[libcu++] Adds a cuda::execution::tie_break requirement

libcudacxx/include/cuda/__execution/tie_break.h

@@ -0,0 +1,97 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of libcu++, the C++ Standard Library for your entire system,
+// 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
+// SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef __CUDA___EXECUTION_TIE_BREAK_H
+#define __CUDA___EXECUTION_TIE_BREAK_H
+
+#include <cuda/std/detail/__config>
+
+#if defined(_CCCL_IMPLICIT_SYSTEM_HEADER_GCC)
+#  pragma GCC system_header
+#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_CLANG)
+#  pragma clang system_header
+#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_MSVC)
+#  pragma system_header
+#endif // no system header
+
+#include <cuda/__execution/require.h>
+#include <cuda/std/__concepts/concept_macros.h>
+#include <cuda/std/__execution/env.h>
+#include <cuda/std/__type_traits/integral_constant.h>
+#include <cuda/std/__type_traits/is_one_of.h>
+
+#include <cuda/std/__cccl/prologue.h>
+
+_CCCL_BEGIN_NAMESPACE_CUDA_EXECUTION
+
+//! @brief Requirement describing how an algorithm breaks ties among elements that compare equal at its selection
+//! boundary (e.g. the K-th element of cub::DeviceBatchedTopK).
+//!
+//! A tie-break requirement only constrains *which* of the equal-comparing elements end up in the result set; it says
+//! nothing about the order in which the results are written (that is controlled independently by
+//! cuda::execution::output_ordering). A tie-break is only meaningful together with a deterministic execution
+//! requirement (cuda::execution::determinism::run_to_run or gpu_to_gpu).
+namespace tie_break
+{
+struct __get_tie_break_t;
+
+enum class __tie_break_t
+{
+  __unspecified, //!< Any (implementation-defined) deterministic tie-break is acceptable.
+  __prefer_smaller_index, //!< Among elements that compare equal, prefer the one(s) with the smaller source index.
+  __prefer_larger_index //!< Among elements that compare equal, prefer the one(s) with the larger source index.
+};
+
+template <__tie_break_t _Preference>
+struct _CCCL_DECLSPEC_EMPTY_BASES __tie_break_holder_t
+    : __requirement
+    , ::cuda::std::integral_constant<__tie_break_t, _Preference>
+{
+  [[nodiscard]] _CCCL_NODEBUG_API constexpr auto query(const __get_tie_break_t&) const noexcept
+    -> __tie_break_holder_t<_Preference>
+  {
+    return *this;
+  }
+};
+
+using unspecified_t          = __tie_break_holder_t<__tie_break_t::__unspecified>;
+using prefer_smaller_index_t = __tie_break_holder_t<__tie_break_t::__prefer_smaller_index>;
+using prefer_larger_index_t  = __tie_break_holder_t<__tie_break_t::__prefer_larger_index>;
+
+_CCCL_GLOBAL_CONSTANT unspecified_t unspecified{};
+_CCCL_GLOBAL_CONSTANT prefer_smaller_index_t prefer_smaller_index{};
+_CCCL_GLOBAL_CONSTANT prefer_larger_index_t prefer_larger_index{};
+
+struct __get_tie_break_t
+{
+  _CCCL_EXEC_CHECK_DISABLE
+  _CCCL_TEMPLATE(class _Env)
+  _CCCL_REQUIRES(::cuda::std::execution::__queryable_with<_Env, __get_tie_break_t>)
+  [[nodiscard]] _CCCL_NODEBUG_API constexpr auto operator()(const _Env& __env) const noexcept
+  {
+    static_assert(noexcept(__env.query(*this)));
+    return __env.query(*this);
+  }
+
+  [[nodiscard]]
+  _CCCL_NODEBUG_API static constexpr auto query(::cuda::std::execution::forwarding_query_t) noexcept -> bool
+  {
+    return true;
+  }
+};
+
+_CCCL_GLOBAL_CONSTANT auto __get_tie_break = __get_tie_break_t{};
+} // namespace tie_break
+
+_CCCL_END_NAMESPACE_CUDA_EXECUTION
+
+#include <cuda/std/__cccl/epilogue.h>
+
+#endif // __CUDA___EXECUTION_TIE_BREAK_H

libcudacxx/include/cuda/execution

@@ -25,6 +25,7 @@
 #include <cuda/__execution/output_ordering.h>
 #include <cuda/__execution/policy.h>
 #include <cuda/__execution/require.h>
+#include <cuda/__execution/tie_break.h>
 #include <cuda/__execution/tune.h>
 #include <cuda/std/execution>
 

libcudacxx/include/cuda/execution.tie_break.h

@@ -0,0 +1,26 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of libcu++, the C++ Standard Library for your entire system,
+// 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
+// SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _CUDA_EXECUTION_EXECUTION_TIE_BREAK_H
+#define _CUDA_EXECUTION_EXECUTION_TIE_BREAK_H
+
+#include <cuda/std/detail/__config>
+
+#if defined(_CCCL_IMPLICIT_SYSTEM_HEADER_GCC)
+#  pragma GCC system_header
+#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_CLANG)
+#  pragma clang system_header
+#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_MSVC)
+#  pragma system_header
+#endif // no system header
+
+#include <cuda/__execution/tie_break.h>
+
+#endif // _CUDA_EXECUTION_EXECUTION_TIE_BREAK_H

libcudacxx/test/libcudacxx/cuda/execution/tie_break.pass.cpp

@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of libcu++, the C++ Standard Library for your entire system,
+// 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
+// SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES.
+//
+//===----------------------------------------------------------------------===//
+
+#include <cuda/execution.tie_break.h>
+
+#include "test_macros.h"
+
+TEST_FUNC void test()
+{
+  namespace exec = cuda::execution;
+  static_assert(cuda::std::is_base_of_v<exec::__requirement, exec::tie_break::unspecified_t>);
+  static_assert(cuda::std::is_base_of_v<exec::__requirement, exec::tie_break::prefer_smaller_index_t>);
+  static_assert(cuda::std::is_base_of_v<exec::__requirement, exec::tie_break::prefer_larger_index_t>);
+
+  static_assert(cuda::std::is_same_v<decltype(exec::tie_break::__get_tie_break(exec::tie_break::unspecified)),
+                                     exec::tie_break::unspecified_t>);
+  static_assert(cuda::std::is_same_v<decltype(exec::tie_break::__get_tie_break(exec::tie_break::prefer_smaller_index)),
+                                     exec::tie_break::prefer_smaller_index_t>);
+  static_assert(cuda::std::is_same_v<decltype(exec::tie_break::__get_tie_break(exec::tie_break::prefer_larger_index)),
+                                     exec::tie_break::prefer_larger_index_t>);
+}
+
+int main(int, char**)
+{
+  test();
+
+  return 0;
+}