// icf.cc -- Identical Code Folding. // // Copyright (C) 2009-2023 Free Software Foundation, Inc. // Written by Sriraman Tallam <tmsriram@google.com>. // This file is part of gold. // This program is free software; you can redistribute it and/or modify // it under the terms of the GNU General Public License as published by // the Free Software Foundation; either version 3 of the License, or // (at your option) any later version. // This program is distributed in the hope that it will be useful, // but WITHOUT ANY WARRANTY; without even the implied warranty of // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the // GNU General Public License for more details. // You should have received a copy of the GNU General Public License // along with this program; if not, write to the Free Software // Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston, // MA 02110-1301, USA. // Identical Code Folding Algorithm // ---------------------------------- // Detecting identical functions is done here and the basic algorithm // is as follows. A checksum is computed on each foldable section using // its contents and relocations. If the symbol name corresponding to // a relocation is known it is used to compute the checksum. If the // symbol name is not known the stringified name of the object and the // section number pointed to by the relocation is used. The checksums // are stored as keys in a hash map and a section is identical to some // other section if its checksum is already present in the hash map. // Checksum collisions are handled by using a multimap and explicitly // checking the contents when two sections have the same checksum. // // However, two functions A and B with identical text but with // relocations pointing to different foldable sections can be identical if // the corresponding foldable sections to which their relocations point to // turn out to be identical. Hence, this checksumming process must be // done repeatedly until convergence is obtained. Here is an example for // the following case : // // int funcA () int funcB () // { { // return foo(); return goo(); // } } // // The functions funcA and funcB are identical if functions foo() and // goo() are identical. // // Hence, as described above, we repeatedly do the checksumming, // assigning identical functions to the same group, until convergence is // obtained. Now, we have two different ways to do this depending on how // we initialize. // // Algorithm I : // ----------- // We can start with marking all functions as different and repeatedly do // the checksumming. This has the advantage that we do not need to wait // for convergence. We can stop at any point and correctness will be // guaranteed although not all cases would have been found. However, this // has a problem that some cases can never be found even if it is run until // convergence. Here is an example with mutually recursive functions : // // int funcA (int a) int funcB (int a) // { { // if (a == 1) if (a == 1) // return 1; return 1; // return 1 + funcB(a - 1); return 1 + funcA(a - 1); // } } // // In this example funcA and funcB are identical and one of them could be // folded into the other. However, if we start with assuming that funcA // and funcB are not identical, the algorithm, even after it is run to // convergence, cannot detect that they are identical. It should be noted // that even if the functions were self-recursive, Algorithm I cannot catch // that they are identical, at least as is. // // Algorithm II : // ------------ // Here we start with marking all functions as identical and then repeat // the checksumming until convergence. This can detect the above case // mentioned above. It can detect all cases that Algorithm I can and more. // However, the caveat is that it has to be run to convergence. It cannot // be stopped arbitrarily like Algorithm I as correctness cannot be // guaranteed. Algorithm II is not implemented. // // Algorithm I is used because experiments show that about three // iterations are more than enough to achieve convergence. Algorithm I can // handle recursive calls if it is changed to use a special common symbol // for recursive relocs. This seems to be the most common case that // Algorithm I could not catch as is. Mutually recursive calls are not // frequent and Algorithm I wins because of its ability to be stopped // arbitrarily. // // Caveat with using function pointers : // ------------------------------------ // // Programs using function pointer comparisons/checks should use function // folding with caution as the result of such comparisons could be different // when folding takes place. This could lead to unexpected run-time // behaviour. // // Safe Folding : // ------------ // // ICF in safe mode folds only ctors and dtors if their function pointers can // never be taken. Also, for X86-64, safe folding uses the relocation // type to determine if a function's pointer is taken or not and only folds // functions whose pointers are definitely not taken. // // Caveat with safe folding : // ------------------------ // // This applies only to x86_64. // // Position independent executables are created from PIC objects (compiled // with -fPIC) and/or PIE objects (compiled with -fPIE). For PIE objects, the // relocation types for function pointer taken and a call are the same. // Now, it is not always possible to tell if an object used in the link of // a pie executable is a PIC object or a PIE object. Hence, for pie // executables, using relocation types to disambiguate function pointers is // currently disabled. // // Further, it is not correct to use safe folding to build non-pie // executables using PIC/PIE objects. PIC/PIE objects have different // relocation types for function pointers than non-PIC objects, and the // current implementation of safe folding does not handle those relocation // types. Hence, if used, functions whose pointers are taken could still be // folded causing unpredictable run-time behaviour if the pointers were used // in comparisons. // // Notes regarding C++ exception handling : // -------------------------------------- // // It is possible for two sections to have identical text, identical // relocations, but different exception handling metadata (unwind // information in the .eh_frame section, and/or handler information in // a .gcc_except_table section). Thus, if a foldable section is // referenced from a .eh_frame FDE, we must include in its checksum // the contents of that FDE as well as of the CIE that the FDE refers // to. The CIE and FDE in turn probably contain relocations to the // personality routine and LSDA, which are handled like any other // relocation for ICF purposes. This logic is helped by the fact that // gcc with -ffunction-sections puts each function's LSDA in its own // .gcc_except_table.<functionname> section. Given sections for two // functions with nontrivial exception handling logic, we will // determine on the first iteration that their .gcc_except_table // sections are identical and can be folded, and on the second // iteration that their .text and .eh_frame contents (including the // now-merged .gcc_except_table relocations for the LSDA) are // identical and can be folded. // // // How to run : --icf=[safe|all|none] // Optional parameters : --icf-iterations <num> --print-icf-sections // // Performance : Less than 20 % link-time overhead on industry strength // applications. Up to 6 % text size reductions. #include "gold.h" #include "object.h" #include "gc.h" #include "icf.h" #include "symtab.h" #include "libiberty.h" #include "demangle.h" #include "elfcpp.h" #include "int_encoding.h" #include <limits> namespace gold { // This function determines if a section or a group of identical // sections has unique contents. Such unique sections or groups can be // declared final and need not be processed any further. // Parameters : // ID_SECTION : Vector mapping a section index to a Section_id pair. // IS_SECN_OR_GROUP_UNIQUE : To check if a section or a group of identical // sections is already known to be unique. // SECTION_CONTENTS : Contains the section's text and relocs to sections // that cannot be folded. SECTION_CONTENTS are NULL // implies that this function is being called for the // first time before the first iteration of icf. static void preprocess_for_unique_sections(const std::vector<Section_id>& id_section, std::vector<bool>* is_secn_or_group_unique, std::vector<std::string>* section_contents) { Unordered_map<uint32_t, unsigned int> uniq_map; std::pair<Unordered_map<uint32_t, unsigned int>::iterator, bool> uniq_map_insert; for (unsigned int i = 0; i < id_section.size(); i++) { if ((*is_secn_or_group_unique)[i]) continue; uint32_t cksum; Section_id secn = id_section[i]; section_size_type plen; if (section_contents == NULL) { // Lock the object so we can read from it. This is only called // single-threaded from queue_middle_tasks, so it is OK to lock. // Unfortunately we have no way to pass in a Task token. const Task* dummy_task = reinterpret_cast<const Task*>(-1); Task_lock_obj<Object> tl(dummy_task, secn.first); const unsigned char* contents; contents = secn.first->section_contents(secn.second, &plen, false); cksum = xcrc32(contents, plen, 0xffffffff); } else { const unsigned char* contents_array = reinterpret_cast <const unsigned char*>((*section_contents)[i].c_str()); cksum = xcrc32(contents_array, (*section_contents)[i].length(), 0xffffffff); } uniq_map_insert = uniq_map.insert(std::make_pair(cksum, i)); if (uniq_map_insert.second) { (*is_secn_or_group_unique)[i] = true; } else { (*is_secn_or_group_unique)[i] = false; (*is_secn_or_group_unique)[uniq_map_insert.first->second] = false; } } } // For SHF_MERGE sections that use REL relocations, the addend is