[libcxx-commits] [libcxx] 410a2a2 - [libc++][pstl] Default implementation of parallel std::reverse_copy (#208085)
via libcxx-commits
libcxx-commits at lists.llvm.org
Sat Jul 11 16:30:02 PDT 2026
Author: Michael G. Kazakov
Date: 2026-07-12T07:29:57+08:00
New Revision: 410a2a236355a432c6a73cbd3338badabd280dc8
URL: https://github.com/llvm/llvm-project/commit/410a2a236355a432c6a73cbd3338badabd280dc8
DIFF: https://github.com/llvm/llvm-project/commit/410a2a236355a432c6a73cbd3338badabd280dc8.diff
LOG: [libc++][pstl] Default implementation of parallel std::reverse_copy (#208085)
This PR adds a default parallel implementation of `std::reverse_copy`
based on `std::reverse_iterator` + parallel `std::copy`.
Effectively it's a one-liner:
```c++
return _Copy()(__policy,
std::reverse_iterator<_BidirectionalIterator>(std::move(__last)),
std::reverse_iterator<_BidirectionalIterator>(std::move(__first)),
std::move(__result));
```
Included tests check that:
- Semantics of the function is correct.
- The function correctly SFINAE out when the first argument is not an
execution policy.
- The `nodiscard` policy is followed.
- The `noexcept` policy is followed.
- `static_assert` verifies iterators' categories (bidirectional and
forward).
Part of #99938.
---------
Co-authored-by: Louis Dionne <ldionne.2 at gmail.com>
Added:
libcxx/test/std/algorithms/alg.modifying.operations/alg.reverse/pstl.reverse_copy.pass.cpp
Modified:
libcxx/include/__algorithm/pstl.h
libcxx/include/__pstl/backend_fwd.h
libcxx/include/__pstl/backends/default.h
libcxx/test/libcxx/algorithms/pstl.iterator-requirements.verify.cpp
libcxx/test/libcxx/algorithms/pstl.nodiscard.verify.cpp
libcxx/test/std/algorithms/pstl.exception_handling.pass.cpp
Removed:
################################################################################
diff --git a/libcxx/include/__algorithm/pstl.h b/libcxx/include/__algorithm/pstl.h
index 3530b3207f59ef..2bf9b4276baab6 100644
--- a/libcxx/include/__algorithm/pstl.h
+++ b/libcxx/include/__algorithm/pstl.h
@@ -395,6 +395,23 @@ generate_n(_ExecutionPolicy&& __policy, _ForwardIterator __first, _Size __n, _Ge
std::forward<_ExecutionPolicy>(__policy), std::move(__first), std::move(__n), std::move(__gen));
}
+template <class _ExecutionPolicy,
+ class _BidirectionalIterator,
+ class _ForwardIterator,
+ class _RawPolicy = __remove_cvref_t<_ExecutionPolicy>,
+ enable_if_t<is_execution_policy_v<_RawPolicy>, int> = 0>
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI _ForwardIterator reverse_copy(
+ _ExecutionPolicy&& __policy,
+ _BidirectionalIterator __first,
+ _BidirectionalIterator __last,
+ _ForwardIterator __result) {
+ _LIBCPP_REQUIRE_CPP17_BIDIRECTIONAL_ITERATOR(_BidirectionalIterator, "reverse_copy requires a BidirectionalIterator");
+ _LIBCPP_REQUIRE_CPP17_FORWARD_ITERATOR(_ForwardIterator, "reverse_copy requires a ForwardIterator");
+ using _Implementation = __pstl::__dispatch<__pstl::__reverse_copy, __pstl::__current_configuration, _RawPolicy>;
+ return __pstl::__handle_exception<_Implementation>(
+ std::forward<_ExecutionPolicy>(__policy), std::move(__first), std::move(__last), std::move(__result));
+}
+
template <class _ExecutionPolicy,
class _ForwardIterator,
class _Predicate,
diff --git a/libcxx/include/__pstl/backend_fwd.h b/libcxx/include/__pstl/backend_fwd.h
index 1ea121367fbc7d..a84a768cad5707 100644
--- a/libcxx/include/__pstl/backend_fwd.h
+++ b/libcxx/include/__pstl/backend_fwd.h
@@ -166,6 +166,13 @@ struct __generate_n;
// optional<__empty>
// operator()(_Policy&&, _ForwardIterator __first, _Size __n, _Generator __gen) const noexcept;
+template <class _Backend, class _ExecutionPolicy>
+struct __reverse_copy;
+// template <class _Policy, class _BidirectionalIterator, class _ForwardIterator>
+// optional<_ForwardIterator>
+// operator()(_Policy&&, _BidirectionalIterator __first, _BidirectionalIterator __last,
+// _ForwardIterator __result) const noexcept;
+
template <class _Backend, class _ExecutionPolicy>
struct __merge;
// template <class _Policy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardOutIterator, class _Comp>
diff --git a/libcxx/include/__pstl/backends/default.h b/libcxx/include/__pstl/backends/default.h
index cb963a130f7432..5c78d4efa412b9 100644
--- a/libcxx/include/__pstl/backends/default.h
+++ b/libcxx/include/__pstl/backends/default.h
@@ -23,6 +23,7 @@
#include <__iterator/concepts.h>
#include <__iterator/iterator_traits.h>
#include <__iterator/next.h>
+#include <__iterator/reverse_iterator.h>
#include <__pstl/backend_fwd.h>
#include <__pstl/dispatch.h>
#include <__type_traits/desugars_to.h>
