gold/stringpool.h

   1 // stringpool.h -- a string pool for gold    -*- C++ -*-
   2
   3 // Copyright (C) 2006-2020 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 // Return the length of a string in units of Char_type.
  36
  37 template<typename Char_type>
  38 inline size_t
  39 string_length(const Char_type* p)
  40 {
  41   size_t len = 0;
  42   for (; *p != 0; ++p)
  43     ++len;
  44   return len;
  45 }
  46
  47 // Specialize string_length for char.  Maybe we could just use
  48 // std::char_traits<>::length?
  49
  50 template<>
  51 inline size_t
  52 string_length(const char* p)
  53 {
  54   return strlen(p);
  55 }
  56
  57 // A Stringpool is a pool of unique strings.  It provides the
  58 // following features:
  59
  60 // Every string in the pool is unique.  Thus, if you have two strings
  61 // in the Stringpool, you can compare them for equality by using
  62 // pointer comparison rather than string comparison.
  63
  64 // There is a key associated with every string in the pool.  If you
  65 // add strings to the Stringpool in the same order, then the key for
  66 // each string will always be the same for any run of the linker.
  67 // This is not true of the string pointers themselves, as they may
  68 // change due to address space randomization.  Some parts of the
  69 // linker (e.g., the symbol table) use the key value instead of the
  70 // string pointer so that repeated runs of the linker will generate
  71 // precisely the same output.
  72
  73 // When you add a string to a Stringpool, Stringpool will optionally
  74 // make a copy of it.  Thus there is no requirement to keep a copy
  75 // elsewhere.
  76
  77 // A Stringpool can be turned into a string table, a sequential series
  78 // of null terminated strings.  The first string may optionally be a
  79 // single zero byte, as required for SHT_STRTAB sections.  This
  80 // conversion is only permitted after all strings have been added to
  81 // the Stringpool.  After doing this conversion, you can ask for the
  82 // offset of any string (or any key) in the stringpool in the string
  83 // table, and you can write the resulting string table to an output
  84 // file.
  85
  86 // When a Stringpool is turned into a string table, then as an
  87 // optimization it will reuse string suffixes to avoid duplicating
  88 // strings.  That is, given the strings "abc" and "bc", only the
  89 // string "abc" will be stored, and "bc" will be represented by an
  90 // offset into the middle of the string "abc".
  91
  92
  93 // A simple chunked vector class--this is a subset of std::vector
  94 // which stores memory in chunks.  We don't provide iterators, because
  95 // we don't need them.
  96
  97 template<typename Element>
  98 class Chunked_vector
  99 {
 100  public:
 101   Chunked_vector()
 102     : chunks_(), size_(0)
 103   { }
 104
 105   // Clear the elements.
 106   void
 107   clear()
 108   {
 109     this->chunks_.clear();
 110     this->size_ = 0;
 111   }
 112
 113   // Reserve elements.
 114   void
 115   reserve(unsigned int n)
 116   {
 117     if (n > this->chunks_.size() * chunk_size)
 118       {
 119         this->chunks_.resize((n + chunk_size - 1) / chunk_size);
 120         // We need to call reserve() of all chunks since changing
 121         // this->chunks_ causes Element_vectors to be copied.  The
 122         // reserved capacity of an Element_vector may be lost in copying.
 123         for (size_t i = 0; i < this->chunks_.size(); ++i)
 124           this->chunks_[i].reserve(chunk_size);
 125       }
 126   }
 127
 128   // Get the number of elements.
 129   size_t
 130   size() const
 131   { return this->size_; }
 132
 133   // Push a new element on the back of the vector.
 134   void
 135   push_back(const Element& element)
 136   {
 137     size_t chunk_index = this->size_ / chunk_size;
 138     if (chunk_index >= this->chunks_.size())
 139       {
 140         this->chunks_.push_back(Element_vector());
 141         this->chunks_.back().reserve(chunk_size);
 142         gold_assert(chunk_index < this->chunks_.size());
 143       }
 144     this->chunks_[chunk_index].push_back(element);
 145     this->size_++;
 146   }
 147
 148   // Return a reference to an entry in the vector.
 149   Element&
 150   operator[](size_t i)
 151   { return this->chunks_[i / chunk_size][i % chunk_size]; }
 152
 153   const Element&
 154   operator[](size_t i) const
 155   { return this->chunks_[i / chunk_size][i % chunk_size]; }
 156
 157  private:
 158   static const unsigned int chunk_size = 8192;
 159
 160   typedef std::vector<Element> Element_vector;
 161   typedef std::vector<Element_vector> Chunk_vector;
 162
 163   Chunk_vector chunks_;
 164   size_t size_;
 165 };
 166
 167
 168 // Stringpools are implemented in terms of Stringpool_template, which
 169 // is generalized on the type of character used for the strings.  Most
 170 // uses will want the Stringpool type which uses char.  Other cases
 171 // are used for merging wide string constants.