stored in // the text section at the relocation offset. Read the addend value given // the pointer to the addend in the text section and the addend size. // Update the addend value if a valid addend is found. // Parameters: // RELOC_ADDEND_PTR : Pointer to the addend in the text section. // ADDEND_SIZE : The size of the addend. // RELOC_ADDEND_VALUE : Pointer to the addend that is updated. inline void get_rel_addend(const unsigned char* reloc_addend_ptr, const unsigned int addend_size, uint64_t* reloc_addend_value) { switch (addend_size) { case 0: break; case 1: *reloc_addend_value = read_from_pointer<8>(reloc_addend_ptr); break; case 2: *reloc_addend_value = read_from_pointer<16>(reloc_addend_ptr); break; case 4: *reloc_addend_value = read_from_pointer<32>(reloc_addend_ptr); break; case 8: *reloc_addend_value = read_from_pointer<64>(reloc_addend_ptr); break; default: gold_unreachable(); } } // 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. // FIXED_CACHE : String that stores the portion of the result that // does not change from iteration to iteration; // written if first_iteration is true, read if it's false. // SECN : Section for which contents are desired. // SELF_SECN : Relocations that target this section will be // considered "relocations to self" so that recursive // functions can be folded. Should normally be the // same as `secn` except when processing extra identity // regions. // 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. // START_OFFSET : Only consider the part of the section at and after // this offset. // END_OFFSET : Only consider the part of the section before this // offset. static std::string get_section_contents(bool first_iteration, std::string* fixed_cache, const Section_id& secn, const Section_id& self_secn, unsigned int* num_tracked_relocs, Symbol_table* symtab, const std::vector<unsigned int>& kept_section_id, section_offset_type start_offset = 0, section_offset_type end_offset = std::numeric_limits<section_offset_type>::max()) { 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; Icf::Reloc_info_list& reloc_info_list = symtab->icf()->reloc_info_list(); Icf::Reloc_info_list::iterator it_reloc_info_list = reloc_info_list.find(secn); buffer.clear(); icf_reloc_buffer.clear(); // Process relocs and put them into the buffer. if (it_reloc_info_list != reloc_info_list.end()) { Icf::Sections_reachable_info &v = (it_reloc_info_list->second).section_info; // Stores the information of the symbol pointed to by the reloc. const Icf::Symbol_info &s = (it_reloc_info_list->second).symbol_info; // Stores the addend and the symbol value. Icf::Addend_info &a = (it_reloc_info_list->second).addend_info; // Stores the offset of the reloc. const Icf::Offset_info &o = (it_reloc_info_list->second).offset_info; const Icf::Reloc_addend_size_info &reloc_addend_size_info = (it_reloc_info_list->second).reloc_addend_size_info; Icf::Sections_reachable_info::iterator it_v = v.begin(); Icf::Symbol_info::const_iterator it_s = s.begin(); Icf::Addend_info::iterator it_a = a.begin(); Icf::Offset_info::const_iterator it_o = o.begin(); Icf::Reloc_addend_size_info::const_iterator it_addend_size = reloc_addend_size_info.begin(); for (; it_v != v.end(); ++it_v, ++it_s, ++it_a, ++it_o, ++it_addend_size) { Symbol* gsym = *it_s; bool is_section_symbol = false; // Ignore relocations outside the region we were told to look at if (static_cast<section_offset_type>(*it_o) < start_offset || static_cast<section_offset_type>(*it_o) >= end_offset) continue; // A -1 value in the symbol vector indicates a local section symbol. if (gsym == reinterpret_cast<Symbol*>(-1)) { is_section_symbol = true; gsym = NULL; } if (first_iteration && it_v->first != NULL) { Symbol_location loc; loc.object = it_v->first; loc.shndx = it_v->second; loc.offset = convert_types<off_t, long long>(it_a->first + it_a->second); // Look through function descriptors parameters->target().function_location(&loc); if (loc.shndx != it_v->second) { it_v->second = loc.shndx; // Modify symvalue/addend to the code entry. it_a->first = loc.offset; it_a->second = 0; } } // ADDEND_STR stores the symbol value and addend and offset, // each at most 16 hex digits long. it_a points to a pair // where first is the symbol value and second is the // addend. char addend_str[50]; // It would be nice if we could use format macros in inttypes.h // here but there are not in ISO/IEC C++ 1998. snprintf(addend_str, sizeof(addend_str), "%llx %llx %llx", static_cast<long long>((*it_a).first), static_cast<long long>((*it_a).second), static_cast<unsigned long long>(*it_o - start_offset)); // If the symbol pointed to by the reloc is not in an ordinary // section or if the symbol type is not FROM_OBJECT, then the // object is NULL. if (it_v->first == NULL) { if (first_iteration) { // If the symbol name is available, use it. if (gsym != NULL) buffer.append(gsym->name()); // Append