gold/stringpool.h

   1 // stringpool.h -- a string pool for gold    -*- C++ -*-
   2
   3 // Copyright 2006, 2007, 2008 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 #include <vector>
  26
  27 #ifndef GOLD_STRINGPOOL_H
  28 #define GOLD_STRINGPOOL_H
  29
  30 namespace gold
  31 {
  32
  33 class Output_file;
  34
  35 // A Stringpool is a pool of unique strings.  It provides the
  36 // following features:
  37
  38 // Every string in the pool is unique.  Thus, if you have two strings
  39 // in the Stringpool, you can compare them for equality by using
  40 // pointer comparison rather than string comparison.
  41
  42 // There is a key associated with every string in the pool.  If you
  43 // add strings to the Stringpool in the same order, then the key for
  44 // each string will always be the same for any run of the linker.
  45 // This is not true of the string pointers themselves, as they may
  46 // change due to address space randomization.  Some parts of the
  47 // linker (e.g., the symbol table) use the key value instead of the
  48 // string pointer so that repeated runs of the linker will generate
  49 // precisely the same output.
  50
  51 // When you add a string to a Stringpool, Stringpool will optionally
  52 // make a copy of it.  Thus there is no requirement to keep a copy
  53 // elsewhere.
  54
  55 // A Stringpool can be turned into a string table, a sequential series
  56 // of null terminated strings.  The first string may optionally be a
  57 // single zero byte, as required for SHT_STRTAB sections.  This
  58 // conversion is only permitted after all strings have been added to
  59 // the Stringpool.  After doing this conversion, you can ask for the
  60 // offset of any string (or any key) in the stringpool in the string
  61 // table, and you can write the resulting string table to an output
  62 // file.
  63
  64 // When a Stringpool is turned into a string table, then as an
  65 // optimization it will reuse string suffixes to avoid duplicating
  66 // strings.  That is, given the strings "abc" and "bc", only the
  67 // string "abc" will be stored, and "bc" will be represented by an
  68 // offset into the middle of the string "abc".
  69
  70
  71 // A simple chunked vector class--this is a subset of std::vector
  72 // which stores memory in chunks.  We don't provide iterators, because
  73 // we don't need them.
  74
  75 template<typename Element>
  76 class Chunked_vector
  77 {
  78  public:
  79   Chunked_vector()
  80     : chunks_(), size_(0)
  81   { }
  82
  83   // Clear the elements.
  84   void
  85   clear()
  86   {
  87     this->chunks_.clear();
  88     this->size_ = 0;
  89   }
  90
  91   // Reserve elements.
  92   void
  93   reserve(unsigned int n)
  94   {
  95     if (n > this->chunks_.size() * chunk_size)
  96       {
  97         this->chunks_.resize((n + chunk_size - 1) / chunk_size);
  98         // We need to call reserve() of all chunks since changing
  99         // this->chunks_ casues Element_vectors to be copied.  The
 100         // reserved capacity of an Element_vector may be lost in copying.
 101         for (size_t i = 0; i < this->chunks_.size(); ++i)
 102           this->chunks_[i].reserve(chunk_size);
 103       }
 104   }
 105
 106   // Get the number of elements.
 107   size_t
 108   size() const
 109   { return this->size_; }
 110
 111   // Push a new element on the back of the vector.
 112   void
 113   push_back(const Element& element)
 114   {
 115     size_t chunk_index = this->size_ / chunk_size;
 116     if (chunk_index >= this->chunks_.size())
 117       {
 118         this->chunks_.push_back(Element_vector());
 119         this->chunks_.back().reserve(chunk_size);
 120         gold_assert(chunk_index < this->chunks_.size());
 121       }
 122     this->chunks_[chunk_index].push_back(element);
 123     this->size_++;
 124   }
 125
 126   // Return a reference to an entry in the vector.
 127   Element&
 128   operator[](size_t i)
 129   { return this->chunks_[i / chunk_size][i % chunk_size]; }
 130
 131   const Element&
 132   operator[](size_t i) const
 133   { return this->chunks_[i / chunk_size][i % chunk_size]; }
 134
 135  private:
 136   static const unsigned int chunk_size = 8192;
 137
 138   typedef std::vector<Element> Element_vector;
 139   typedef std::vector<Element_vector> Chunk_vector;
 140
 141   Chunk_vector chunks_;
 142   size_t size_;
 143 };
 144
 145
 146 // Stringpools are implemented in terms of Stringpool_template, which
 147 // is generalized on the type of character used for the strings.  Most
 148 // uses will want the Stringpool type which uses char.  Other cases
 149 // are used for merging wide string constants.
