1 // stringpool.h -- a string pool for gold -*- C++ -*-
3 // Copyright 2006, 2007 Free Software Foundation, Inc.
4 // Written by Ian Lance Taylor <iant@google.com>.
6 // This file is part of gold.
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.
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.
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.
26 #ifndef GOLD_STRINGPOOL_H
27 #define GOLD_STRINGPOOL_H
34 // A Stringpool is a pool of unique strings. It provides the
35 // following features:
37 // Every string in the pool is unique. Thus, if you have two strings
38 // in the Stringpool, you can compare them for equality by using
39 // pointer comparison rather than string comparison.
41 // There is a key associated with every string in the pool. If you
42 // add strings to the Stringpool in the same order, then the key for
43 // each string will always be the same for any run of the linker.
44 // This is not true of the string pointers themselves, as they may
45 // change due to address space randomization. Some parts of the
46 // linker (e.g., the symbol table) use the key value instead of the
47 // string pointer so that repeated runs of the linker will generate
48 // precisely the same output.
50 // When you add a string to a Stringpool, Stringpool will optionally
51 // make a copy of it. Thus there is no requirement to keep a copy
54 // A Stringpool can be turned into a string table, a sequential series
55 // of null terminated strings. The first string may optionally be a
56 // single zero byte, as required for SHT_STRTAB sections. This
57 // conversion is only permitted after all strings have been added to
58 // the Stringpool. After doing this conversion, you can ask for the
59 // offset of any string in the stringpool in the string table, and you
60 // can write the resulting string table to an output file.
62 // When a Stringpool is turned into a string table, then as an
63 // optimization it will reuse string suffixes to avoid duplicating
64 // strings. That is, given the strings "abc" and "bc", only the
65 // string "abc" will be stored, and "bc" will be represented by an
66 // offset into the middle of the string "abc".
68 // Stringpools are implemented in terms of Stringpool_template, which
69 // is generalized on the type of character used for the strings. Most
70 // uses will want the Stringpool type which uses char. Other cases
71 // are used for merging wide string constants.
73 template<typename Stringpool_char>
74 class Stringpool_template
77 // The type of a key into the stringpool. As described above, a key
78 // value will always be the same during any run of the linker. Zero
79 // is never a valid key value.
82 // Create a Stringpool.
83 Stringpool_template();
85 ~Stringpool_template();
87 // Clear all the data from the stringpool.
91 // Hint to the stringpool class that you intend to insert n additional
92 // elements. The stringpool class can use this info however it likes;
93 // in practice it will resize its internal hashtables to make room.
95 reserve(unsigned int n);
97 // Indicate that we should not reserve offset 0 to hold the empty
98 // string when converting the stringpool to a string table. This
99 // should not be called for a proper ELF SHT_STRTAB section.
102 { this->zero_null_ = false; }
104 // Add the string S to the pool. This returns a canonical permanent
105 // pointer to the string in the pool. If COPY is true, the string
106 // is copied into permanent storage. If PKEY is not NULL, this sets
107 // *PKEY to the key for the string.
108 const Stringpool_char*
109 add(const Stringpool_char* s, bool copy, Key* pkey);
111 // Add the prefix of length LEN of string S to the pool.
112 const Stringpool_char*
113 add_prefix(const Stringpool_char* s, size_t len, Key* pkey);
115 // If the string S is present in the pool, return the canonical
116 // string pointer. Otherwise, return NULL. If PKEY is not NULL,
117 // set *PKEY to the key.
118 const Stringpool_char*
119 find(const Stringpool_char* s, Key* pkey) const;
121 // Turn the stringpool into a string table: determine the offsets of
122 // all the strings. After this is called, no more strings may be
123 // added to the stringpool.
125 set_string_offsets();
127 // Get the offset of the string S in the string table. This returns
128 // the offset in bytes, not in units of Stringpool_char. This may
129 // only be called after set_string_offsets has been called.
