llvm/llvm/include/llvm/ADT/LazyAtomicPointer.h

//===- LazyAtomicPointer.----------------------------------------*- 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_ADT_LAZYATOMICPOINTER_H
#define LLVM_ADT_LAZYATOMICPOINTER_H

#include "llvm/ADT/STLFunctionalExtras.h"
#include "llvm/Support/Compiler.h"
#include <assert.h>
#include <atomic>

namespace llvm {

/// Atomic pointer that's lock-free, but that can coordinate concurrent writes
/// from a lazy generator. Should be reserved for cases where concurrent uses of
/// a generator for the same storage is unlikely.
///
/// The laziness comes in with \a loadOrGenerate(), which lazily calls the
/// provided generator ONLY when the value is currently \c nullptr. With
/// concurrent calls, only one generator is called and the rest see that value.
///
/// Most other APIs treat an in-flight \a loadOrGenerate() as if \c nullptr
/// were stored. APIs that are required to write a value will spin.
///
/// The underlying storage is \a std::atomic<uintptr_t>.
///
/// TODO: In C++20, use std::atomic<T>::wait() instead of spinning and call
/// std::atomic<T>::notify_all() in \a loadOrGenerate().
template <class T> class LazyAtomicPointer { … };

} // end namespace llvm

#endif // LLVM_ADT_LAZYATOMICPOINTER_H