gold/gc.h

   1 // gc.h -- garbage collection of unused sections
   2
   3 // Copyright 2009, 2010 Free Software Foundation, Inc.
   4 // Written by Sriraman Tallam <tmsriram@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 #ifndef GOLD_GC_H
  24 #define GOLD_GC_H
  25
  26 #include <queue>
  27 #include <vector>
  28
  29 #include "elfcpp.h"
  30 #include "symtab.h"
  31 #include "object.h"
  32 #include "icf.h"
  33
  34 namespace gold
  35 {
  36
  37 class Object;
  38
  39 template<int size, bool big_endian>
  40 class Sized_relobj_file;
  41
  42 template<int sh_type, int size, bool big_endian>
  43 class Reloc_types;
  44
  45 class Output_section;
  46 class General_options;
  47 class Layout;
  48
  49 class Garbage_collection
  50 {
  51  public:
  52
  53   typedef Unordered_set<Section_id, Section_id_hash> Sections_reachable;
  54   typedef std::map<Section_id, Sections_reachable> Section_ref;
  55   typedef std::queue<Section_id> Worklist_type;
  56   // This maps the name of the section which can be represented as a C
  57   // identifier (cident) to the list of sections that have that name.
  58   // Different object files can have cident sections with the same name.
  59   typedef std::map<std::string, Sections_reachable> Cident_section_map;
  60
  61   Garbage_collection()
  62   : is_worklist_ready_(false)
  63   { }
  64
  65   // Accessor methods for the private members.
  66
  67   Sections_reachable&
  68   referenced_list()
  69   { return referenced_list_; }
  70
  71   Section_ref&
  72   section_reloc_map()
  73   { return this->section_reloc_map_; }
  74
  75   Worklist_type&
  76   worklist()
  77   { return this->work_list_; }
  78
  79   bool
  80   is_worklist_ready()
  81   { return this->is_worklist_ready_; }
  82
  83   void
  84   worklist_ready()
  85   { this->is_worklist_ready_ = true; }
  86
  87   void
  88   do_transitive_closure();
  89
  90   bool
  91   is_section_garbage(Object* obj, unsigned int shndx)
  92   { return (this->referenced_list().find(Section_id(obj, shndx))
  93             == this->referenced_list().end()); }
  94
  95   Cident_section_map*
  96   cident_sections()
  97   { return &cident_sections_; }
  98
  99   void
 100   add_cident_section(std::string section_name,
 101                      Section_id secn)
 102   { this->cident_sections_[section_name].insert(secn); }
 103
 104   // Add a reference from the SRC_SHNDX-th section of SRC_OBJECT to
 105   // DST_SHNDX-th section of DST_OBJECT.
 106   void
 107   add_reference(Object* src_object, unsigned int src_shndx,
 108                 Object* dst_object, unsigned int dst_shndx)
 109   {
 110     Section_id src_id(src_object, src_shndx);
 111     Section_id dst_id(dst_object, dst_shndx);
 112     Section_ref::iterator p = this->section_reloc_map_.find(src_id);
 113     if (p == this->section_reloc_map_.end())
 114       this->section_reloc_map_[src_id].insert(dst_id);
 115     else
 116       p->second.insert(dst_id);
 117   }
 118
 119  private:
 120
 121   Worklist_type work_list_;
 122   bool is_worklist_ready_;
 123   Section_ref section_reloc_map_;
 124   Sections_reachable referenced_list_;
 125   Cident_section_map cident_sections_;
 126 };
 127
 128 // Data to pass between successive invocations of do_layout
 129 // in object.cc while garbage collecting.  This data structure
 130 // is filled by using the data from Read_symbols_data.
 131
 132 struct Symbols_data
 133 {
 134   // Section headers.
 135   unsigned char* section_headers_data;
 136   // Section names.
 137   unsigned char* section_names_data;
 138   // Size of section name data in bytes.
 139   section_size_type section_names_size;
 140   // Symbol data.
 141   unsigned char* symbols_data;
 142   // Size of symbol data in bytes.
 143   section_size_type symbols_size;
 144   // Offset of external symbols within symbol data.  This structure
 145   // sometimes contains only external symbols, in which case this will
 146   // be zero.  Sometimes it contains all symbols.
 147   section_offset_type external_symbols_offset;
 148   // Symbol names.
 149   unsigned char* symbol_names_data;
 150   // Size of symbol name data in bytes.
 151   section_size_type symbol_names_size;
 152 };
 153
 154 // Relocations of type SHT_REL store the addend value in their bytes.
 155 // This function returns the size of the embedded addend which is
 156 // nothing but the size of the relocation.
