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// icf.cc -- Identical Code Folding.
//
// Copyright 2009 Free Software Foundation, Inc.
// Written by Sriraman Tallam <tmsriram@google.com>.
// This file is part of gold.
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation; either version 3 of the License, or
// (at your option) any later version.
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software
// Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston,
// MA 02110-1301, USA.
// Identical Code Folding Algorithm
// ----------------------------------
// Detecting identical functions is done here and the basic algorithm
// is as follows. A checksum is computed on each .text section using
// its contents and relocations. If the symbol name corresponding to
// a relocation is known it is used to compute the checksum. If the
// symbol name is not known the stringified name of the object and the
// section number pointed to by the relocation is used. The checksums
// are stored as keys in a hash map and a section is identical to some
// other section if its checksum is already present in the hash map.
// Checksum collisions are handled by using a multimap and explicitly
// checking the contents when two sections have the same checksum.
//
// However, two functions A and B with identical text but with
// relocations pointing to different .text sections can be identical if
// the corresponding .text sections to which their relocations point to
// turn out to be identical. Hence, this checksumming process must be
// done repeatedly until convergence is obtained. Here is an example for
// the following case :
//
// int funcA () int funcB ()
// { {
// return foo(); return goo();
// } }
//
// The functions funcA and funcB are identical if functions foo() and
// goo() are identical.
//
// Hence, as described above, we repeatedly do the checksumming,
// assigning identical functions to the same group, until convergence is
// obtained. Now, we have two different ways to do this depending on how
// we initialize.
//
// Algorithm I :
// -----------
// We can start with marking all functions as different and repeatedly do
// the checksumming. This has the advantage that we do not need to wait
// for convergence. We can stop at any point and correctness will be
// guaranteed although not all cases would have been found. However, this
// has a problem that some cases can never be found even if it is run until
// convergence. Here is an example with mutually recursive functions :
//
// int funcA (int a) int funcB (int a)
// { {
// if (a == 1) if (a == 1)
// return 1; return 1;
// return 1 + funcB(a - 1); return 1 + funcA(a - 1);
// } }
//
// In this example funcA and funcB are identical and one of them could be
// folded into the other. However, if we start with assuming that funcA
// and funcB are not identical, the algorithm, even after it is run to
// convergence, cannot detect that they are identical. It should be noted
// that even if the functions were self-recursive, Algorithm I cannot catch
// that they are identical, at least as is.
//
// Algorithm II :
// ------------
// Here we start with marking all functions as identical and then repeat
// the checksumming until convergence. This can detect the above case
// mentioned above. It can detect all cases that Algorithm I can and more.
// However, the caveat is that it has to be run to convergence. It cannot
// be stopped arbitrarily like Algorithm I as correctness cannot be
// guaranteed. Algorithm II is not implemented.
//
// Algorithm I is used because experiments show that about three
// iterations are more than enough to achieve convergence. Algorithm I can
// handle recursive calls if it is changed to use a special common symbol
// for recursive relocs. This seems to be the most common case that
// Algorithm I could not catch as is. Mutually recursive calls are not
// frequent and Algorithm I wins because of its ability to be stopped
// arbitrarily.
//
// Caveat with using function pointers :
// ------------------------------------
//
// Programs using function pointer comparisons/checks should use function
// folding with caution as the result of such comparisons could be different
// when folding takes place. This could lead to unexpected run-time
// behaviour.
//
//
// How to run : --icf
// Optional parameters : --icf-iterations <num> --print-icf-sections
//
// Performance : Less than 20 % link-time overhead on industry strength
// applications. Up to 6 % text size reductions.
#include "gold.h"
#include "object.h"
#include "gc.h"
#include "icf.h"
#include "symtab.h"
#include "libiberty.h"
#include "demangle.h"
namespace gold
{
// This function determines if a section or a group of identical
// sections has unique contents. Such unique sections or groups can be
// declared final and need not be processed any further.
// Parameters :
// ID_SECTION : Vector mapping a section index to a Section_id pair.
// IS_SECN_OR_GROUP_UNIQUE : To check if a section or a group of identical
// sections is already known to be unique.
