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1 // icf.cc -- Identical Code Folding.
2 //
3 // Copyright (C) 2009-2019 Free Software Foundation, Inc.
4 // Written by Sriraman Tallam <tmsriram@google.com>.
5
6 // This file is part of gold.
7
8 // This program is free software; you can redistribute it and/or modify
9 // it under the terms of the GNU General Public License as published by
10 // the Free Software Foundation; either version 3 of the License, or
11 // (at your option) any later version.
12
13 // This program is distributed in the hope that it will be useful,
14 // but WITHOUT ANY WARRANTY; without even the implied warranty of
15 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
16 // GNU General Public License for more details.
17
18 // You should have received a copy of the GNU General Public License
19 // along with this program; if not, write to the Free Software
20 // Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston,
21 // MA 02110-1301, USA.
22
23 // Identical Code Folding Algorithm
24 // ----------------------------------
25 // Detecting identical functions is done here and the basic algorithm
26 // is as follows. A checksum is computed on each foldable section using
27 // its contents and relocations. If the symbol name corresponding to
28 // a relocation is known it is used to compute the checksum. If the
29 // symbol name is not known the stringified name of the object and the
30 // section number pointed to by the relocation is used. The checksums
31 // are stored as keys in a hash map and a section is identical to some
32 // other section if its checksum is already present in the hash map.
33 // Checksum collisions are handled by using a multimap and explicitly
34 // checking the contents when two sections have the same checksum.
35 //
36 // However, two functions A and B with identical text but with
37 // relocations pointing to different foldable sections can be identical if
38 // the corresponding foldable sections to which their relocations point to
39 // turn out to be identical. Hence, this checksumming process must be
40 // done repeatedly until convergence is obtained. Here is an example for
41 // the following case :
42 //
43 // int funcA () int funcB ()
44 // { {
45 // return foo(); return goo();
46 // } }
47 //
48 // The functions funcA and funcB are identical if functions foo() and
49 // goo() are identical.
50 //
51 // Hence, as described above, we repeatedly do the checksumming,
52 // assigning identical functions to the same group, until convergence is
53 // obtained. Now, we have two different ways to do this depending on how
54 // we initialize.
55 //
56 // Algorithm I :
57 // -----------
58 // We can start with marking all functions as different and repeatedly do
59 // the checksumming. This has the advantage that we do not need to wait
60 // for convergence. We can stop at any point and correctness will be
61 // guaranteed although not all cases would have been found. However, this
62 // has a problem that some cases can never be found even if it is run until
63 // convergence. Here is an example with mutually recursive functions :
64 //
65 // int funcA (int a) int funcB (int a)
66 // { {
67 // if (a == 1) if (a == 1)
68 // return 1; return 1;
69 // return 1 + funcB(a - 1); return 1 + funcA(a - 1);
70 // } }
71 //
72 // In this example funcA and funcB are identical and one of them could be
73 // folded into the other. However, if we start with assuming that funcA
74 // and funcB are not identical, the algorithm, even after it is run to
75 // convergence, cannot detect that they are identical. It should be noted
76 // that even if the functions were self-recursive, Algorithm I cannot catch
77 // that they are identical, at least as is.
78 //
79 // Algorithm II :
80 // ------------
81 // Here we start with marking all functions as identical and then repeat
82 // the checksumming until convergence. This can detect the above case
83 // mentioned above. It can detect all cases that Algorithm I can and more.
84 // However, the caveat is that it has to be run to convergence. It cannot
85 // be stopped arbitrarily like Algorithm I as correctness cannot be
86 // guaranteed. Algorithm II is not implemented.
87 //
88 // Algorithm I is used because experiments show that about three
89 // iterations are more than enough to achieve convergence. Algorithm I can
90 // handle recursive calls if it is changed to use a special common symbol
91 // for recursive relocs. This seems to be the most common case that
92 // Algorithm I could not catch as is. Mutually recursive calls are not
93 // frequent and Algorithm I wins because of its ability to be stopped
94 // arbitrarily.
95 //
96 // Caveat with using function pointers :
97 // ------------------------------------
98 //
99 // Programs using function pointer comparisons/checks should use function
100 // folding with caution as the result of such comparisons could be different
101 // when folding takes place. This could lead to unexpected run-time
102 // behaviour.
