1 // stringpool.h -- a string pool for gold    -*- C++ -*-
2 
3 // Copyright (C) 2006-2016 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
string_length(const Char_type * p)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
string_length(const char * p)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:
Chunked_vector()101   Chunked_vector()
102     : chunks_(), size_(0)
103   { }
104 
105   // Clear the elements.
106   void
clear()107   clear()
108   {
109     this->chunks_.clear();
110     this->size_ = 0;
111   }
112 
113   // Reserve elements.
114   void
reserve(unsigned int n)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
size()130   size() const
131   { return this->size_; }
132 
133   // Push a new element on the back of the vector.
134   void
push_back(const Element & element)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
set_no_zero_null()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
set_optimize()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*
add(const std::basic_string<Stringpool_char> & s,bool copy,Key * pkey)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
get_offset(const std::basic_string<Stringpool_char> & s)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
get_offset_from_key(Key k)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
get_strtab_size()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.
HashkeyHashkey343     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.
HashkeyHashkey349     explicit Hashkey(const Stringpool_char* s)
350       : string(s), length(string_length(s)), hash_code(string_hash(s, length))
351     { }
352 
HashkeyHashkey353     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
operatorStringpool_hash363     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