// SECTION_CONTENTS : Contains the section's text and relocs to sections
// that cannot be folded. SECTION_CONTENTS are NULL
// implies that this function is being called for the
// first time before the first iteration of icf.
static void
preprocess_for_unique_sections(const std::vector<Section_id>& id_section,
std::vector<bool>* is_secn_or_group_unique,
std::vector<std::string>* section_contents)
{
Unordered_map<uint32_t, unsigned int> uniq_map;
std::pair<Unordered_map<uint32_t, unsigned int>::iterator, bool>
uniq_map_insert;
for (unsigned int i = 0; i < id_section.size(); i++)
{
if ((*is_secn_or_group_unique)[i])
continue;
uint32_t cksum;
Section_id secn = id_section[i];
section_size_type plen;
if (section_contents == NULL)
{
const unsigned char* contents;
contents = secn.first->section_contents(secn.second,
&plen,
false);
cksum = xcrc32(contents, plen, 0xffffffff);
}
else
{
const unsigned char* contents_array = reinterpret_cast
<const unsigned char*>((*section_contents)[i].c_str());
cksum = xcrc32(contents_array, (*section_contents)[i].length(),
0xffffffff);
}
uniq_map_insert = uniq_map.insert(std::make_pair(cksum, i));
if (uniq_map_insert.second)
{
(*is_secn_or_group_unique)[i] = true;
}
else
{
(*is_secn_or_group_unique)[i] = false;
(*is_secn_or_group_unique)[uniq_map_insert.first->second] = false;
}
}
}
// This returns the buffer containing the section's contents, both
// text and relocs. Relocs are differentiated as those pointing to
// sections that could be folded and those that cannot. Only relocs
// pointing to sections that could be folded are recomputed on
// subsequent invocations of this function.
// Parameters :
// FIRST_ITERATION : true if it is the first invocation.
// SECN : Section for which contents are desired.
// SECTION_NUM : Unique section number of this section.
// NUM_TRACKED_RELOCS : Vector reference to store the number of relocs
// to ICF sections.
// KEPT_SECTION_ID : Vector which maps folded sections to kept sections.
// SECTION_CONTENTS : Store the section's text and relocs to non-ICF
// sections.
static std::string
get_section_contents(bool first_iteration,
const Section_id& secn,
unsigned int section_num,
unsigned int* num_tracked_relocs,
Symbol_table* symtab,
const std::vector<unsigned int>& kept_section_id,
std::vector<std::string>* section_contents)
{
section_size_type plen;
const unsigned char* contents = NULL;
if (first_iteration)
{
contents = secn.first->section_contents(secn.second,
&plen,
false);
}
// The buffer to hold all the contents including relocs. A checksum
// is then computed on this buffer.
std::string buffer;
std::string icf_reloc_buffer;
if (num_tracked_relocs)
*num_tracked_relocs = 0;
Icf::Section_list& seclist = symtab->icf()->section_reloc_list();
Icf::Symbol_list& symlist = symtab->icf()->symbol_reloc_list();
Icf::Addend_list& addendlist = symtab->icf()->addend_reloc_list();
Icf::Section_list::iterator it_seclist = seclist.find(secn);
Icf::Symbol_list::iterator it_symlist = symlist.find(secn);
Icf::Addend_list::iterator it_addendlist = addendlist.find(secn);
buffer.clear();
icf_reloc_buffer.clear();
// Process relocs and put them into the buffer.
if (it_seclist != seclist.end())
{
gold_assert(it_symlist != symlist.end());
gold_assert(it_addendlist != addendlist.end());
Icf::Sections_reachable_list v = it_seclist->second;
Icf::Symbol_info s = it_symlist->second;
Icf::Addend_info a = it_addendlist->second;
Icf::Sections_reachable_list::iterator it_v = v.begin();
Icf::Symbol_info::iterator it_s = s.begin();
Icf::Addend_info::iterator it_a = a.begin();
for (; it_v != v.end(); ++it_v, ++it_s, ++it_a)
{
// ADDEND_STR stores the symbol value and addend, each
// atmost 16 hex digits long. it_v points to a pair
// where first is the symbol value and second is the
// addend.
