gold/gc.h

   1 // gc.h -- garbage collection of unused sections
   2
   3 // Copyright (C) 2009-2020 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 <vector>
  27
  28 #include "elfcpp.h"
  29 #include "symtab.h"
  30 #include "object.h"
  31 #include "icf.h"
  32
  33 namespace gold
  34 {
  35
  36 class Object;
  37
  38 template<int size, bool big_endian>
  39 class Sized_relobj_file;
  40
  41 class Output_section;
  42 class General_options;
  43 class Layout;
  44
  45 class Garbage_collection
  46 {
  47  public:
  48
  49   typedef Unordered_set<Section_id, Section_id_hash> Sections_reachable;
  50   typedef std::map<Section_id, Sections_reachable> Section_ref;
  51   typedef std::vector<Section_id> Worklist_type;
  52   // This maps the name of the section which can be represented as a C
  53   // identifier (cident) to the list of sections that have that name.
  54   // Different object files can have cident sections with the same name.
  55   typedef std::map<std::string, Sections_reachable> Cident_section_map;
  56
  57   Garbage_collection()
  58   : is_worklist_ready_(false)
  59   { }
  60
  61   // Accessor methods for the private members.
  62
  63   Sections_reachable&
  64   referenced_list()
  65   { return referenced_list_; }
  66
  67   Section_ref&
  68   section_reloc_map()
  69   { return this->section_reloc_map_; }
  70
  71   Worklist_type&
  72   worklist()
  73   { return this->work_list_; }
  74
  75   bool
  76   is_worklist_ready()
  77   { return this->is_worklist_ready_; }
  78
  79   void
  80   worklist_ready()
  81   { this->is_worklist_ready_ = true; }
  82
  83   void
  84   do_transitive_closure();
  85
  86   bool
  87   is_section_garbage(Relobj* obj, unsigned int shndx)
  88   { return (this->referenced_list().find(Section_id(obj, shndx))
  89             == this->referenced_list().end()); }
  90
  91   Cident_section_map*
  92   cident_sections()
  93   { return &cident_sections_; }
  94
  95   void
  96   add_cident_section(std::string section_name,
  97                      Section_id secn)
  98   { this->cident_sections_[section_name].insert(secn); }
  99
 100   // Add a reference from the SRC_SHNDX-th section of SRC_OBJECT to
 101   // DST_SHNDX-th section of DST_OBJECT.
 102   void
 103   add_reference(Relobj* src_object, unsigned int src_shndx,
 104                 Relobj* dst_object, unsigned int dst_shndx)
 105   {
 106     Section_id src_id(src_object, src_shndx);
 107     Section_id dst_id(dst_object, dst_shndx);
 108     Sections_reachable& reachable = this->section_reloc_map_[src_id];
 109     reachable.insert(dst_id);
 110   }
 111
 112  private:
 113
 114   Worklist_type work_list_;
 115   bool is_worklist_ready_;
 116   Section_ref section_reloc_map_;
 117   Sections_reachable referenced_list_;
 118   Cident_section_map cident_sections_;
 119 };
 120
 121 // Data to pass between successive invocations of do_layout
 122 // in object.cc while garbage collecting.  This data structure
 123 // is filled by using the data from Read_symbols_data.
 124
 125 struct Symbols_data
 126 {
 127   // Section headers.
 128   unsigned char* section_headers_data;
 129   // Section names.
 130   unsigned char* section_names_data;
 131   // Size of section name data in bytes.
 132   section_size_type section_names_size;
 133   // Symbol data.
 134   unsigned char* symbols_data;
 135   // Size of symbol data in bytes.
 136   section_size_type symbols_size;
 137   // Offset of external symbols within symbol data.  This structure
 138   // sometimes contains only external symbols, in which case this will
 139   // be zero.  Sometimes it contains all symbols.
 140   section_offset_type external_symbols_offset;
 141   // Symbol names.
 142   unsigned char* symbol_names_data;
 143   // Size of symbol name data in bytes.
 144   section_size_type symbol_names_size;
 145 };
 146
 147 // Relocations of type SHT_REL store the addend value in their bytes.
 148 // This function returns the size of the embedded addend which is
 149 // nothing but the size of the relocation.
 150
 151 template<typename Classify_reloc>
 152 inline unsigned int
 153 get_embedded_addend_size(int r_type, Relobj* obj)
 154 {
 155   if (Classify_reloc::sh_type == elfcpp::SHT_REL)
 156     return Classify_reloc::get_size_for_reloc(r_type, obj);
 157   return 0;
 158 }
 159
 160 // This function implements the generic part of reloc
 161 // processing to map a section to all the sections it
 162 // references through relocs.  It is called only during
 163 // garbage collection (--gc-sections) and identical code
 164 // folding (--icf).