 172
 173 template<typename Stringpool_char>
 174 class Stringpool_template
 175 {
 176  public:
 177   // The type of a key into the stringpool.  As described above, a key
 178   // value will always be the same during any run of the linker.  Zero
 179   // is never a valid key value.
 180   typedef size_t Key;
 181
 182   // Create a Stringpool.
 183   Stringpool_template(uint64_t addralign = 1);
 184
 185   ~Stringpool_template();
 186
 187   // Clear all the data from the stringpool.
 188   void
 189   clear();
 190
 191   // Hint to the stringpool class that you intend to insert n additional
 192   // elements.  The stringpool class can use this info however it likes;
 193   // in practice it will resize its internal hashtables to make room.
 194   void
 195   reserve(unsigned int n);
 196
 197   // Indicate that we should not reserve offset 0 to hold the empty
 198   // string when converting the stringpool to a string table.  This
 199   // should not be called for a proper ELF SHT_STRTAB section.
 200   void
 201   set_no_zero_null()
 202   {
 203     gold_assert(this->string_set_.empty()
 204                 && this->offset_ == sizeof(Stringpool_char));
 205     this->zero_null_ = false;
 206     this->offset_ = 0;
 207   }
 208
 209   // Indicate that this string pool should be optimized, even if not
 210   // running with -O2.
 211   void
 212   set_optimize()
 213   { this->optimize_ = true; }
 214
 215   // Add the string S to the pool.  This returns a canonical permanent
 216   // pointer to the string in the pool.  If COPY is true, the string
 217   // is copied into permanent storage.  If PKEY is not NULL, this sets
 218   // *PKEY to the key for the string.
 219   const Stringpool_char*
 220   add(const Stringpool_char* s, bool copy, Key* pkey);
 221
 222   // Add the string S to the pool.
 223   const Stringpool_char*
 224   add(const std::basic_string<Stringpool_char>& s, bool copy, Key* pkey)
 225   { return this->add_with_length(s.data(), s.size(), copy, pkey); }
 226
 227   // Add string S of length LEN characters to the pool.  If COPY is
 228   // true, S need not be null terminated.
 229   const Stringpool_char*
 230   add_with_length(const Stringpool_char* s, size_t len, bool copy, Key* pkey);
 231
 232   // If the string S is present in the pool, return the canonical
 233   // string pointer.  Otherwise, return NULL.  If PKEY is not NULL,
 234   // set *PKEY to the key.
 235   const Stringpool_char*
 236   find(const Stringpool_char* s, Key* pkey) const;
 237
 238   // Turn the stringpool into a string table: determine the offsets of
 239   // all the strings.  After this is called, no more strings may be
 240   // added to the stringpool.
 241   void
 242   set_string_offsets();
 243
 244   // Get the offset of the string S in the string table.  This returns
 245   // the offset in bytes, not in units of Stringpool_char.  This may
 246   // only be called after set_string_offsets has been called.
 247   section_offset_type
 248   get_offset(const Stringpool_char* s) const;
 249
 250   // Get the offset of the string S in the string table.
 251   section_offset_type
 252   get_offset(const std::basic_string<Stringpool_char>& s) const
 253   { return this->get_offset_with_length(s.c_str(), s.size()); }
 254
 255   // Get the offset of string S, with length LENGTH characters, in the
 256   // string table.
 257   section_offset_type
 258   get_offset_with_length(const Stringpool_char* s, size_t length) const;
 259
 260   // Get the offset of the string with key K.
 261   section_offset_type
 262   get_offset_from_key(Key k) const
 263   {
 264     gold_assert(k <= this->key_to_offset_.size());
 265     return this->key_to_offset_[k - 1];
 266   }
 267
 268   // Get the size of the string table.  This returns the number of
 269   // bytes, not in units of Stringpool_char.
 270   section_size_type
 271   get_strtab_size() const
 272   {
 273     gold_assert(this->strtab_size_ != 0);
 274     return this->strtab_size_;
 275   }
 276
 277   // Write the string table into the output file at the specified
 278   // offset.
 279   void
 280   write(Output_file*, off_t offset);
 281
 282   // Write the string table into the specified buffer, of the
 283   // specified size.  buffer_size should be at least
 284   // get_strtab_size().
