[libc++][PSTL] Implement std::transform

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D149615

13 files changed, 388 insertions, 44 deletions

diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index 4dd363de4d17..ead9693b5f7d 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -78,10 +78,12 @@ set(files
   __algorithm/pstl_backends/cpu_backends/find_if.h
   __algorithm/pstl_backends/cpu_backends/for_each.h
   __algorithm/pstl_backends/cpu_backends/serial.h
+  __algorithm/pstl_backends/cpu_backends/transform.h
   __algorithm/pstl_fill.h
   __algorithm/pstl_find.h
   __algorithm/pstl_for_each.h
   __algorithm/pstl_frontend_dispatch.h
+  __algorithm/pstl_transform.h
   __algorithm/push_heap.h
   __algorithm/ranges_adjacent_find.h
   __algorithm/ranges_all_of.h
diff --git a/libcxx/include/__algorithm/pstl_backend.h b/libcxx/include/__algorithm/pstl_backend.h
index 55f2dc4b9f4e..9ac8002a3d27 100644
--- a/libcxx/include/__algorithm/pstl_backend.h
+++ b/libcxx/include/__algorithm/pstl_backend.h
@@ -32,6 +32,16 @@ A PSTL parallel backend is a tag type to which the following functions are assoc
   template <class _ExecutionPolicy, class _Iterator, class _Predicate>
   _Iterator __pstl_find_if(_Backend, _Iterator __first, _Iterator __last, _Predicate __pred);
 
+  template <class _ExecutionPolicy, class _InIterator, class _OutIterator, class _UnaryOperation>
+  _OutIterator __pstl_transform(_InIterator __first, _InIterator __last, _OutIterator __result, _UnaryOperation __op);
+
+  template <class _ExecutionPolicy, class _InIterator1, class _InIterator2, class _OutIterator, class _BinaryOperation>
+  _OutIterator __pstl_transform(_InIterator1 __first1,
+                                _InIterator1 __last1,
+                                _InIterator2 __first2,
+                                _OutIterator __result,
+                                _BinaryOperation __op);
+
 // TODO: Complete this list
 
 The following functions are optional but can be provided. If provided, they are used by the corresponding
diff --git a/libcxx/include/__algorithm/pstl_backends/cpu_backend.h b/libcxx/include/__algorithm/pstl_backends/cpu_backend.h
index 98e6eb76df4c..a15e7f8f1acd 100644
--- a/libcxx/include/__algorithm/pstl_backends/cpu_backend.h
+++ b/libcxx/include/__algorithm/pstl_backends/cpu_backend.h
@@ -27,5 +27,6 @@
 #include <__algorithm/pstl_backends/cpu_backends/fill.h>
 #include <__algorithm/pstl_backends/cpu_backends/find_if.h>
 #include <__algorithm/pstl_backends/cpu_backends/for_each.h>
+#include <__algorithm/pstl_backends/cpu_backends/transform.h>
 
