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// icf.cc -- Identical Code Folding.

//

// Copyright 2009, 2010 Free Software Foundation, Inc.

// Written by Sriraman Tallam <tmsriram@google.com>.

// 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

// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

// 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.

// Identical Code Folding Algorithm

// ----------------------------------

// Detecting identical functions is done here and the basic algorithm

// is as follows.  A checksum is computed on each foldable section using

// its contents and relocations.  If the symbol name corresponding to

// a relocation is known it is used to compute the checksum.  If the

// symbol name is not known the stringified name of the object and the

// section number pointed to by the relocation is used.  The checksums

// are stored as keys in a hash map and a section is identical to some

// other section if its checksum is already present in the hash map.

// Checksum collisions are handled by using a multimap and explicitly

// checking the contents when two sections have the same checksum.

//

// However, two functions A and B with identical text but with

// relocations pointing to different foldable sections can be identical if

// the corresponding foldable sections to which their relocations point to

// turn out to be identical.  Hence, this checksumming process must be

// done repeatedly until convergence is obtained.  Here is an example for

// the following case :

//

// int funcA ()               int funcB ()

// {                          {

//   return foo();              return goo();

// }                          }

//

// The functions funcA and funcB are identical if functions foo() and

// goo() are identical.

//

// Hence, as described above, we repeatedly do the checksumming,

// assigning identical functions to the same group, until convergence is

// obtained.  Now, we have two different ways to do this depending on how

// we initialize.

//

// Algorithm I :

// -----------

// We can start with marking all functions as different and repeatedly do

// the checksumming.  This has the advantage that we do not need to wait

// for convergence. We can stop at any point and correctness will be

// guaranteed although not all cases would have been found.  However, this

// has a problem that some cases can never be found even if it is run until

// convergence.  Here is an example with mutually recursive functions :

//

// int funcA (int a)            int funcB (int a)

// {                            {

//   if (a == 1)                  if (a == 1)

//     return 1;                    return 1;

//   return 1 + funcB(a - 1);     return 1 + funcA(a - 1);

// }                            }

//

// In this example funcA and funcB are identical and one of them could be

// folded into the other.  However, if we start with assuming that funcA

// and funcB are not identical, the algorithm, even after it is run to

// convergence, cannot detect that they are identical.  It should be noted

// that even if the functions were self-recursive, Algorithm I cannot catch

// that they are identical, at least as is.

//

// Algorithm II :

// ------------

// Here we start with marking all functions as identical and then repeat

// the checksumming until convergence.  This can detect the above case

// mentioned above.  It can detect all cases that Algorithm I can and more.

// However, the caveat is that it has to be run to convergence.  It cannot

// be stopped arbitrarily like Algorithm I as correctness cannot be

// guaranteed.  Algorithm II is not implemented.

//

// Algorithm I is used because experiments show that about three

// iterations are more than enough to achieve convergence. Algorithm I can

// handle recursive calls if it is changed to use a special common symbol

// for recursive relocs.  This seems to be the most common case that

// Algorithm I could not catch as is.  Mutually recursive calls are not

// frequent and Algorithm I wins because of its ability to be stopped

// arbitrarily.

//

// Caveat with using function pointers :

// ------------------------------------

//

// Programs using function pointer comparisons/checks should use function

// folding with caution as the result of such comparisons could be different

// when folding takes place.  This could lead to unexpected run-time

// behaviour.

//

// Safe Folding :

// ------------

//

// ICF in safe mode folds only ctors and dtors if their function pointers can

// never be taken.  Also, for X86-64, safe folding uses the relocation

// type to determine if a function's pointer is taken or not and only folds

// functions whose pointers are definitely not taken.

//

// Caveat with safe folding :

// ------------------------

//

// This applies only to x86_64.

//

// Position independent executables are created from PIC objects (compiled

// with -fPIC) and/or PIE objects (compiled with -fPIE).  For PIE objects, the

// relocation types for function pointer taken and a call are the same.

// Now, it is not always possible to tell if an object used in the link of

// a pie executable is a PIC object or a PIE object.  Hence, for pie

// executables, using relocation types to disambiguate function pointers is

// currently disabled.

//

// Further, it is not correct to use safe folding to build non-pie

// executables using PIC/PIE objects.  PIC/PIE objects have different

// relocation types for function pointers than non-PIC objects, and the

// current implementation of safe folding does not handle those relocation

// types.  Hence, if used, functions whose pointers are taken could still be

// folded causing unpredictable run-time behaviour if the pointers were used

// in comparisons.

