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