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_() 81 { } 82 83 // Clear the elements. 84 void 85 clear() 86 { this->chunks_.clear(); } 87 88 // Reserve elements. 89 void 90 reserve(unsigned int n) 91 { 92 n += chunk_size - 1; 93 while (n >= chunk_size) 94 { 95 this->chunks_.push_back(Element_vector()); 96 this->chunks_.back().reserve(chunk_size); 97 n -= chunk_size; 98 } 99 } 100 101 // Get the number of elements. 102 size_t 103 size() const 104 { 105 if (this->chunks_.empty()) 106 return 0; 107 else 108 return ((this->chunks_.size() - 1) * chunk_size 109 + this->chunks_.back().size()); 110 } 111 112 // Push a new element on the back of the vector. 113 void 114 push_back(const Element& element) 115 { 116 if (this->chunks_.empty() || this->chunks_.back().size() == chunk_size) 117 { 118 this->chunks_.push_back(Element_vector()); 119 this->chunks_.back().reserve(chunk_size); 120 } 121 this->chunks_.back().push_back(element); 122 } 123 124 // Return a reference to an entry in the vector. 125 Element& 126 operator[](size_t i) 127 { return this->chunks_[i / chunk_size][i % chunk_size]; } 128 129 const Element& 130 operator[](size_t i) const 131 { return this->chunks_[i / chunk_size][i % chunk_size]; } 132 133 private: 134 static const unsigned int chunk_size = 8192; 135 136 typedef std::vector<Element> Element_vector; 137 typedef std::vector<Element_vector> Chunk_vector; 138 139 Chunk_vector chunks_; 140 }; 141 142 143 // Stringpools are implemented in terms of Stringpool_template, which 144 // is generalized on the type of character used for the strings. Most 145 // uses will want the Stringpool type which uses char. Other cases 146 // are used for merging wide string constants. 147 148 template<typename Stringpool_char> 149 class Stringpool_template 150 { 151 public: 152 // The type of a key into the stringpool. As described above, a key 153 // value will always be the same during any run of the linker. Zero 154 // is never a valid key value. 155 typedef size_t Key; 156 157 // Create a Stringpool. 158 Stringpool_template(); 159 160 ~Stringpool_template(); 161 162 // Clear all the data from the stringpool. 163 void 164 clear(); 165 166 // Hint to the stringpool class that you intend to insert n additional 167 // elements. The stringpool class can use this info however it likes; 168 // in practice it will resize its internal hashtables to make room. 169 void 170 reserve(unsigned int n); 171 172 // Indicate that we should not reserve offset 0 to hold the empty 173 // string when converting the stringpool to a string table. This 174 // should not be called for a proper ELF SHT_STRTAB section. 175 void 176 set_no_zero_null() 177 { this->zero_null_ = false; } 178 179 // Add the string S to the pool. This returns a canonical permanent 180 // pointer to the string in the pool. If COPY is true, the string 181 // is copied into permanent storage. If PKEY is not NULL, this sets 182 // *PKEY to the key for the string. 183 const Stringpool_char* 184 add(const Stringpool_char* s, bool copy, Key* pkey); 185 186 // Add string S of length LEN characters to the pool. If COPY is 187 // true, S need not be null terminated. 188 const Stringpool_char* 189 add_with_length(const Stringpool_char* s, size_t len, bool copy, Key* pkey); 190 191 // If the string S is present in the pool, return the canonical 192 // string pointer. Otherwise, return NULL. If PKEY is not NULL, 193 // set *PKEY to the key. 194 const Stringpool_char* 195 find(const Stringpool_char* s, Key* pkey) const; 196 197 // Turn the stringpool into a string table: determine the offsets of 198 // all the strings. After this is called, no more strings may be 199 // added to the stringpool. 200 void 201 set_string_offsets(); 202 203 // Get the offset of the string S in the string table. This returns 204 // the offset in bytes, not in units of Stringpool_char. This may 205 // only be called after set_string_offsets has been called. 206 section_offset_type 207 get_offset(const Stringpool_char* s) const; 208 209 // Get the offset of the string S in the string table. 210 section_offset_type 211 get_offset(const std::basic_string<Stringpool_char>& s) const 212 { return this->get_offset_with_length(s.c_str(), s.size()); } 213 214 // Get the offset of string S, with length LENGTH characters, in the 215 // string table. 