 #endif // _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKEND_H
diff --git a/libcxx/include/__algorithm/pstl_backends/cpu_backends/transform.h b/libcxx/include/__algorithm/pstl_backends/cpu_backends/transform.h
new file mode 100644
index 000000000000..f1aac24607c2
--- /dev/null
+++ b/libcxx/include/__algorithm/pstl_backends/cpu_backends/transform.h
@@ -0,0 +1,132 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKENDS_TRANSFORM_H
+#define _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKENDS_TRANSFORM_H
+
+#include <__algorithm/pstl_backends/cpu_backends/backend.h>
+#include <__algorithm/transform.h>
+#include <__config>
+#include <__iterator/iterator_traits.h>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/is_execution_policy.h>
+#include <__type_traits/remove_cvref.h>
+#include <__utility/terminate_on_exception.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _Iterator1, class _DifferenceType, class _Iterator2, class _Function>
+_LIBCPP_HIDE_FROM_ABI _Iterator2
+__simd_walk_2(_Iterator1 __first1, _DifferenceType __n, _Iterator2 __first2, _Function __f) noexcept {
+  _PSTL_PRAGMA_SIMD
+  for (_DifferenceType __i = 0; __i < __n; ++__i)
+    __f(__first1[__i], __first2[__i]);
+  return __first2 + __n;
+}
+
+template <class _ExecutionPolicy, class _ForwardIterator, class _ForwardOutIterator, class _UnaryOperation>
+_LIBCPP_HIDE_FROM_ABI _ForwardOutIterator __pstl_transform(
+    __cpu_backend_tag,
+    _ForwardIterator __first,
+    _ForwardIterator __last,
+    _ForwardOutIterator __result,
+    _UnaryOperation __op) {
+  if constexpr (__is_parallel_execution_policy_v<_ExecutionPolicy> &&
+                __is_cpp17_random_access_iterator<_ForwardIterator>::value &&
+                __is_cpp17_random_access_iterator<_ForwardOutIterator>::value) {
+    std::__terminate_on_exception([&] {
+      std::__par_backend::__parallel_for(
+          __first, __last, [__op, __first, __result](_ForwardIterator __brick_first, _ForwardIterator __brick_last) {
+            return std::__pstl_transform<__remove_parallel_policy_t<_ExecutionPolicy>>(
+                __cpu_backend_tag{}, __brick_first, __brick_last, __result + (__brick_first - __first), __op);
+          });
+    });
+    return __result + (__last - __first);
+  } else if constexpr (__is_unsequenced_execution_policy_v<_ExecutionPolicy> &&
+                       __is_cpp17_random_access_iterator<_ForwardIterator>::value &&
+                       __is_cpp17_random_access_iterator<_ForwardOutIterator>::value) {
+    return std::__simd_walk_2(
+        __first,
+        __last - __first,
+        __result,
+        [&](__iter_reference<_ForwardIterator> __in_value, __iter_reference<_ForwardOutIterator> __out_value) {
+          __out_value = __op(__in_value);
+        });
+  } else {
+    return std::transform(__first, __last, __result, __op);
+  }
+}
+
+template <class _Iterator1, class _DifferenceType, class _Iterator2, class _Iterator3, class _Function>
+_LIBCPP_HIDE_FROM_ABI _Iterator3 __simd_walk_3(
+    _Iterator1 __first1, _DifferenceType __n, _Iterator2 __first2, _Iterator3 __first3, _Function __f) noexcept {
+  _PSTL_PRAGMA_SIMD
+  for (_DifferenceType __i = 0; __i < __n; ++__i)
+    __f(__first1[__i], __first2[__i], __first3[__i]);
+  return __first3 + __n;
+}
+template <class _ExecutionPolicy,
+          class _ForwardIterator1,
+          class _ForwardIterator2,
+          class _ForwardOutIterator,
+          class _BinaryOperation,
+          enable_if_t<is_execution_policy_v<__remove_cvref_t<_ExecutionPolicy>>, int> = 0>
+_LIBCPP_HIDE_FROM_ABI _ForwardOutIterator __pstl_transform(
+    __cpu_backend_tag,
+    _ForwardIterator1 __first1,
+    _ForwardIterator1 __last1,
+    _ForwardIterator2 __first2,
+    _ForwardOutIterator __result,
+    _BinaryOperation __op) {
+  if constexpr (__is_parallel_execution_policy_v<_ExecutionPolicy> &&
+                __is_cpp17_random_access_iterator<_ForwardIterator1>::value &&
+                __is_cpp17_random_access_iterator<_ForwardIterator2>::value &&
+                __is_cpp17_random_access_iterator<_ForwardOutIterator>::value) {
+    std::__terminate_on_exception([&] {
+      std::__par_backend::__parallel_for(