103 //
104 // Safe Folding :
105 // ------------
106 //
107 // ICF in safe mode folds only ctors and dtors if their function pointers can
108 // never be taken. Also, for X86-64, safe folding uses the relocation
109 // type to determine if a function's pointer is taken or not and only folds
110 // functions whose pointers are definitely not taken.
111 //
112 // Caveat with safe folding :
113 // ------------------------
114 //
115 // This applies only to x86_64.
116 //
117 // Position independent executables are created from PIC objects (compiled
118 // with -fPIC) and/or PIE objects (compiled with -fPIE). For PIE objects, the
119 // relocation types for function pointer taken and a call are the same.
120 // Now, it is not always possible to tell if an object used in the link of
121 // a pie executable is a PIC object or a PIE object. Hence, for pie
122 // executables, using relocation types to disambiguate function pointers is
123 // currently disabled.
124 //
125 // Further, it is not correct to use safe folding to build non-pie
126 // executables using PIC/PIE objects. PIC/PIE objects have different
127 // relocation types for function pointers than non-PIC objects, and the
128 // current implementation of safe folding does not handle those relocation
129 // types. Hence, if used, functions whose pointers are taken could still be
130 // folded causing unpredictable run-time behaviour if the pointers were used
131 // in comparisons.
132 //
133 //
134 //
135 // How to run : --icf=[safe|all|none]
136 // Optional parameters : --icf-iterations <num> --print-icf-sections
137 //
138 // Performance : Less than 20 % link-time overhead on industry strength
139 // applications. Up to 6 % text size reductions.
140
141 #include "gold.h"
142 #include "object.h"
143 #include "gc.h"
144 #include "icf.h"
145 #include "symtab.h"
146 #include "libiberty.h"
147 #include "demangle.h"
148 #include "elfcpp.h"
149 #include "int_encoding.h"
150
151 namespace gold
152 {
153
154 // This function determines if a section or a group of identical
155 // sections has unique contents. Such unique sections or groups can be
156 // declared final and need not be processed any further.
157 // Parameters :
158 // ID_SECTION : Vector mapping a section index to a Section_id pair.
159 // IS_SECN_OR_GROUP_UNIQUE : To check if a section or a group of identical
160 // sections is already known to be unique.
161 // SECTION_CONTENTS : Contains the section's text and relocs to sections
162 // that cannot be folded. SECTION_CONTENTS are NULL
163 // implies that this function is being called for the
164 // first time before the first iteration of icf.
165
166 static void
167 preprocess_for_unique_sections(const std::vector<Section_id>& id_section,
168 std::vector<bool>* is_secn_or_group_unique,
169 std::vector<std::string>* section_contents)
170 {
171 Unordered_map<uint32_t, unsigned int> uniq_map;
172 std::pair<Unordered_map<uint32_t, unsigned int>::iterator, bool>
173 uniq_map_insert;
174
175 for (unsigned int i = 0; i < id_section.size(); i++)
176 {
177 if ((*is_secn_or_group_unique)[i])
178 continue;
179
180 uint32_t cksum;
181 Section_id secn = id_section[i];
182 section_size_type plen;
183 if (section_contents == NULL)
184 {
185 // Lock the object so we can read from it. This is only called
186 // single-threaded from queue_middle_tasks, so it is OK to lock.
187 // Unfortunately we have no way to pass in a Task token.
188 const Task* dummy_task = reinterpret_cast<const Task*>(-1);
189 Task_lock_obj<Object> tl(dummy_task, secn.first);
190 const unsigned char* contents;
191 contents = secn.first->section_contents(secn.second,
192 &plen,
193 false);
194 cksum = xcrc32(contents, plen, 0xffffffff);
195 }
196 else
197 {
198 const unsigned char* contents_array = reinterpret_cast
199 <const unsigned char*>((*section_contents)[i].c_str());
200 cksum = xcrc32(contents_array, (*section_contents)[i].length(),
201 0xffffffff);
202 }
203 uniq_map_insert = uniq_map.insert(std::make_pair(cksum, i));
204 if (uniq_map_insert.second)
205 {
206 (*is_secn_or_group_unique)[i] = true;
207 }
208 else
209 {
210 (*is_secn_or_group_unique)[i] = false;
211 (*is_secn_or_group_unique)[uniq_map_insert.first->second] = false;
212 }
213 }
214 }
215
216 // For SHF_MERGE sections that use REL relocations, the addend is stored in
217 // the text section at the relocation offset. Read the addend value given
218 // the pointer to the addend in the text section and the addend size.
