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 30namespace gold 31{ 32 33class Output_file; 34 35// Return the length of a string in units of Char_type. 36 37template<typename Char_type> 38inline size_t 39string_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 50template<> 51inline size_t 52string_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 97template<typename Element> 98class 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 173template<typename Stringpool_char> 174class 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(); 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 string S of length LEN characters to the pool. If COPY is 223 // true, S need not be null terminated. 224 const Stringpool_char* 225 add_with_length(const Stringpool_char* s, size_t len, bool copy, Key* pkey); 226 227 // If the string S is present in the pool, return the canonical 228 // string pointer. Otherwise, return NULL. If PKEY is not NULL, 229 // set *PKEY to the key. 230 const Stringpool_char* 231 find(const Stringpool_char* s, Key* pkey) const; 232 233 // Turn the stringpool into a string table: determine the offsets of 234 // all the strings. After this is called, no more strings may be 235 // added to the stringpool. 236 void 237 set_string_offsets(); 238 239 // Get the offset of the string S in the string table. This returns 240 // the offset in bytes, not in units of Stringpool_char. This may 241 // only be called after set_string_offsets has been called. 242 section_offset_type 243 get_offset(const Stringpool_char* s) const; 244 245 // Get the offset of the string S in the string table. 246 section_offset_type 247 get_offset(const std::basic_string<Stringpool_char>& s) const 248 { return this->get_offset_with_length(s.c_str(), s.size()); } 249 250 // Get the offset of string S, with length LENGTH characters, in the 251 // string table. 252 section_offset_type 253 get_offset_with_length(const Stringpool_char* s, size_t length) const; 254 255 // Get the offset of the string with key K. 256 section_offset_type 257 get_offset_from_key(Key k) const 258 { 259 gold_assert(k <= this->key_to_offset_.size()); 260 return this->key_to_offset_[k - 1]; 261 } 262 263 // Get the size of the string table. This returns the number of 264 // bytes, not in units of Stringpool_char. 265 section_size_type 266 get_strtab_size() const 267 { 268 gold_assert(this->strtab_size_ != 0); 269 return this->strtab_size_; 270 } 271 272 // Write the string table into the output file at the specified 273 // offset. 274 void 275 write(Output_file*, off_t offset); 276 277 // Write the string table into the specified buffer, of the 278 // specified size. buffer_size should be at least 279 // get_strtab_size(). 280 void 281 write_to_buffer(unsigned char* buffer, section_size_type buffer_size); 282 283 // Dump statistical information to stderr. 284 void 285 print_stats(const char*) const; 286 287 private: 288 Stringpool_template(const Stringpool_template&); 289 Stringpool_template& operator=(const Stringpool_template&); 290 291 // Return whether two strings are equal. 292 static bool 293 string_equal(const Stringpool_char*, const Stringpool_char*); 294 295 // Compute a hash code for a string. LENGTH is the length of the 296 // string in characters. 297 static size_t 298 string_hash(const Stringpool_char*, size_t length); 299 300 // We store the actual data in a list of these buffers. 301 struct Stringdata 302 { 303 // Length of data in buffer. 304 size_t len; 305 // Allocated size of buffer. 306 size_t alc; 307 // Buffer. 308 char data[1]; 309 }; 310 311 // Add a new key offset entry. 312 void 313 new_key_offset(size_t); 314 315 // Copy a string into the buffers, returning a canonical string. 316 const Stringpool_char* 317 add_string(const Stringpool_char*, size_t); 318 319 // Return whether s1 is a suffix of s2. 320 static bool 321 is_suffix(const Stringpool_char* s1, size_t len1, 322 const Stringpool_char* s2, size_t len2); 323 324 // The hash table key includes the string, the length of the string, 325 // and the hash code for the string. We put the hash code 326 // explicitly into the key so that we can do a find()/insert() 327 // sequence without having to recompute the hash. Computing the 328 // hash code is a significant user of CPU time in the linker. 329 struct Hashkey 330 { 331 const Stringpool_char* string; 332 // Length is in characters, not bytes. 333 size_t length; 334 size_t hash_code; 335 336 // This goes in an STL container, so we need a default 337 // constructor. 338 Hashkey() 339 : string(NULL), length(0), hash_code(0) 340 { } 341 342 // Note that these constructors are relatively expensive, because 343 // they compute the hash code. 344 explicit Hashkey(const Stringpool_char* s) 345 : string(s), length(string_length(s)), hash_code(string_hash(s, length)) 346 { } 347 348 Hashkey(const Stringpool_char* s, size_t len) 349 : string(s), length(len), hash_code(string_hash(s, len)) 350 { } 351 }; 352 353 // Hash function. This is trivial, since we have already computed 354 // the hash. 355 struct Stringpool_hash 356 { 357 size_t 358 operator()(const Hashkey& hk) const 359 { return hk.hash_code; } 360 }; 361 362 // Equality comparison function for hash table. 363 struct Stringpool_eq 364 { 365 bool 366 operator()(const Hashkey&, const Hashkey&) const; 367 }; 368 369 // The hash table is a map from strings to Keys. 370 371 typedef Key Hashval; 372 373 typedef Unordered_map<Hashkey, Hashval, Stringpool_hash, 374 Stringpool_eq> String_set_type; 375 376 // Comparison routine used when sorting into a string table. 377 378 typedef typename String_set_type::iterator Stringpool_sort_info; 379 380 struct Stringpool_sort_comparison 381 { 382 bool 383 operator()(const Stringpool_sort_info&, const Stringpool_sort_info&) const; 384 }; 385 386 // Keys map to offsets via a Chunked_vector. We only use the 387 // offsets if we turn this into an string table section. 388 typedef Chunked_vector<section_offset_type> Key_to_offset; 389 390 // List of Stringdata structures. 391 typedef std::list<Stringdata*> Stringdata_list; 392 393 // Mapping from const char* to namepool entry. 394 String_set_type string_set_; 395 // Mapping from Key to string table offset. 396 Key_to_offset key_to_offset_; 397 // List of buffers. 398 Stringdata_list strings_; 399 // Size of string table. 400 section_size_type strtab_size_; 401 // Whether to reserve offset 0 to hold the null string. 402 bool zero_null_; 403 // Whether to optimize the string table. 404 bool optimize_; 405 // offset of the next string. 406 section_offset_type offset_; 407}; 408 409// The most common type of Stringpool. 410typedef Stringpool_template<char> Stringpool; 411 412} // End namespace gold. 413 414#endif // !defined(GOLD_STRINGPOOL_H) 415