+          __first1,
+          __last1,
+          [__op, __first1, __first2, __result](_ForwardIterator1 __brick_first, _ForwardIterator1 __brick_last) {
+            return std::__pstl_transform<__remove_parallel_policy_t<_ExecutionPolicy>>(
+                __cpu_backend_tag{},
+                __brick_first,
+                __brick_last,
+                __first2 + (__brick_first - __first1),
+                __result + (__brick_first - __first1),
+                __op);
+          });
+    });
+    return __result + (__last1 - __first1);
+  } else if constexpr (__is_unsequenced_execution_policy_v<_ExecutionPolicy> &&
+                       __is_cpp17_random_access_iterator<_ForwardIterator1>::value &&
+                       __is_cpp17_random_access_iterator<_ForwardIterator2>::value &&
+                       __is_cpp17_random_access_iterator<_ForwardOutIterator>::value) {
+    return std::__simd_walk_3(
+        __first1,
+        __last1 - __first1,
+        __first2,
+        __result,
+        [&](__iter_reference<_ForwardIterator1> __in1,
+            __iter_reference<_ForwardIterator2> __in2,
+            __iter_reference<_ForwardOutIterator> __out_value) { __out_value = __op(__in1, __in2); });
+  } else {
+    return std::transform(__first1, __last1, __first2, __result, __op);
+  }
+}
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
+
+#endif // _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKENDS_TRANSFORM_H
diff --git a/libcxx/include/__algorithm/pstl_transform.h b/libcxx/include/__algorithm/pstl_transform.h
new file mode 100644
index 000000000000..9d2d731eeb15
--- /dev/null
+++ b/libcxx/include/__algorithm/pstl_transform.h
@@ -0,0 +1,66 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___ALGORITHM_PSTL_TRANSFORM_H
+#define _LIBCPP___ALGORITHM_PSTL_TRANSFORM_H
+
+#include <__algorithm/pstl_backend.h>
+#include <__config>
+#include <__iterator/iterator_traits.h>
+#include <__type_traits/is_execution_policy.h>
+#include <__utility/terminate_on_exception.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _ExecutionPolicy,
+          class _ForwardIterator,
+          class _ForwardOutIterator,
+          class _UnaryOperation,
+          class _RawPolicy                                    = __remove_cvref_t<_ExecutionPolicy>,
+          enable_if_t<is_execution_policy_v<_RawPolicy>, int> = 0>
+_LIBCPP_HIDE_FROM_ABI _ForwardOutIterator transform(
+    _ExecutionPolicy&&,
+    _ForwardIterator __first,
+    _ForwardIterator __last,
+    _ForwardOutIterator __result,
+    _UnaryOperation __op) {
+  using _Backend = typename __select_backend<_RawPolicy>::type;
+  return std::__pstl_transform<_RawPolicy>(
+      _Backend{}, std::move(__first), std::move(__last), std::move(__result), std::move(__op));
+}
+
+template <class _ExecutionPolicy,
+          class _ForwardIterator1,
+          class _ForwardIterator2,
+          class _ForwardOutIterator,
+          class _BinaryOperation,
+          class _RawPolicy                                    = __remove_cvref_t<_ExecutionPolicy>,
+          enable_if_t<is_execution_policy_v<_RawPolicy>, int> = 0>
+_LIBCPP_HIDE_FROM_ABI _ForwardOutIterator transform(
+    _ExecutionPolicy&&,
+    _ForwardIterator1 __first1,
+    _ForwardIterator1 __last1,
+    _ForwardIterator2 __first2,
+    _ForwardOutIterator __result,
+    _BinaryOperation __op) {
+  using _Backend = typename __select_backend<_RawPolicy>::type;
+  return std::__pstl_transform<_RawPolicy>(
+      _Backend{}, std::move(__first1), std::move(__last1), std::move(__first2), std::move(__result), std::move(__op));
+}
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
+
+#endif // _LIBCPP___ALGORITHM_PSTL_TRANSFORM_H
diff --git a/libcxx/include/__pstl/internal/glue_algorithm_defs.h b/libcxx/include/__pstl/internal/glue_algorithm_defs.h
index 82bb3f508d5a..de4501e56b2c 100644
--- a/libcxx/include/__pstl/internal/glue_algorithm_defs.h
+++ b/libcxx/include/__pstl/internal/glue_algorithm_defs.h
@@ -134,29 +134,6 @@ template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterato
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2> swap_ranges(
     _ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2);
 