219 // Update the addend value if a valid addend is found.
220 // Parameters:
221 // RELOC_ADDEND_PTR : Pointer to the addend in the text section.
222 // ADDEND_SIZE : The size of the addend.
223 // RELOC_ADDEND_VALUE : Pointer to the addend that is updated.
224
225 inline void
226 get_rel_addend(const unsigned char* reloc_addend_ptr,
227 const unsigned int addend_size,
228 uint64_t* reloc_addend_value)
229 {
230 switch (addend_size)
231 {
232 case 0:
233 break;
234 case 1:
235 *reloc_addend_value =
236 read_from_pointer<8>(reloc_addend_ptr);
237 break;
238 case 2:
239 *reloc_addend_value =
240 read_from_pointer<16>(reloc_addend_ptr);
241 break;
242 case 4:
243 *reloc_addend_value =
244 read_from_pointer<32>(reloc_addend_ptr);
245 break;
246 case 8:
247 *reloc_addend_value =
248 read_from_pointer<64>(reloc_addend_ptr);
249 break;
250 default:
251 gold_unreachable();
252 }
253 }
254
255 // This returns the buffer containing the section's contents, both
256 // text and relocs. Relocs are differentiated as those pointing to
257 // sections that could be folded and those that cannot. Only relocs
258 // pointing to sections that could be folded are recomputed on
259 // subsequent invocations of this function.
260 // Parameters :
261 // FIRST_ITERATION : true if it is the first invocation.
262 // SECN : Section for which contents are desired.
263 // SECTION_NUM : Unique section number of this section.
264 // NUM_TRACKED_RELOCS : Vector reference to store the number of relocs
265 // to ICF sections.
266 // KEPT_SECTION_ID : Vector which maps folded sections to kept sections.
267 // SECTION_CONTENTS : Store the section's text and relocs to non-ICF
268 // sections.
269
270 static std::string
271 get_section_contents(bool first_iteration,
272 const Section_id& secn,
273 unsigned int section_num,
274 unsigned int* num_tracked_relocs,
275 Symbol_table* symtab,
276 const std::vector<unsigned int>& kept_section_id,
277 std::vector<std::string>* section_contents)
278 {
279 // Lock the object so we can read from it. This is only called
280 // single-threaded from queue_middle_tasks, so it is OK to lock.
281 // Unfortunately we have no way to pass in a Task token.
282 const Task* dummy_task = reinterpret_cast<const Task*>(-1);
283 Task_lock_obj<Object> tl(dummy_task, secn.first);
284
285 section_size_type plen;
286 const unsigned char* contents = NULL;
287 if (first_iteration)
288 contents = secn.first->section_contents(secn.second, &plen, false);
289
290 // The buffer to hold all the contents including relocs. A checksum
291 // is then computed on this buffer.
292 std::string buffer;
293 std::string icf_reloc_buffer;
294
295 if (num_tracked_relocs)
296 *num_tracked_relocs = 0;
297
298 Icf::Reloc_info_list& reloc_info_list =
299 symtab->icf()->reloc_info_list();
300
301 Icf::Reloc_info_list::iterator it_reloc_info_list =
302 reloc_info_list.find(secn);
303
304 buffer.clear();
305 icf_reloc_buffer.clear();
306
307 // Process relocs and put them into the buffer.
308
309 if (it_reloc_info_list != reloc_info_list.end())
310 {
311 Icf::Sections_reachable_info &v =
312 (it_reloc_info_list->second).section_info;
313 // Stores the information of the symbol pointed to by the reloc.
314 const Icf::Symbol_info &s = (it_reloc_info_list->second).symbol_info;
315 // Stores the addend and the symbol value.
316 Icf::Addend_info &a = (it_reloc_info_list->second).addend_info;
317 // Stores the offset of the reloc.