char addend_str[34];
snprintf(addend_str, sizeof(addend_str), "%llx %llx",
(*it_a).first, (*it_a).second);
Section_id reloc_secn(it_v->first, it_v->second);
// If this reloc turns back and points to the same section,
// like a recursive call, use a special symbol to mark this.
if (reloc_secn.first == secn.first
&& reloc_secn.second == secn.second)
{
if (first_iteration)
{
buffer.append("R");
buffer.append(addend_str);
buffer.append("@");
}
continue;
}
Icf::Uniq_secn_id_map& section_id_map =
symtab->icf()->section_to_int_map();
Icf::Uniq_secn_id_map::iterator section_id_map_it =
section_id_map.find(reloc_secn);
if (section_id_map_it != section_id_map.end())
{
// This is a reloc to a section that might be folded.
if (num_tracked_relocs)
(*num_tracked_relocs)++;
char kept_section_str[10];
unsigned int secn_id = section_id_map_it->second;
snprintf(kept_section_str, sizeof(kept_section_str), "%u",
kept_section_id[secn_id]);
if (first_iteration)
{
buffer.append("ICF_R");
buffer.append(addend_str);
}
icf_reloc_buffer.append(kept_section_str);
// Append the addend.
icf_reloc_buffer.append(addend_str);
icf_reloc_buffer.append("@");
}
else
{
// This is a reloc to a section that cannot be folded.
// Process it only in the first iteration.
if (!first_iteration)
continue;
uint64_t secn_flags = (it_v->first)->section_flags(it_v->second);
// This reloc points to a merge section. Hash the
// contents of this section.
if ((secn_flags & elfcpp::SHF_MERGE) != 0)
{
uint64_t entsize =
(it_v->first)->section_entsize(it_v->second);
long long offset = it_a->first + it_a->second;
section_size_type secn_len;
const unsigned char* str_contents =
(it_v->first)->section_contents(it_v->second,
&secn_len,
false) + offset;
if ((secn_flags & elfcpp::SHF_STRINGS) != 0)
{
// String merge section.
const char* str_char =
reinterpret_cast<const char*>(str_contents);
switch(entsize)
{
case 1:
{
buffer.append(str_char);
break;
}
case 2:
{
const uint16_t* ptr_16 =
reinterpret_cast<const uint16_t*>(str_char);
unsigned int strlen_16 = 0;
// Find the NULL character.
while(*(ptr_16 + strlen_16) != 0)
strlen_16++;
buffer.append(str_char, strlen_16 * 2);
}
break;
case 4:
{
const uint32_t* ptr_32 =
reinterpret_cast<const uint32_t*>(str_char);
unsigned int strlen_32 = 0;
// Find the NULL character.
while(*(ptr_32 + strlen_32) != 0)
strlen_32++;
buffer.append(str_char, strlen_32 * 4);
}
break;
default:
gold_unreachable();
}
}
else
{
// Use the entsize to determine the length.
buffer.append(reinterpret_cast<const
char*>(str_contents),
entsize);
}
}
else if ((*it_s) != NULL)
{
// If symbol name is available use that.
const char *sym_name = (*it_s)->name();
buffer.append(sym_name);
// Append the addend.
buffer.append(addend_str);
buffer.append("@");
}
else
{
// Symbol name is not available, like for a local symbol,
// use object and section id.
buffer.append(it_v->first->name());
char secn_id[10];
snprintf(secn_id, sizeof(secn_id), "%u",it_v->second);
buffer.append(secn_id);
// Append the addend.
buffer.append(addend_str);
buffer.append("@");
}
}
}
}
if (first_iteration)
{
buffer.append("Contents = ");
buffer.append(reinterpret_cast<const char*>(contents), plen);
// Store the section contents that dont change to avoid recomputing
// during the next call to this function.
(*section_contents)[section_num] = buffer;
}
else
{
gold_assert(buffer.empty());
// Reuse the contents computed in the previous iteration.
buffer.append((*section_contents)[section_num]);
}
buffer.append(icf_reloc_buffer);
return buffer;
}
// This function computes a checksum on each section to detect and form
// groups of identical sections. The first iteration does this for all
// sections.
// Further iterations do this only for the kept sections from each group to
// determine if larger groups of identical sections could be formed. The
// first section in each group is the kept section for that group.
