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+// 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"
+
+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;
+}
+
+
+// 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)
+        {
+          // Only looking to fold functions, so just look at .text sections.
+          if (!is_prefix_of(".text.", (*p)->section_name(i).c_str()))
+            continue;
+          if (!(*p)->is_section_included(i))
+            continue;
+          if (parameters->options().gc_sections()
+              && symtab->gc()->is_section_garbage(*p, i))
+              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);
+    }
+
+  this->icf_ready();
+}
+
+// This function determines if the section corresponding to the
+// given object and index is folded based on if the kept section
+// is different from this section.
+
+bool
+Icf::is_section_folded(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())
+    return false;
+  unsigned int section_num = it->second;
+  unsigned int kept_section_id = this->kept_section_id_[section_num];
+  return kept_section_id != section_num;
+}
+
+// This function returns the folded section for the given section.
+
+Section_id
+Icf::get_folded_section(Object* dup_obj, unsigned int dup_shndx)
+{
+  Section_id dup_secn(dup_obj, dup_shndx);
+  Uniq_secn_id_map::iterator it = this->section_id_.find(dup_secn);
+  gold_assert(it != this->section_id_.end());
+  unsigned int section_num = it->second;
+  unsigned int kept_section_id = this->kept_section_id_[section_num];
+  Section_id folded_section = this->id_section_[kept_section_id];
+  return folded_section;
+}
+
+} // End of namespace gold.