318 const Icf::Offset_info &o = (it_reloc_info_list->second).offset_info;
319 const Icf::Reloc_addend_size_info &reloc_addend_size_info =
320 (it_reloc_info_list->second).reloc_addend_size_info;
321 Icf::Sections_reachable_info::iterator it_v = v.begin();
322 Icf::Symbol_info::const_iterator it_s = s.begin();
323 Icf::Addend_info::iterator it_a = a.begin();
324 Icf::Offset_info::const_iterator it_o = o.begin();
325 Icf::Reloc_addend_size_info::const_iterator it_addend_size =
326 reloc_addend_size_info.begin();
327
328 for (; it_v != v.end(); ++it_v, ++it_s, ++it_a, ++it_o, ++it_addend_size)
329 {
330 Symbol* gsym = *it_s;
331 bool is_section_symbol = false;
332
333 // A -1 value in the symbol vector indicates a local section symbol.
334 if (gsym == reinterpret_cast<Symbol*>(-1))
335 {
336 is_section_symbol = true;
337 gsym = NULL;
338 }
339
340 if (first_iteration
341 && it_v->first != NULL)
342 {
343 Symbol_location loc;
344 loc.object = it_v->first;
345 loc.shndx = it_v->second;
346 loc.offset = convert_types<off_t, long long>(it_a->first
347 + it_a->second);
348 // Look through function descriptors
349 parameters->target().function_location(&loc);
350 if (loc.shndx != it_v->second)
351 {
352 it_v->second = loc.shndx;
353 // Modify symvalue/addend to the code entry.
354 it_a->first = loc.offset;
355 it_a->second = 0;
356 }
357 }
358
359 // ADDEND_STR stores the symbol value and addend and offset,
360 // each at most 16 hex digits long. it_a points to a pair
361 // where first is the symbol value and second is the
362 // addend.
363 char addend_str[50];
364
365 // It would be nice if we could use format macros in inttypes.h
366 // here but there are not in ISO/IEC C++ 1998.
367 snprintf(addend_str, sizeof(addend_str), "%llx %llx %llx",
368 static_cast<long long>((*it_a).first),
369 static_cast<long long>((*it_a).second),
370 static_cast<unsigned long long>(*it_o));
371
372 // If the symbol pointed to by the reloc is not in an ordinary
373 // section or if the symbol type is not FROM_OBJECT, then the
374 // object is NULL.
375 if (it_v->first == NULL)
376 {
377 if (first_iteration)
378 {
379 // If the symbol name is available, use it.
380 if (gsym != NULL)
381 buffer.append(gsym->name());
382 // Append the addend.
383 buffer.append(addend_str);
384 buffer.append("@");
385 }
386 continue;
387 }
388
389 Section_id reloc_secn(it_v->first, it_v->second);
390
391 // If this reloc turns back and points to the same section,
392 // like a recursive call, use a special symbol to mark this.
393 if (reloc_secn.first == secn.first
394 && reloc_secn.second == secn.second)
395 {
396 if (first_iteration)
397 {
398 buffer.append("R");
399 buffer.append(addend_str);
400 buffer.append("@");
401 }
402 continue;
403 }
404 Icf::Uniq_secn_id_map& section_id_map =
405 symtab->icf()->section_to_int_map();
406 Icf::Uniq_secn_id_map::iterator section_id_map_it =
407 section_id_map.find(reloc_secn);
408 bool is_sym_preemptible = (gsym != NULL
409 && !gsym->is_from_dynobj()
410 && !gsym->is_undefined()
411 && gsym->is_preemptible());
412 if (!is_sym_preemptible
413 && section_id_map_it != section_id_map.end())
414 {
415 // This is a reloc to a section that might be folded.
416 if (num_tracked_relocs)
417 (*num_tracked_relocs)++;
418
419 char kept_section_str[10];
420 unsigned int secn_id = section_id_map_it->second;
421 snprintf(kept_section_str, sizeof(kept_section_str), "%u",
422 kept_section_id[secn_id]);
423 if (first_iteration)
424 {
425 buffer.append("ICF_R");
426 buffer.append(addend_str);
427 }
428 icf_reloc_buffer.append(kept_section_str);
429 // Append the addend.
430 icf_reloc_buffer.append(addend_str);
431 icf_reloc_buffer.append("@");
432 }
433 else
434 {
435 // This is a reloc to a section that cannot be folded.
436 // Process it only in the first iteration.
437 if (!first_iteration)
438 continue;
439
440 uint64_t secn_flags = (it_v->first)->section_flags(it_v->second);
441 // This reloc points to a merge section. Hash the
442 // contents of this section.
