1 //===- ICF.cpp ------------------------------------------------------------===//
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7 //===----------------------------------------------------------------------===//
9 // ICF is short for Identical Code Folding. This is a size optimization to
10 // identify and merge two or more read-only sections (typically functions)
11 // that happened to have the same contents. It usually reduces output size
14 // In ICF, two sections are considered identical if they have the same
15 // section flags, section data, and relocations. Relocations are tricky,
16 // because two relocations are considered the same if they have the same
17 // relocation types, values, and if they point to the same sections *in
20 // Here is an example. If foo and bar defined below are compiled to the
21 // same machine instructions, ICF can and should merge the two, although
22 // their relocations point to each other.
24 // void foo() { bar(); }
25 // void bar() { foo(); }
27 // If you merge the two, their relocations point to the same section and
28 // thus you know they are mergeable, but how do you know they are
29 // mergeable in the first place? This is not an easy problem to solve.
31 // What we are doing in LLD is to partition sections into equivalence
32 // classes. Sections in the same equivalence class when the algorithm
33 // terminates are considered identical. Here are details:
35 // 1. First, we partition sections using their hash values as keys. Hash
36 // values contain section types, section contents and numbers of
37 // relocations. During this step, relocation targets are not taken into
38 // account. We just put sections that apparently differ into different
39 // equivalence classes.
41 // 2. Next, for each equivalence class, we visit sections to compare
42 // relocation targets. Relocation targets are considered equivalent if
43 // their targets are in the same equivalence class. Sections with
44 // different relocation targets are put into different equivalence
47 // 3. If we split an equivalence class in step 2, two relocations
48 // previously target the same equivalence class may now target
49 // different equivalence classes. Therefore, we repeat step 2 until a
50 // convergence is obtained.
52 // 4. For each equivalence class C, pick an arbitrary section in C, and
53 // merge all the other sections in C with it.
55 // For small programs, this algorithm needs 3-5 iterations. For large
56 // programs such as Chromium, it takes more than 20 iterations.
58 // This algorithm was mentioned as an "optimistic algorithm" in [1],
59 // though gold implements a different algorithm than this.
61 // We parallelize each step so that multiple threads can work on different
62 // equivalence classes concurrently. That gave us a large performance
63 // boost when applying ICF on large programs. For example, MSVC link.exe
64 // or GNU gold takes 10-20 seconds to apply ICF on Chromium, whose output
65 // size is about 1.5 GB, but LLD can finish it in less than 2 seconds on a
66 // 2.8 GHz 40 core machine. Even without threading, LLD's ICF is still
67 // faster than MSVC or gold though.
69 // [1] Safe ICF: Pointer Safe and Unwinding aware Identical Code Folding
71 // http://static.googleusercontent.com/media/research.google.com/en//pubs/archive/36912.pdf
73 //===----------------------------------------------------------------------===//
77 #include "LinkerScript.h"
78 #include "OutputSections.h"
79 #include "SymbolTable.h"
81 #include "SyntheticSections.h"
83 #include "lld/Common/Threads.h"
84 #include "llvm/ADT/StringExtras.h"
85 #include "llvm/BinaryFormat/ELF.h"
86 #include "llvm/Object/ELF.h"
87 #include "llvm/Support/xxhash.h"
92 using namespace llvm::ELF;
93 using namespace llvm::object;
98 template <class ELFT> class ICF {
103 void segregate(size_t begin, size_t end, bool constant);
105 template <class RelTy>
106 bool constantEq(const InputSection *a, ArrayRef<RelTy> relsA,
107 const InputSection *b, ArrayRef<RelTy> relsB);
109 template <class RelTy>
110 bool variableEq(const InputSection *a, ArrayRef<RelTy> relsA,
111 const InputSection *b, ArrayRef<RelTy> relsB);
113 bool equalsConstant(const InputSection *a, const InputSection *b);
114 bool equalsVariable(const InputSection *a, const InputSection *b);
116 size_t findBoundary(size_t begin, size_t end);
118 void forEachClassRange(size_t begin, size_t end,
119 llvm::function_ref<void(size_t, size_t)> fn);
121 void forEachClass(llvm::function_ref<void(size_t, size_t)> fn);
123 std::vector<InputSection *> sections;
125 // We repeat the main loop while `Repeat` is true.
126 std::atomic<bool> repeat;
128 // The main loop counter.
131 // We have two locations for equivalence classes. On the first iteration
132 // of the main loop, Class[0] has a valid value, and Class[1] contains
133 // garbage. We read equivalence classes from slot 0 and write to slot 1.
