gold/stringpool.h

   1 // stringpool.h -- a string pool for gold    -*- C++ -*-
   2
   3 // Copyright 2006, 2007 Free Software Foundation, Inc.
   4 // Written by Ian Lance Taylor <iant@google.com>.
   5
   6 // This file is part of gold.
   7
   8 // This program is free software; you can redistribute it and/or modify
   9 // it under the terms of the GNU General Public License as published by
  10 // the Free Software Foundation; either version 3 of the License, or
  11 // (at your option) any later version.
  12
  13 // This program is distributed in the hope that it will be useful,
  14 // but WITHOUT ANY WARRANTY; without even the implied warranty of
  15 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  16 // GNU General Public License for more details.
  17
  18 // You should have received a copy of the GNU General Public License
  19 // along with this program; if not, write to the Free Software
  20 // Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston,
  21 // MA 02110-1301, USA.
  22
  23 #include <string>
  24 #include <list>
  25
  26 #ifndef GOLD_STRINGPOOL_H
  27 #define GOLD_STRINGPOOL_H
  28
  29 namespace gold
  30 {
  31
  32 class Output_file;
  33
  34 // A Stringpool is a pool of unique strings.  It provides the
  35 // following features:
  36
  37 // Every string in the pool is unique.  Thus, if you have two strings
  38 // in the Stringpool, you can compare them for equality by using
  39 // pointer comparison rather than string comparison.
  40
  41 // There is a key associated with every string in the pool.  If you
  42 // add strings to the Stringpool in the same order, then the key for
  43 // each string will always be the same for any run of the linker.
  44 // This is not true of the string pointers themselves, as they may
  45 // change due to address space randomization.  Some parts of the
  46 // linker (e.g., the symbol table) use the key value instead of the
  47 // string pointer so that repeated runs of the linker will generate
  48 // precisely the same output.
  49
  50 // When you add a string to a Stringpool, Stringpool will make a copy
  51 // of it.  Thus there is no need to keep a copy elsewhere.
  52
  53 // A Stringpool can be turned into a string table, a sequential series
  54 // of null terminated strings.  The first string may optionally be a
  55 // single zero byte, as required for SHT_STRTAB sections.  This
  56 // conversion is only permitted after all strings have been added to
  57 // the Stringpool.  After doing this conversion, you can ask for the
  58 // offset of any string in the stringpool in the string table, and you
  59 // can write the resulting string table to an output file.
  60
  61 // When a Stringpool is turned into a string table, then as an
  62 // optimization it will reuse string suffixes to avoid duplicating
  63 // strings.  That is, given the strings "abc" and "bc", only the
  64 // string "abc" will be stored, and "bc" will be represented by an
  65 // offset into the middle of the string "abc".
  66
  67 // Stringpools are implemented in terms of Stringpool_template, which
  68 // is generalized on the type of character used for the strings.  Most
  69 // uses will want the Stringpool type which uses char.  Other cases
  70 // are used for merging wide string constants.
  71
  72 template<typename Stringpool_char>
  73 class Stringpool_template
  74 {
  75  public:
  76   // The type of a key into the stringpool.  As described above, a key
  77   // value will always be the same during any run of the linker.  Zero
  78   // is never a valid key value.
  79   typedef size_t Key;
  80
  81   // Create a Stringpool.
  82   Stringpool_template();
  83
  84   ~Stringpool_template();
  85
  86   // Indicate that we should not reserve offset 0 to hold the empty
  87   // string when converting the stringpool to a string table.  This
  88   // should not be called for a proper ELF SHT_STRTAB section.
  89   void
  90   set_no_zero_null()
  91   { this->zero_null_ = false; }
  92
  93   // Add the string S to the pool.  This returns a canonical permanent
  94   // pointer to the string in the pool.  If PKEY is not NULL, this
  95   // sets *PKEY to the key for the string.
  96   const Stringpool_char*
  97   add(const Stringpool_char* s, Key* pkey);
  98
  99   // Add the string S to the pool.
 100   const Stringpool_char*
 101   add(const std::basic_string<Stringpool_char>& s, Key* pkey)
 102   { return this->add(s.c_str(), pkey); }
 103
 104   // Add the prefix of length LEN of string S to the pool.
 105   const Stringpool_char*
 106   add(const Stringpool_char* s, size_t len, Key* pkey);
 107
 108   // If the string S is present in the pool, return the canonical
 109   // string pointer.  Otherwise, return NULL.  If PKEY is not NULL,
 110   // set *PKEY to the key.