-// [alg.transform]
-
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _UnaryOperation>
-__pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
-transform(_ExecutionPolicy&& __exec,
-          _ForwardIterator1 __first,
-          _ForwardIterator1 __last,
-          _ForwardIterator2 __result,
-          _UnaryOperation __op);
-
-template <class _ExecutionPolicy,
-          class _ForwardIterator1,
-          class _ForwardIterator2,
-          class _ForwardIterator,
-          class _BinaryOperation>
-__pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
-transform(_ExecutionPolicy&& __exec,
-          _ForwardIterator1 __first1,
-          _ForwardIterator1 __last1,
-          _ForwardIterator2 __first2,
-          _ForwardIterator __result,
-          _BinaryOperation __op);
-
 // [alg.replace]
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate, class _Tp>
diff --git a/libcxx/include/__pstl/internal/glue_algorithm_impl.h b/libcxx/include/__pstl/internal/glue_algorithm_impl.h
index db62705233b9..bae5efa7d057 100644
--- a/libcxx/include/__pstl/internal/glue_algorithm_impl.h
+++ b/libcxx/include/__pstl/internal/glue_algorithm_impl.h
@@ -251,27 +251,6 @@ __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardItera
 
 // [alg.transform]
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _UnaryOperation>
-__pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
-transform(_ExecutionPolicy&& __exec,
-          _ForwardIterator1 __first,
-          _ForwardIterator1 __last,
-          _ForwardIterator2 __result,
-          _UnaryOperation __op) {
-  typedef typename iterator_traits<_ForwardIterator1>::reference _InputType;
-  typedef typename iterator_traits<_ForwardIterator2>::reference _OutputType;
-
-  auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __result);
-
-  return __pstl::__internal::__pattern_walk2(
-      __dispatch_tag,
-      std::forward<_ExecutionPolicy>(__exec),
-      __first,
-      __last,
-      __result,
-      [__op](_InputType __x, _OutputType __y) mutable { __y = __op(__x); });
-}
-
 template <class _ExecutionPolicy,
           class _ForwardIterator1,
           class _ForwardIterator2,
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
index 469bf1706628..18a89eb1a4dc 100644
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -1792,6 +1792,7 @@ template <class BidirectionalIterator, class Compare>
 #include <__algorithm/pstl_fill.h>
 #include <__algorithm/pstl_find.h>
 #include <__algorithm/pstl_for_each.h>
+#include <__algorithm/pstl_transform.h>
 #include <__algorithm/push_heap.h>
 #include <__algorithm/ranges_adjacent_find.h>
 #include <__algorithm/ranges_all_of.h>
diff --git a/libcxx/include/module.modulemap.in b/libcxx/include/module.modulemap.in
index fcf793f0e63f..d6419daff675 100644
--- a/libcxx/include/module.modulemap.in
+++ b/libcxx/include/module.modulemap.in
@@ -341,6 +341,9 @@ module std [system] {
       module pstl_backends_cpu_backends_serial {
         private header "__algorithm/pstl_backends/cpu_backends/serial.h"
       }
+      module pstl_backends_cpu_backends_transform {
+        private header "__algorithm/pstl_backends/cpu_backends/transform.h"
+      }
       module push_heap                       { private header "__algorithm/push_heap.h" }
       module ranges_adjacent_find            { private header "__algorithm/ranges_adjacent_find.h" }
       module ranges_all_of                   { private header "__algorithm/ranges_all_of.h" }
diff --git a/libcxx/test/libcxx/private_headers.verify.cpp b/libcxx/test/libcxx/private_headers.verify.cpp
index 33cc828b9218..cb80edfd0694 100644
--- a/libcxx/test/libcxx/private_headers.verify.cpp
+++ b/libcxx/test/libcxx/private_headers.verify.cpp
@@ -121,6 +121,7 @@ END-SCRIPT
 #include <__algorithm/pstl_backends/cpu_backends/find_if.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/pstl_backends/cpu_backends/find_if.h'}}
 #include <__algorithm/pstl_backends/cpu_backends/for_each.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/pstl_backends/cpu_backends/for_each.h'}}
 #include <__algorithm/pstl_backends/cpu_backends/serial.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/pstl_backends/cpu_backends/serial.h'}}
+#include <__algorithm/pstl_backends/cpu_backends/transform.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/pstl_backends/cpu_backends/transform.h'}}
 #include <__algorithm/push_heap.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/push_heap.h'}}
 #include <__algorithm/ranges_adjacent_find.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_adjacent_find.h'}}
 #include <__algorithm/ranges_all_of.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_all_of.h'}}
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/pstl.transform.binary.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/pstl.transform.binary.pass.cpp
new file mode 100644
index 000000000000..b2b98619fb96
--- /dev/null
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/pstl.transform.binary.pass.cpp
@@ -0,0 +1,79 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14
+
+// UNSUPPORTED: libcpp-has-no-incomplete-pstl
+
+// <algorithm>
+
+// template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2,
