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// stringpool.h -- a string pool for gold -*- C++ -*-
// Copyright (C) 2006-2016 Free Software Foundation, Inc.
// Written by Ian Lance Taylor <>.
// This file is part of gold.
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation; either version 3 of the License, or
// (at your option) any later version.
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// GNU General Public License for more details.
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software
// Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston,
// MA 02110-1301, USA.
#include <string>
#include <list>
#include <vector>
namespace gold
class Output_file;
// Return the length of a string in units of Char_type.
template<typename Char_type>
inline size_t
string_length(const Char_type* p)
size_t len = 0;
for (; *p != 0; ++p)
return len;
// Specialize string_length for char. Maybe we could just use
// std::char_traits<>::length?
inline size_t
string_length(const char* p)
return strlen(p);
// A Stringpool is a pool of unique strings. It provides the
// following features:
// Every string in the pool is unique. Thus, if you have two strings
// in the Stringpool, you can compare them for equality by using
// pointer comparison rather than string comparison.
// There is a key associated with every string in the pool. If you
// add strings to the Stringpool in the same order, then the key for
// each string will always be the same for any run of the linker.
// This is not true of the string pointers themselves, as they may
// change due to address space randomization. Some parts of the
// linker (e.g., the symbol table) use the key value instead of the
// string pointer so that repeated runs of the linker will generate
// precisely the same output.
// When you add a string to a Stringpool, Stringpool will optionally
// make a copy of it. Thus there is no requirement to keep a copy
// elsewhere.
// A Stringpool can be turned into a string table, a sequential series
// of null terminated strings. The first string may optionally be a
// single zero byte, as required for SHT_STRTAB sections. This
// conversion is only permitted after all strings have been added to
// the Stringpool. After doing this conversion, you can ask for the
// offset of any string (or any key) in the stringpool in the string
// table, and you can write the resulting string table to an output
// file.
// When a Stringpool is turned into a string table, then as an
// optimization it will reuse string suffixes to avoid duplicating
// strings. That is, given the strings "abc" and "bc", only the
// string "abc" will be stored, and "bc" will be represented by an
// offset into the middle of the string "abc".
// A simple chunked vector class--this is a subset of std::vector
// which stores memory in chunks. We don't provide iterators, because
// we don't need them.
template<typename Element>
class Chunked_vector
: chunks_(), size_(0)
{ }
// Clear the elements.
this->size_ = 0;
// Reserve elements.
reserve(unsigned int n)
if (n > this->chunks_.size() * chunk_size)
this->chunks_.resize((n + chunk_size - 1) / chunk_size);
// We need to call reserve() of all chunks since changing
// this->chunks_ casues Element_vectors to be copied. The
// reserved capacity of an Element_vector may be lost in copying.
for (size_t i = 0; i < this->chunks_.size(); ++i)
// Get the number of elements.
size() const
{ return this->size_; }
// Push a new element on the back of the vector.
push_back(const Element& element)
size_t chunk_index = this->size_ / chunk_size;
if (chunk_index >= this->chunks_.size())
gold_assert(chunk_index < this->chunks_.size());
// Return a reference to an entry in the vector.
operator[](size_t i)
{ return this->chunks_[i / chunk_size][i % chunk_size]; }
const Element&
operator[](size_t i) const
{ return this->chunks_[i / chunk_size][i % chunk_size]; }
static const unsigned int chunk_size = 8192;
typedef std::vector<Element> Element_vector;
typedef std::vector<Element_vector> Chunk_vector;
Chunk_vector chunks_;
size_t size_;
// Stringpools are implemented in terms of Stringpool_template, which
// is generalized on the type of character used for the strings. Most
// uses will want the Stringpool type which uses char. Other cases
// are used for merging wide string constants.
template<typename Stringpool_char>
class Stringpool_template
// The type of a key into the stringpool. As described above, a key
// value will always be the same during any run of the linker. Zero
// is never a valid key value.
typedef size_t Key;
// Create a Stringpool.
Stringpool_template(uint64_t addralign = 1);
// Clear all the data from the stringpool.
// Hint to the stringpool class that you intend to insert n additional
// elements. The stringpool class can use this info however it likes;
// in practice it will resize its internal hashtables to make room.
reserve(unsigned int n);
// Indicate that we should not reserve offset 0 to hold the empty
// string when converting the stringpool to a string table. This
// should not be called for a proper ELF SHT_STRTAB section.
&& this->offset_ == sizeof(Stringpool_char));
this->zero_null_ = false;
this->offset_ = 0;
// Indicate that this string pool should be optimized, even if not
// running with -O2.
