//===- llvm/ADT/EpochTracker.h - ADT epoch tracking --------------*- C++ -*-==//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines the DebugEpochBase and DebugEpochBase::HandleBase classes.
// These can be used to write iterators that are fail-fast when LLVM is built
// with asserts enabled.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_ADT_EPOCH_TRACKER_H
#define LLVM_ADT_EPOCH_TRACKER_H

#include <cstdint>

namespace llvm {

/// \brief A base class for data structure classes wishing to make iterators
/// ("handles") pointing into themselves fail-fast.  When building without
/// asserts, this class is empty and does nothing.
///
/// DebugEpochBase does not by itself track handles pointing into itself.  The
/// expectation is that routines touching the handles will poll on
/// isHandleInSync at appropriate points to assert that the handle they're using
/// is still valid.
///
class DebugEpochBase {
  uint64_t Epoch;

public:
  DebugEpochBase() : Epoch(0) {}

  /// \brief Calling incrementEpoch invalidates all handles pointing into the
  /// calling instance.
  void incrementEpoch() { ++Epoch; }

  /// \brief The destructor calls incrementEpoch to make use-after-free bugs
  /// more likely to crash deterministically.
  ~DebugEpochBase() { incrementEpoch(); }

  /// \brief A base class for iterator classes ("handles") that wish to poll for
  /// iterator invalidating modifications in the underlying data structure.
  /// When LLVM is built without asserts, this class is empty and does nothing.
  ///
  /// HandleBase does not track the parent data structure by itself.  It expects
  /// the routines modifying the data structure to call incrementEpoch when they
  /// make an iterator-invalidating modification.
  ///
  class HandleBase {
    const uint64_t *EpochAddress;
    uint64_t EpochAtCreation;

  public:
    HandleBase() : EpochAddress(nullptr), EpochAtCreation(UINT64_MAX) {}

    explicit HandleBase(const DebugEpochBase *Parent)
        : EpochAddress(&Parent->Epoch), EpochAtCreation(Parent->Epoch) {}

    /// \brief Returns true if the DebugEpochBase this Handle is linked to has
    /// not called incrementEpoch on itself since the creation of this
    /// HandleBase instance.
    bool isHandleInSync() const { return *EpochAddress == EpochAtCreation; }

    /// \brief Returns a pointer to the epoch word stored in the data structure
    /// this handle points into.  Can be used to check if two iterators point
    /// into the same data structure.
    const void *getEpochAddress() const { return EpochAddress; }
  };
};

} // namespace llvm

#endif