@@ -96,6 +97,7 @@ namespace __pstl {
// - move
// - copy
// - copy_n
+// - reverse_copy
// - rotate_copy
//
@@ -557,6 +559,22 @@ struct __rotate_copy<__default_backend_tag, _ExecutionPolicy> {
}
};
+template <class _ExecutionPolicy>
+struct __reverse_copy<__default_backend_tag, _ExecutionPolicy> {
+ template <class _Policy, class _BidirectionalIterator, class _ForwardIterator>
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<_ForwardIterator>
+ operator()(_Policy&& __policy,
+ _BidirectionalIterator __first,
+ _BidirectionalIterator __last,
+ _ForwardIterator __result) const noexcept {
+ using _Copy = __dispatch<__copy, __current_configuration, _ExecutionPolicy>;
+ return _Copy()(__policy,
+ std::reverse_iterator<_BidirectionalIterator>(std::move(__last)),
+ std::reverse_iterator<_BidirectionalIterator>(std::move(__first)),
+ std::move(__result));
+ }
+};
+
} // namespace __pstl
_LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/test/libcxx/algorithms/pstl.iterator-requirements.verify.cpp b/libcxx/test/libcxx/algorithms/pstl.iterator-requirements.verify.cpp
index aee3f97019071d..fbe3460b67f174 100644
--- a/libcxx/test/libcxx/algorithms/pstl.iterator-requirements.verify.cpp
+++ b/libcxx/test/libcxx/algorithms/pstl.iterator-requirements.verify.cpp
@@ -31,12 +31,16 @@
#include "test_iterators.h"
-using non_forward_iterator = cpp17_input_iterator<int*>;
+using non_forward_iterator = cpp17_input_iterator<int*>;
+using non_bidirectional_iterator = forward_iterator<int*>;
struct non_output_iterator : forward_iterator<int*> {
constexpr int const& operator*() const; // prevent it from being an output iterator
};
-void f(non_forward_iterator non_fwd, non_output_iterator non_output, std::execution::sequenced_policy pol) {
+void f(non_forward_iterator non_fwd,
+ non_output_iterator non_output,
+ non_bidirectional_iterator non_bidir,
+ std::execution::sequenced_policy pol) {
auto pred = [](auto&&...) -> bool { return true; };
auto func = [](auto&&...) -> int { return 1; };
int* it = nullptr;
@@ -110,6 +114,12 @@ void f(non_forward_iterator non_fwd, non_output_iterator non_output, std::execut
(void)std::generate_n(pol, non_fwd, n, func); // expected-error@*:* {{static assertion failed: generate_n}}
}
+ {
+ (void)std::reverse_copy(
+ pol, non_bidir, non_bidir, it); // expected-error@*:* {{static assertion failed: reverse_copy}}
+ (void)std::reverse_copy(pol, it, it, non_fwd); // expected-error@*:* {{static assertion failed: reverse_copy}}
+ }
+
{
(void)std::is_partitioned(
pol, non_fwd, non_fwd, pred); // expected-error@*:* {{static assertion failed: is_partitioned}}
diff --git a/libcxx/test/libcxx/algorithms/pstl.nodiscard.verify.cpp b/libcxx/test/libcxx/algorithms/pstl.nodiscard.verify.cpp
index 747d62bc668379..049c8dc88c2da2 100644
--- a/libcxx/test/libcxx/algorithms/pstl.nodiscard.verify.cpp
+++ b/libcxx/test/libcxx/algorithms/pstl.nodiscard.verify.cpp
@@ -56,4 +56,6 @@ void test() {
std::is_sorted(std::execution::par, std::begin(a), std::end(a));
// expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
std::is_sorted(std::execution::par, std::begin(a), std::end(a), pred2);
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::reverse_copy(std::execution::par, std::begin(a), std::end(a), std::begin(b));
}
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.reverse/pstl.reverse_copy.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.reverse/pstl.reverse_copy.pass.cpp
new file mode 100644
index 00000000000000..d81dde163cf023
--- /dev/null
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.reverse/pstl.reverse_copy.pass.cpp
@@ -0,0 +1,116 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++17
+
+// UNSUPPORTED: libcpp-has-no-incomplete-pstl
+
+// template <class ExecutionPolicy,
+// class BidirectionalIterator,
+// class ForwardIterator>
+// ForwardIterator reverse_copy(ExecutionPolicy&& exec,
+// BidirectionalIterator first,
+// BidirectionalIterator last,
+// ForwardIterator result);
+
+#include <algorithm>
+#include <cassert>
+#include <functional>
+#include <iterator>
+#include <limits>
+#include <numeric>
+
+#include "test_execution_policies.h"
+#include "test_iterators.h"
+#include "test_macros.h"
+#include "type_algorithms.h"
+
+EXECUTION_POLICY_SFINAE_TEST(reverse_copy);
+
+static_assert(sfinae_test_reverse_copy<int, int*, int*, int*>);
+static_assert(!sfinae_test_reverse_copy<std::execution::parallel_policy, int*, int*, int*>);
+
+template <class Callable>
+void runway_sample(size_t size, Callable callable) {
+ constexpr size_t affix = 16;
+ // 0, 1, 2, ..., 15, 16, 50, 157, 493, 1548, ...