the addend. buffer.append(addend_str); buffer.append("@"); } continue; } Section_id reloc_secn(it_v->first, it_v->second); // If this reloc turns back and points to the same section, // like a recursive call, use a special symbol to mark this. if (reloc_secn.first == self_secn.first && reloc_secn.second == self_secn.second) { if (first_iteration) { buffer.append("R"); buffer.append(addend_str); buffer.append("@"); } continue; } Icf::Uniq_secn_id_map& section_id_map = symtab->icf()->section_to_int_map(); Icf::Uniq_secn_id_map::iterator section_id_map_it = section_id_map.find(reloc_secn); bool is_sym_preemptible = (gsym != NULL && !gsym->is_from_dynobj() && !gsym->is_undefined() && gsym->is_preemptible()); if (!is_sym_preemptible && section_id_map_it != section_id_map.end()) { // This is a reloc to a section that might be folded. if (num_tracked_relocs) (*num_tracked_relocs)++; char kept_section_str[10]; unsigned int secn_id = section_id_map_it->second; snprintf(kept_section_str, sizeof(kept_section_str), "%u", kept_section_id[secn_id]); if (first_iteration) { buffer.append("ICF_R"); buffer.append(addend_str); } icf_reloc_buffer.append(kept_section_str); // Append the addend. icf_reloc_buffer.append(addend_str); icf_reloc_buffer.append("@"); } else { // This is a reloc to a section that cannot be folded. // Process it only in the first iteration. if (!first_iteration) continue; uint64_t secn_flags = (it_v->first)->section_flags(it_v->second); // This reloc points to a merge section. Hash the // contents of this section. if ((secn_flags & elfcpp::SHF_MERGE) != 0 && parameters->target().can_icf_inline_merge_sections()) { uint64_t entsize = (it_v->first)->section_entsize(it_v->second); long long offset = it_a->first; // Handle SHT_RELA and SHT_REL addends. Only one of these // addends exists. When pointing to a merge section, the // addend only matters if it's relative to a section // symbol. In order to unambiguously identify the target // of the relocation, the compiler (and assembler) must use // a local non-section symbol unless Symbol+Addend does in // fact point directly to the target. (In other words, // a bias for a pc-relative reference or a non-zero based // access forces the use of a local symbol, and the addend // is used only to provide that bias.) uint64_t reloc_addend_value = 0; if (is_section_symbol) { // Get the SHT_RELA addend. For RELA relocations, // we have the addend from the relocation. reloc_addend_value = it_a->second; // Handle SHT_REL addends. // For REL relocations, we need to fetch the addend // from the section contents. const unsigned char* reloc_addend_ptr = contents + static_cast<unsigned long long>(*it_o); // Update the addend value with the SHT_REL addend if // available. get_rel_addend(reloc_addend_ptr, *it_addend_size, &reloc_addend_value); // Ignore the addend when it is a negative value. // See the comments in Merged_symbol_value::value // in object.h. if (reloc_addend_value < 0xffffff00) offset = offset + reloc_addend_value; } section_size_type secn_len; const unsigned char* str_contents = (it_v->first)->section_contents(it_v->second, &secn_len, false) + offset; gold_assert (offset < (long long) secn_len); 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 to copy. uint64_t bufsize = entsize; // If entsize is too big, copy all the remaining bytes. if ((offset + entsize) > secn_len) bufsize = secn_len - offset; buffer.append(reinterpret_cast<const char*>(str_contents), bufsize); } buffer.append("@"); } else if (gsym != NULL) { // If symbol name is available use that. buffer.append(gsym->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 = "); const unsigned char* slice_end = contents + std::min<section_offset_type>(plen, end_offset); if (contents + start_offset < slice_end) { buffer.append(reinterpret_cast<const char*>(contents + start_offset), slice_end - (contents + start_offset)); } } // Add any extra identity regions. std::pair<Icf::Extra_identity_list::const_iterator, Icf::Extra_identity_list::const_iterator> extra_range = symtab->icf()->extra_identity_list().equal_range(secn); for (Icf::Extra_identity_list::const_iterator it_ext = extra_range.first; it_ext != extra_range.second; ++it_ext) { std::string external_fixed; std::string external_all = get_section_contents(first_iteration, &external_fixed, it_ext->second.section, self_secn, num_tracked_relocs, symtab, kept_section_id, it_ext->second.offset, it_ext->second.offset + it_ext->second.length); buffer.append(external_fixed); icf_reloc_buffer.append(external_all, external_fixed.length(), std::string::npos); } if (first_iteration) { // Store the section contents that don't change to avoid recomputing // during the next call to this function. *fixed_cache = buffer; } else { gold_assert(buffer.empty()); // Reuse the contents computed in the previous iteration. buffer.append(*fixed_cache); } buffer.append(icf_reloc_buffer); return