131 get_offset(const Stringpool_char* s) const;
133 // Get the offset of the string S in the string table.
135 get_offset(const std::basic_string<Stringpool_char>& s) const
136 { return this->get_offset(s.c_str()); }
138 // Get the size of the string table. This returns the number of
139 // bytes, not in units of Stringpool_char.
141 get_strtab_size() const
143 gold_assert(this->strtab_size_ != 0);
144 return this->strtab_size_;
147 // Write the string table into the output file at the specified
150 write(Output_file*, off_t offset);
152 // Write the string table into the specified buffer, of the
153 // specified size. buffer_size should be at least
154 // get_strtab_size().
156 write_to_buffer(unsigned char* buffer, size_t buffer_size);
158 // Dump statistical information to stderr.
160 print_stats(const char*) const;
163 Stringpool_template(const Stringpool_template&);
164 Stringpool_template& operator=(const Stringpool_template&);
166 // Return the length of a string in units of Stringpool_char.
168 string_length(const Stringpool_char*);
170 // Return whether two strings are equal.
172 string_equal(const Stringpool_char*, const Stringpool_char*);
174 // Compute a hash code for a string. LENGTH is the length of the
175 // string in characters.
177 string_hash(const Stringpool_char*, size_t length);
179 // We store the actual data in a list of these buffers.
182 // Length of data in buffer.
184 // Allocated size of buffer.
192 // Copy a string into the buffers, returning a canonical string.
193 const Stringpool_char*
194 add_string(const Stringpool_char*, size_t, Key*);
196 // Return whether s1 is a suffix of s2.
198 is_suffix(const Stringpool_char* s1, size_t len1,
199 const Stringpool_char* s2, size_t len2);
201 // The hash table key includes the string, the length of the string,
202 // and the hash code for the string. We put the hash code
203 // explicitly into the key so that we can do a find()/insert()
204 // sequence without having to recompute the hash. Computing the
205 // hash code is a significant user of CPU time in the linker.
208 const Stringpool_char* string;
209 // Length is in characters, not bytes.
213 // This goes in an STL container, so we need a default
216 : string(NULL), length(0), hash_code(0)
219 // Note that these constructors are relatively expensive, because
220 // they compute the hash code.
221 Hashkey(const Stringpool_char* s)
222 : string(s), length(string_length(s)), hash_code(string_hash(s, length))
225 Hashkey(const Stringpool_char* s, size_t len)
226 : string(s), length(len), hash_code(string_hash(s, len))
230 // Hash function. This is trivial, since we have already computed
232 struct Stringpool_hash
235 operator()(const Hashkey& hk) const
236 { return hk.hash_code; }
239 // Equality comparison function for hash table.
243 operator()(const Hashkey&, const Hashkey&) const;
246 // The hash table is a map from strings to a pair of Key and string
247 // table offsets. We only use the offsets if we turn this into an
248 // string table section.
250 typedef std::pair<Key, off_t> Hashval;
252 typedef Unordered_map<Hashkey, Hashval, Stringpool_hash,
253 Stringpool_eq> String_set_type;
255 // Comparison routine used when sorting into a string table.
257 typedef typename String_set_type::iterator Stringpool_sort_info;
259 struct Stringpool_sort_comparison
262 operator()(const Stringpool_sort_info&, const Stringpool_sort_info&) const;
265 // List of Stringdata structures.
266 typedef std::list<Stringdata*> Stringdata_list;
268 // Mapping from const char* to namepool entry.
269 String_set_type string_set_;
271 Stringdata_list strings_;
272 // Size of string table.
274 // Next Stringdata index.
275 unsigned int next_index_;
276 // Next key value for a string we don't copy.
277 int next_uncopied_key_;
278 // Whether to reserve offset 0 to hold the null string.
282 // The most common type of Stringpool.
283 typedef Stringpool_template<char> Stringpool;
285 } // End namespace gold.
287 #endif // !defined(GOLD_STRINGPOOL_H)