// icf.cc -- Identical Code Folding. // // Copyright 2009, 2010 Free Software Foundation, Inc. // Written by Sriraman Tallam . // 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 --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& id_section, std::vector* is_secn_or_group_unique, std::vector* section_contents) { Unordered_map uniq_map; std::pair::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 ((*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& kept_section_id, std::vector* 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(str_contents); switch(entsize) { case 1: { buffer.append(str_char); break; } case 2: { const uint16_t* ptr_16 = reinterpret_cast(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(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(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(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* num_tracked_relocs, std::vector* kept_section_id, const std::vector& id_section, std::vector* is_secn_or_group_unique, std::vector* section_contents) { Unordered_multimap section_cksum; std::pair::iterator, Unordered_multimap::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 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(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::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 num_tracked_relocs; std::vector is_secn_or_group_unique; std::vector section_contents; const Target& target = parameters->target(); // 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(); if (!is_section_foldable_candidate(section_name)) continue; if (!(*p)->is_section_included(i)) continue; if (parameters->options().gc_sections() && symtab->gc()->is_section_garbage(*p, i)) continue; const char* mangled_func_name = strrchr(section_name, '.'); gold_assert(mangled_func_name != NULL); // With --icf=safe, check if the mangled function name is a ctor // or a dtor. The mangled function name can be obtained from the // section name by stripping the section prefix. if (parameters->options().icf_safe_folding() && !is_function_ctor_or_dtor(mangled_func_name + 1) && (!target.can_check_for_function_pointers() || section_has_function_pointers(*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, §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.