gold/gc.h

   1 // gc.h -- garbage collection of unused sections
   2
   3 // Copyright (C) 2009-2015 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 struct 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     Sections_reachable& reachable = this->section_reloc_map_[src_id];
 113     reachable.insert(dst_id);
 114   }
 115
 116  private:
 117
 118   Worklist_type work_list_;
 119   bool is_worklist_ready_;
 120   Section_ref section_reloc_map_;
 121   Sections_reachable referenced_list_;
 122   Cident_section_map cident_sections_;
 123 };
 124
 125 // Data to pass between successive invocations of do_layout
 126 // in object.cc while garbage collecting.  This data structure
 127 // is filled by using the data from Read_symbols_data.
 128
 129 struct Symbols_data
 130 {
 131   // Section headers.
 132   unsigned char* section_headers_data;
 133   // Section names.
 134   unsigned char* section_names_data;
 135   // Size of section name data in bytes.
 136   section_size_type section_names_size;
 137   // Symbol data.
 138   unsigned char* symbols_data;
 139   // Size of symbol data in bytes.
 140   section_size_type symbols_size;
 141   // Offset of external symbols within symbol data.  This structure
 142   // sometimes contains only external symbols, in which case this will
 143   // be zero.  Sometimes it contains all symbols.
 144   section_offset_type external_symbols_offset;
 145   // Symbol names.
 146   unsigned char* symbol_names_data;
 147   // Size of symbol name data in bytes.
 148   section_size_type symbol_names_size;
 149 };
 150
 151 // Relocations of type SHT_REL store the addend value in their bytes.
 152 // This function returns the size of the embedded addend which is
 153 // nothing but the size of the relocation.
 154
 155 template<typename Classify_reloc>
 156 inline unsigned int
 157 get_embedded_addend_size(int sh_type, int r_type, Relobj* obj)
 158 {
 159   if (sh_type != elfcpp::SHT_REL)
 160     return 0;
 161   Classify_reloc classify_reloc;
 162   return classify_reloc.get_size_for_reloc(r_type, obj);
 163 }
 164
 165 // This function implements the generic part of reloc
 166 // processing to map a section to all the sections it
 167 // references through relocs.  It is called only during
 168 // garbage collection (--gc-sections) and identical code
 169 // folding (--icf).
 170
 171 template<int size, bool big_endian, typename Target_type, int sh_type,
 172          typename Scan, typename Classify_reloc>
 173 inline void
 174 gc_process_relocs(
 175     Symbol_table* symtab,
 176     Layout*,
 177     Target_type* target,
 178     Sized_relobj_file<size, big_endian>* src_obj,
 179     unsigned int src_indx,
 180     const unsigned char* prelocs,
 181     size_t reloc_count,
 182     Output_section*,
 183     bool,
 184     size_t local_count,
 185     const unsigned char* plocal_syms)
 186 {
 187   Scan scan;
 188
 189   typedef typename Reloc_types<sh_type, size, big_endian>::Reloc Reltype;
 190   const int reloc_size = Reloc_types<sh_type, size, big_endian>::reloc_size;
 191   const int sym_size = elfcpp::Elf_sizes<size>::sym_size;
 192
 193   Icf::Sections_reachable_info* secvec = NULL;
 194   Icf::Symbol_info* symvec = NULL;
 195   Icf::Addend_info* addendvec = NULL;
 196   Icf::Offset_info* offsetvec = NULL;
 197   Icf::Reloc_addend_size_info* reloc_addend_size_vec = NULL;
 198   bool is_icf_tracked = false;
 199   const char* cident_section_name = NULL;
 200
 201   std::string src_section_name = (parameters->options().icf_enabled()
 202                                   ? src_obj->section_name(src_indx)
 203                                   : "");
 204
 205   bool check_section_for_function_pointers = false;
 206
 207   if (parameters->options().icf_enabled()
 208       && is_section_foldable_candidate(src_section_name.c_str()))
 209     {
 210       is_icf_tracked = true;
 211       Section_id src_id(src_obj, src_indx);
 212       Icf::Reloc_info* reloc_info =
 213         &symtab->icf()->reloc_info_list()[src_id];
 214       secvec = &reloc_info->section_info;
 215       symvec = &reloc_info->symbol_info;