443 if ((secn_flags & elfcpp::SHF_MERGE) != 0
444 && parameters->target().can_icf_inline_merge_sections())
445 {
446 uint64_t entsize =
447 (it_v->first)->section_entsize(it_v->second);
448 long long offset = it_a->first;
449
450 // Handle SHT_RELA and SHT_REL addends. Only one of these
451 // addends exists. When pointing to a merge section, the
452 // addend only matters if it's relative to a section
453 // symbol. In order to unambiguously identify the target
454 // of the relocation, the compiler (and assembler) must use
455 // a local non-section symbol unless Symbol+Addend does in
456 // fact point directly to the target. (In other words,
457 // a bias for a pc-relative reference or a non-zero based
458 // access forces the use of a local symbol, and the addend
459 // is used only to provide that bias.)
460 uint64_t reloc_addend_value = 0;
461 if (is_section_symbol)
462 {
463 // Get the SHT_RELA addend. For RELA relocations,
464 // we have the addend from the relocation.
465 reloc_addend_value = it_a->second;
466
467 // Handle SHT_REL addends.
468 // For REL relocations, we need to fetch the addend
469 // from the section contents.
470 const unsigned char* reloc_addend_ptr =
471 contents + static_cast<unsigned long long>(*it_o);
472
473 // Update the addend value with the SHT_REL addend if
474 // available.
475 get_rel_addend(reloc_addend_ptr, *it_addend_size,
476 &reloc_addend_value);
477
478 // Ignore the addend when it is a negative value.
479 // See the comments in Merged_symbol_value::value
480 // in object.h.
481 if (reloc_addend_value < 0xffffff00)
482 offset = offset + reloc_addend_value;
483 }
484
485 section_size_type secn_len;
486
487 const unsigned char* str_contents =
488 (it_v->first)->section_contents(it_v->second,
489 &secn_len,
490 false) + offset;
491 gold_assert (offset < (long long) secn_len);
492
493 if ((secn_flags & elfcpp::SHF_STRINGS) != 0)
494 {
495 // String merge section.
496 const char* str_char =
497 reinterpret_cast<const char*>(str_contents);
498 switch(entsize)
499 {
500 case 1:
501 {
502 buffer.append(str_char);
503 break;
504 }
505 case 2:
506 {
507 const uint16_t* ptr_16 =
508 reinterpret_cast<const uint16_t*>(str_char);
509 unsigned int strlen_16 = 0;
510 // Find the NULL character.
511 while(*(ptr_16 + strlen_16) != 0)
512 strlen_16++;
513 buffer.append(str_char, strlen_16 * 2);
514 }
515 break;
516 case 4:
517 {
518 const uint32_t* ptr_32 =
519 reinterpret_cast<const uint32_t*>(str_char);
520 unsigned int strlen_32 = 0;
521 // Find the NULL character.
522 while(*(ptr_32 + strlen_32) != 0)
523 strlen_32++;
524 buffer.append(str_char, strlen_32 * 4);
525 }
526 break;
527 default:
528 gold_unreachable();
529 }
530 }
531 else
532 {
533 // Use the entsize to determine the length to copy.
534 uint64_t bufsize = entsize;
535 // If entsize is too big, copy all the remaining bytes.
536 if ((offset + entsize) > secn_len)
537 bufsize = secn_len - offset;
538 buffer.append(reinterpret_cast<const
539 char*>(str_contents),
540 bufsize);
541 }
542 buffer.append("@");
543 }
544 else if (gsym != NULL)
545 {
546 // If symbol name is available use that.
547 buffer.append(gsym->name());
548 // Append the addend.
549 buffer.append(addend_str);
550 buffer.append("@");
551 }
552 else
553 {
554 // Symbol name is not available, like for a local symbol,
555 // use object and section id.
556 buffer.append(it_v->first->name());
557 char secn_id[10];
558 snprintf(secn_id, sizeof(secn_id), "%u",it_v->second);
559 buffer.append(secn_id);
560 // Append the addend.
561 buffer.append(addend_str);
562 buffer.append("@");
563 }
564 }
565 }
566 }
567
568 if (first_iteration)
569 {
570 buffer.append("Contents = ");
571 buffer.append(reinterpret_cast<const char*>(contents), plen);
572 // Store the section contents that don't change to avoid recomputing
573 // during the next call to this function.
