1 // stringpool.h -- a string pool for gold -*- C++ -*- 2 3 // Copyright (C) 2006-2014 Free Software Foundation, Inc. 4 // Written by Ian Lance Taylor <iant (at) 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_ casues 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) 422