 216       addendvec = &reloc_info->addend_info;
 217       offsetvec = &reloc_info->offset_info;
 218       reloc_addend_size_vec = &reloc_info->reloc_addend_size_info;
 219     }
 220
 221   check_section_for_function_pointers =
 222     symtab->icf()->check_section_for_function_pointers(src_section_name,
 223                                                        target);
 224
 225   for (size_t i = 0; i < reloc_count; ++i, prelocs += reloc_size)
 226     {
 227       Reltype reloc(prelocs);
 228       typename elfcpp::Elf_types<size>::Elf_WXword r_info = reloc.get_r_info();
 229       unsigned int r_sym = elfcpp::elf_r_sym<size>(r_info);
 230       unsigned int r_type = elfcpp::elf_r_type<size>(r_info);
 231       typename elfcpp::Elf_types<size>::Elf_Swxword addend =
 232       Reloc_types<sh_type, size, big_endian>::get_reloc_addend_noerror(&reloc);
 233       Object* dst_obj;
 234       unsigned int dst_indx;
 235       typedef typename elfcpp::Elf_types<size>::Elf_Addr Address;
 236       Address dst_off;
 237
 238       if (r_sym < local_count)
 239         {
 240           gold_assert(plocal_syms != NULL);
 241           typename elfcpp::Sym<size, big_endian> lsym(plocal_syms
 242                                                       + r_sym * sym_size);
 243           dst_indx = lsym.get_st_shndx();
 244           bool is_ordinary;
 245           dst_indx = src_obj->adjust_sym_shndx(r_sym, dst_indx, &is_ordinary);
 246           dst_obj = src_obj;
 247           dst_off = lsym.get_st_value() + addend;
 248
 249           if (is_icf_tracked)
 250             {
 251               Address symvalue = dst_off - addend;
 252               if (is_ordinary)
 253                 (*secvec).push_back(Section_id(dst_obj, dst_indx));
 254               else
 255                 (*secvec).push_back(Section_id(NULL, 0));
 256               (*symvec).push_back(NULL);
 257               (*addendvec).push_back(std::make_pair(
 258                                         static_cast<long long>(symvalue),
 259                                         static_cast<long long>(addend)));
 260               uint64_t reloc_offset =
 261                 convert_to_section_size_type(reloc.get_r_offset());
 262               (*offsetvec).push_back(reloc_offset);
 263               (*reloc_addend_size_vec).push_back(
 264                 get_embedded_addend_size<Classify_reloc>(sh_type, r_type,
 265                                                          src_obj));
 266             }
 267
 268           // When doing safe folding, check to see if this relocation is that
 269           // of a function pointer being taken.
 270           if (is_ordinary
 271               && check_section_for_function_pointers
 272               && lsym.get_st_type() != elfcpp::STT_OBJECT
 273               && scan.local_reloc_may_be_function_pointer(symtab, NULL, NULL,
 274                                                           src_obj, src_indx,
 275                                                           NULL, reloc, r_type,
 276                                                           lsym))
 277             symtab->icf()->set_section_has_function_pointers(
 278               src_obj, lsym.get_st_shndx());
 279
 280           if (!is_ordinary || dst_indx == src_indx)
 281             continue;
 282         }
 283       else
 284         {
 285           Symbol* gsym = src_obj->global_symbol(r_sym);
 286           gold_assert(gsym != NULL);
 287           if (gsym->is_forwarder())
 288             gsym = symtab->resolve_forwards(gsym);
 289
 290           dst_obj = NULL;
 291           dst_indx = 0;
 292           bool is_ordinary = false;
 293           if (gsym->source() == Symbol::FROM_OBJECT)
 294             {
 295               dst_obj = gsym->object();
 296               dst_indx = gsym->shndx(&is_ordinary);
 297             }
 298           dst_off = static_cast<const Sized_symbol<size>*>(gsym)->value();
 299           dst_off += addend;
 300
 301           // When doing safe folding, check to see if this relocation is that
 302           // of a function pointer being taken.