 111   const Stringpool_char*
 112   find(const Stringpool_char* s, Key* pkey) const;
 113
 114   // Turn the stringpool into a string table: determine the offsets of
 115   // all the strings.  After this is called, no more strings may be
 116   // added to the stringpool.
 117   void
 118   set_string_offsets();
 119
 120   // Get the offset of the string S in the string table.  This returns
 121   // the offset in bytes, not in units of Stringpool_char.  This may
 122   // only be called after set_string_offsets has been called.
 123   off_t
 124   get_offset(const Stringpool_char* s) const;
 125
 126   // Get the offset of the string S in the string table.
 127   off_t
 128   get_offset(const std::basic_string<Stringpool_char>& s) const
 129   { return this->get_offset(s.c_str()); }
 130
 131   // Get the size of the string table.  This returns the number of
 132   // bytes, not in units of Stringpool_char.
 133   off_t
 134   get_strtab_size() const
 135   {
 136     gold_assert(this->strtab_size_ != 0);
 137     return this->strtab_size_;
 138   }
 139
 140   // Write the string table into the output file at the specified
 141   // offset.
 142   void
 143   write(Output_file*, off_t offset);
 144
 145  private:
 146   Stringpool_template(const Stringpool_template&);
 147   Stringpool_template& operator=(const Stringpool_template&);
 148
 149   // Return the length of a string in units of Stringpool_char.
 150   static size_t
 151   string_length(const Stringpool_char*);
 152
 153   // We store the actual data in a list of these buffers.
 154   struct Stringdata
 155   {
 156     // Length of data in buffer.
 157     size_t len;
 158     // Allocated size of buffer.
 159     size_t alc;
 160     // Buffer index.
 161     unsigned int index;
 162     // Buffer.
 163     char data[1];
 164   };
 165
 166   // Copy a string into the buffers, returning a canonical string.
 167   const Stringpool_char*
 168   add_string(const Stringpool_char*, Key*);
 169
 170   // Hash function.
 171   struct Stringpool_hash
 172   {
 173     size_t
 174     operator()(const Stringpool_char*) const;
 175   };
 176
 177   // Equality comparison function for hash table.
 178   struct Stringpool_eq
 179   {
 180     bool
 181     operator()(const Stringpool_char* p1, const Stringpool_char* p2) const;
 182   };
 183
 184   // Return whether s1 is a suffix of s2.
 185   static bool
 186   is_suffix(const Stringpool_char* s1, size_t len1,
 187             const Stringpool_char* s2, size_t len2);
 188
 189   // The hash table is a map from string names to a pair of Key and
 190   // string table offsets.  We only use the offsets if we turn this
 191   // into an string table section.
 192
 193   typedef std::pair<Key, off_t> Val;
 194
 195 #ifdef HAVE_TR1_UNORDERED_SET
 196   typedef Unordered_map<const Stringpool_char*, Val, Stringpool_hash,
 197                         Stringpool_eq,
 198                         std::allocator<std::pair<const Stringpool_char* const,
 199                                                  Val> >,
 200                         true> String_set_type;
 201 #else
 202   typedef Unordered_map<const Stringpool_char*, Val, Stringpool_hash,
 203                         Stringpool_eq> String_set_type;
 204 #endif
 205
 206   // Comparison routine used when sorting into a string table.  We
 207   // store string-sizes in the sort-vector so we don't have to
 208   // recompute them log(n) times as we sort.
 209   struct Stringpool_sort_info
 210   {
 211     typename String_set_type::iterator it;
 212     size_t string_length;
 213     Stringpool_sort_info(typename String_set_type::iterator i, size_t s)
 214       : it(i), string_length(s)
 215     { }
 216   };
 217
 218   struct Stringpool_sort_comparison
 219   {
 220     bool
 221     operator()(const Stringpool_sort_info&, const Stringpool_sort_info&) const;
 222   };
 223
 224   // List of Stringdata structures.
 225   typedef std::list<Stringdata*> Stringdata_list;
 226
 227   // Mapping from const char* to namepool entry.
 228   String_set_type string_set_;
 229   // List of buffers.
 230   Stringdata_list strings_;
 231   // Size of string table.
 232   off_t strtab_size_;
 233   // Next Stringdata index.
 234   unsigned int next_index_;
 235   // Whether to reserve offset 0 to hold the null string.
 236   bool zero_null_;
 237 };
 238
 239 // The most common type of Stringpool.
 240 typedef Stringpool_template<char> Stringpool;
 241
 242 } // End namespace gold.
 243
 244 #endif // !defined(GOLD_STRINGPOOL_H)