//

//

//

// How to run  : --icf=[safe|all|none]

// Optional parameters : --icf-iterations <num> --print-icf-sections

//

// Performance : Less than 20 % link-time overhead on industry strength

// applications.  Up to 6 %  text size reductions.

#include "gold.h"

#include "object.h"

#include "gc.h"

#include "icf.h"

#include "symtab.h"

#include "libiberty.h"

#include "demangle.h"

#include "elfcpp.h"

#include "int_encoding.h"

namespace gold

{

// This function determines if a section or a group of identical

// sections has unique contents.  Such unique sections or groups can be

// declared final and need not be processed any further.

// Parameters :

// ID_SECTION : Vector mapping a section index to a Section_id pair.

// IS_SECN_OR_GROUP_UNIQUE : To check if a section or a group of identical

//                            sections is already known to be unique.

// SECTION_CONTENTS : Contains the section's text and relocs to sections

//                    that cannot be folded.   SECTION_CONTENTS are NULL

//                    implies that this function is being called for the

//                    first time before the first iteration of icf.

static void

preprocess_for_unique_sections(const std::vector<Section_id>& id_section,

                               std::vector<bool>* is_secn_or_group_unique,

                               std::vector<std::string>* section_contents)

{

  Unordered_map<uint32_t, unsigned int> uniq_map;

  std::pair<Unordered_map<uint32_t, unsigned int>::iterator, bool>

    uniq_map_insert;

  for (unsigned int i = 0; i < id_section.size(); i++)

    {

      if ((*is_secn_or_group_unique)[i])

        continue;

      uint32_t cksum;

      Section_id secn = id_section[i];

      section_size_type plen;

      if (section_contents == NULL)

        {

          // Lock the object so we can read from it.  This is only called

          // single-threaded from queue_middle_tasks, so it is OK to lock.

          // Unfortunately we have no way to pass in a Task token.

          const Task* dummy_task = reinterpret_cast<const Task*>(-1);

          Task_lock_obj<Object> tl(dummy_task, secn.first);

          const unsigned char* contents;

          contents = secn.first->section_contents(secn.second,

                                                  &plen,

                                                  false);

          cksum = xcrc32(contents, plen, 0xffffffff);

        }

      else

        {

          const unsigned char* contents_array = reinterpret_cast

            <const unsigned char*>((*section_contents)[i].c_str());

          cksum = xcrc32(contents_array, (*section_contents)[i].length(),

                         0xffffffff);

        }

      uniq_map_insert = uniq_map.insert(std::make_pair(cksum, i));

      if (uniq_map_insert.second)

        {

          (*is_secn_or_group_unique)[i] = true;

        }

      else

        {

          (*is_secn_or_group_unique)[i] = false;

          (*is_secn_or_group_unique)[uniq_map_insert.first->second] = false;

        }

    }

}

// This returns the buffer containing the section's contents, both

// text and relocs.  Relocs are differentiated as those pointing to

// sections that could be folded and those that cannot.  Only relocs

// pointing to sections that could be folded are recomputed on

// subsequent invocations of this function.

// Parameters  :

// FIRST_ITERATION    : true if it is the first invocation.

// SECN               : Section for which contents are desired.

// SECTION_NUM        : Unique section number of this section.

// NUM_TRACKED_RELOCS : Vector reference to store the number of relocs

//                      to ICF sections.

// KEPT_SECTION_ID    : Vector which maps folded sections to kept sections.

// SECTION_CONTENTS   : Store the section's text and relocs to non-ICF

//                      sections.

static std::string

get_section_contents(bool first_iteration,

                     const Section_id& secn,

                     unsigned int section_num,

                     unsigned int* num_tracked_relocs,

                     Symbol_table* symtab,

                     const std::vector<unsigned int>& kept_section_id,

                     std::vector<std::string>* section_contents)

{

  // Lock the object so we can read from it.  This is only called

  // single-threaded from queue_middle_tasks, so it is OK to lock.

  // Unfortunately we have no way to pass in a Task token.

  const Task* dummy_task = reinterpret_cast<const Task*>(-1);

  Task_lock_obj<Object> tl(dummy_task, secn.first);

  section_size_type plen;

  const unsigned char* contents = NULL;

  if (first_iteration)

    contents = secn.first->section_contents(secn.second, &plen, false);

  // The buffer to hold all the contents including relocs.  A checksum

  // is then computed on this buffer.

  std::string buffer;

  std::string icf_reloc_buffer;

  if (num_tracked_relocs)

    *num_tracked_relocs = 0;

  Icf::Reloc_info_list& reloc_info_list =

    symtab->icf()->reloc_info_list();

  Icf::Reloc_info_list::iterator it_reloc_info_list =

    reloc_info_list.find(secn);

  buffer.clear();

  icf_reloc_buffer.clear();

  // Process relocs and put them into the buffer.

  if (it_reloc_info_list != reloc_info_list.end())

    {

      Icf::Sections_reachable_info v =

        (it_reloc_info_list->second).section_info;

      // Stores the information of the symbol pointed to by the reloc.