 303           if (gsym->source() == Symbol::FROM_OBJECT
 304               && check_section_for_function_pointers
 305               && gsym->type() != elfcpp::STT_OBJECT
 306               && (!is_ordinary
 307                   || scan.global_reloc_may_be_function_pointer(
 308                        symtab, NULL, NULL, src_obj, src_indx, NULL, reloc,
 309                        r_type, gsym)))
 310             symtab->icf()->set_section_has_function_pointers(dst_obj, dst_indx);
 311
 312           // If the symbol name matches '__start_XXX' then the section with
 313           // the C identifier like name 'XXX' should not be garbage collected.
 314           // A similar treatment to symbols with the name '__stop_XXX'.
 315           if (is_prefix_of(cident_section_start_prefix, gsym->name()))
 316             {
 317               cident_section_name = (gsym->name()
 318                                      + strlen(cident_section_start_prefix));
 319             }
 320           else if (is_prefix_of(cident_section_stop_prefix, gsym->name()))
 321             {
 322               cident_section_name = (gsym->name()
 323                                      + strlen(cident_section_stop_prefix));
 324             }
 325           if (is_icf_tracked)
 326             {
 327               Address symvalue = dst_off - addend;
 328               if (is_ordinary && gsym->source() == Symbol::FROM_OBJECT)
 329                 (*secvec).push_back(Section_id(dst_obj, dst_indx));
 330               else
 331                 (*secvec).push_back(Section_id(NULL, 0));
 332               (*symvec).push_back(gsym);
 333               (*addendvec).push_back(std::make_pair(
 334                                         static_cast<long long>(symvalue),
 335                                         static_cast<long long>(addend)));
 336               uint64_t reloc_offset =
 337                 convert_to_section_size_type(reloc.get_r_offset());
 338               (*offsetvec).push_back(reloc_offset);
 339               (*reloc_addend_size_vec).push_back(
 340                 get_embedded_addend_size<Classify_reloc>(sh_type, r_type,
 341                                                          src_obj));
 342             }
 343
 344           if (gsym->source() != Symbol::FROM_OBJECT)
 345             continue;
 346           if (!is_ordinary)
 347             continue;
 348         }
 349       if (parameters->options().gc_sections())
 350         {
 351           symtab->gc()->add_reference(src_obj, src_indx, dst_obj, dst_indx);
 352           parameters->sized_target<size, big_endian>()
 353             ->gc_add_reference(symtab, src_obj, src_indx,
 354                                dst_obj, dst_indx, dst_off);
 355           if (cident_section_name != NULL)
 356             {
 357               Garbage_collection::Cident_section_map::iterator ele =
 358                 symtab->gc()->cident_sections()->find(std::string(cident_section_name));
 359               if (ele == symtab->gc()->cident_sections()->end())
 360                 continue;
 361               Section_id src_id(src_obj, src_indx);
 362               Garbage_collection::Sections_reachable&
 363                 v(symtab->gc()->section_reloc_map()[src_id]);
 364               Garbage_collection::Sections_reachable& cident_secn(ele->second);
 365               for (Garbage_collection::Sections_reachable::iterator it_v
 366                      = cident_secn.begin();
 367                    it_v != cident_secn.end();
 368                    ++it_v)
 369                 {
 370                   v.insert(*it_v);
 371                 }
 372             }
 373         }
 374     }
 375   return;
 376 }
 377
 378 } // End of namespace gold.
 379
 380 #endif