 150
 151 template<typename Stringpool_char>
 152 class Stringpool_template
 153 {
 154  public:
 155   // The type of a key into the stringpool.  As described above, a key
 156   // value will always be the same during any run of the linker.  Zero
 157   // is never a valid key value.
 158   typedef size_t Key;
 159
 160   // Create a Stringpool.
 161   Stringpool_template();
 162
 163   ~Stringpool_template();
 164
 165   // Clear all the data from the stringpool.
 166   void
 167   clear();
 168
 169   // Hint to the stringpool class that you intend to insert n additional
 170   // elements.  The stringpool class can use this info however it likes;
 171   // in practice it will resize its internal hashtables to make room.
 172   void
 173   reserve(unsigned int n);
 174
 175   // Indicate that we should not reserve offset 0 to hold the empty
 176   // string when converting the stringpool to a string table.  This
 177   // should not be called for a proper ELF SHT_STRTAB section.
 178   void
 179   set_no_zero_null()
 180   {
 181     gold_assert(this->string_set_.empty()
 182                 && this->offset_ == sizeof(Stringpool_char));
 183     this->zero_null_ = false;
 184     this->offset_ = 0;
 185   }
 186
 187   // Indicate that this string pool should be optimized, even if not
 188   // running with -O2.
 189   void
 190   set_optimize()
 191   { this->optimize_ = true; }
 192
 193   // Add the string S to the pool.  This returns a canonical permanent
 194   // pointer to the string in the pool.  If COPY is true, the string
 195   // is copied into permanent storage.  If PKEY is not NULL, this sets
 196   // *PKEY to the key for the string.
 197   const Stringpool_char*
 198   add(const Stringpool_char* s, bool copy, Key* pkey);
 199
 200   // Add string S of length LEN characters to the pool.  If COPY is
 201   // true, S need not be null terminated.
 202   const Stringpool_char*
 203   add_with_length(const Stringpool_char* s, size_t len, bool copy, Key* pkey);
 204
 205   // If the string S is present in the pool, return the canonical
 206   // string pointer.  Otherwise, return NULL.  If PKEY is not NULL,
 207   // set *PKEY to the key.
 208   const Stringpool_char*
 209   find(const Stringpool_char* s, Key* pkey) const;
 210
 211   // Turn the stringpool into a string table: determine the offsets of
 212   // all the strings.  After this is called, no more strings may be
 213   // added to the stringpool.
 214   void
 215   set_string_offsets();
 216
 217   // Get the offset of the string S in the string table.  This returns
 218   // the offset in bytes, not in units of Stringpool_char.  This may
 219   // only be called after set_string_offsets has been called.
 220   section_offset_type
 221   get_offset(const Stringpool_char* s) const;
 222
 223   // Get the offset of the string S in the string table.
 224   section_offset_type
 225   get_offset(const std::basic_string<Stringpool_char>& s) const
 226   { return this->get_offset_with_length(s.c_str(), s.size()); }
 227
 228   // Get the offset of string S, with length LENGTH characters, in the
 229   // string table.
 230   section_offset_type
 231   get_offset_with_length(const Stringpool_char* s, size_t length) const;
 232
 233   // Get the offset of the string with key K.
 234   section_offset_type
 235   get_offset_from_key(Key k) const
 236   {
 237     gold_assert(k <= this->key_to_offset_.size());
 238     return this->key_to_offset_[k - 1];
 239   }
 240
 241   // Get the size of the string table.  This returns the number of
 242   // bytes, not in units of Stringpool_char.
 243   section_size_type
 244   get_strtab_size() const
 245   {
 246     gold_assert(this->strtab_size_ != 0);
 247     return this->strtab_size_;
 248   }
 249
 250   // Write the string table into the output file at the specified
 251   // offset.
 252   void
 253   write(Output_file*, off_t offset);
 254
 255   // Write the string table into the specified buffer, of the
 256   // specified size.  buffer_size should be at least
 257   // get_strtab_size().
 258   void
 259   write_to_buffer(unsigned char* buffer, section_size_type buffer_size);
 260
 261   // Dump statistical information to stderr.