134 // So, Class[0] represents the current class, and Class[1] represents
135 // the next class. On each iteration, we switch their roles and use them
138 // Why are we doing this? Recall that other threads may be working on
139 // other equivalence classes in parallel. They may read sections that we
140 // are updating. We cannot update equivalence classes in place because
141 // it breaks the invariance that all possibly-identical sections must be
142 // in the same equivalence class at any moment. In other words, the for
143 // loop to update equivalence classes is not atomic, and that is
144 // observable from other threads. By writing new classes to other
145 // places, we can keep the invariance.
147 // Below, `Current` has the index of the current class, and `Next` has
148 // the index of the next class. If threading is enabled, they are either
151 // Note on single-thread: if that's the case, they are always (0, 0)
152 // because we can safely read the next class without worrying about race
153 // conditions. Using the same location makes this algorithm converge
154 // faster because it uses results of the same iteration earlier.
160 // Returns true if section S is subject of ICF.
161 static bool isEligible(InputSection *s) {
162 if (!s->isLive() || s->keepUnique || !(s->flags & SHF_ALLOC))
165 // Don't merge writable sections. .data.rel.ro sections are marked as writable
166 // but are semantically read-only.
167 if ((s->flags & SHF_WRITE) && s->name != ".data.rel.ro" &&
168 !s->name.startswith(".data.rel.ro."))
171 // SHF_LINK_ORDER sections are ICF'd as a unit with their dependent sections,
172 // so we don't consider them for ICF individually.
173 if (s->flags & SHF_LINK_ORDER)
176 // Don't merge synthetic sections as their Data member is not valid and empty.
177 // The Data member needs to be valid for ICF as it is used by ICF to determine
178 // the equality of section contents.
179 if (isa<SyntheticSection>(s))
182 // .init and .fini contains instructions that must be executed to initialize
183 // and finalize the process. They cannot and should not be merged.
184 if (s->name == ".init" || s->name == ".fini")
187 // A user program may enumerate sections named with a C identifier using
188 // __start_* and __stop_* symbols. We cannot ICF any such sections because
189 // that could change program semantics.
190 if (isValidCIdentifier(s->name))
196 // Split an equivalence class into smaller classes.
197 template <class ELFT>
198 void ICF<ELFT>::segregate(size_t begin, size_t end, bool constant) {
199 // This loop rearranges sections in [Begin, End) so that all sections
200 // that are equal in terms of equals{Constant,Variable} are contiguous
203 // The algorithm is quadratic in the worst case, but that is not an
204 // issue in practice because the number of the distinct sections in
205 // each range is usually very small.
207 while (begin < end) {
208 // Divide [Begin, End) into two. Let Mid be the start index of the
211 std::stable_partition(sections.begin() + begin + 1,
212 sections.begin() + end, [&](InputSection *s) {
214 return equalsConstant(sections[begin], s);
215 return equalsVariable(sections[begin], s);
217 size_t mid = bound - sections.begin();
219 // Now we split [Begin, End) into [Begin, Mid) and [Mid, End) by
220 // updating the sections in [Begin, Mid). We use Mid as an equivalence
221 // class ID because every group ends with a unique index.
222 for (size_t i = begin; i < mid; ++i)
223 sections[i]->eqClass[next] = mid;
225 // If we created a group, we need to iterate the main loop again.
233 // Compare two lists of relocations.
234 template <class ELFT>
235 template <class RelTy>
236 bool ICF<ELFT>::constantEq(const InputSection *secA, ArrayRef<RelTy> ra,
237 const InputSection *secB, ArrayRef<RelTy> rb) {
238 for (size_t i = 0; i < ra.size(); ++i) {
239 if (ra[i].r_offset != rb[i].r_offset ||
240 ra[i].getType(config->isMips64EL) != rb[i].getType(config->isMips64EL))
243 uint64_t addA = getAddend<ELFT>(ra[i]);
244 uint64_t addB = getAddend<ELFT>(rb[i]);
246 Symbol &sa = secA->template getFile<ELFT>()->getRelocTargetSym(ra[i]);
247 Symbol &sb = secB->template getFile<ELFT>()->getRelocTargetSym(rb[i]);
254 auto *da = dyn_cast<Defined>(&sa);
255 auto *db = dyn_cast<Defined>(&sb);
257 // Placeholder symbols generated by linker scripts look the same now but
258 // may have different values later.
259 if (!da || !db || da->scriptDefined || db->scriptDefined)
262 // When comparing a pair of relocations, if they refer to different symbols,
263 // and either symbol is preemptible, the containing sections should be
264 // considered different. This is because even if the sections are identical
265 // in this DSO, they may not be after preemption.
