--- /dev/null
+// 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,
+ §ion_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())
+ return;
+ unsigned int section_num = it->second;
+ unsigned int kept_section_id = this->kept_section_id_[section_num];
+ if (kept_section_id != section_num)
+ this->kept_section_id_[section_num] = section_num;
+}
+
+// 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.
projects / msp430-binutils.git / blobdiff