 157
 158 template<typename Classify_reloc>
 159 inline unsigned int
 160 get_embedded_addend_size(int sh_type, int r_type, Relobj* obj)
 161 {
 162   if (sh_type != elfcpp::SHT_REL)
 163     return 0;
 164   Classify_reloc classify_reloc;
 165   return classify_reloc.get_size_for_reloc(r_type, obj);
 166 }
 167
 168 // This function implements the generic part of reloc
 169 // processing to map a section to all the sections it
 170 // references through relocs.  It is called only during
 171 // garbage collection (--gc-sections) and identical code
 172 // folding (--icf).
 173
 174 template<int size, bool big_endian, typename Target_type, int sh_type,
 175          typename Scan, typename Classify_reloc>
 176 inline void
 177 gc_process_relocs(
 178     Symbol_table* symtab,
 179     Layout*,
 180     Target_type* target,
 181     Sized_relobj_file<size, big_endian>* src_obj,
 182     unsigned int src_indx,
 183     const unsigned char* prelocs,
 184     size_t reloc_count,
 185     Output_section*,
 186     bool,
 187     size_t local_count,
 188     const unsigned char* plocal_syms)
 189 {
 190   Object* dst_obj;
 191   unsigned int dst_indx;
 192   Scan scan;
 193
 194   typedef typename Reloc_types<sh_type, size, big_endian>::Reloc Reltype;
 195   const int reloc_size = Reloc_types<sh_type, size, big_endian>::reloc_size;
 196   const int sym_size = elfcpp::Elf_sizes<size>::sym_size;
 197
 198   Icf::Sections_reachable_info* secvec = NULL;
 199   Icf::Symbol_info* symvec = NULL;
 200   Icf::Addend_info* addendvec = NULL;
 201   Icf::Offset_info* offsetvec = NULL;
 202   Icf::Reloc_addend_size_info* reloc_addend_size_vec = NULL;
 203   bool is_icf_tracked = false;
 204   const char* cident_section_name = NULL;
 205
 206   std::string src_section_name = (parameters->options().icf_enabled()
 207                                   ? src_obj->section_name(src_indx)
 208                                   : "");
 209
 210   bool check_section_for_function_pointers = false;
 211
 212   if (parameters->options().icf_enabled()
 213       && is_section_foldable_candidate(src_section_name.c_str()))
 214     {
 215       is_icf_tracked = true;
 216       Section_id src_id(src_obj, src_indx);
 217       Icf::Reloc_info* reloc_info =
 218         &symtab->icf()->reloc_info_list()[src_id];
 219       secvec = &reloc_info->section_info;
 220       symvec = &reloc_info->symbol_info;
 221       addendvec = &reloc_info->addend_info;
 222       offsetvec = &reloc_info->offset_info;
 223       reloc_addend_size_vec = &reloc_info->reloc_addend_size_info;
 224     }
 225
 226   check_section_for_function_pointers =
 227     symtab->icf()->check_section_for_function_pointers(src_section_name,
 228                                                        target);
 229
 230   for (size_t i = 0; i < reloc_count; ++i, prelocs += reloc_size)
 231     {
 232       Reltype reloc(prelocs);
 233       typename elfcpp::Elf_types<size>::Elf_WXword r_info = reloc.get_r_info();
 234       unsigned int r_sym = elfcpp::elf_r_sym<size>(r_info);
 235       unsigned int r_type = elfcpp::elf_r_type<size>(r_info);
 236       typename elfcpp::Elf_types<size>::Elf_Swxword addend =
 237       Reloc_types<sh_type, size, big_endian>::get_reloc_addend_noerror(&reloc);
 238
 239       if (r_sym < local_count)
 240         {
 241           gold_assert(plocal_syms != NULL);
 242           typename elfcpp::Sym<size, big_endian> lsym(plocal_syms
 243                                                       + r_sym * sym_size);
 244           unsigned int shndx = lsym.get_st_shndx();
 245           bool is_ordinary;
 246           shndx = src_obj->adjust_sym_shndx(r_sym, shndx, &is_ordinary);
 247           dst_obj = src_obj;
 248           dst_indx = shndx;
 249           if (is_icf_tracked)
 250             {
 251               if (is_ordinary)
 252                 (*secvec).push_back(Section_id(dst_obj, dst_indx));
 253               else
 254                 (*secvec).push_back(Section_id(NULL, 0));
 255               (*symvec).push_back(NULL);
 256               long long symvalue = static_cast<long long>(lsym.get_st_value());
 257               (*addendvec).push_back(std::make_pair(symvalue,
 258                                               static_cast<long long>(addend)));
 259               uint64_t reloc_offset =
 260                 convert_to_section_size_type(reloc.get_r_offset());
 261               (*offsetvec).push_back(reloc_offset);
 262               (*reloc_addend_size_vec).push_back(
 263                 get_embedded_addend_size<Classify_reloc>(sh_type, r_type,
 264                                                          src_obj));
 265             }
 266
 267           // When doing safe folding, check to see if this relocation is that
 268           // of a function pointer being taken.
