EpochTracker.h

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//===- llvm/ADT/EpochTracker.h - ADT epoch tracking --------------*- 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
//
//===----------------------------------------------------------------------===//
///
/// \file
/// 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_EPOCHTRACKER_H
#define LLVM_ADT_EPOCHTRACKER_H
 
#include "llvm/Config/abi-breaking.h"
 
#include <cstdint>
 
namespace llvm {
 
#if LLVM_ENABLE_ABI_BREAKING_CHECKS
 
/// 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 = 0;
 
public:
  DebugEpochBase() = default;
 
  /// Calling incrementEpoch invalidates all handles pointing into the
  /// calling instance.
  void incrementEpoch() { ++Epoch; }
 
  /// The destructor calls incrementEpoch to make use-after-free bugs
  /// more likely to crash deterministically.
  ~DebugEpochBase() { incrementEpoch(); }
 
  /// 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) {}
 
    /// 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; }
 
    /// 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; }
  };
};
 
#else
 
class DebugEpochBase {
public:
  void incrementEpoch() {}
 
  class HandleBase {
  public:
    HandleBase() = default;
    explicit HandleBase(const DebugEpochBase *) {}
    bool isHandleInSync() const { return true; }
    const void *getEpochAddress() const { return nullptr; }
  };
};
 
#endif // LLVM_ENABLE_ABI_BREAKING_CHECKS
 
} // namespace llvm
 
#endif