 262   void
 263   print_stats(const char*) const;
 264
 265  private:
 266   Stringpool_template(const Stringpool_template&);
 267   Stringpool_template& operator=(const Stringpool_template&);
 268
 269   // Return the length of a string in units of Stringpool_char.
 270   static size_t
 271   string_length(const Stringpool_char*);
 272
 273   // Return whether two strings are equal.
 274   static bool
 275   string_equal(const Stringpool_char*, const Stringpool_char*);
 276
 277   // Compute a hash code for a string.  LENGTH is the length of the
 278   // string in characters.
 279   static size_t
 280   string_hash(const Stringpool_char*, size_t length);
 281
 282   // We store the actual data in a list of these buffers.
 283   struct Stringdata
 284   {
 285     // Length of data in buffer.
 286     size_t len;
 287     // Allocated size of buffer.
 288     size_t alc;
 289     // Buffer.
 290     char data[1];
 291   };
 292
 293   // Add a new key offset entry.
 294   void
 295   new_key_offset(size_t);
 296
 297   // Copy a string into the buffers, returning a canonical string.
 298   const Stringpool_char*
 299   add_string(const Stringpool_char*, size_t);
 300
 301   // Return whether s1 is a suffix of s2.
 302   static bool
 303   is_suffix(const Stringpool_char* s1, size_t len1,
 304             const Stringpool_char* s2, size_t len2);
 305
 306   // The hash table key includes the string, the length of the string,
 307   // and the hash code for the string.  We put the hash code
 308   // explicitly into the key so that we can do a find()/insert()
 309   // sequence without having to recompute the hash.  Computing the
 310   // hash code is a significant user of CPU time in the linker.
 311   struct Hashkey
 312   {
 313     const Stringpool_char* string;
 314     // Length is in characters, not bytes.
 315     size_t length;
 316     size_t hash_code;
 317
 318     // This goes in an STL container, so we need a default
 319     // constructor.
 320     Hashkey()
 321       : string(NULL), length(0), hash_code(0)
 322     { }
 323
 324     // Note that these constructors are relatively expensive, because
 325     // they compute the hash code.
 326     explicit Hashkey(const Stringpool_char* s)
 327       : string(s), length(string_length(s)), hash_code(string_hash(s, length))
 328     { }
 329
 330     Hashkey(const Stringpool_char* s, size_t len)
 331       : string(s), length(len), hash_code(string_hash(s, len))
 332     { }
 333   };
 334
 335   // Hash function.  This is trivial, since we have already computed
 336   // the hash.
 337   struct Stringpool_hash
 338   {
 339     size_t
 340     operator()(const Hashkey& hk) const
 341     { return hk.hash_code; }
 342   };
 343
 344   // Equality comparison function for hash table.
 345   struct Stringpool_eq
 346   {
 347     bool
 348     operator()(const Hashkey&, const Hashkey&) const;
 349   };
 350
 351   // The hash table is a map from strings to Keys.
 352
 353   typedef Key Hashval;
 354
 355   typedef Unordered_map<Hashkey, Hashval, Stringpool_hash,
 356                         Stringpool_eq> String_set_type;
 357
 358   // Comparison routine used when sorting into a string table.
 359
 360   typedef typename String_set_type::iterator Stringpool_sort_info;
 361
 362   struct Stringpool_sort_comparison
 363   {
 364     bool
 365     operator()(const Stringpool_sort_info&, const Stringpool_sort_info&) const;
 366   };
 367
 368   // Keys map to offsets via a Chunked_vector.  We only use the
 369   // offsets if we turn this into an string table section.
 370   typedef Chunked_vector<section_offset_type> Key_to_offset;
 371
 372   // List of Stringdata structures.
 373   typedef std::list<Stringdata*> Stringdata_list;
 374
 375   // Mapping from const char* to namepool entry.
 376   String_set_type string_set_;
 377   // Mapping from Key to string table offset.
 378   Key_to_offset key_to_offset_;
 379   // List of buffers.
 380   Stringdata_list strings_;
 381   // Size of string table.
 382   section_size_type strtab_size_;
 383   // Whether to reserve offset 0 to hold the null string.
 384   bool zero_null_;
 385   // Whether to optimize the string table.
 386   bool optimize_;
 387   // offset of the next string.
 388   section_offset_type offset_;
 389 };
 390
 391 // The most common type of Stringpool.
 392 typedef Stringpool_template<char> Stringpool;
 393
 394 } // End namespace gold.
 395
 396 #endif // !defined(GOLD_STRINGPOOL_H)