Subversion Repositories openrisc_2011-10-31

// icf.cc -- Identical Code Folding.

//

// Copyright 2009 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 .text 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 .text sections can be identical if

// the corresponding .text 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.

//

//

// How to run  : --icf

// 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"

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)

        {

          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)

{

  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::Section_list& seclist = symtab->icf()->section_reloc_list();

  Icf::Symbol_list& symlist = symtab->icf()->symbol_reloc_list();

  Icf::Addend_list& addendlist = symtab->icf()->addend_reloc_list();

  Icf::Section_list::iterator it_seclist = seclist.find(secn);

  Icf::Symbol_list::iterator it_symlist = symlist.find(secn);

  Icf::Addend_list::iterator it_addendlist = addendlist.find(secn);

  buffer.clear();

  icf_reloc_buffer.clear();

  // Process relocs and put them into the buffer.

  if (it_seclist != seclist.end())

    {

      gold_assert(it_symlist != symlist.end());

      gold_assert(it_addendlist != addendlist.end());

      Icf::Sections_reachable_list v = it_seclist->second;

      Icf::Symbol_info s = it_symlist->second;

      Icf::Addend_info a = it_addendlist->second;

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

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

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

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

        {

          // ADDEND_STR stores the symbol value and addend, each

          // atmost 16 hex digits long.  it_v points to a pair

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

          // addend.

          char addend_str[34];

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

                   (*it_a).first, (*it_a).second);

          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);

          if (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)

                {

                  uint64_t entsize =

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

                  long long offset = it_a->first + it_a->second;

                  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);

                    }

                }

              else if ((*it_s) != NULL)

                {

                  // If symbol name is available use that.

                  const char *sym_name = (*it_s)->name();

                  buffer.append(sym_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

//                            sectionsis 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 mangled function name is a ctor or a

// dtor.

static bool

is_function_ctor_or_dtor(const char* mangled_func_name)

{

  if ((is_prefix_of("_ZN", mangled_func_name)

       || is_prefix_of("_ZZ", mangled_func_name))

      && (is_gnu_v3_mangled_ctor(mangled_func_name)

          || is_gnu_v3_mangled_dtor(mangled_func_name)))

    {

      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.

void

Icf::find_identical_sections(const Input_objects* input_objects,

                             Symbol_table* symtab)

{

  unsigned int section_num = 0;

  std::vector<unsigned int> num_tracked_relocs;

  std::vector<bool> is_secn_or_group_unique;

  std::vector<std::string> section_contents;

  // Decide which sections are possible candidates first.

  for (Input_objects::Relobj_iterator p = input_objects->relobj_begin();

       p != input_objects->relobj_end();

       ++p)

    {

      for (unsigned int i = 0;i < (*p)->shnum(); ++i)

        {

          const char* section_name = (*p)->section_name(i).c_str();

          // Only looking to fold functions, so just look at .text sections.

          if (!is_prefix_of(".text.", section_name))

            continue;

          if (!(*p)->is_section_included(i))

            continue;

          if (parameters->options().gc_sections()

              && symtab->gc()->is_section_garbage(*p, i))

              continue;

          // With --icf=safe, check if mangled name is a ctor or a dtor.

          if (parameters->options().icf_safe_folding()

              && !is_function_ctor_or_dtor(section_name + 6))

            continue;

          this->id_section_.push_back(Section_id(*p, i));

          this->section_id_[Section_id(*p, i)] = section_num;

          this->kept_section_id_.push_back(section_num);

          num_tracked_relocs.push_back(0);

          is_secn_or_group_unique.push_back(false);

          section_contents.push_back("");

          section_num++;

        }

    }

  unsigned int num_iterations = 0;

  // Default number of iterations to run ICF is 2.

  unsigned int max_iterations = (parameters->options().icf_iterations() > 0)

                            ? parameters->options().icf_iterations()

                            : 2;

  bool converged = false;

  while (!converged && (num_iterations < max_iterations))

    {

      num_iterations++;

      converged = match_sections(num_iterations, symtab,

                                 &num_tracked_relocs, &this->kept_section_id_,

                                 this->id_section_, &is_secn_or_group_unique,

                                 &section_contents);

    }

  if (parameters->options().print_icf_sections())

    {

      if (converged)

        gold_info(_("%s: ICF Converged after %u iteration(s)"),

                  program_name, num_iterations);

      else

        gold_info(_("%s: ICF stopped after %u iteration(s)"),

                  program_name, num_iterations);

    }

  // Unfold --keep-unique symbols.

  for (options::String_set::const_iterator p =

         parameters->options().keep_unique_begin();

       p != parameters->options().keep_unique_end();

       ++p)

    {

      const char* name = p->c_str();

      Symbol* sym = symtab->lookup(name);

      if (sym == NULL)

        {

          gold_warning(_("Could not find symbol %s to unfold\n"), name);

        }

      else if (sym->source() == Symbol::FROM_OBJECT

               && !sym->object()->is_dynamic())

        {

          Object* obj = sym->object();

          bool is_ordinary;

          unsigned int shndx = sym->shndx(&is_ordinary);

          if (is_ordinary)

            {

              this->unfold_section(obj, shndx);

            }

        }

    }

  this->icf_ready();

}

// Unfolds the section denoted by OBJ and SHNDX if folded.

void

Icf::unfold_section(Object* obj, unsigned int shndx)

{

  Section_id secn(obj, shndx);

  Uniq_secn_id_map::iterator it = this->section_id_.find(secn);

  if (it == this->section_id_.end())