574 (*section_contents)[section_num] = buffer;
575 }
576 else
577 {
578 gold_assert(buffer.empty());
579 // Reuse the contents computed in the previous iteration.
580 buffer.append((*section_contents)[section_num]);
581 }
582
583 buffer.append(icf_reloc_buffer);
584 return buffer;
585 }
586
587 // This function computes a checksum on each section to detect and form
588 // groups of identical sections. The first iteration does this for all
589 // sections.
590 // Further iterations do this only for the kept sections from each group to
591 // determine if larger groups of identical sections could be formed. The
592 // first section in each group is the kept section for that group.
593 //
594 // CRC32 is the checksumming algorithm and can have collisions. That is,
595 // two sections with different contents can have the same checksum. Hence,
596 // a multimap is used to maintain more than one group of checksum
597 // identical sections. A section is added to a group only after its
598 // contents are explicitly compared with the kept section of the group.
599 //
600 // Parameters :
601 // ITERATION_NUM : Invocation instance of this function.
602 // NUM_TRACKED_RELOCS : Vector reference to store the number of relocs
603 // to ICF sections.
604 // KEPT_SECTION_ID : Vector which maps folded sections to kept sections.
605 // ID_SECTION : Vector mapping a section to an unique integer.
606 // IS_SECN_OR_GROUP_UNIQUE : To check if a section or a group of identical
607 // sections is already known to be unique.
608 // SECTION_CONTENTS : Store the section's text and relocs to non-ICF
609 // sections.
610
611 static bool
612 match_sections(unsigned int iteration_num,
613 Symbol_table* symtab,
614 std::vector<unsigned int>* num_tracked_relocs,
615 std::vector<unsigned int>* kept_section_id,
616 const std::vector<Section_id>& id_section,
617 const std::vector<uint64_t>& section_addraligns,
618 std::vector<bool>* is_secn_or_group_unique,
619 std::vector<std::string>* section_contents)
620 {
621 Unordered_multimap<uint32_t, unsigned int> section_cksum;
622 std::pair<Unordered_multimap<uint32_t, unsigned int>::iterator,
623 Unordered_multimap<uint32_t, unsigned int>::iterator> key_range;
624 bool converged = true;
625
626 if (iteration_num == 1)
627 preprocess_for_unique_sections(id_section,
628 is_secn_or_group_unique,
629 NULL);
630 else
631 preprocess_for_unique_sections(id_section,
632 is_secn_or_group_unique,
633 section_contents);
634
635 std::vector<std::string> full_section_contents;
636
637 for (unsigned int i = 0; i < id_section.size(); i++)
638 {
639 full_section_contents.push_back("");
640 if ((*is_secn_or_group_unique)[i])
641 continue;
642
643 Section_id secn = id_section[i];
644 std::string this_secn_contents;
645 uint32_t cksum;
646 if (iteration_num == 1)
647 {
648 unsigned int num_relocs = 0;
649 this_secn_contents = get_section_contents(true, secn, i, &num_relocs,
650 symtab, (*kept_section_id),
651 section_contents);
652 (*num_tracked_relocs)[i] = num_relocs;
653 }
654 else
655 {
656 if ((*kept_section_id)[i] != i)
657 {
658 // This section is already folded into something.
659 continue;
660 }
661 this_secn_contents = get_section_contents(false, secn, i, NULL,
662 symtab, (*kept_section_id),
663 section_contents);
664 }
665
666 const unsigned char* this_secn_contents_array =
667 reinterpret_cast<const unsigned char*>(this_secn_contents.c_str());
668 cksum = xcrc32(this_secn_contents_array, this_secn_contents.length(),
669 0xffffffff);
670 size_t count = section_cksum.count(cksum);
671
672 if (count == 0)
673 {
674 // Start a group with this cksum.
675 section_cksum.insert(std::make_pair(cksum, i));
676 full_section_contents[i] = this_secn_contents;
677 }
678 else
679 {
680 key_range = section_cksum.equal_range(cksum);
681 Unordered_multimap<uint32_t, unsigned int>::iterator it;
682 // Search all the groups with this cksum for a match.
