1//===-- StringPool.h - Interned string pool ---------------------*- 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 declares an interned string pool, which helps reduce the cost of
11// strings by using the same storage for identical strings.
12//
13// To intern a string:
14//
15//   StringPool Pool;
16//   PooledStringPtr Str = Pool.intern("wakka wakka");
17//
18// To use the value of an interned string, use operator bool and operator*:
19//
20//   if (Str)
21//     cerr << "the string is" << *Str << "\n";
22//
23// Pooled strings are immutable, but you can change a PooledStringPtr to point
24// to another instance. So that interned strings can eventually be freed,
25// strings in the string pool are reference-counted (automatically).
26//
27//===----------------------------------------------------------------------===//
28
29#ifndef LLVM_SUPPORT_STRINGPOOL_H
30#define LLVM_SUPPORT_STRINGPOOL_H
31
32#include "llvm/ADT/StringMap.h"
33#include <cassert>
34
35namespace llvm {
36
37  class PooledStringPtr;
38
39  /// StringPool - An interned string pool. Use the intern method to add a
40  /// string. Strings are removed automatically as PooledStringPtrs are
41  /// destroyed.
42  class StringPool {
43    /// PooledString - This is the value of an entry in the pool's interning
44    /// table.
45    struct PooledString {
46      StringPool *Pool;  ///< So the string can remove itself.
47      unsigned Refcount; ///< Number of referencing PooledStringPtrs.
48
49    public:
50      PooledString() : Pool(nullptr), Refcount(0) { }
51    };
52
53    friend class PooledStringPtr;
54
55    typedef StringMap<PooledString> table_t;
56    typedef StringMapEntry<PooledString> entry_t;
57    table_t InternTable;
58
59  public:
60    StringPool();
61    ~StringPool();
62
63    /// intern - Adds a string to the pool and returns a reference-counted
64    /// pointer to it. No additional memory is allocated if the string already
65    /// exists in the pool.
66    PooledStringPtr intern(StringRef Str);
67
68    /// empty - Checks whether the pool is empty. Returns true if so.
69    ///
70    inline bool empty() const { return InternTable.empty(); }
71  };
72
73  /// PooledStringPtr - A pointer to an interned string. Use operator bool to
74  /// test whether the pointer is valid, and operator * to get the string if so.
75  /// This is a lightweight value class with storage requirements equivalent to
76  /// a single pointer, but it does have reference-counting overhead when
77  /// copied.
78  class PooledStringPtr {
79    typedef StringPool::entry_t entry_t;
80    entry_t *S;
81
82  public:
83    PooledStringPtr() : S(nullptr) {}
84
85    explicit PooledStringPtr(entry_t *E) : S(E) {
86      if (S) ++S->getValue().Refcount;
87    }
88
89    PooledStringPtr(const PooledStringPtr &That) : S(That.S) {
90      if (S) ++S->getValue().Refcount;
91    }
92
93    PooledStringPtr &operator=(const PooledStringPtr &That) {
94      if (S != That.S) {
95        clear();
96        S = That.S;
97        if (S) ++S->getValue().Refcount;
98      }
99      return *this;
100    }
101
102    void clear() {
103      if (!S)
104        return;
105      if (--S->getValue().Refcount == 0) {
106        S->getValue().Pool->InternTable.remove(S);
107        S->Destroy();
108      }
109      S = nullptr;
110    }
111
112    ~PooledStringPtr() { clear(); }
113
114    inline const char *begin() const {
115      assert(*this && "Attempt to dereference empty PooledStringPtr!");
116      return S->getKeyData();
117    }
118
119    inline const char *end() const {
120      assert(*this && "Attempt to dereference empty PooledStringPtr!");
121      return S->getKeyData() + S->getKeyLength();
122    }
123
124    inline unsigned size() const {
125      assert(*this && "Attempt to dereference empty PooledStringPtr!");
126      return S->getKeyLength();
127    }
128
129    inline const char *operator*() const { return begin(); }
130    inline explicit operator bool() const { return S != nullptr; }
131
132    inline bool operator==(const PooledStringPtr &That) const { return S == That.S; }
133    inline bool operator!=(const PooledStringPtr &That) const { return S != That.S; }
134  };
135
136} // End llvm namespace
137
138#endif
139