 269           if (is_ordinary
 270               && check_section_for_function_pointers
 271               && lsym.get_st_type() != elfcpp::STT_OBJECT
 272               && scan.local_reloc_may_be_function_pointer(symtab, NULL, NULL,
 273                                                           src_obj, src_indx,
 274                                                           NULL, reloc, r_type,
 275                                                           lsym))
 276             symtab->icf()->set_section_has_function_pointers(
 277               src_obj, lsym.get_st_shndx());
 278
 279           if (!is_ordinary || shndx == src_indx)
 280             continue;
 281         }
 282       else
 283         {
 284           Symbol* gsym = src_obj->global_symbol(r_sym);
 285           gold_assert(gsym != NULL);
 286           if (gsym->is_forwarder())
 287             gsym = symtab->resolve_forwards(gsym);
 288
 289           dst_obj = NULL;
 290           dst_indx = 0;
 291           bool is_ordinary = false;
 292           if (gsym->source() == Symbol::FROM_OBJECT)
 293             {
 294               dst_obj = gsym->object();
 295               dst_indx = gsym->shndx(&is_ordinary);
 296             }
 297
 298           // When doing safe folding, check to see if this relocation is that
 299           // of a function pointer being taken.
 300           if (gsym->source() == Symbol::FROM_OBJECT
 301               && check_section_for_function_pointers
 302               && gsym->type() != elfcpp::STT_OBJECT
 303               && (!is_ordinary
 304                   || scan.global_reloc_may_be_function_pointer(
 305                        symtab, NULL, NULL, src_obj, src_indx, NULL, reloc,
 306                        r_type, gsym)))
 307             symtab->icf()->set_section_has_function_pointers(dst_obj, dst_indx);
 308
 309           // If the symbol name matches '__start_XXX' then the section with
 310           // the C identifier like name 'XXX' should not be garbage collected.
 311           // A similar treatment to symbols with the name '__stop_XXX'.
 312           if (is_prefix_of(cident_section_start_prefix, gsym->name()))
 313             {
 314               cident_section_name = (gsym->name()
 315                                      + strlen(cident_section_start_prefix));
 316             }
 317           else if (is_prefix_of(cident_section_stop_prefix, gsym->name()))
 318             {
 319               cident_section_name = (gsym->name()
 320                                      + strlen(cident_section_stop_prefix));
 321             }
 322           if (is_icf_tracked)
 323             {
 324               if (is_ordinary && gsym->source() == Symbol::FROM_OBJECT)
 325                 (*secvec).push_back(Section_id(dst_obj, dst_indx));
 326               else
 327                 (*secvec).push_back(Section_id(NULL, 0));
 328               (*symvec).push_back(gsym);
 329               Sized_symbol<size>* sized_gsym =
 330                         static_cast<Sized_symbol<size>* >(gsym);
 331               long long symvalue =
 332                         static_cast<long long>(sized_gsym->value());
 333               (*addendvec).push_back(std::make_pair(symvalue,
 334                                         static_cast<long long>(addend)));
 335               uint64_t reloc_offset =
 336                 convert_to_section_size_type(reloc.get_r_offset());
 337               (*offsetvec).push_back(reloc_offset);
 338               (*reloc_addend_size_vec).push_back(
 339                 get_embedded_addend_size<Classify_reloc>(sh_type, r_type,
 340                                                          src_obj));
 341             }
 342
 343           if (gsym->source() != Symbol::FROM_OBJECT)
 344             continue;
 345           if (!is_ordinary)
 346             continue;
 347         }
 348       if (parameters->options().gc_sections())
 349         {
 350           symtab->gc()->add_reference(src_obj, src_indx, dst_obj, dst_indx);
 351           if (cident_section_name != NULL)
 352             {
 353               Garbage_collection::Cident_section_map::iterator ele =
 354                 symtab->gc()->cident_sections()->find(std::string(cident_section_name));
 355               if (ele == symtab->gc()->cident_sections()->end())
 356                 continue;
 357               Section_id src_id(src_obj, src_indx);
 358               Garbage_collection::Sections_reachable&
 359                 v(symtab->gc()->section_reloc_map()[src_id]);
 360               Garbage_collection::Sections_reachable& cident_secn(ele->second);
 361               for (Garbage_collection::Sections_reachable::iterator it_v
 362                      = cident_secn.begin();
 363                    it_v != cident_secn.end();
 364                    ++it_v)
 365                 {
 366                   v.insert(*it_v);
 367                 }
 368             }
 369         }
 370     }
 371   return;
 372 }
 373
 374 } // End of namespace gold.
 375
 376 #endif