//
// CRC32 is the checksumming algorithm and can have collisions. That is,
// two sections with different contents can have the same checksum. Hence,
// a multimap is used to maintain more than one group of checksum
// identical sections. A section is added to a group only after its
// contents are explicitly compared with the kept section of the group.
//
// Parameters :
// ITERATION_NUM : Invocation instance of this function.
// NUM_TRACKED_RELOCS : Vector reference to store the number of relocs
// to ICF sections.
// KEPT_SECTION_ID : Vector which maps folded sections to kept sections.
// ID_SECTION : Vector mapping a section to an unique integer.
// IS_SECN_OR_GROUP_UNIQUE : To check if a section or a group of identical
// sectionsis already known to be unique.
// SECTION_CONTENTS : Store the section's text and relocs to non-ICF
// sections.
static bool
match_sections(unsigned int iteration_num,
Symbol_table* symtab,
std::vector<unsigned int>* num_tracked_relocs,
std::vector<unsigned int>* kept_section_id,
const std::vector<Section_id>& id_section,
std::vector<bool>* is_secn_or_group_unique,
std::vector<std::string>* section_contents)
{
Unordered_multimap<uint32_t, unsigned int> section_cksum;
std::pair<Unordered_multimap<uint32_t, unsigned int>::iterator,
Unordered_multimap<uint32_t, unsigned int>::iterator> key_range;
bool converged = true;
if (iteration_num == 1)
preprocess_for_unique_sections(id_section,
is_secn_or_group_unique,
NULL);
else
preprocess_for_unique_sections(id_section,
is_secn_or_group_unique,
section_contents);
std::vector<std::string> full_section_contents;
for (unsigned int i = 0; i < id_section.size(); i++)
{
full_section_contents.push_back("");
if ((*is_secn_or_group_unique)[i])
continue;
Section_id secn = id_section[i];
std::string this_secn_contents;
uint32_t cksum;
if (iteration_num == 1)
{
unsigned int num_relocs = 0;
this_secn_contents = get_section_contents(true, secn, i, &num_relocs,
symtab, (*kept_section_id),
section_contents);
(*num_tracked_relocs)[i] = num_relocs;
}
else
{
if ((*kept_section_id)[i] != i)
{
// This section is already folded into something. See
// if it should point to a different kept section.
unsigned int kept_section = (*kept_section_id)[i];
if (kept_section != (*kept_section_id)[kept_section])
{
(*kept_section_id)[i] = (*kept_section_id)[kept_section];
}
continue;
}
this_secn_contents = get_section_contents(false, secn, i, NULL,
symtab, (*kept_section_id),
section_contents);
}
const unsigned char* this_secn_contents_array =
reinterpret_cast<const unsigned char*>(this_secn_contents.c_str());
cksum = xcrc32(this_secn_contents_array, this_secn_contents.length(),
0xffffffff);
size_t count = section_cksum.count(cksum);
if (count == 0)
{
// Start a group with this cksum.
section_cksum.insert(std::make_pair(cksum, i));
full_section_contents[i] = this_secn_contents;
}
else
{
key_range = section_cksum.equal_range(cksum);
Unordered_multimap<uint32_t, unsigned int>::iterator it;
// Search all the groups with this cksum for a match.
for (it = key_range.first; it != key_range.second; ++it)
{
unsigned int kept_section = it->second;
if (full_section_contents[kept_section].length()
!= this_secn_contents.length())
continue;
if (memcmp(full_section_contents[kept_section].c_str(),
this_secn_contents.c_str(),
this_secn_contents.length()) != 0)
continue;
(*kept_section_id)[i] = kept_section;
converged = false;
break;
}
if (it == key_range.second)
{
// Create a new group for this cksum.
section_cksum.insert(std::make_pair(cksum, i));
full_section_contents[i] = this_secn_contents;
}
}
// If there are no relocs to foldable sections do not process
// this section any further.
if (iteration_num == 1 && (*num_tracked_relocs)[i] == 0)
(*is_secn_or_group_unique)[i] = true;
}
return converged;
}
// During safe icf (--icf=safe), only fold functions that are ctors or dtors.