      Icf::Symbol_info s = (it_reloc_info_list->second).symbol_info;

      // Stores the addend and the symbol value.

      Icf::Addend_info a = (it_reloc_info_list->second).addend_info;

      // Stores the offset of the reloc.

      Icf::Offset_info o = (it_reloc_info_list->second).offset_info;

      Icf::Reloc_addend_size_info reloc_addend_size_info =

        (it_reloc_info_list->second).reloc_addend_size_info;

      Icf::Sections_reachable_info::iterator it_v = v.begin();

      Icf::Symbol_info::iterator it_s = s.begin();

      Icf::Addend_info::iterator it_a = a.begin();

      Icf::Offset_info::iterator it_o = o.begin();

      Icf::Reloc_addend_size_info::iterator it_addend_size =

        reloc_addend_size_info.begin();

      for (; it_v != v.end(); ++it_v, ++it_s, ++it_a, ++it_o, ++it_addend_size)

        {

          // ADDEND_STR stores the symbol value and addend and offset,

          // each at most 16 hex digits long.  it_a points to a pair

          // where first is the symbol value and second is the

          // addend.

          char addend_str[50];

          // It would be nice if we could use format macros in inttypes.h

          // here but there are not in ISO/IEC C++ 1998.

          snprintf(addend_str, sizeof(addend_str), "%llx %llx %llux",

                   static_cast<long long>((*it_a).first),

                   static_cast<long long>((*it_a).second),

                   static_cast<unsigned long long>(*it_o));

          // If the symbol pointed to by the reloc is not in an ordinary

          // section or if the symbol type is not FROM_OBJECT, then the

          // object is NULL.

          if (it_v->first == NULL)

            {

              if (first_iteration)

                {

                  // If the symbol name is available, use it.

                  if ((*it_s) != NULL)

                      buffer.append((*it_s)->name());

                  // Append the addend.

                  buffer.append(addend_str);

                  buffer.append("@");

                }

              continue;

            }

          Section_id reloc_secn(it_v->first, it_v->second);

          // If this reloc turns back and points to the same section,

          // like a recursive call, use a special symbol to mark this.

          if (reloc_secn.first == secn.first

              && reloc_secn.second == secn.second)

            {

              if (first_iteration)

                {

                  buffer.append("R");

                  buffer.append(addend_str);

                  buffer.append("@");

                }

              continue;

            }

          Icf::Uniq_secn_id_map& section_id_map =

            symtab->icf()->section_to_int_map();

          Icf::Uniq_secn_id_map::iterator section_id_map_it =

            section_id_map.find(reloc_secn);

          bool is_sym_preemptible = (*it_s != NULL

                                     && !(*it_s)->is_from_dynobj()

                                     && !(*it_s)->is_undefined()

                                     && (*it_s)->is_preemptible());

          if (!is_sym_preemptible

              && section_id_map_it != section_id_map.end())

            {

              // This is a reloc to a section that might be folded.

              if (num_tracked_relocs)

                (*num_tracked_relocs)++;

              char kept_section_str[10];

              unsigned int secn_id = section_id_map_it->second;

              snprintf(kept_section_str, sizeof(kept_section_str), "%u",

                       kept_section_id[secn_id]);

              if (first_iteration)

                {

                  buffer.append("ICF_R");

                  buffer.append(addend_str);

                }

              icf_reloc_buffer.append(kept_section_str);

              // Append the addend.

              icf_reloc_buffer.append(addend_str);

              icf_reloc_buffer.append("@");

            }

          else

            {

              // This is a reloc to a section that cannot be folded.

