[libc-commits] [libc] [libc][arc4random][patch 2/n] add ABA protected MPMCStack (PR #151361)

[Joseph Huber via libc-commits]

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@@ -0,0 +1,107 @@
+//===-- Simple Lock-free MPMC Stack -----------------------------*- 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 LLVM_LIBC_SRC___SUPPORT_MPMC_STACK_H
+#define LLVM_LIBC_SRC___SUPPORT_MPMC_STACK_H
+
+#include "src/__support/CPP/atomic.h"
+#include "src/__support/CPP/new.h"
+#include "src/__support/CPP/optional.h"
+#include "src/__support/aba_ptr.h"
+
+namespace LIBC_NAMESPACE_DECL {
+template <class T> class MPMCStack {
+  struct Node {
+    cpp::Atomic<size_t> visitor;
+    Node *next;
+    T value;
+
+    LIBC_INLINE Node(T val) : visitor(0), next(nullptr), value(val) {}
+  };
+  AbaPtr<Node> head;
+
+public:
+  static_assert(cpp::is_copy_constructible<T>::value,
+                "T must be copy constructible");
+  LIBC_INLINE constexpr MPMCStack() : head(nullptr) {}
+  LIBC_INLINE bool push(T value) {
+    AllocChecker ac;
+    Node *new_node = new (ac) Node(value);
+    if (!ac)
+      return false;
+    head.transaction([new_node](Node *old_head) {
+      new_node->next = old_head;
+      return new_node;
+    });
+    return true;
+  }
+  LIBC_INLINE bool push_all(T values[], size_t count) {
+    struct Guard {
+      size_t count;
+      Node **allocated;
+      LIBC_INLINE Guard(Node *allocated[]) : count(0), allocated(allocated) {}
+      LIBC_INLINE ~Guard() {
+        for (size_t i = 0; i < count; ++i)
+          delete allocated[i];
+      }
+      LIBC_INLINE void add(Node *node) { allocated[count++] = node; }
+      LIBC_INLINE void clear() { count = 0; }
+    };
+    // Variable sized array is a GNU extension.
+    __extension__ Node *allocated[count];
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jhuber6 wrote:

We we do this without VLAs?

https://github.com/llvm/llvm-project/pull/151361