gold/stringpool.h

   1 // stringpool.h -- a string pool for gold    -*- C++ -*-
   2
   3 #include <string>
   4 #include <list>
   5
   6 #ifndef GOLD_STRINGPOOL_H
   7 #define GOLD_STRINGPOOL_H
   8
   9 namespace gold
  10 {
  11
  12 class Output_file;
  13
  14 // A Stringpool is a pool of unique strings.  It provides the
  15 // following features:
  16
  17 // Every string in the pool is unique.  Thus, if you have two strings
  18 // in the Stringpool, you can compare them for equality by using
  19 // pointer comparison rather than string comparison.
  20
  21 // There is a key associated with every string in the pool.  If you
  22 // add strings to the Stringpool in the same order, then the key for
  23 // each string will always be the same for any run of the linker.
  24 // This is not true of the string pointers themselves, as they may
  25 // change due to address space randomization.  Some parts of the
  26 // linker (e.g., the symbol table) use the key value instead of the
  27 // string pointer so that repeated runs of the linker will generate
  28 // precisely the same output.
  29
  30 // When you add a string to a Stringpool, Stringpool will make a copy
  31 // of it.  Thus there is no need to keep a copy elsewhere.
  32
  33 // A Stringpool can be turned into a string table, a sequential series
  34 // of null terminated strings.  The first string may optionally be a
  35 // single zero byte, as required for SHT_STRTAB sections.  This
  36 // conversion is only permitted after all strings have been added to
  37 // the Stringpool.  After doing this conversion, you can ask for the
  38 // offset of any string in the stringpool in the string table, and you
  39 // can write the resulting string table to an output file.
  40
  41 // When a Stringpool is turned into a string table, then as an
  42 // optimization it will reuse string suffixes to avoid duplicating
  43 // strings.  That is, given the strings "abc" and "bc", only the
  44 // string "abc" will be stored, and "bc" will be represented by an
  45 // offset into the middle of the string "abc".
  46
  47 // Stringpools are implemented in terms of Stringpool_template, which
  48 // is generalized on the type of character used for the strings.  Most
  49 // uses will want the Stringpool type which uses char.  Other cases
  50 // are used for merging wide string constants.
  51
  52 template<typename Stringpool_char>
  53 class Stringpool_template
  54 {
  55  public:
  56   // The type of a key into the stringpool.  As described above, a key
  57   // value will always be the same during any run of the linker.  Zero
  58   // is never a valid key value.
  59   typedef size_t Key;
  60
  61   // Create a Stringpool.  ZERO_NULL is true if we should reserve
  62   // offset 0 to hold the empty string when converting the stringpool
  63   // to a string table.  ZERO_NULL should be true if you want a proper
  64   // ELF SHT_STRTAB section.
  65   Stringpool_template(bool zero_null = true);
  66
  67   ~Stringpool_template();
  68
  69   // Add the string S to the pool.  This returns a canonical permanent
  70   // pointer to the string in the pool.  If PKEY is not NULL, this
  71   // sets *PKEY to the key for the string.
  72   const Stringpool_char*
  73   add(const Stringpool_char* s, Key* pkey);
  74
  75   // Add the string S to the pool.
  76   const Stringpool_char*
  77   add(const std::basic_string<Stringpool_char>& s, Key* pkey)
  78   { return this->add(s.c_str(), pkey); }
  79
  80   // Add the prefix of length LEN of string S to the pool.
  81   const Stringpool_char*
  82   add(const Stringpool_char* s, size_t len, Key* pkey);
  83
  84   // If the string S is present in the pool, return the canonical
  85   // string pointer.  Otherwise, return NULL.  If PKEY is not NULL,
  86   // set *PKEY to the key.
  87   const Stringpool_char*
  88   find(const Stringpool_char* s, Key* pkey) const;
  89
  90   // Turn the stringpool into a string table: determine the offsets of
  91   // all the strings.  After this is called, no more strings may be
  92   // added to the stringpool.
  93   void
  94   set_string_offsets();
  95
  96   // Get the offset of the string S in the string table.  This returns
  97   // the offset in bytes, not in units of Stringpool_char.  This may
  98   // only be called after set_string_offsets has been called.