266 if (da->isPreemptible || db->isPreemptible)
269 // Relocations referring to absolute symbols are constant-equal if their
271 if (!da->section && !db->section && da->value + addA == db->value + addB)
273 if (!da->section || !db->section)
276 if (da->section->kind() != db->section->kind())
279 // Relocations referring to InputSections are constant-equal if their
280 // section offsets are equal.
281 if (isa<InputSection>(da->section)) {
282 if (da->value + addA == db->value + addB)
287 // Relocations referring to MergeInputSections are constant-equal if their
288 // offsets in the output section are equal.
289 auto *x = dyn_cast<MergeInputSection>(da->section);
292 auto *y = cast<MergeInputSection>(db->section);
293 if (x->getParent() != y->getParent())
297 sa.isSection() ? x->getOffset(addA) : x->getOffset(da->value) + addA;
299 sb.isSection() ? y->getOffset(addB) : y->getOffset(db->value) + addB;
300 if (offsetA != offsetB)
307 // Compare "non-moving" part of two InputSections, namely everything
308 // except relocation targets.
309 template <class ELFT>
310 bool ICF<ELFT>::equalsConstant(const InputSection *a, const InputSection *b) {
311 if (a->numRelocations != b->numRelocations || a->flags != b->flags ||
312 a->getSize() != b->getSize() || a->data() != b->data())
315 // If two sections have different output sections, we cannot merge them.
316 assert(a->getParent() && b->getParent());
317 if (a->getParent() != b->getParent())
320 if (a->areRelocsRela)
321 return constantEq(a, a->template relas<ELFT>(), b,
322 b->template relas<ELFT>());
323 return constantEq(a, a->template rels<ELFT>(), b, b->template rels<ELFT>());
326 // Compare two lists of relocations. Returns true if all pairs of
327 // relocations point to the same section in terms of ICF.
328 template <class ELFT>
329 template <class RelTy>
330 bool ICF<ELFT>::variableEq(const InputSection *secA, ArrayRef<RelTy> ra,
331 const InputSection *secB, ArrayRef<RelTy> rb) {
332 assert(ra.size() == rb.size());
334 for (size_t i = 0; i < ra.size(); ++i) {
335 // The two sections must be identical.
336 Symbol &sa = secA->template getFile<ELFT>()->getRelocTargetSym(ra[i]);
337 Symbol &sb = secB->template getFile<ELFT>()->getRelocTargetSym(rb[i]);
341 auto *da = cast<Defined>(&sa);
342 auto *db = cast<Defined>(&sb);
344 // We already dealt with absolute and non-InputSection symbols in
345 // constantEq, and for InputSections we have already checked everything
346 // except the equivalence class.
349 auto *x = dyn_cast<InputSection>(da->section);
352 auto *y = cast<InputSection>(db->section);
354 // Ineligible sections are in the special equivalence class 0.
355 // They can never be the same in terms of the equivalence class.
356 if (x->eqClass[current] == 0)
358 if (x->eqClass[current] != y->eqClass[current])
365 // Compare "moving" part of two InputSections, namely relocation targets.
366 template <class ELFT>
367 bool ICF<ELFT>::equalsVariable(const InputSection *a, const InputSection *b) {
368 if (a->areRelocsRela)
369 return variableEq(a, a->template relas<ELFT>(), b,
370 b->template relas<ELFT>());
371 return variableEq(a, a->template rels<ELFT>(), b, b->template rels<ELFT>());
374 template <class ELFT> size_t ICF<ELFT>::findBoundary(size_t begin, size_t end) {
375 uint32_t eqClass = sections[begin]->eqClass[current];
376 for (size_t i = begin + 1; i < end; ++i)
377 if (eqClass != sections[i]->eqClass[current])
382 // Sections in the same equivalence class are contiguous in Sections
383 // vector. Therefore, Sections vector can be considered as contiguous
384 // groups of sections, grouped by the class.
386 // This function calls Fn on every group within [Begin, End).
387 template <class ELFT>
388 void ICF<ELFT>::forEachClassRange(size_t begin, size_t end,
389 llvm::function_ref<void(size_t, size_t)> fn) {
390 while (begin < end) {
391 size_t mid = findBoundary(begin, end);
397 // Call Fn on each equivalence class.
398 template <class ELFT>
399 void ICF<ELFT>::forEachClass(llvm::function_ref<void(size_t, size_t)> fn) {
400 // If threading is disabled or the number of sections are
401 // too small to use threading, call Fn sequentially.
402 if (!threadsEnabled || sections.size() < 1024) {
403 forEachClassRange(0, sections.size(), fn);
409 next = (cnt + 1) % 2;
411 // Shard into non-overlapping intervals, and call Fn in parallel.