683 for (it = key_range.first; it != key_range.second; ++it)
684 {
685 unsigned int kept_section = it->second;
686 if (full_section_contents[kept_section].length()
687 != this_secn_contents.length())
688 continue;
689 if (memcmp(full_section_contents[kept_section].c_str(),
690 this_secn_contents.c_str(),
691 this_secn_contents.length()) != 0)
692 continue;
693
694 // Check section alignment here.
695 // The section with the larger alignment requirement
696 // should be kept. We assume alignment can only be
697 // zero or positive integral powers of two.
698 uint64_t align_i = section_addraligns[i];
699 uint64_t align_kept = section_addraligns[kept_section];
700 if (align_i <= align_kept)
701 {
702 (*kept_section_id)[i] = kept_section;
703 }
704 else
705 {
706 (*kept_section_id)[kept_section] = i;
707 it->second = i;
708 full_section_contents[kept_section].swap(
709 full_section_contents[i]);
710 }
711
712 converged = false;
713 break;
714 }
715 if (it == key_range.second)
716 {
717 // Create a new group for this cksum.
718 section_cksum.insert(std::make_pair(cksum, i));
719 full_section_contents[i] = this_secn_contents;
720 }
721 }
722 // If there are no relocs to foldable sections do not process
723 // this section any further.
724 if (iteration_num == 1 && (*num_tracked_relocs)[i] == 0)
725 (*is_secn_or_group_unique)[i] = true;
726 }
727
728 // If a section was folded into another section that was later folded
729 // again then the former has to be updated.
730 for (unsigned int i = 0; i < id_section.size(); i++)
731 {
732 // Find the end of the folding chain
733 unsigned int kept = i;
734 while ((*kept_section_id)[kept] != kept)
735 {
736 kept = (*kept_section_id)[kept];
737 }
738 // Update every element of the chain
739 unsigned int current = i;
740 while ((*kept_section_id)[current] != kept)
741 {
742 unsigned int next = (*kept_section_id)[current];
743 (*kept_section_id)[current] = kept;
744 current = next;
745 }
746 }
747
748 return converged;
749 }
750
751 // During safe icf (--icf=safe), only fold functions that are ctors or dtors.
752 // This function returns true if the section name is that of a ctor or a dtor.
753
754 static bool
755 is_function_ctor_or_dtor(const std::string& section_name)
756 {
757 const char* mangled_func_name = strrchr(section_name.c_str(), '.');
758 gold_assert(mangled_func_name != NULL);
759 if ((is_prefix_of("._ZN", mangled_func_name)
760 || is_prefix_of("._ZZ", mangled_func_name))
761 && (is_gnu_v3_mangled_ctor(mangled_func_name + 1)
762 || is_gnu_v3_mangled_dtor(mangled_func_name + 1)))
763 {
764 return true;
765 }
766 return false;
767 }
768
769 // This is the main ICF function called in gold.cc. This does the
770 // initialization and calls match_sections repeatedly (twice by default)
771 // which computes the crc checksums and detects identical functions.
772
773 void
774 Icf::find_identical_sections(const Input_objects* input_objects,
775 Symbol_table* symtab)
776 {
777 unsigned int section_num = 0;
778 std::vector<unsigned int> num_tracked_relocs;
779 std::vector<uint64_t> section_addraligns;
780 std::vector<bool> is_secn_or_group_unique;
781 std::vector<std::string> section_contents;
782 const Target& target = parameters->target();
783
784 // Decide which sections are possible candidates first.
785
786 for (Input_objects::Relobj_iterator p = input_objects->relobj_begin();
787 p != input_objects->relobj_end();
788 ++p)
789 {
790 // Lock the object so we can read from it. This is only called
791 // single-threaded from queue_middle_tasks, so it is OK to lock.
792 // Unfortunately we have no way to pass in a Task token.
793 const Task* dummy_task = reinterpret_cast<const Task*>(-1);
794 Task_lock_obj<Object> tl(dummy_task, *p);
795
796 for (unsigned int i = 0;i < (*p)->shnum(); ++i)
797 {
798 const std::string section_name = (*p)->section_name(i);
799 if (!is_section_foldable_candidate(section_name))
800 continue;
801 if (!(*p)->is_section_included(i))
802 continue;
803 if (parameters->options().gc_sections()
804 && symtab->gc()->is_section_garbage(*p, i))
805 continue;
806 // With --icf=safe, check if the mangled function name is a ctor
807 // or a dtor. The mangled function name can be obtained from the
808 // section name by stripping the section prefix.