  99   off_t
 100   get_offset(const Stringpool_char* s) const;
 101
 102   // Get the offset of the string S in the string table.
 103   off_t
 104   get_offset(const std::basic_string<Stringpool_char>& s) const
 105   { return this->get_offset(s.c_str()); }
 106
 107   // Get the size of the string table.  This returns the number of
 108   // bytes, not in units of Stringpool_char.
 109   off_t
 110   get_strtab_size() const
 111   {
 112     gold_assert(this->strtab_size_ != 0);
 113     return this->strtab_size_;
 114   }
 115
 116   // Write the string table into the output file at the specified
 117   // offset.
 118   void
 119   write(Output_file*, off_t offset);
 120
 121  private:
 122   Stringpool_template(const Stringpool_template&);
 123   Stringpool_template& operator=(const Stringpool_template&);
 124
 125   // Return the length of a string in units of Stringpool_char.
 126   static size_t
 127   string_length(const Stringpool_char*);
 128
 129   // We store the actual data in a list of these buffers.
 130   struct Stringdata
 131   {
 132     // Length of data in buffer.
 133     size_t len;
 134     // Allocated size of buffer.
 135     size_t alc;
 136     // Buffer index.
 137     unsigned int index;
 138     // Buffer.
 139     char data[1];
 140   };
 141
 142   // Copy a string into the buffers, returning a canonical string.
 143   const Stringpool_char*
 144   add_string(const Stringpool_char*, Key*);
 145
 146   // Hash function.
 147   struct Stringpool_hash
 148   {
 149     size_t
 150     operator()(const Stringpool_char*) const;
 151   };
 152
 153   // Equality comparison function for hash table.
 154   struct Stringpool_eq
 155   {
 156     bool
 157     operator()(const Stringpool_char* p1, const Stringpool_char* p2) const;
 158   };
 159
 160   // Return whether s1 is a suffix of s2.
 161   static bool
 162   is_suffix(const Stringpool_char* s1, size_t len1,
 163             const Stringpool_char* s2, size_t len2);
 164
 165   // The hash table is a map from string names to a pair of Key and
 166   // string table offsets.  We only use the offsets if we turn this
 167   // into an string table section.
 168
 169   typedef std::pair<Key, off_t> Val;
 170
 171 #ifdef HAVE_TR1_UNORDERED_SET
 172   typedef Unordered_map<const Stringpool_char*, Val, Stringpool_hash,
 173                         Stringpool_eq,
 174                         std::allocator<std::pair<const Stringpool_char* const,
 175                                                  Val> >,
 176                         true> String_set_type;
 177 #else
 178   typedef Unordered_map<const Stringpool_char*, Val, Stringpool_hash,
 179                         Stringpool_eq> String_set_type;
 180 #endif
 181
 182   // Comparison routine used when sorting into a string table.  We
 183   // store string-sizes in the sort-vector so we don't have to
 184   // recompute them log(n) times as we sort.
 185   struct Stringpool_sort_info
 186   {
 187     typename String_set_type::iterator it;
 188     size_t string_length;
 189     Stringpool_sort_info(typename String_set_type::iterator i, size_t s)
 190       : it(i), string_length(s)
 191     { }
 192   };
 193
 194   struct Stringpool_sort_comparison
 195   {
 196     bool
 197     operator()(const Stringpool_sort_info&, const Stringpool_sort_info&) const;
 198   };
 199
 200   // List of Stringdata structures.
 201   typedef std::list<Stringdata*> Stringdata_list;
 202
 203   // Mapping from const char* to namepool entry.
 204   String_set_type string_set_;
 205   // List of buffers.
 206   Stringdata_list strings_;
 207   // Size of string table.
 208   off_t strtab_size_;
 209   // Next Stringdata index.
 210   unsigned int next_index_;
 211   // Whether to reserve offset 0 to hold the null string.
 212   bool zero_null_;
 213 };
 214
 215 // The most common type of Stringpool.
 216 typedef Stringpool_template<char> Stringpool;
 217
 218 } // End namespace gold.
 219
 220 #endif // !defined(GOLD_STRINGPOOL_H)