              // Process it only in the first iteration.

              if (!first_iteration)

                continue;

              uint64_t secn_flags = (it_v->first)->section_flags(it_v->second);

              // This reloc points to a merge section.  Hash the

              // contents of this section.

              if ((secn_flags & elfcpp::SHF_MERGE) != 0

                  && parameters->target().can_icf_inline_merge_sections ())

                {

                  uint64_t entsize =

                    (it_v->first)->section_entsize(it_v->second);

                  long long offset = it_a->first;

                  unsigned long long addend = it_a->second;

                  // Ignoring the addend when it is a negative value.  See the 

                  // comments in Merged_symbol_value::Value in object.h.

                  if (addend < 0xffffff00)

                    offset = offset + addend;

                  // For SHT_REL relocation sections, the addend is stored in the

                  // text section at the relocation offset.

                  uint64_t reloc_addend_value = 0;

                  const unsigned char* reloc_addend_ptr =

                    contents + static_cast<unsigned long long>(*it_o);

                  switch(*it_addend_size)

                    {

                      case 0:

                        {

                          break;

                        }

                      case 1:

                        {

                          reloc_addend_value =

                            read_from_pointer<8>(reloc_addend_ptr);

                          break;

                        }

                      case 2:

                        {

                          reloc_addend_value =

                            read_from_pointer<16>(reloc_addend_ptr);

                          break;

                        }

                      case 4:

                        {

                          reloc_addend_value =

                            read_from_pointer<32>(reloc_addend_ptr);

                          break;

                        }

                      case 8:

                        {

                          reloc_addend_value =

                            read_from_pointer<64>(reloc_addend_ptr);

                          break;

                        }

                      default:

                        gold_unreachable();

                    }

                  offset = offset + reloc_addend_value;

                  section_size_type secn_len;

                  const unsigned char* str_contents =

                  (it_v->first)->section_contents(it_v->second,

                                                  &secn_len,

                                                  false) + offset;

                  if ((secn_flags & elfcpp::SHF_STRINGS) != 0)

                    {

                      // String merge section.

                      const char* str_char =

                        reinterpret_cast<const char*>(str_contents);

                      switch(entsize)

                        {

                        case 1:

                          {

                            buffer.append(str_char);

                            break;

                          }

                        case 2:

                          {

                            const uint16_t* ptr_16 =

                              reinterpret_cast<const uint16_t*>(str_char);

                            unsigned int strlen_16 = 0;

                            // Find the NULL character.

                            while(*(ptr_16 + strlen_16) != 0)

                                strlen_16++;

                            buffer.append(str_char, strlen_16 * 2);

                          }

                          break;

                        case 4:

                          {

                            const uint32_t* ptr_32 =

                              reinterpret_cast<const uint32_t*>(str_char);

                            unsigned int strlen_32 = 0;

                            // Find the NULL character.

                            while(*(ptr_32 + strlen_32) != 0)

                                strlen_32++;

                            buffer.append(str_char, strlen_32 * 4);

                          }

                          break;

                        default:

                          gold_unreachable();

                        }

                    }

                  else

                    {

                      // Use the entsize to determine the length.

                      buffer.append(reinterpret_cast<const

                                                     char*>(str_contents),

                                    entsize);

                    }

                  buffer.append("@");

                }

              else if ((*it_s) != NULL)

                {

                  // If symbol name is available use that.

                  buffer.append((*it_s)->name());

                  // Append the addend.

                  buffer.append(addend_str);

                  buffer.append("@");

                }

              else

                {

                  // Symbol name is not available, like for a local symbol,

                  // use object and section id.

                  buffer.append(it_v->first->name());

                  char secn_id[10];

                  snprintf(secn_id, sizeof(secn_id), "%u",it_v->second);

                  buffer.append(secn_id);

                  // Append the addend.

                  buffer.append(addend_str);

                  buffer.append("@");

                }

            }

        }

    }

  if (first_iteration)

    {

      buffer.append("Contents = ");

      buffer.append(reinterpret_cast<const char*>(contents), plen);

      // Store the section contents that dont change to avoid recomputing

      // during the next call to this function.

      (*section_contents)[section_num] = buffer;

    }

  else

    {

      gold_assert(buffer.empty());

      // Reuse the contents computed in the previous iteration.

      buffer.append((*section_contents)[section_num]);

    }

  buffer.append(icf_reloc_buffer);

  return buffer;

}

// This function computes a checksum on each section to detect and form

// groups of identical sections.  The first iteration does this for all 

// sections.

// Further iterations do this only for the kept sections from each group to

// determine if larger groups of identical sections could be formed.  The

// first section in each group is the kept section for that group.

//

// CRC32 is the checksumming algorithm and can have collisions.  That is,

// two sections with different contents can have the same checksum. Hence,

// a multimap is used to maintain more than one group of checksum

// identical sections.  A section is added to a group only after its

// contents are explicitly compared with the kept section of the group.

//

// Parameters  :

// ITERATION_NUM           : Invocation instance of this function.

// NUM_TRACKED_RELOCS : Vector reference to store the number of relocs

//                      to ICF sections.

// KEPT_SECTION_ID    : Vector which maps folded sections to kept sections.

// ID_SECTION         : Vector mapping a section to an unique integer.

