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 optionally
  51 // make a copy of it.  Thus there is no requirement to keep a copy
  52 // elsewhere.
  53
  54 // A Stringpool can be turned into a string table, a sequential series
  55 // of null terminated strings.  The first string may optionally be a
  56 // single zero byte, as required for SHT_STRTAB sections.  This
  57 // conversion is only permitted after all strings have been added to
  58 // the Stringpool.  After doing this conversion, you can ask for the
  59 // offset of any string in the stringpool in the string table, and you
  60 // can write the resulting string table to an output file.
  61
  62 // When a Stringpool is turned into a string table, then as an
  63 // optimization it will reuse string suffixes to avoid duplicating
  64 // strings.  That is, given the strings "abc" and "bc", only the
  65 // string "abc" will be stored, and "bc" will be represented by an
  66 // offset into the middle of the string "abc".
  67
  68 // Stringpools are implemented in terms of Stringpool_template, which
  69 // is generalized on the type of character used for the strings.  Most
  70 // uses will want the Stringpool type which uses char.  Other cases
  71 // are used for merging wide string constants.
  72
  73 template<typename Stringpool_char>
  74 class Stringpool_template
  75 {
  76  public:
  77   // The type of a key into the stringpool.  As described above, a key
  78   // value will always be the same during any run of the linker.  Zero
  79   // is never a valid key value.
  80   typedef size_t Key;
  81
  82   // Create a Stringpool.
  83   Stringpool_template();
  84
  85   ~Stringpool_template();
  86
  87   // Indicate that we should not reserve offset 0 to hold the empty
  88   // string when converting the stringpool to a string table.  This
  89   // should not be called for a proper ELF SHT_STRTAB section.
  90   void
  91   set_no_zero_null()
  92   { this->zero_null_ = false; }
  93
  94   // Add the string S to the pool.  This returns a canonical permanent
  95   // pointer to the string in the pool.  If COPY is true, the string
  96   // is copied into permanent storage.  If PKEY is not NULL, this sets
  97   // *PKEY to the key for the string.
  98   const Stringpool_char*
  99   add(const Stringpool_char* s, bool copy, Key* pkey);
 100
 101   // Add the prefix of length LEN of string S to the pool.
 102   const Stringpool_char*
 103   add_prefix(const Stringpool_char* s, size_t len, Key* pkey);
 104
 105   // If the string S is present in the pool, return the canonical
 106   // string pointer.  Otherwise, return NULL.  If PKEY is not NULL,
 107   // set *PKEY to the key.
 108   const Stringpool_char*
 109   find(const Stringpool_char* s, Key* pkey) const;
 110
 111   // Turn the stringpool into a string table: determine the offsets of
 112   // all the strings.  After this is called, no more strings may be
 113   // added to the stringpool.
 114   void
 115   set_string_offsets();
 116
 117   // Get the offset of the string S in the string table.  This returns
 118   // the offset in bytes, not in units of Stringpool_char.  This may
 119   // only be called after set_string_offsets has been called.
 120   off_t
 121   get_offset(const Stringpool_char* s) const;
 122
 123   // Get the offset of the string S in the string table.
 124   off_t
 125   get_offset(const std::basic_string<Stringpool_char>& s) const
 126   { return this->get_offset(s.c_str()); }
 127
 128   // Get the size of the string table.  This returns the number of
 129   // bytes, not in units of Stringpool_char.
 130   off_t
 131   get_strtab_size() const
 132   {
 133     gold_assert(this->strtab_size_ != 0);
 134     return this->strtab_size_;
 135   }
 136
 137   // Write the string table into the output file at the specified
 138   // offset.
 139   void
 140   write(Output_file*, off_t offset);
 141
 142  private:
 143   Stringpool_template(const Stringpool_template&);
 144   Stringpool_template& operator=(const Stringpool_template&);
 145
 146   // Return the length of a string in units of Stringpool_char.
 147   static size_t
 148   string_length(const Stringpool_char*);
 149
 150   // We store the actual data in a list of these buffers.
 151   struct Stringdata
 152   {
 153     // Length of data in buffer.
 154     size_t len;
 155     // Allocated size of buffer.
 156     size_t alc;
 157     // Buffer index.
 158     unsigned int index;
 159     // Buffer.
 160     char data[1];
 161   };
 162
 163   // Copy a string into the buffers, returning a canonical string.
 164   const Stringpool_char*
 165   add_string(const Stringpool_char*, Key*);
 166
 167   // Hash function.
 168   struct Stringpool_hash
 169   {
 170     size_t
 171     operator()(const Stringpool_char*) const;
 172   };
 173
 174   // Equality comparison function for hash table.
 175   struct Stringpool_eq
 176   {
 177     bool
 178     operator()(const Stringpool_char* p1, const Stringpool_char* p2) const;
 179   };
 180
 181   // Return whether s1 is a suffix of s2.
 182   static bool
 183   is_suffix(const Stringpool_char* s1, size_t len1,
 184             const Stringpool_char* s2, size_t len2);
 185
 186   // The hash table is a map from string names to a pair of Key and
 187   // string table offsets.  We only use the offsets if we turn this
 188   // into an string table section.
 189
 190   typedef std::pair<Key, off_t> Val;
 191
 192 #ifdef HAVE_TR1_UNORDERED_SET
 193   typedef Unordered_map<const Stringpool_char*, Val, Stringpool_hash,
 194                         Stringpool_eq,
 195                         std::allocator<std::pair<const Stringpool_char* const,
 196                                                  Val> >,
 197                         true> String_set_type;
 198 #else
 199   typedef Unordered_map<const Stringpool_char*, Val, Stringpool_hash,
 200                         Stringpool_eq> String_set_type;
 201 #endif
 202
 203   // Comparison routine used when sorting into a string table.  We
 204   // store string-sizes in the sort-vector so we don't have to
 205   // recompute them log(n) times as we sort.
 206   struct Stringpool_sort_info
 207   {
 208     typename String_set_type::iterator it;
 209     size_t string_length;
 210     Stringpool_sort_info(typename String_set_type::iterator i, size_t s)
 211       : it(i), string_length(s)
 212     { }
 213   };
 214
 215   struct Stringpool_sort_comparison
 216   {
 217     bool
 218     operator()(const Stringpool_sort_info&, const Stringpool_sort_info&) const;
 219   };
 220
 221   // List of Stringdata structures.
 222   typedef std::list<Stringdata*> Stringdata_list;
 223
 224   // Mapping from const char* to namepool entry.
 225   String_set_type string_set_;
 226   // List of buffers.
 227   Stringdata_list strings_;
 228   // Size of string table.
 229   off_t strtab_size_;
 230   // Next Stringdata index.
 231   unsigned int next_index_;
 232   // Next key value for a string we don't copy.
 233   int next_uncopied_key_;
 234   // Whether to reserve offset 0 to hold the null string.
 235   bool zero_null_;
 236 };
 237
 238 // The most common type of Stringpool.
 239 typedef Stringpool_template<char> Stringpool;
 240
 241 } // End namespace gold.
 242
 243 #endif // !defined(GOLD_STRINGPOOL_H)