{ this->optimize_ = true; }
// Add the string S to the pool. This returns a canonical permanent
// pointer to the string in the pool. If COPY is true, the string
// is copied into permanent storage. If PKEY is not NULL, this sets
// *PKEY to the key for the string.
const Stringpool_char*
add(const Stringpool_char* s, bool copy, Key* pkey);
// Add the string S to the pool.
const Stringpool_char*
add(const std::basic_string<Stringpool_char>& s, bool copy, Key* pkey)
{ return this->add_with_length(, s.size(), copy, pkey); }
// Add string S of length LEN characters to the pool. If COPY is
// true, S need not be null terminated.
const Stringpool_char*
add_with_length(const Stringpool_char* s, size_t len, bool copy, Key* pkey);
// If the string S is present in the pool, return the canonical
// string pointer. Otherwise, return NULL. If PKEY is not NULL,
// set *PKEY to the key.
const Stringpool_char*
find(const Stringpool_char* s, Key* pkey) const;
// Turn the stringpool into a string table: determine the offsets of
// all the strings. After this is called, no more strings may be
// added to the stringpool.
// Get the offset of the string S in the string table. This returns
// the offset in bytes, not in units of Stringpool_char. This may
// only be called after set_string_offsets has been called.
get_offset(const Stringpool_char* s) const;
// Get the offset of the string S in the string table.
get_offset(const std::basic_string<Stringpool_char>& s) const
{ return this->get_offset_with_length(s.c_str(), s.size()); }
// Get the offset of string S, with length LENGTH characters, in the
// string table.
get_offset_with_length(const Stringpool_char* s, size_t length) const;
// Get the offset of the string with key K.
get_offset_from_key(Key k) const
gold_assert(k <= this->key_to_offset_.size());
return this->key_to_offset_[k - 1];
// Get the size of the string table. This returns the number of
// bytes, not in units of Stringpool_char.
get_strtab_size() const
gold_assert(this->strtab_size_ != 0);
return this->strtab_size_;
// Write the string table into the output file at the specified
// offset.
write(Output_file*, off_t offset);
// Write the string table into the specified buffer, of the
// specified size. buffer_size should be at least
// get_strtab_size().
write_to_buffer(unsigned char* buffer, section_size_type buffer_size);
// Dump statistical information to stderr.
print_stats(const char*) const;
Stringpool_template(const Stringpool_template&);
Stringpool_template& operator=(const Stringpool_template&);
// Return whether two strings are equal.
static bool
string_equal(const Stringpool_char*, const Stringpool_char*);
// Compute a hash code for a string. LENGTH is the length of the
// string in characters.
static size_t
string_hash(const Stringpool_char*, size_t length);
// We store the actual data in a list of these buffers.
struct Stringdata
// Length of data in buffer.
size_t len;
// Allocated size of buffer.
size_t alc;
// Buffer.
char data[1];
// Add a new key offset entry.
// Copy a string into the buffers, returning a canonical string.
const Stringpool_char*
add_string(const Stringpool_char*, size_t);
// Return whether s1 is a suffix of s2.
static bool
is_suffix(const Stringpool_char* s1, size_t len1,
const Stringpool_char* s2, size_t len2);
// The hash table key includes the string, the length of the string,
// and the hash code for the string. We put the hash code
// explicitly into the key so that we can do a find()/insert()
// sequence without having to recompute the hash. Computing the
// hash code is a significant user of CPU time in the linker.
struct Hashkey
const Stringpool_char* string;
// Length is in characters, not bytes.
size_t length;
size_t hash_code;
// This goes in an STL container, so we need a default
// constructor.
: string(NULL), length(0), hash_code(0)
{ }
// Note that these constructors are relatively expensive, because
// they compute the hash code.
explicit Hashkey(const Stringpool_char* s)
: string(s), length(string_length(s)), hash_code(string_hash(s, length))
{ }
Hashkey(const Stringpool_char* s, size_t len)
: string(s), length(len), hash_code(string_hash(s, len))
{ }
// Hash function. This is trivial, since we have already computed
// the hash.
struct Stringpool_hash
operator()(const Hashkey& hk) const
{ return hk.hash_code; }
// Equality comparison function for hash table.
struct Stringpool_eq
operator()(const Hashkey&, const Hashkey&) const;
// The hash table is a map from strings to Keys.
typedef Key Hashval;
typedef Unordered_map<Hashkey, Hashval, Stringpool_hash,
Stringpool_eq> String_set_type;
// Comparison routine used when sorting into a string table.
typedef typename String_set_type::iterator Stringpool_sort_info;
struct Stringpool_sort_comparison
operator()(const Stringpool_sort_info&, const Stringpool_sort_info&) const;
// Keys map to offsets via a Chunked_vector. We only use the
// offsets if we turn this into an string table section.
typedef Chunked_vector<section_offset_type> Key_to_offset;
// List of Stringdata structures.
typedef std::list<Stringdata*> Stringdata_list;
// Mapping from const char* to namepool entry.
String_set_type string_set_;
// Mapping from Key to string table offset.
Key_to_offset key_to_offset_;
// List of buffers.
Stringdata_list strings_;
// Size of string table.
section_size_type strtab_size_;
// Whether to reserve offset 0 to hold the null string.
bool zero_null_;
// Whether to optimize the string table.
bool optimize_;
// offset of the next string.
section_offset_type offset_;
// The alignment of strings in the stringpool.
uint64_t addralign_;
// The most common type of Stringpool.
typedef Stringpool_template<char> Stringpool;
} // End namespace gold.
#endif // !defined(GOLD_STRINGPOOL_H)