// IS_SECN_OR_GROUP_UNIQUE : To check if a section or a group of identical

//                            sections is already known to be unique.

// SECTION_CONTENTS   : Store the section's text and relocs to non-ICF

//                      sections.

static bool

match_sections(unsigned int iteration_num,

               Symbol_table* symtab,

               std::vector<unsigned int>* num_tracked_relocs,

               std::vector<unsigned int>* kept_section_id,

               const std::vector<Section_id>& id_section,

               std::vector<bool>* is_secn_or_group_unique,

               std::vector<std::string>* section_contents)

{

  Unordered_multimap<uint32_t, unsigned int> section_cksum;

  std::pair<Unordered_multimap<uint32_t, unsigned int>::iterator,

            Unordered_multimap<uint32_t, unsigned int>::iterator> key_range;

  bool converged = true;

  if (iteration_num == 1)

    preprocess_for_unique_sections(id_section,

                                   is_secn_or_group_unique,

                                   NULL);

  else

    preprocess_for_unique_sections(id_section,

                                   is_secn_or_group_unique,

                                   section_contents);

  std::vector<std::string> full_section_contents;

  for (unsigned int i = 0; i < id_section.size(); i++)

    {

      full_section_contents.push_back("");

      if ((*is_secn_or_group_unique)[i])

        continue;

      Section_id secn = id_section[i];

      std::string this_secn_contents;

      uint32_t cksum;

      if (iteration_num == 1)

        {

          unsigned int num_relocs = 0;

          this_secn_contents = get_section_contents(true, secn, i, &num_relocs,

                                                    symtab, (*kept_section_id),

                                                    section_contents);

          (*num_tracked_relocs)[i] = num_relocs;

        }

      else

        {

          if ((*kept_section_id)[i] != i)

            {

              // This section is already folded into something.  See

              // if it should point to a different kept section.

              unsigned int kept_section = (*kept_section_id)[i];

              if (kept_section != (*kept_section_id)[kept_section])

                {

                  (*kept_section_id)[i] = (*kept_section_id)[kept_section];

                }

              continue;

            }

          this_secn_contents = get_section_contents(false, secn, i, NULL,

                                                    symtab, (*kept_section_id),

                                                    section_contents);

        }

      const unsigned char* this_secn_contents_array =

            reinterpret_cast<const unsigned char*>(this_secn_contents.c_str());

      cksum = xcrc32(this_secn_contents_array, this_secn_contents.length(),

                     0xffffffff);

      size_t count = section_cksum.count(cksum);

      if (count == 0)

        {

          // Start a group with this cksum.

          section_cksum.insert(std::make_pair(cksum, i));

          full_section_contents[i] = this_secn_contents;

        }

      else

        {

          key_range = section_cksum.equal_range(cksum);

          Unordered_multimap<uint32_t, unsigned int>::iterator it;

          // Search all the groups with this cksum for a match.

          for (it = key_range.first; it != key_range.second; ++it)

            {

              unsigned int kept_section = it->second;

              if (full_section_contents[kept_section].length()

                  != this_secn_contents.length())

                  continue;

              if (memcmp(full_section_contents[kept_section].c_str(),

                         this_secn_contents.c_str(),

                         this_secn_contents.length()) != 0)

                  continue;

              (*kept_section_id)[i] = kept_section;

              converged = false;

              break;

            }

          if (it == key_range.second)

            {

              // Create a new group for this cksum.

              section_cksum.insert(std::make_pair(cksum, i));

              full_section_contents[i] = this_secn_contents;

            }

        }

      // If there are no relocs to foldable sections do not process

      // this section any further.

      if (iteration_num == 1 && (*num_tracked_relocs)[i] == 0)

        (*is_secn_or_group_unique)[i] = true;

    }

  return converged;

}

// During safe icf (--icf=safe), only fold functions that are ctors or dtors.

// This function returns true if the section name is that of a ctor or a dtor.

static bool

is_function_ctor_or_dtor(const std::string& section_name)

{

  const char* mangled_func_name = strrchr(section_name.c_str(), '.');

  gold_assert(mangled_func_name != NULL);

  if ((is_prefix_of("._ZN", mangled_func_name)

       || is_prefix_of("._ZZ", mangled_func_name))

      && (is_gnu_v3_mangled_ctor(mangled_func_name + 1)

          || is_gnu_v3_mangled_dtor(mangled_func_name + 1)))

    {

      return true;

    }

  return false;

}

// This is the main ICF function called in gold.cc.  This does the

// initialization and calls match_sections repeatedly (twice by default)

// which computes the crc checksums and detects identical functions.