1 //===- llvm/ADT/EpochTracker.h - ADT epoch tracking --------------*- C++ -*-==//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 ///
9 /// \file
10 /// This file defines the DebugEpochBase and DebugEpochBase::HandleBase classes.
11 /// These can be used to write iterators that are fail-fast when LLVM is built
12 /// with asserts enabled.
13 ///
14 //===----------------------------------------------------------------------===//
15 
16 #ifndef LLVM_ADT_EPOCHTRACKER_H
17 #define LLVM_ADT_EPOCHTRACKER_H
18 
19 #include "llvm/Config/abi-breaking.h"
20 
21 #include <cstdint>
22 
23 namespace llvm {
24 
25 #if LLVM_ENABLE_ABI_BREAKING_CHECKS
26 
27 /// A base class for data structure classes wishing to make iterators
28 /// ("handles") pointing into themselves fail-fast.  When building without
29 /// asserts, this class is empty and does nothing.
30 ///
31 /// DebugEpochBase does not by itself track handles pointing into itself.  The
32 /// expectation is that routines touching the handles will poll on
33 /// isHandleInSync at appropriate points to assert that the handle they're using
34 /// is still valid.
35 ///
36 class DebugEpochBase {
37   uint64_t Epoch = 0;
38 
39 public:
40   DebugEpochBase() = default;
41 
42   /// Calling incrementEpoch invalidates all handles pointing into the
43   /// calling instance.
44   void incrementEpoch() { ++Epoch; }
45 
46   /// The destructor calls incrementEpoch to make use-after-free bugs
47   /// more likely to crash deterministically.
48   ~DebugEpochBase() { incrementEpoch(); }
49 
50   /// A base class for iterator classes ("handles") that wish to poll for
51   /// iterator invalidating modifications in the underlying data structure.
52   /// When LLVM is built without asserts, this class is empty and does nothing.
53   ///
54   /// HandleBase does not track the parent data structure by itself.  It expects
55   /// the routines modifying the data structure to call incrementEpoch when they
56   /// make an iterator-invalidating modification.
57   ///
58   class HandleBase {
59     const uint64_t *EpochAddress = nullptr;
60     uint64_t EpochAtCreation = UINT64_MAX;
61 
62   public:
63     HandleBase() = default;
64 
65     explicit HandleBase(const DebugEpochBase *Parent)
66         : EpochAddress(&Parent->Epoch), EpochAtCreation(Parent->Epoch) {}
67 
68     /// Returns true if the DebugEpochBase this Handle is linked to has
69     /// not called incrementEpoch on itself since the creation of this
70     /// HandleBase instance.
71     bool isHandleInSync() const { return *EpochAddress == EpochAtCreation; }
72 
73     /// Returns a pointer to the epoch word stored in the data structure
74     /// this handle points into.  Can be used to check if two iterators point
75     /// into the same data structure.
76     const void *getEpochAddress() const { return EpochAddress; }
77   };
78 };
79 
80 #else
81 
82 class DebugEpochBase {
83 public:
84   void incrementEpoch() {}
85 
86   class HandleBase {
87   public:
88     HandleBase() = default;
89     explicit HandleBase(const DebugEpochBase *) {}
90     bool isHandleInSync() const { return true; }
91     const void *getEpochAddress() const { return nullptr; }
92   };
93 };
94 
95 #endif // LLVM_ENABLE_ABI_BREAKING_CHECKS
96 
97 } // namespace llvm
98 
99 #endif
100