216 section_offset_type 217 get_offset_with_length(const Stringpool_char* s, size_t length) const; 218 219 // Get the offset of the string with key K. 220 section_offset_type 221 get_offset_from_key(Key k) const 222 { 223 gold_assert(k <= this->key_to_offset_.size()); 224 return this->key_to_offset_[k - 1]; 225 } 226 227 // Get the size of the string table. This returns the number of 228 // bytes, not in units of Stringpool_char. 229 section_size_type 230 get_strtab_size() const 231 { 232 gold_assert(this->strtab_size_ != 0); 233 return this->strtab_size_; 234 } 235 236 // Write the string table into the output file at the specified 237 // offset. 238 void 239 write(Output_file*, off_t offset); 240 241 // Write the string table into the specified buffer, of the 242 // specified size. buffer_size should be at least 243 // get_strtab_size(). 244 void 245 write_to_buffer(unsigned char* buffer, section_size_type buffer_size); 246 247 // Dump statistical information to stderr. 248 void 249 print_stats(const char*) const; 250 251 private: 252 Stringpool_template(const Stringpool_template&); 253 Stringpool_template& operator=(const Stringpool_template&); 254 255 // Return the length of a string in units of Stringpool_char. 256 static size_t 257 string_length(const Stringpool_char*); 258 259 // Return whether two strings are equal. 260 static bool 261 string_equal(const Stringpool_char*, const Stringpool_char*); 262 263 // Compute a hash code for a string. LENGTH is the length of the 264 // string in characters. 265 static size_t 266 string_hash(const Stringpool_char*, size_t length); 267 268 // We store the actual data in a list of these buffers. 269 struct Stringdata 270 { 271 // Length of data in buffer. 272 size_t len; 273 // Allocated size of buffer. 274 size_t alc; 275 // Buffer. 276 char data[1]; 277 }; 278 279 // Copy a string into the buffers, returning a canonical string. 280 const Stringpool_char* 281 add_string(const Stringpool_char*, size_t); 282 283 // Return whether s1 is a suffix of s2. 284 static bool 285 is_suffix(const Stringpool_char* s1, size_t len1, 286 const Stringpool_char* s2, size_t len2); 287 288 // The hash table key includes the string, the length of the string, 289 // and the hash code for the string. We put the hash code 290 // explicitly into the key so that we can do a find()/insert() 291 // sequence without having to recompute the hash. Computing the 292 // hash code is a significant user of CPU time in the linker. 293 struct Hashkey 294 { 295 const Stringpool_char* string; 296 // Length is in characters, not bytes. 297 size_t length; 298 size_t hash_code; 299 300 // This goes in an STL container, so we need a default 301 // constructor. 302 Hashkey() 303 : string(NULL), length(0), hash_code(0) 304 { } 305 306 // Note that these constructors are relatively expensive, because 307 // they compute the hash code. 308 explicit Hashkey(const Stringpool_char* s) 309 : string(s), length(string_length(s)), hash_code(string_hash(s, length)) 310 { } 311 312 Hashkey(const Stringpool_char* s, size_t len) 313 : string(s), length(len), hash_code(string_hash(s, len)) 314 { } 315 }; 316 317 // Hash function. This is trivial, since we have already computed 318 // the hash. 319 struct Stringpool_hash 320 { 321 size_t 322 operator()(const Hashkey& hk) const 323 { return hk.hash_code; } 324 }; 325 326 // Equality comparison function for hash table. 327 struct Stringpool_eq 328 { 329 bool 330 operator()(const Hashkey&, const Hashkey&) const; 331 }; 332 333 // The hash table is a map from strings to Keys. 334 335 typedef Key Hashval; 336 337 typedef Unordered_map<Hashkey, Hashval, Stringpool_hash, 338 Stringpool_eq> String_set_type; 339 340 // Comparison routine used when sorting into a string table. 341 342 typedef typename String_set_type::iterator Stringpool_sort_info; 343 344 struct Stringpool_sort_comparison 345 { 346 bool 347 operator()(const Stringpool_sort_info&, const Stringpool_sort_info&) const; 348 }; 349 350 // Keys map to offsets via a Chunked_vector. We only use the 351 // offsets if we turn this into an string table section. 352 typedef Chunked_vector<section_offset_type> Key_to_offset; 353 354 // List of Stringdata structures. 355 typedef std::list<Stringdata*> Stringdata_list; 356 357 // Mapping from const char* to namepool entry. 358 String_set_type string_set_; 359 // Mapping from Key to string table offset. 360 Key_to_offset key_to_offset_; 361 // List of buffers. 362 Stringdata_list strings_; 363 // Size of string table. 364 section_size_type strtab_size_; 365 // Whether to reserve offset 0 to hold the null string. 366 bool zero_null_; 367 }; 368 369 // The most common type of Stringpool. 370 typedef Stringpool_template<char> Stringpool; 371 372 } // End namespace gold. 373 374 #endif // !defined(GOLD_STRINGPOOL_H) 375