+//          class ForwardIterator, class BinaryOperation>
+//   ForwardIterator
+//     transform(ExecutionPolicy&& exec,
+//               ForwardIterator1 first1, ForwardIterator1 last1,
+//               ForwardIterator2 first2, ForwardIterator result,
+//               BinaryOperation binary_op);
+
+#include <algorithm>
+#include <vector>
+
+#include "test_macros.h"
+#include "test_execution_policies.h"
+#include "test_iterators.h"
+
+EXECUTION_POLICY_SFINAE_TEST(transform);
+
+static_assert(sfinae_test_transform<int, int*, int*, int*, int*, bool (*)(int)>);
+static_assert(!sfinae_test_transform<std::execution::parallel_policy, int*, int*, int*, int*, int (*)(int, int)>);
+
+template <class Iter1, class Iter2, class Iter3>
+struct Test {
+  template <class Policy>
+  void operator()(Policy&& policy) {
+    // simple test
+    for (const int size : {0, 1, 2, 100, 350}) {
+      std::vector<int> a(size);
+      std::vector<int> b(size);
+      for (int i = 0; i != size; ++i) {
+        a[i] = i + 1;
+        b[i] = i - 3;
+      }
+
+      std::vector<int> out(std::size(a));
+      decltype(auto) ret = std::transform(
+          policy,
+          Iter1(std::data(a)),
+          Iter1(std::data(a) + std::size(a)),
+          Iter2(std::data(b)),
+          Iter3(std::data(out)),
+          [](int i, int j) { return i + j + 3; });
+      static_assert(std::is_same_v<decltype(ret), Iter3>);
+      assert(base(ret) == std::data(out) + std::size(out));
+      for (int i = 0; i != size; ++i) {
+        assert(out[i] == i * 2 + 1);
+      }
+    }
+  }
+};
+
+int main(int, char**) {
+  types::for_each(
+      types::forward_iterator_list<int*>{}, types::apply_type_identity{[](auto v) {
+        using Iter3 = typename decltype(v)::type;
+        types::for_each(
+            types::forward_iterator_list<int*>{}, types::apply_type_identity{[](auto v2) {
+              using Iter2 = typename decltype(v2)::type;
+              types::for_each(
+                  types::forward_iterator_list<int*>{},
+                  TestIteratorWithPolicies<types::partial_instantiation<Test, Iter2, Iter3>::template apply>{});
+            }});
+      }});
+
+  return 0;
+}
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/pstl.transform.unary.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/pstl.transform.unary.pass.cpp
new file mode 100644
index 000000000000..03985c7d3673
--- /dev/null
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/pstl.transform.unary.pass.cpp
@@ -0,0 +1,65 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14
+
+// UNSUPPORTED: libcpp-has-no-incomplete-pstl
+
+// <algorithm>
+
+// template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2,
+//          class UnaryOperation>
+//   ForwardIterator2
+//     transform(ExecutionPolicy&& exec,
+//               ForwardIterator1 first1, ForwardIterator1 last1,
+//               ForwardIterator2 result, UnaryOperation op);
+
+#include <algorithm>
+#include <cassert>
+#include <vector>
+
+#include "test_macros.h"
+#include "test_execution_policies.h"
+#include "test_iterators.h"
+
+// We can't test the constraint on the execution policy, because that would conflict with the binary
+// transform algorithm that doesn't take an execution policy, which is not constrained at all.
+
+template <class Iter1, class Iter2>
+struct Test {
+  template <class Policy>
+  void operator()(Policy&& policy) {
+    // simple test
+    for (const int size : {0, 1, 2, 100, 350}) {
+      std::vector<int> a(size);
+      for (int i = 0; i != size; ++i)
+        a[i] = i + 1;
+
+      std::vector<int> out(std::size(a));
+      decltype(auto) ret = std::transform(
+          policy, Iter1(std::data(a)), Iter1(std::data(a) + std::size(a)), Iter2(std::data(out)), [](int i) {
+            return i + 3;
+          });
+      static_assert(std::is_same_v<decltype(ret), Iter2>);
+      assert(base(ret) == std::data(out) + std::size(out));
+      for (int i = 0; i != size; ++i)
+        assert(out[i] == i + 4);
+    }
+  }
+};
+
+int main(int, char**) {
+  types::for_each(types::forward_iterator_list<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>{});
+                  }});
+
+  return 0;
+}
diff --git a/libcxx/test/support/type_algorithms.h b/libcxx/test/support/type_algorithms.h
index 95a282b7b0bc..46e658624f82 100644
--- a/libcxx/test/support/type_algorithms.h
+++ b/libcxx/test/support/type_algorithms.h
@@ -52,6 +52,34 @@ TEST_CONSTEXPR_CXX14 void for_each(type_list<Types...>, Functor f) {
   swallow((f.template operator()<Types>(), 0)...);
 }
 
+template <class T>
+struct type_identity {
+  using type = T;
+};
+
+#if TEST_STD_VER >= 17
+template <class Func>
+struct apply_type_identity {
+  Func func_;
+
+  apply_type_identity(Func func) : func_(func) {}
+
+  template <class... Args>
+  decltype(auto) operator()() const {
+    return func_(type_identity<Args>{}...);
+  }
+};
+
+template <class T>
+apply_type_identity(T) -> apply_type_identity<T>;
+
+#endif
+template <template <class...> class T, class... Args>
+struct partial_instantiation {
+  template <class Other>
+  using apply = T<Args..., Other>;
+};
+
 // type categories defined in [basic.fundamental] plus extensions (without CV-qualifiers)
 
 using character_types =