[libcxx-commits] [libcxx] [libc++] Implement P2545R4 Read-Copy Update (RCU) (PR #175451)

[via libcxx-commits]

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+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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___RCU_RCU_LIST_H
+#define _LIBCPP___RCU_RCU_LIST_H
+
+#include <__config>
+#include <__functional/function.h>
+#include <__rcu/rcu_domain.h>
+#include <atomic>
+
+#include "thread_local_container.h"
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 26 && _LIBCPP_HAS_THREADS && _LIBCPP_HAS_EXPERIMENTAL_RCU
+
+class rcu_thread_local_list_view;
+
+class rcu_singly_list_view {
+private:
+  __rcu_node* head_ = nullptr;
+  __rcu_node* tail_ = nullptr;
+
+public:
+  void splice_back(rcu_singly_list_view& other) noexcept {
+    if (other.head_ == nullptr) {
+      return;
+    }
+    if (head_ == nullptr) {
+      head_ = other.head_;
+      tail_ = other.tail_;
+    } else {
+      tail_->__next_ = other.head_;
+      tail_          = other.tail_;
+    }
+    other.head_ = nullptr;
+    other.tail_ = nullptr;
+  }
+
+  void splice_back(rcu_thread_local_list_view& __other) noexcept;
+
+  template <class Func>
+  void for_each(Func&& f) noexcept {
+    __rcu_node* current = head_;
+    while (current != nullptr) {
+      // __f could delete __current, so we need to get the next pointer first
+      auto __next = current->__next_;
+      f(current);
+      current = __next;
+    }
+  }
+};
+
+class rcu_thread_local_list_view {
+  struct alignas(2 * sizeof(void*)) thread_entry {
+    __rcu_node* head_ = nullptr;
+    __rcu_node* tail_ = nullptr;
+  };
+
+  using per_thread_entries = thread_local_container<thread_entry>;
+
+  friend class rcu_singly_list_view;
+
+public:
+  void push_front(__rcu_node* node) noexcept {
+    atomic_ref<thread_entry> entry_ref = per_thread_entries::get_current_thread_instance();
+    auto expected_entry                = entry_ref.load(std::memory_order_relaxed);
+    auto original_next                 = node->__next_;
+    while (true) {
+      auto new_entry = [&] {
+        if (expected_entry.head_ == nullptr) {
+          return thread_entry{node, node};
+        } else {
+          node->__next_ = expected_entry.head_;
+          return thread_entry{node, expected_entry.tail_};
+        }
+      }();
+      if (entry_ref.compare_exchange_weak(
+              expected_entry, new_entry, std::memory_order_acq_rel, std::memory_order_relaxed)) {
+        break;
+      } else {
+        node->__next_ = original_next;
+      }
+    }
+  }
+};
+
+void rcu_singly_list_view::splice_back(rcu_thread_local_list_view& __other) noexcept {
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huixie90 wrote:

I did some refactoring and let the list owns the `atomic` and removed the `atomic_ref` from the `thread_local_container` 

https://github.com/llvm/llvm-project/pull/175451