 285   void
 286   write_to_buffer(unsigned char* buffer, section_size_type buffer_size);
 287
 288   // Dump statistical information to stderr.
 289   void
 290   print_stats(const char*) const;
 291
 292  private:
 293   Stringpool_template(const Stringpool_template&);
 294   Stringpool_template& operator=(const Stringpool_template&);
 295
 296   // Return whether two strings are equal.
 297   static bool
 298   string_equal(const Stringpool_char*, const Stringpool_char*);
 299
 300   // Compute a hash code for a string.  LENGTH is the length of the
 301   // string in characters.
 302   static size_t
 303   string_hash(const Stringpool_char*, size_t length);
 304
 305   // We store the actual data in a list of these buffers.
 306   struct Stringdata
 307   {
 308     // Length of data in buffer.
 309     size_t len;
 310     // Allocated size of buffer.
 311     size_t alc;
 312     // Buffer.
 313     char data[1];
 314   };
 315
 316   // Add a new key offset entry.
 317   void
 318   new_key_offset(size_t);
 319
 320   // Copy a string into the buffers, returning a canonical string.
 321   const Stringpool_char*
 322   add_string(const Stringpool_char*, size_t);
 323
 324   // Return whether s1 is a suffix of s2.
 325   static bool
 326   is_suffix(const Stringpool_char* s1, size_t len1,
 327             const Stringpool_char* s2, size_t len2);
 328
 329   // The hash table key includes the string, the length of the string,
 330   // and the hash code for the string.  We put the hash code
 331   // explicitly into the key so that we can do a find()/insert()
 332   // sequence without having to recompute the hash.  Computing the
 333   // hash code is a significant user of CPU time in the linker.
 334   struct Hashkey
 335   {
 336     const Stringpool_char* string;
 337     // Length is in characters, not bytes.
 338     size_t length;
 339     size_t hash_code;
 340
 341     // This goes in an STL container, so we need a default
 342     // constructor.
 343     Hashkey()
 344       : string(NULL), length(0), hash_code(0)
 345     { }
 346
 347     // Note that these constructors are relatively expensive, because
 348     // they compute the hash code.
 349     explicit Hashkey(const Stringpool_char* s)
 350       : string(s), length(string_length(s)), hash_code(string_hash(s, length))
 351     { }
 352
 353     Hashkey(const Stringpool_char* s, size_t len)
 354       : string(s), length(len), hash_code(string_hash(s, len))
 355     { }
 356   };
 357
 358   // Hash function.  This is trivial, since we have already computed
 359   // the hash.
 360   struct Stringpool_hash
 361   {
 362     size_t
 363     operator()(const Hashkey& hk) const
 364     { return hk.hash_code; }
 365   };
 366
 367   // Equality comparison function for hash table.
 368   struct Stringpool_eq
 369   {
 370     bool
 371     operator()(const Hashkey&, const Hashkey&) const;
 372   };
 373
 374   // The hash table is a map from strings to Keys.
 375
 376   typedef Key Hashval;
 377
 378   typedef Unordered_map<Hashkey, Hashval, Stringpool_hash,
 379                         Stringpool_eq> String_set_type;
 380
 381   // Comparison routine used when sorting into a string table.
 382
 383   typedef typename String_set_type::iterator Stringpool_sort_info;
 384
 385   struct Stringpool_sort_comparison
 386   {
 387     bool
 388     operator()(const Stringpool_sort_info&, const Stringpool_sort_info&) const;
 389   };
 390
 391   // Keys map to offsets via a Chunked_vector.  We only use the
 392   // offsets if we turn this into an string table section.
 393   typedef Chunked_vector<section_offset_type> Key_to_offset;
 394
 395   // List of Stringdata structures.
 396   typedef std::list<Stringdata*> Stringdata_list;
 397
 398   // Mapping from const char* to namepool entry.
 399   String_set_type string_set_;
 400   // Mapping from Key to string table offset.
 401   Key_to_offset key_to_offset_;
 402   // List of buffers.
 403   Stringdata_list strings_;
 404   // Size of string table.
 405   section_size_type strtab_size_;
 406   // Whether to reserve offset 0 to hold the null string.
 407   bool zero_null_;
 408   // Whether to optimize the string table.
 409   bool optimize_;
 410   // offset of the next string.
 411   section_offset_type offset_;
 412   // The alignment of strings in the stringpool.
 413   uint64_t addralign_;
 414 };
 415
 416 // The most common type of Stringpool.
 417 typedef Stringpool_template<char> Stringpool;
 418
 419 } // End namespace gold.
 420
 421 #endif // !defined(GOLD_STRINGPOOL_H)