 165
 166 template<int size, bool big_endian, typename Target_type,
 167          typename Scan, typename Classify_reloc>
 168 inline void
 169 gc_process_relocs(
 170     Symbol_table* symtab,
 171     Layout*,
 172     Target_type* target,
 173     Sized_relobj_file<size, big_endian>* src_obj,
 174     unsigned int src_indx,
 175     const unsigned char* prelocs,
 176     size_t reloc_count,
 177     Output_section*,
 178     bool,
 179     size_t local_count,
 180     const unsigned char* plocal_syms)
 181 {
 182   Scan scan;
 183
 184   typedef typename Classify_reloc::Reltype Reltype;
 185   const int reloc_size = Classify_reloc::reloc_size;
 186   const int sym_size = elfcpp::Elf_sizes<size>::sym_size;
 187
 188   Icf::Sections_reachable_info* secvec = NULL;
 189   Icf::Symbol_info* symvec = NULL;
 190   Icf::Addend_info* addendvec = NULL;
 191   Icf::Offset_info* offsetvec = NULL;
 192   Icf::Reloc_addend_size_info* reloc_addend_size_vec = NULL;
 193   bool is_icf_tracked = false;
 194   const char* cident_section_name = NULL;
 195
 196   std::string src_section_name = (parameters->options().icf_enabled()
 197                                   ? src_obj->section_name(src_indx)
 198                                   : "");
 199
 200   bool check_section_for_function_pointers = false;
 201
 202   if (parameters->options().icf_enabled()
 203       && (is_section_foldable_candidate(src_section_name)
 204           || is_prefix_of(".eh_frame", src_section_name.c_str())))
 205     {
 206       is_icf_tracked = true;
 207       Section_id src_id(src_obj, src_indx);
 208       Icf::Reloc_info* reloc_info =
 209         &symtab->icf()->reloc_info_list()[src_id];
 210       secvec = &reloc_info->section_info;
 211       symvec = &reloc_info->symbol_info;
 212       addendvec = &reloc_info->addend_info;
 213       offsetvec = &reloc_info->offset_info;
 214       reloc_addend_size_vec = &reloc_info->reloc_addend_size_info;
 215     }
 216
 217   check_section_for_function_pointers =
 218     symtab->icf()->check_section_for_function_pointers(src_section_name,
 219                                                        target);
 220
 221   for (size_t i = 0; i < reloc_count; ++i, prelocs += reloc_size)
 222     {
 223       Reltype reloc(prelocs);
 224       unsigned int r_sym = Classify_reloc::get_r_sym(&reloc);
 225       unsigned int r_type = Classify_reloc::get_r_type(&reloc);
 226       typename elfcpp::Elf_types<size>::Elf_Swxword addend =
 227           Classify_reloc::get_r_addend(&reloc);
 228       Relobj* dst_obj;
 229       unsigned int dst_indx;
 230       typedef typename elfcpp::Elf_types<size>::Elf_Addr Address;
 231       Address dst_off;
 232
 233       if (r_sym < local_count)
 234         {
 235           gold_assert(plocal_syms != NULL);
 236           typename elfcpp::Sym<size, big_endian> lsym(plocal_syms
 237                                                       + r_sym * sym_size);
 238           dst_indx = lsym.get_st_shndx();
 239           bool is_ordinary;
 240           dst_indx = src_obj->adjust_sym_shndx(r_sym, dst_indx, &is_ordinary);
 241           dst_obj = src_obj;
 242           dst_off = lsym.get_st_value() + addend;
 243
 244           if (is_icf_tracked)
 245             {
 246               Address symvalue = dst_off - addend;
 247               if (is_ordinary)
 248                 (*secvec).push_back(Section_id(src_obj, dst_indx));
 249               else
 250                 (*secvec).push_back(Section_id(NULL, 0));
 251               // If the target of the relocation is an STT_SECTION symbol,
 252               // make a note of that by storing -1 in the symbol vector.
 253               if (lsym.get_st_type() == elfcpp::STT_SECTION)
 254                 (*symvec).push_back(reinterpret_cast<Symbol*>(-1));
 255               else
 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>(r_type, 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, target,
 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 || dst_indx == 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               && !gsym->object()->is_dynamic())
 294             {
 295               dst_obj = static_cast<Relobj*>(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               && gsym->type() == elfcpp::STT_FUNC
 305               && check_section_for_function_pointers
 306               && dst_obj != NULL
 307               && (!is_ordinary
 308                   || scan.global_reloc_may_be_function_pointer(
 309                        symtab, NULL, target, src_obj, src_indx, NULL, reloc,
 310                        r_type, gsym)))
 311             symtab->icf()->set_section_has_function_pointers(dst_obj, dst_indx);
 312
 313           // If the symbol name matches '__start_XXX' then the section with
 314           // the C identifier like name 'XXX' should not be garbage collected.
 315           // A similar treatment to symbols with the name '__stop_XXX'.
 316           if (is_prefix_of(cident_section_start_prefix, gsym->name()))
 317             {
 318               cident_section_name = (gsym->name()
 319                                      + strlen(cident_section_start_prefix));
 320             }
 321           else if (is_prefix_of(cident_section_stop_prefix, gsym->name()))
 322             {
 323               cident_section_name = (gsym->name()
 324                                      + strlen(cident_section_stop_prefix));
 325             }
 326           if (is_icf_tracked)
 327             {
 328               Address symvalue = dst_off - addend;
 329               if (is_ordinary && dst_obj != NULL)
 330                 (*secvec).push_back(Section_id(dst_obj, dst_indx));
 331               else
 332                 (*secvec).push_back(Section_id(NULL, 0));
 333               (*symvec).push_back(gsym);
 334               (*addendvec).push_back(std::make_pair(
 335                                         static_cast<long long>(symvalue),
 336                                         static_cast<long long>(addend)));
 337               uint64_t reloc_offset =
 338                 convert_to_section_size_type(reloc.get_r_offset());
 339               (*offsetvec).push_back(reloc_offset);
 340               (*reloc_addend_size_vec).push_back(
 341                 get_embedded_addend_size<Classify_reloc>(r_type, src_obj));
 342             }
 343
 344           if (dst_obj == NULL)
 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, dst_obj, dst_indx,
 354                                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