809 if (parameters->options().icf_safe_folding()
810 && !is_function_ctor_or_dtor(section_name)
811 && (!target.can_check_for_function_pointers()
812 || section_has_function_pointers(*p, i)))
813 {
814 continue;
815 }
816 this->id_section_.push_back(Section_id(*p, i));
817 this->section_id_[Section_id(*p, i)] = section_num;
818 this->kept_section_id_.push_back(section_num);
819 num_tracked_relocs.push_back(0);
820 section_addraligns.push_back((*p)->section_addralign(i));
821 is_secn_or_group_unique.push_back(false);
822 section_contents.push_back("");
823 section_num++;
824 }
825 }
826
827 unsigned int num_iterations = 0;
828
829 // Default number of iterations to run ICF is 2.
830 unsigned int max_iterations = (parameters->options().icf_iterations() > 0)
831 ? parameters->options().icf_iterations()
832 : 2;
833
834 bool converged = false;
835
836 while (!converged && (num_iterations < max_iterations))
837 {
838 num_iterations++;
839 converged = match_sections(num_iterations, symtab,
840 &num_tracked_relocs, &this->kept_section_id_,
841 this->id_section_, section_addraligns,
842 &is_secn_or_group_unique, &section_contents);
843 }
844
845 if (parameters->options().print_icf_sections())
846 {
847 if (converged)
848 gold_info(_("%s: ICF Converged after %u iteration(s)"),
849 program_name, num_iterations);
850 else
851 gold_info(_("%s: ICF stopped after %u iteration(s)"),
852 program_name, num_iterations);
853 }
854
855 // Unfold --keep-unique symbols.
856 for (options::String_set::const_iterator p =
857 parameters->options().keep_unique_begin();
858 p != parameters->options().keep_unique_end();
859 ++p)
860 {
861 const char* name = p->c_str();
862 Symbol* sym = symtab->lookup(name);
863 if (sym == NULL)
864 {
865 gold_warning(_("Could not find symbol %s to unfold\n"), name);
866 }
867 else if (sym->source() == Symbol::FROM_OBJECT
868 && !sym->object()->is_dynamic())
869 {
870 Relobj* obj = static_cast<Relobj*>(sym->object());
871 bool is_ordinary;
872 unsigned int shndx = sym->shndx(&is_ordinary);
873 if (is_ordinary)
874 {
875 this->unfold_section(obj, shndx);
876 }
877 }
878
879 }
880
881 this->icf_ready();
882 }
883
884 // Unfolds the section denoted by OBJ and SHNDX if folded.
885
886 void
887 Icf::unfold_section(Relobj* obj, unsigned int shndx)
888 {
889 Section_id secn(obj, shndx);
890 Uniq_secn_id_map::iterator it = this->section_id_.find(secn);
891 if (it == this->section_id_.end())
892 return;
893 unsigned int section_num = it->second;
894 unsigned int kept_section_id = this->kept_section_id_[section_num];
895 if (kept_section_id != section_num)
896 this->kept_section_id_[section_num] = section_num;
897 }
898
899 // This function determines if the section corresponding to the
900 // given object and index is folded based on if the kept section
901 // is different from this section.
902
903 bool
904 Icf::is_section_folded(Relobj* obj, unsigned int shndx)
905 {
906 Section_id secn(obj, shndx);
907 Uniq_secn_id_map::iterator it = this->section_id_.find(secn);
908 if (it == this->section_id_.end())
909 return false;
910 unsigned int section_num = it->second;
911 unsigned int kept_section_id = this->kept_section_id_[section_num];
912 return kept_section_id != section_num;
913 }
914
915 // This function returns the folded section for the given section.
916
917 Section_id
918 Icf::get_folded_section(Relobj* dup_obj, unsigned int dup_shndx)
919 {
920 Section_id dup_secn(dup_obj, dup_shndx);
921 Uniq_secn_id_map::iterator it = this->section_id_.find(dup_secn);
922 gold_assert(it != this->section_id_.end());
923 unsigned int section_num = it->second;
924 unsigned int kept_section_id = this->kept_section_id_[section_num];
925 Section_id folded_section = this->id_section_[kept_section_id];
926 return folded_section;
927 }
928
929 } // End of namespace gold.