+ for (size_t i = 0; i < size; i = i < affix ? i + 1 : size_t(3.1415 * i)) {
+ callable(i);
+ }
+ if (size <= affix)
+ return;
+ // size - 16, size - 15, ..., size - 1
+ for (size_t i = size - affix; i < size; ++i) {
+ callable(i);
+ }
+}
+
+template <class Iter1, class Iter2>
+struct Test {
+ template <class ExecutionPolicy>
+ void operator()(ExecutionPolicy&& policy) {
+ {
+ int ia[] = {0};
+ int ib[std::size(ia)] = {-1};
+ Iter2 r = std::reverse_copy(policy, Iter1(std::begin(ia)), Iter1(std::begin(ia)), Iter2(std::begin(ib)));
+ assert(r == Iter2(std::begin(ib)));
+ assert(ib[0] == -1);
+ r = std::reverse_copy(policy, Iter1(std::begin(ia)), Iter1(std::end(ia)), Iter2(std::begin(ib)));
+ assert(ia[0] == 0);
+ }
+ {
+ int ia[] = {0, 1};
+ int ib[std::size(ia)] = {-1};
+ Iter2 r = std::reverse_copy(policy, Iter1(std::begin(ia)), Iter1(std::end(ia)), Iter2(std::begin(ib)));
+ assert(r == Iter2(std::end(ib)));
+ assert(ib[0] == 1);
+ assert(ib[1] == 0);
+ }
+ {
+ int ia[] = {0, 1, 2};
+ int ib[std::size(ia)] = {-1};
+ Iter2 r = std::reverse_copy(policy, Iter1(std::begin(ia)), Iter1(std::end(ia)), Iter2(std::begin(ib)));
+ assert(r == Iter2(std::end(ib)));
+ assert(ib[0] == 2);
+ assert(ib[1] == 1);
+ assert(ib[2] == 0);
+ }
+ {
+ int ia[] = {0, 1, 2, 3};
+ int ib[std::size(ia)] = {-1};
+ Iter2 r = std::reverse_copy(policy, Iter1(std::begin(ia)), Iter1(std::end(ia)), Iter2(std::begin(ib)));
+ assert(r == Iter2(std::end(ib)));
+ assert(ib[0] == 3);
+ assert(ib[1] == 2);
+ assert(ib[2] == 1);
+ assert(ib[3] == 0);
+ }
+ {
+ int ia[1073];
+ int ib[1073];
+ std::iota(std::begin(ia), std::end(ia), 1);
+ runway_sample(std::size(ia) + 1, [&](size_t i) {
+ Iter2 r = std::reverse_copy(policy, Iter1(std::begin(ia)), Iter1(std::begin(ia) + i), Iter2(std::begin(ib)));
+ assert(r == Iter2(std::begin(ib) + i));
+ for (size_t j = 0; j < i; ++j) {
+ assert(ib[j] == static_cast<int>(i - j));
+ }
+ });
+ }
+ }
+};
+
+int main(int, char**) {
+ types::for_each(
+ types::concatenate_t<types::bidirectional_iterator_list<int*>, types::bidirectional_iterator_list<const int*>>{},
+ types::apply_type_identity{[](auto v) {
+ using Iter = typename decltype(v)::type;
+ types::for_each(types::forward_iterator_list<int*>{},
+ TestIteratorWithPolicies<types::partial_instantiation<Test, Iter>::template apply>{});
+ }});
+}
diff --git a/libcxx/test/std/algorithms/pstl.exception_handling.pass.cpp b/libcxx/test/std/algorithms/pstl.exception_handling.pass.cpp
index 9e6c8f5b8ce409..8af2a0a15fd5cc 100644
--- a/libcxx/test/std/algorithms/pstl.exception_handling.pass.cpp
+++ b/libcxx/test/std/algorithms/pstl.exception_handling.pass.cpp
@@ -210,6 +210,13 @@ int main(int, char**) {
assert_non_throwing([=, &policy] { (void)std::generate_n(policy, std::move(first1), n, gen); });
}
+ {
+ // reverse_copy(first, last, dest)
+ assert_non_throwing([=, &policy] {
+ (void)std::reverse_copy(policy, std::move(first1), std::move(last1), std::move(dest));
+ });
+ }
+
{
auto pred = maybe_throw(tokens[5], [](int x) -> bool { return x % 2 == 0; });
More information about the libcxx-commits
mailing list