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binutils-gdb/gold/icf.cc

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// 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, &section_contents);
}
if (parameters->options().print_icf_sections())
{
if (converged)
gold_info(_("%s: ICF Converged after %u iteration(s)"),
program_name, num_iterations);
else
gold_info(_("%s: ICF stopped after %u iteration(s)"),
program_name, num_iterations);
}
// 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.
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