,7 +22,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
template <class _ForwardIterator, class _Sent, class _Generator>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
-__generate(_ForwardIterator __first, _Sent __last, _Generator __gen) {
+__generate(_ForwardIterator __first, _Sent __last, _Generator& __gen) {
for (; __first != __last; ++__first)
*__first = __gen();
}
diff --git a/libcxx/test/libcxx/algorithms/robust_against_copying_comparators.pass.cpp b/libcxx/test/libcxx/algorithms/robust_against_copying_comparators.pass.cpp
index 7a3b17b8a6246..256251686bb3e 100644
--- a/libcxx/test/libcxx/algorithms/robust_against_copying_comparators.pass.cpp
+++ b/libcxx/test/libcxx/algorithms/robust_against_copying_comparators.pass.cpp
@@ -149,8 +149,7 @@ TEST_CONSTEXPR_CXX20 bool all_the_algorithms() {
assert(copies == 0);
#endif
(void)std::generate(first, last, NullaryValue<T>(&copies));
- assert(copies == 1);
- copies = 0;
+ assert(copies == 0);
(void)std::generate_n(first, count, NullaryValue<T>(&copies));
assert(copies == 0);
(void)std::includes(first, last, first2, last2, Less<T>(&copies));
>From 4589580d9b0a7bb7b9906394e5e5e5b5b7b0eb4e Mon Sep 17 00:00:00 2001
From: c8ef <c8ef at outlook.com>
Date: Mon, 13 Oct 2025 23:14:54 +0800
Subject: [PATCH 4/6] forward std::generate to std::for_each
---
libcxx/include/__algorithm/generate.h | 32 +++++----------------------
1 file changed, 5 insertions(+), 27 deletions(-)
diff --git a/libcxx/include/__algorithm/generate.h b/libcxx/include/__algorithm/generate.h
index 188f10111d454..87483d45e7482 100644
--- a/libcxx/include/__algorithm/generate.h
+++ b/libcxx/include/__algorithm/generate.h
@@ -9,10 +9,9 @@
#ifndef _LIBCPP___ALGORITHM_GENERATE_H
#define _LIBCPP___ALGORITHM_GENERATE_H
-#include <__algorithm/for_each_segment.h>
+#include <__algorithm/for_each.h>
#include <__config>
-#include <__iterator/segmented_iterator.h>
-#include <__type_traits/enable_if.h>
+
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
# pragma GCC system_header
@@ -20,31 +19,10 @@
_LIBCPP_BEGIN_NAMESPACE_STD
-template <class _ForwardIterator, class _Sent, class _Generator>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
-__generate(_ForwardIterator __first, _Sent __last, _Generator& __gen) {
- for (; __first != __last; ++__first)
- *__first = __gen();
-}
-
-#ifndef _LIBCPP_CXX03_LANG
-template <class _SegmentedIterator,
- class _Generator,
- __enable_if_t<__is_segmented_iterator_v<_SegmentedIterator>, int> = 0>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
-_SegmentedIterator __generate(_SegmentedIterator __first, _SegmentedIterator __last, _Generator& __gen) {
- using __local_iterator_t = typename __segmented_iterator_traits<_SegmentedIterator>::__local_iterator;
- std::__for_each_segment(__first, __last, [&](__local_iterator_t __lfirst, __local_iterator_t __llast) {
- std::__generate(__lfirst, __llast, __gen);
- });
- return __last;
-}
-#endif // !_LIBCPP_CXX03_LANG
-
template <class _ForwardIterator, class _Generator>
-inline _LIBCPP_HIDE_FROM_ABI
-_LIBCPP_CONSTEXPR_SINCE_CXX20 void generate(_ForwardIterator __first, _ForwardIterator __last, _Generator __gen) {
- std::__generate(__first, __last, __gen);
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
+generate(_ForwardIterator __first, _ForwardIterator __last, _Generator __gen) {
+ std::for_each(__first, __last, [&](auto& __element) { __element = __gen(); });
}
_LIBCPP_END_NAMESPACE_STD
>From bb58a986d2ee0373caf144f914af9c453f8fcc4e Mon Sep 17 00:00:00 2001
From: c8ef <c8ef at outlook.com>
Date: Mon, 13 Oct 2025 23:17:14 +0800
Subject: [PATCH 5/6] forward std::generate to std::for_each
---
libcxx/include/__algorithm/generate.h | 1 -
1 file changed, 1 deletion(-)
diff --git a/libcxx/include/__algorithm/generate.h b/libcxx/include/__algorithm/generate.h
index 87483d45e7482..18af2a1b85434 100644
--- a/libcxx/include/__algorithm/generate.h
+++ b/libcxx/include/__algorithm/generate.h
@@ -12,7 +12,6 @@
#include <__algorithm/for_each.h>
#include <__config>
-
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
# pragma GCC system_header
#endif
>From f1e883ff9bbb2fbc4c8187dd4a12cac10466fa72 Mon Sep 17 00:00:00 2001
From: c8ef <c8ef at outlook.com>
Date: Wed, 15 Oct 2025 22:54:07 +0800
Subject: [PATCH 6/6] use std::forward
---
libcxx/docs/ReleaseNotes/22.rst | 2 ++
libcxx/include/__algorithm/generate.h | 3 ++-
2 files changed, 4 insertions(+), 1 deletion(-)
diff --git a/libcxx/docs/ReleaseNotes/22.rst b/libcxx/docs/ReleaseNotes/22.rst
index ec23ba9d1e3a1..5ca106125fb22 100644
--- a/libcxx/docs/ReleaseNotes/22.rst
+++ b/libcxx/docs/ReleaseNotes/22.rst
@@ -67,6 +67,8 @@ Improvements and New Features
reduced debug information.
- The performance of ``std::find`` has been improved by up to 2x for integral types
+- The ``std::generate`` algorithm has been optimized for segmented iterators, resulting in a performance improvement for
+ ``std::deque<short>`` and ``std::join_view<vector<vector<short>>>`` iterators.
Deprecations and Removals
-------------------------
diff --git a/libcxx/include/__algorithm/generate.h b/libcxx/include/__algorithm/generate.h
index 18af2a1b85434..8f27dbeef4667 100644
--- a/libcxx/include/__algorithm/generate.h
+++ b/libcxx/include/__algorithm/generate.h
@@ -11,6 +11,7 @@
#include <__algorithm/for_each.h>
#include <__config>
+#include <__utility/forward.h>
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
# pragma GCC system_header
@@ -21,7 +22,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
template <class _ForwardIterator, class _Generator>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
generate(_ForwardIterator __first, _ForwardIterator __last, _Generator __gen) {
- std::for_each(__first, __last, [&](auto& __element) { __element = __gen(); });
+ std::for_each(__first, __last, [&](auto&& __element) { std::forward<decltype(__element)>(__element) = __gen(); });
}
_LIBCPP_END_NAMESPACE_STD
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