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/abi-breaking.h"
20 #include "llvm/Config/llvm-config.h"
21 
22 #include <cstdint>
23 
24 namespace llvm {
25 
26 #if LLVM_ENABLE_ABI_BREAKING_CHECKS
27 
28 /// \brief A base class for data structure classes wishing to make iterators
29 /// ("handles") pointing into themselves fail-fast.  When building without
30 /// asserts, this class is empty and does nothing.
31 ///
32 /// DebugEpochBase does not by itself track handles pointing into itself.  The
33 /// expectation is that routines touching the handles will poll on
34 /// isHandleInSync at appropriate points to assert that the handle they're using
35 /// is still valid.
36 ///
37 class DebugEpochBase {
38   uint64_t Epoch;
39 
40 public:
DebugEpochBase()41   DebugEpochBase() : Epoch(0) {}
42 
43   /// \brief Calling incrementEpoch invalidates all handles pointing into the
44   /// calling instance.
incrementEpoch()45   void incrementEpoch() { ++Epoch; }
46 
47   /// \brief The destructor calls incrementEpoch to make use-after-free bugs
48   /// more likely to crash deterministically.
~DebugEpochBase()49   ~DebugEpochBase() { incrementEpoch(); }
50 
51   /// \brief A base class for iterator classes ("handles") that wish to poll for
52   /// iterator invalidating modifications in the underlying data structure.
53   /// When LLVM is built without asserts, this class is empty and does nothing.
54   ///
55   /// HandleBase does not track the parent data structure by itself.  It expects
56   /// the routines modifying the data structure to call incrementEpoch when they
57   /// make an iterator-invalidating modification.
58   ///
59   class HandleBase {
60     const uint64_t *EpochAddress;
61     uint64_t EpochAtCreation;
62 
63   public:
HandleBase()64     HandleBase() : EpochAddress(nullptr), EpochAtCreation(UINT64_MAX) {}
65 
HandleBase(const DebugEpochBase * Parent)66     explicit HandleBase(const DebugEpochBase *Parent)
67         : EpochAddress(&Parent->Epoch), EpochAtCreation(Parent->Epoch) {}
68 
69     /// \brief Returns true if the DebugEpochBase this Handle is linked to has
70     /// not called incrementEpoch on itself since the creation of this
71     /// HandleBase instance.
isHandleInSync()72     bool isHandleInSync() const { return *EpochAddress == EpochAtCreation; }
73 
74     /// \brief Returns a pointer to the epoch word stored in the data structure
75     /// this handle points into.  Can be used to check if two iterators point
76     /// into the same data structure.
getEpochAddress()77     const void *getEpochAddress() const { return EpochAddress; }
78   };
79 };
80 
81 #else
82 
83 class DebugEpochBase {
84 public:
85   void incrementEpoch() {}
86 
87   class HandleBase {
88   public:
89     HandleBase() = default;
90     explicit HandleBase(const DebugEpochBase *) {}
91     bool isHandleInSync() const { return true; }
92     const void *getEpochAddress() const { return nullptr; }
93   };
94 };
95 
96 #endif // LLVM_ENABLE_ABI_BREAKING_CHECKS
97 
98 } // namespace llvm
99 
100 #endif
101