412 // The sharding must be completed before any calls to Fn are made
413 // so that Fn can modify the Chunks in its shard without causing data
415 const size_t numShards = 256;
416 size_t step = sections.size() / numShards;
417 size_t boundaries[numShards + 1];
419 boundaries[numShards] = sections.size();
421 parallelForEachN(1, numShards, [&](size_t i) {
422 boundaries[i] = findBoundary((i - 1) * step, sections.size());
425 parallelForEachN(1, numShards + 1, [&](size_t i) {
426 if (boundaries[i - 1] < boundaries[i])
427 forEachClassRange(boundaries[i - 1], boundaries[i], fn);
432 // Combine the hashes of the sections referenced by the given section into its
434 template <class ELFT, class RelTy>
435 static void combineRelocHashes(unsigned cnt, InputSection *isec,
436 ArrayRef<RelTy> rels) {
437 uint32_t hash = isec->eqClass[cnt % 2];
438 for (RelTy rel : rels) {
439 Symbol &s = isec->template getFile<ELFT>()->getRelocTargetSym(rel);
440 if (auto *d = dyn_cast<Defined>(&s))
441 if (auto *relSec = dyn_cast_or_null<InputSection>(d->section))
442 hash += relSec->eqClass[cnt % 2];
444 // Set MSB to 1 to avoid collisions with non-hash IDs.
445 isec->eqClass[(cnt + 1) % 2] = hash | (1U << 31);
448 static void print(const Twine &s) {
449 if (config->printIcfSections)
453 // The main function of ICF.
454 template <class ELFT> void ICF<ELFT>::run() {
455 // Compute isPreemptible early. We may add more symbols later, so this loop
456 // cannot be merged with the later computeIsPreemptible() pass which is used
457 // by scanRelocations().
458 for (Symbol *sym : symtab->symbols())
459 sym->isPreemptible = computeIsPreemptible(*sym);
461 // Collect sections to merge.
462 for (InputSectionBase *sec : inputSections) {
463 auto *s = cast<InputSection>(sec);
465 sections.push_back(s);
468 // Initially, we use hash values to partition sections.
469 parallelForEach(sections, [&](InputSection *s) {
470 s->eqClass[0] = xxHash64(s->data());
473 for (unsigned cnt = 0; cnt != 2; ++cnt) {
474 parallelForEach(sections, [&](InputSection *s) {
475 if (s->areRelocsRela)
476 combineRelocHashes<ELFT>(cnt, s, s->template relas<ELFT>());
478 combineRelocHashes<ELFT>(cnt, s, s->template rels<ELFT>());
482 // From now on, sections in Sections vector are ordered so that sections
483 // in the same equivalence class are consecutive in the vector.
484 llvm::stable_sort(sections, [](const InputSection *a, const InputSection *b) {
485 return a->eqClass[0] < b->eqClass[0];
488 // Compare static contents and assign unique IDs for each static content.
489 forEachClass([&](size_t begin, size_t end) { segregate(begin, end, true); });
491 // Split groups by comparing relocations until convergence is obtained.
495 [&](size_t begin, size_t end) { segregate(begin, end, false); });
498 log("ICF needed " + Twine(cnt) + " iterations");
500 // Merge sections by the equivalence class.
501 forEachClassRange(0, sections.size(), [&](size_t begin, size_t end) {
502 if (end - begin == 1)
504 print("selected section " + toString(sections[begin]));
505 for (size_t i = begin + 1; i < end; ++i) {
506 print(" removing identical section " + toString(sections[i]));
507 sections[begin]->replace(sections[i]);
509 // At this point we know sections merged are fully identical and hence
510 // we want to remove duplicate implicit dependencies such as link order
511 // and relocation sections.
512 for (InputSection *isec : sections[i]->dependentSections)
517 // InputSectionDescription::sections is populated by processSectionCommands().
518 // ICF may fold some input sections assigned to output sections. Remove them.
519 for (BaseCommand *base : script->sectionCommands)
520 if (auto *sec = dyn_cast<OutputSection>(base))
521 for (BaseCommand *sub_base : sec->sectionCommands)
522 if (auto *isd = dyn_cast<InputSectionDescription>(sub_base))
523 llvm::erase_if(isd->sections,
524 [](InputSection *isec) { return !isec->isLive(); });
527 // ICF entry point function.
528 template <class ELFT> void doIcf() { ICF<ELFT>().run(); }
530 template void doIcf<ELF32LE>();
531 template void doIcf<ELF32BE>();
532 template void doIcf<ELF64LE>();
533 template void doIcf<ELF64BE>();