// icf.cc -- Identical Code Folding.
//
-// Copyright 2009 Free Software Foundation, Inc.
+// Copyright (C) 2009-2016 Free Software Foundation, Inc.
// Written by Sriraman Tallam <tmsriram@google.com>.
// This file is part of gold.
// 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
+// 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
// 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
+// 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 :
// 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.
//
-// How to run : --icf
+// Further, it is not correct to use safe folding to build non-pie
+// executables using PIC/PIE objects. PIC/PIE objects have different
+// relocation types for function pointers than non-PIC objects, and the
+// current implementation of safe folding does not handle those relocation
+// types. Hence, if used, functions whose pointers are taken could still be
+// folded causing unpredictable run-time behaviour if the pointers were used
+// in comparisons.
+//
+//
+//
+// How to run : --icf=[safe|all|none]
// Optional parameters : --icf-iterations <num> --print-icf-sections
//
// Performance : Less than 20 % link-time overhead on industry strength
#include "icf.h"
#include "symtab.h"
#include "libiberty.h"
+#include "demangle.h"
+#include "elfcpp.h"
+#include "int_encoding.h"
namespace gold
{
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,
}
}
+// 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
const std::vector<unsigned int>& kept_section_id,
std::vector<std::string>* section_contents)
{
+ // 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);
+
section_size_type plen;
const unsigned char* contents = NULL;
-
if (first_iteration)
- {
- contents = secn.first->section_contents(secn.second,
- &plen,
- false);
- }
+ 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.
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::Reloc_info_list& reloc_info_list =
+ symtab->icf()->reloc_info_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);
+ 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_seclist != seclist.end())
+ if (it_reloc_info_list != reloc_info_list.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::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)
+ for (; it_v != v.end(); ++it_v, ++it_s, ++it_a, ++it_o, ++it_addend_size)
{
- // ADDEND_STR stores the symbol value and addend, each
- // atmost 16 hex digits long. it_v points to a pair
+ 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[34];
- snprintf(addend_str, sizeof(addend_str), "%llx %llx",
- (*it_a).first, (*it_a).second);
+ 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 %llux",
+ static_cast<long long>((*it_a).first),
+ static_cast<long long>((*it_a).second),
+ static_cast<unsigned long long>(*it_o));
+
+ // 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 ((*it_s) != NULL)
+ buffer.append((*it_s)->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,
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())
+ bool is_sym_preemptible = (*it_s != NULL
+ && !(*it_s)->is_from_dynobj()
+ && !(*it_s)->is_undefined()
+ && (*it_s)->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)
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)
+ 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 + it_a->second;
+ long long offset = it_a->first;
+ // Handle SHT_RELA and SHT_REL addends, only one of these
+ // addends exists.
+ // Get the SHT_RELA addend. For RELA relocations, we have
+ // the addend from the relocation.
+ uint64_t 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.
}
else
{
- // Use the entsize to determine the length.
- buffer.append(reinterpret_cast<const
+ // 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),
- entsize);
+ bufsize);
}
+ buffer.append("@");
}
else if ((*it_s) != NULL)
{
// If symbol name is available use that.
- const char *sym_name = (*it_s)->name();
- buffer.append(sym_name);
+ buffer.append((*it_s)->name());
// Append the addend.
buffer.append(addend_str);
buffer.append("@");
// 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.
+// sections is already known to be unique.
// SECTION_CONTENTS : Store the section's text and relocs to non-ICF
// sections.
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;
+}
// This is the main ICF function called in gold.cc. This does the
// initialization and calls match_sections repeatedly (twice by default)
std::vector<unsigned int> num_tracked_relocs;
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.
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);
+
for (unsigned int i = 0;i < (*p)->shnum(); ++i)
{
- // Only looking to fold functions, so just look at .text sections.
- if (!is_prefix_of(".text.", (*p)->section_name(i).c_str()))
+ const std::string section_name = (*p)->section_name(i);
+ if (!is_section_foldable_candidate(section_name))
continue;
if (!(*p)->is_section_included(i))
continue;
if (parameters->options().gc_sections()
&& symtab->gc()->is_section_garbage(*p, i))
continue;
+ // 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);
{
const char* name = p->c_str();
Symbol* sym = symtab->lookup(name);
- if (sym != NULL
- && sym->source() == Symbol::FROM_OBJECT
- && !sym->object()->is_dynamic())
+ 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();
+ Relobj* obj = static_cast<Relobj*>(sym->object());
bool is_ordinary;
unsigned int shndx = sym->shndx(&is_ordinary);
if (is_ordinary)
// Unfolds the section denoted by OBJ and SHNDX if folded.
void
-Icf::unfold_section(Object* obj, unsigned int shndx)
+Icf::unfold_section(Relobj* obj, unsigned int shndx)
{
Section_id secn(obj, shndx);
Uniq_secn_id_map::iterator it = this->section_id_.find(secn);
// is different from this section.
bool
-Icf::is_section_folded(Object* obj, unsigned int shndx)
+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);
// This function returns the folded section for the given section.
Section_id
-Icf::get_folded_section(Object* dup_obj, unsigned int dup_shndx)
+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);
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