[INFRA] Import NVIDIA/CCCL upstream as optimization reference library

CCCL (CUDA C++ Core Libraries) provides:
- CUB: device/block/warp-level GPU primitives (reduce, scan, sort, topk)
- Thrust: high-level parallel algorithms (transform_reduce, sort, scan)
- libcudacxx: CUDA C++ standard library (atomics, barriers, memory)
- cudax: experimental features (memory resources, allocators)
- Tuning policies: per-SM hardware-specific algorithm parameters

Competition optimization vectors mapped to CCCL:
- Output TPS (83% weight): warp_reduce, block_reduce, device_topk
- Input TPS (14% weight): device_scan, block_load, prefetch
- Cache TPS (3% weight): prefix caching strategy patterns
- Memory (0.9 util): pooled/cached/buddy allocators

Source: https://github.com/NVIDIA/cccl (shallow clone, HEAD only)
License: Apache-2.0

cccl_upstream/libcudacxx/codegen/generators/fence.h

@@ -0,0 +1,110 @@
+//===----------------------------------------------------------------------===//
+//
+// 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) 2024 NVIDIA CORPORATION & AFFILIATES.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef FENCE_H
+#define FENCE_H
+
+#include <format>
+#include <string>
+
+#include "definitions.h"
+
+inline std::string membar_scope(Scope sco)
+{
+  static std::map scope_map{
+    std::pair{Scope::GPU, ".gl"},
+    std::pair{Scope::System, ".sys"},
+    std::pair{Scope::CTA, ".cta"},
+  };
+
+  return scope_map[sco];
+}
+
+inline void FormatFence(std::ostream& out)
+{
+  // Argument ID Reference
+  // 0 - Membar scope tag
+  // 1 - Membar scope
+  constexpr auto intrinsic_membar = R"XXX(
+static inline _CCCL_DEVICE void __cuda_atomic_membar({0})
+{{ asm volatile("membar{1};" ::: "memory"); }})XXX";
+
+  const std::map membar_scopes{
+    std::pair{Scope::GPU, ".gl"},
+    std::pair{Scope::System, ".sys"},
+    std::pair{Scope::CTA, ".cta"},
+  };
+
+  for (const auto& sco : membar_scopes)
+  {
+    out << std::format(intrinsic_membar, scope_tag(sco.first), sco.second);
+  }
+
+  // Argument ID Reference
+  // 0 - Fence scope tag
+  // 1 - Fence scope
+  // 2 - Fence order tag
+  // 3 - Fence order
+  constexpr auto intrinsic_fence = R"XXX(
+static inline _CCCL_DEVICE void __cuda_atomic_fence({0}, {2})
+{{ asm volatile("fence{1}{3};" ::: "memory"); }})XXX";
+
+  const Scope fence_scopes[] = {
+    Scope::CTA,
+    Scope::Cluster,
+    Scope::GPU,
+    Scope::System,
+  };
+
+  const Semantic fence_semantics[] = {
+    Semantic::Acq_Rel,
+    Semantic::Seq_Cst,
+  };
+
+  for (const auto& sco : fence_scopes)
+  {
+    for (const auto& sem : fence_semantics)
+    {
+      out << std::format(intrinsic_fence, scope_tag(sco), semantic(sem), semantic_tag(sem), scope(sco));
+    }
+  }
+  out << "\n"
+      << R"XXX(
+template <typename _Sco>
+static inline _CCCL_DEVICE void __atomic_thread_fence_cuda(int __memorder, _Sco) {
+  NV_DISPATCH_TARGET(
+    NV_PROVIDES_SM_70, (
+      switch (__memorder) {
+        case __ATOMIC_SEQ_CST: __cuda_atomic_fence(_Sco{}, __atomic_cuda_seq_cst{}); break;
+        case __ATOMIC_CONSUME: [[fallthrough]];
+        case __ATOMIC_ACQUIRE: [[fallthrough]];
+        case __ATOMIC_ACQ_REL: [[fallthrough]];
+        case __ATOMIC_RELEASE: __cuda_atomic_fence(_Sco{}, __atomic_cuda_acq_rel{}); break;
+        case __ATOMIC_RELAXED: break;
+        default: _CCCL_ASSERT(false, "invalid memory order");
+      }
+    ),
+    NV_IS_DEVICE, (
+      switch (__memorder) {
+        case __ATOMIC_SEQ_CST: [[fallthrough]];
+        case __ATOMIC_CONSUME: [[fallthrough]];
+        case __ATOMIC_ACQUIRE: [[fallthrough]];
+        case __ATOMIC_ACQ_REL: [[fallthrough]];
+        case __ATOMIC_RELEASE: __cuda_atomic_membar(_Sco{}); break;
+        case __ATOMIC_RELAXED: break;
+        default: _CCCL_ASSERT(false, "invalid memory order");
+      }
+    )
+  )
+}
+)XXX";
+}
+
+#endif // FENCE_H