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.
  62   Stringpool_template();
  63
  64   ~Stringpool_template();
  65
  66   // Indicate that we should not reserve offset 0 to hold the empty
  67   // string when converting the stringpool to a string table.  This
  68   // should not be called for a proper ELF SHT_STRTAB section.
  69   void
  70   set_no_zero_null()
  71   { this->zero_null_ = false; }
  72
  73   // Add the string S to the pool.  This returns a canonical permanent
  74   // pointer to the string in the pool.  If PKEY is not NULL, this
  75   // sets *PKEY to the key for the string.
  76   const Stringpool_char*
  77   add(const Stringpool_char* s, Key* pkey);
  78
  79   // Add the string S to the pool.
  80   const Stringpool_char*
  81   add(const std::basic_string<Stringpool_char>& s, Key* pkey)
  82   { return this->add(s.c_str(), pkey); }
  83
  84   // Add the prefix of length LEN of string S to the pool.
  85   const Stringpool_char*
  86   add(const Stringpool_char* s, size_t len, Key* pkey);
  87
  88   // If the string S is present in the pool, return the canonical
  89   // string pointer.  Otherwise, return NULL.  If PKEY is not NULL,
  90   // set *PKEY to the key.
  91   const Stringpool_char*
  92   find(const Stringpool_char* s, Key* pkey) const;
  93
  94   // Turn the stringpool into a string table: determine the offsets of
  95   // all the strings.  After this is called, no more strings may be
  96   // added to the stringpool.
  97   void
  98   set_string_offsets();
  99
 100   // Get the offset of the string S in the string table.  This returns
 101   // the offset in bytes, not in units of Stringpool_char.  This may
 102   // only be called after set_string_offsets has been called.
 103   off_t
 104   get_offset(const Stringpool_char* s) const;
 105
 106   // Get the offset of the string S in the string table.
 107   off_t
 108   get_offset(const std::basic_string<Stringpool_char>& s) const
 109   { return this->get_offset(s.c_str()); }
 110
 111   // Get the size of the string table.  This returns the number of
 112   // bytes, not in units of Stringpool_char.
 113   off_t
 114   get_strtab_size() const
 115   {
 116     gold_assert(this->strtab_size_ != 0);
 117     return this->strtab_size_;
 118   }
 119
 120   // Write the string table into the output file at the specified
 121   // offset.
 122   void
 123   write(Output_file*, off_t offset);
 124
 125  private:
 126   Stringpool_template(const Stringpool_template&);
 127   Stringpool_template& operator=(const Stringpool_template&);
 128
 129   // Return the length of a string in units of Stringpool_char.
 130   static size_t
 131   string_length(const Stringpool_char*);
 132
 133   // We store the actual data in a list of these buffers.
 134   struct Stringdata
 135   {
 136     // Length of data in buffer.
 137     size_t len;
 138     // Allocated size of buffer.
 139     size_t alc;
 140     // Buffer index.
 141     unsigned int index;
 142     // Buffer.
 143     char data[1];
 144   };
 145
 146   // Copy a string into the buffers, returning a canonical string.
 147   const Stringpool_char*
 148   add_string(const Stringpool_char*, Key*);
 149
 150   // Hash function.
 151   struct Stringpool_hash
 152   {
 153     size_t
 154     operator()(const Stringpool_char*) const;
 155   };
 156
 157   // Equality comparison function for hash table.
 158   struct Stringpool_eq
 159   {
 160     bool
 161     operator()(const Stringpool_char* p1, const Stringpool_char* p2) const;
 162   };
 163
 164   // Return whether s1 is a suffix of s2.
 165   static bool
 166   is_suffix(const Stringpool_char* s1, size_t len1,
 167             const Stringpool_char* s2, size_t len2);
 168
 169   // The hash table is a map from string names to a pair of Key and
 170   // string table offsets.  We only use the offsets if we turn this
 171   // into an string table section.
 172
 173   typedef std::pair<Key, off_t> Val;
 174
 175 #ifdef HAVE_TR1_UNORDERED_SET
 176   typedef Unordered_map<const Stringpool_char*, Val, Stringpool_hash,
 177                         Stringpool_eq,
 178                         std::allocator<std::pair<const Stringpool_char* const,
 179                                                  Val> >,
 180                         true> String_set_type;
 181 #else
 182   typedef Unordered_map<const Stringpool_char*, Val, Stringpool_hash,
 183                         Stringpool_eq> String_set_type;
 184 #endif
 185
 186   // Comparison routine used when sorting into a string table.  We
 187   // store string-sizes in the sort-vector so we don't have to
 188   // recompute them log(n) times as we sort.
 189   struct Stringpool_sort_info
 190   {
 191     typename String_set_type::iterator it;
 192     size_t string_length;
 193     Stringpool_sort_info(typename String_set_type::iterator i, size_t s)
 194       : it(i), string_length(s)
 195     { }
 196   };
 197
 198   struct Stringpool_sort_comparison
 199   {
 200     bool
 201     operator()(const Stringpool_sort_info&, const Stringpool_sort_info&) const;
 202   };
 203
 204   // List of Stringdata structures.
 205   typedef std::list<Stringdata*> Stringdata_list;
 206
 207   // Mapping from const char* to namepool entry.
 208   String_set_type string_set_;
 209   // List of buffers.
 210   Stringdata_list strings_;
 211   // Size of string table.
 212   off_t strtab_size_;
 213   // Next Stringdata index.
 214   unsigned int next_index_;
 215   // Whether to reserve offset 0 to hold the null string.
 216   bool zero_null_;
 217 };
 218
 219 // The most common type of Stringpool.
 220 typedef Stringpool_template<char> Stringpool;
 221
 222 } // End namespace gold.
 223
 224 #endif // !defined(GOLD_STRINGPOOL_H)