buffer; } // This function computes a checksum on each section to detect and form // groups of identical sections. The first iteration does this for all // sections. // Further iterations do this only for the kept sections from each group to // determine if larger groups of identical sections could be formed. The // first section in each group is the kept section for that group. // // CRC32 is the checksumming algorithm and can have collisions. That is, // two sections with different contents can have the same checksum. Hence, // a multimap is used to maintain more than one group of checksum // identical sections. A section is added to a group only after its // contents are explicitly compared with the kept section of the group. // // Parameters : // ITERATION_NUM : Invocation instance of this function. // NUM_TRACKED_RELOCS : Vector reference to store the number of relocs // to ICF sections. // KEPT_SECTION_ID : Vector which maps folded sections to kept sections. // ID_SECTION : Vector mapping a section to an unique integer. // IS_SECN_OR_GROUP_UNIQUE : To check if a section or a group of identical // sections is already known to be unique. // SECTION_CONTENTS : Store the section's text and relocs to non-ICF // sections. static bool match_sections(unsigned int iteration_num, Symbol_table* symtab, std::vector<unsigned int>* num_tracked_relocs, std::vector<unsigned int>* kept_section_id, const std::vector<Section_id>& id_section, const std::vector<uint64_t>& section_addraligns, 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]; // Lock the object so we can read from it. This is only called // single-threaded from queue_middle_tasks, so it is OK to lock. // Unfortunately we have no way to pass in a Task token. const Task* dummy_task = reinterpret_cast<const Task*>(-1); Task_lock_obj<Object> tl(dummy_task, secn.first); std::string this_secn_contents; uint32_t cksum; std::string* this_secn_cache = &((*section_contents)[i]); if (iteration_num == 1) { unsigned int num_relocs = 0; this_secn_contents = get_section_contents(true, this_secn_cache, secn, secn, &num_relocs, symtab, (*kept_section_id)); (*num_tracked_relocs)[i] = num_relocs; } else { if ((*kept_section_id)[i] != i) { // This section is already folded into something. continue; } this_secn_contents = get_section_contents(false, this_secn_cache, secn, secn, NULL, symtab, (*kept_section_id)); } 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; // Check section alignment here. // The section with the larger alignment requirement // should be kept. We assume alignment can only be // zero or positive integral powers of two. uint64_t align_i = section_addraligns[i]; uint64_t align_kept = section_addraligns[kept_section]; if (align_i <= align_kept) { (*kept_section_id)[i] = kept_section; } else { (*kept_section_id)[kept_section] = i; it->second = i; full_section_contents[kept_section].swap( full_section_contents[i]); } 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; } // If a section was folded into another section that was later folded // again then the former has to be updated. for (unsigned int i = 0; i < id_section.size(); i++) { // Find the end of the folding chain unsigned int kept = i; while ((*kept_section_id)[kept] != kept) { kept = (*kept_section_id)[kept]; } // Update every element of the chain unsigned int current = i; while ((*kept_section_id)[current] != kept) { unsigned int next = (*kept_section_id)[current]; (*kept_section_id)[current] = kept; current = next; } } return converged; } // During safe icf (--icf=safe), only fold functions that are ctors or dtors. // This function returns true if the section name is that of a ctor or a dtor. static bool is_function_ctor_or_dtor(const std::string& section_name) { const char* mangled_func_name = strrchr(section_name.c_str(), '.'); gold_assert(mangled_func_name != NULL); if ((is_prefix_of("._ZN", mangled_func_name) || is_prefix_of("._ZZ", mangled_func_name)) && (is_gnu_v3_mangled_ctor(mangled_func_name + 1) || is_gnu_v3_mangled_dtor(mangled_func_name + 1))) { return true; } return false; } // Iterate through the .eh_frame section that has index // `ehframe_shndx` in `object`, adding entries to extra_identity_list_ // that will cause the contents of each FDE and its CIE to be included // in the logical ICF identity of the function that the FDE refers to. bool Icf::add_ehframe_links(Relobj* object, unsigned int ehframe_shndx, Reloc_info& relocs) { section_size_type contents_len; const unsigned char* pcontents = object->section_contents(ehframe_shndx, &contents_len, false); const unsigned char* p = pcontents; const unsigned char* pend = pcontents + contents_len; Sections_reachable_info::iterator it_target = relocs.section_info.begin(); Sections_reachable_info::iterator it_target_end = relocs.section_info.end(); Offset_info::iterator it_offset = relocs.offset_info.begin(); Offset_info::iterator it_offset_end = relocs.offset_info.end(); // Maps section offset to the length of the CIE defined at that offset. typedef Unordered_map<section_offset_type, section_size_type> Cie_map; Cie_map cies; uint32_t (*read_swap_32)(const unsigned char*); if (object->is_big_endian()) read_swap_32 = &elfcpp::Swap<32, true>::readval; else read_swap_32 = &elfcpp::Swap<32, false>::readval; // TODO: The logic for parsing the CIE/FDE framing is copied from // Eh_frame::do_add_ehframe_input_section() and might want to be // factored into a shared helper function. while (p < pend) { if (pend - p < 4) return false; unsigned int len = read_swap_32(p); p += 4; if (len == 0) { // We should only find a zero-length entry at the end of the // section. if (p < pend) return false; break; } // We don't support a 64-bit .eh_frame. if (len == 0xffffffff) return false; if (static_cast<unsigned int>(pend - p) < len) return false; const unsigned char* const pentend = p + len; if (pend - p < 4) return false; unsigned int id = read_swap_32(p); p += 4; if (id == 0) { // CIE. cies.insert(std::make_pair(p - pcontents, len - 4)); } else { // FDE. Cie_map::const_iterator it; it = cies.find((p - pcontents) - (id - 4)); if (it == cies.end()) return false; // Figure out which section this FDE refers into. The word at `p` // is an address, and we expect to see a relocation there. If not, // this FDE isn't ICF-relevant. while (it_offset != it_offset_end && it_target != it_target_end && static_cast<ptrdiff_t>(*it_offset) < (p - pcontents)) { ++it_offset; ++it_target; } if (it_offset != it_offset_end && it_target != it_target_end && static_cast<ptrdiff_t>(*it_offset) == (p - pcontents)) { // Found a reloc. Add this FDE and its CIE as extra identity // info for the section it refers to. Extra_identity_info rec_fde = {Section_id(object, ehframe_shndx), p - pcontents, len - 4}; Extra_identity_info rec_cie = {Section_id(object, ehframe_shndx), it->first, it->second}; extra_identity_list_.insert(std::make_pair(*it_target, rec_fde)); extra_identity_list_.insert(std::make_pair(*it_target, rec_cie)); } } p = pentend; } return true; } // This is the main ICF function called in gold.cc. This does the // initialization and calls match_sections repeatedly (thrice 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<uint64_t> section_addraligns; std::vector<bool> is_secn_or_group_unique; std::vector<std::string> 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) { // Lock the object so we can read from it. This is only called // single-threaded from queue_middle_tasks, so it is OK to lock. // Unfortunately we have no way to pass in a Task token. const Task* dummy_task = reinterpret_cast<const Task*>(-1); Task_lock_obj<Object> tl(dummy_task, *p); std::vector<unsigned int> eh_frame_ind; for (unsigned int i = 0; i < (*p)->shnum(); ++i) { if ((*p)->section_size(i) == 0) continue; const std::string section_name = (*p)->section_name(i); if (!is_section_foldable_candidate(section_name)) { if (is_prefix_of(".eh_frame", section_name.c_str())) eh_frame_ind.push_back(i); 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 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(section_name) && (!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); section_addraligns.push_back((*p)->section_addralign(i)); is_secn_or_group_unique.push_back(false); section_contents.push_back(""); section_num++; } for (std::vector<unsigned int>::iterator it_eh_ind = eh_frame_ind.begin(); it_eh_ind != eh_frame_ind.end(); ++it_eh_ind) { // gc_process_relocs() recorded relocations for this // section even though we can't fold it. We need to // use those relocations to associate other foldable // sections with the FDEs and CIEs that are relevant // to them, so we can avoid merging sections that // don't have identical exception-handling behavior. Section_id sect(*p, *it_eh_ind); Reloc_info_list::iterator it_rel = this->reloc_info_list().find(sect); if (it_rel != this->reloc_info_list().end()) { if (!add_ehframe_links(*p, *it_eh_ind, it_rel->second)) { gold_warning(_("could not parse eh_frame section %s(%s); ICF " "might not preserve exception handling " "behavior"), (*p)->name().c_str(), (*p)->section_name(*it_eh_ind).c_str()); } } } } unsigned int num_iterations = 0; // Default number of iterations to run ICF is 3. unsigned int max_iterations = (parameters->options().icf_iterations() > 0) ? parameters->options().icf_iterations() : 3; 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_, section_addraligns, &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()) { Relobj* obj = static_cast<Relobj*>(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(Relobj* 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(Relobj* 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(Relobj* 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.