// This function returns true if the mangled function name is a ctor or a
// dtor.
static bool
is_function_ctor_or_dtor(const char* mangled_func_name)
{
if ((is_prefix_of("_ZN", mangled_func_name)
|| is_prefix_of("_ZZ", mangled_func_name))
&& (is_gnu_v3_mangled_ctor(mangled_func_name)
|| is_gnu_v3_mangled_dtor(mangled_func_name)))
{
return true;
}
return false;
}
// This is the main ICF function called in gold.cc. This does the
// initialization and calls match_sections repeatedly (twice by default)
// which computes the crc checksums and detects identical functions.
void
Icf::find_identical_sections(const Input_objects* input_objects,
Symbol_table* symtab)
{
unsigned int section_num = 0;
std::vector<unsigned int> num_tracked_relocs;
std::vector<bool> is_secn_or_group_unique;
std::vector<std::string> section_contents;
// Decide which sections are possible candidates first.
for (Input_objects::Relobj_iterator p = input_objects->relobj_begin();
p != input_objects->relobj_end();
++p)
{
for (unsigned int i = 0;i < (*p)->shnum(); ++i)
{
const char* section_name = (*p)->section_name(i).c_str();
// Only looking to fold functions, so just look at .text sections.
if (!is_prefix_of(".text.", section_name))
continue;
if (!(*p)->is_section_included(i))
continue;
if (parameters->options().gc_sections()
&& symtab->gc()->is_section_garbage(*p, i))
continue;
// With --icf=safe, check if mangled name is a ctor or a dtor.
if (parameters->options().icf_safe_folding()
&& !is_function_ctor_or_dtor(section_name + 6))
continue;
this->id_section_.push_back(Section_id(*p, i));
this->section_id_[Section_id(*p, i)] = section_num;
this->kept_section_id_.push_back(section_num);
num_tracked_relocs.push_back(0);
is_secn_or_group_unique.push_back(false);
section_contents.push_back("");
section_num++;
}
}
unsigned int num_iterations = 0;
// Default number of iterations to run ICF is 2.
unsigned int max_iterations = (parameters->options().icf_iterations() > 0)
? parameters->options().icf_iterations()
: 2;
bool converged = false;
while (!converged && (num_iterations < max_iterations))
{
num_iterations++;
converged = match_sections(num_iterations, symtab,
&num_tracked_relocs, &this->kept_section_id_,
this->id_section_, &is_secn_or_group_unique,
&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())
{
Object* obj = sym->object();
bool is_ordinary;
unsigned int shndx = sym->shndx(&is_ordinary);
if (is_ordinary)
{
this->unfold_section(obj, shndx);
}
}
}
this->icf_ready();
}
// Unfolds the section denoted by OBJ and SHNDX if folded.
void
Icf::unfold_section(Object* obj, unsigned int shndx)
{
Section_id secn(obj, shndx);
Uniq_secn_id_map::iterator it = this->section_id_.find(secn);
if (it == this->section_id_.end())
return;
unsigned int section_num = it->second;
unsigned int kept_section_id = this->kept_section_id_[section_num];
if (kept_section_id != section_num)
this->kept_section_id_[section_num] = section_num;
}
// This function determines if the section corresponding to the
// given object and index is folded based on if the kept section
// is different from this section.
bool
Icf::is_section_folded(Object* obj, unsigned int shndx)
{
Section_id secn(obj, shndx);
Uniq_secn_id_map::iterator it = this->section_id_.find(secn);
if (it == this->section_id_.end())
return false;
unsigned int section_num = it->second;
unsigned int kept_section_id = this->kept_section_id_[section_num];
return kept_section_id != section_num;
}
// This function returns the folded section for the given section.
Section_id
Icf::get_folded_section(Object* dup_obj, unsigned int dup_shndx)
{
Section_id dup_secn(dup_obj, dup_shndx);
Uniq_secn_id_map::iterator it = this->section_id_.find(dup_secn);
gold_assert(it != this->section_id_.end());
unsigned int section_num = it->second;
unsigned int kept_section_id = this->kept_section_id_[section_num];
Section_id folded_section = this->id_section_[kept_section_id];
return folded_section;
}
} // End of namespace gold.
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