| // icf.cc -- Identical Code Folding. |
| // |
| // Copyright (C) 2009-2021 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. |
