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

[Schrodinger ZHU Yifan via libc-commits]

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@@ -0,0 +1,83 @@
+//===-- Transactional Ptr for ABA prevention --------------------*- 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_TAGGED_POINTER_H
+#define LLVM_LIBC_SRC___SUPPORT_TAGGED_POINTER_H
+
+#include "hdr/types/size_t.h"
+#include "src/__support/common.h"
+#include "src/__support/threads/sleep.h"
+
+#ifdef __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16
+#define LIBC_ABA_PTR_IS_ATOMIC true
+#else
+#define LIBC_ABA_PTR_IS_ATOMIC false
+#endif
+
+namespace LIBC_NAMESPACE_DECL {
+
+template <class T, bool IsAtomic> struct AbaPtrImpl {
+  union Impl {
+    struct alignas(2 * alignof(void *)) Atomic {
+      T *ptr;
+      size_t tag;
+    } atomic;
+    struct Mutex {
+      T *ptr;
+      bool locked;
+    } mutex;
+  } impl;
+
+  LIBC_INLINE constexpr AbaPtrImpl(T *ptr)
+      : impl(IsAtomic ? Impl{.atomic{ptr, 0}} : Impl{.mutex{ptr, false}}) {}
+
+  /// User must guarantee that operation is redoable.
+  template <class Op> LIBC_INLINE void transaction(Op &&op) {
+    if constexpr (IsAtomic) {
+      for (;;) {
+        typename Impl::Atomic snapshot, next;
+        __atomic_load(&impl.atomic, &snapshot, __ATOMIC_RELAXED);
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SchrodingerZhu wrote:

Nice point. I thought our atomic reference does not support custom type but we fixed that already.

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