1 //===- MarkLive.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 // This file implements --gc-sections, which is a feature to remove unused
10 // sections from output. Unused sections are sections that are not reachable
11 // from known GC-root symbols or sections. Naturally the feature is
12 // implemented as a mark-sweep garbage collector.
14 // Here's how it works. Each InputSectionBase has a "Live" bit. The bit is off
15 // by default. Starting with GC-root symbols or sections, markLive function
16 // defined in this file visits all reachable sections to set their Live
17 // bits. Writer will then ignore sections whose Live bits are off, so that
18 // such sections are not included into output.
20 //===----------------------------------------------------------------------===//
23 #include "InputSection.h"
24 #include "LinkerScript.h"
25 #include "OutputSections.h"
26 #include "SymbolTable.h"
28 #include "SyntheticSections.h"
30 #include "lld/Common/Memory.h"
31 #include "lld/Common/Strings.h"
32 #include "llvm/ADT/STLExtras.h"
33 #include "llvm/Object/ELF.h"
38 using namespace llvm::ELF;
39 using namespace llvm::object;
40 using namespace llvm::support::endian;
43 using namespace lld::elf;
46 template <class ELFT> class MarkLive {
48 MarkLive(unsigned partition) : partition(partition) {}
54 void enqueue(InputSectionBase *sec, uint64_t offset);
55 void markSymbol(Symbol *sym);
58 template <class RelTy>
59 void resolveReloc(InputSectionBase &sec, RelTy &rel, bool isLSDA);
61 template <class RelTy>
62 void scanEhFrameSection(EhInputSection &eh, ArrayRef<RelTy> rels);
64 // The index of the partition that we are currently processing.
67 // A list of sections to visit.
68 SmallVector<InputSection *, 256> queue;
70 // There are normally few input sections whose names are valid C
71 // identifiers, so we just store a std::vector instead of a multimap.
72 DenseMap<StringRef, std::vector<InputSectionBase *>> cNamedSections;
77 static uint64_t getAddend(InputSectionBase &sec,
78 const typename ELFT::Rel &rel) {
79 return target->getImplicitAddend(sec.data().begin() + rel.r_offset,
80 rel.getType(config->isMips64EL));
84 static uint64_t getAddend(InputSectionBase &sec,
85 const typename ELFT::Rela &rel) {
90 template <class RelTy>
91 void MarkLive<ELFT>::resolveReloc(InputSectionBase &sec, RelTy &rel,
93 Symbol &sym = sec.getFile<ELFT>()->getRelocTargetSym(rel);
95 // If a symbol is referenced in a live section, it is used.
98 if (auto *d = dyn_cast<Defined>(&sym)) {
99 auto *relSec = dyn_cast_or_null<InputSectionBase>(d->section);
103 uint64_t offset = d->value;
105 offset += getAddend<ELFT>(sec, rel);
107 if (!isLSDA || !(relSec->flags & SHF_EXECINSTR))
108 enqueue(relSec, offset);
112 if (auto *ss = dyn_cast<SharedSymbol>(&sym))
114 ss->getFile().isNeeded = true;
116 for (InputSectionBase *sec : cNamedSections.lookup(sym.getName()))
120 // The .eh_frame section is an unfortunate special case.
121 // The section is divided in CIEs and FDEs and the relocations it can have are
122 // * CIEs can refer to a personality function.
123 // * FDEs can refer to a LSDA
124 // * FDEs refer to the function they contain information about
125 // The last kind of relocation cannot keep the referred section alive, or they
126 // would keep everything alive in a common object file. In fact, each FDE is
127 // alive if the section it refers to is alive.
128 // To keep things simple, in here we just ignore the last relocation kind. The
129 // other two keep the referred section alive.
131 // A possible improvement would be to fully process .eh_frame in the middle of
132 // the gc pass. With that we would be able to also gc some sections holding
133 // LSDAs and personality functions if we found that they were unused.
134 template <class ELFT>
135 template <class RelTy>
136 void MarkLive<ELFT>::scanEhFrameSection(EhInputSection &eh,
137 ArrayRef<RelTy> rels) {
138 for (size_t i = 0, end = eh.pieces.size(); i < end; ++i) {
139 EhSectionPiece &piece = eh.pieces[i];
140 size_t firstRelI = piece.firstRelocation;
141 if (firstRelI == (unsigned)-1)
144 if (read32<ELFT::TargetEndianness>(piece.data().data() + 4) == 0) {
145 // This is a CIE, we only need to worry about the first relocation. It is
146 // known to point to the personality function.
147 resolveReloc(eh, rels[firstRelI], false);
151 // This is a FDE. The relocations point to the described function or to
152 // a LSDA. We only need to keep the LSDA alive, so ignore anything that
153 // points to executable sections.
154 uint64_t pieceEnd = piece.inputOff + piece.size;
155 for (size_t j = firstRelI, end2 = rels.size(); j < end2; ++j)
156 if (rels[j].r_offset < pieceEnd)
157 resolveReloc(eh, rels[j], true);
161 // Some sections are used directly by the loader, so they should never be
162 // garbage-collected. This function returns true if a given section is such
164 static bool isReserved(InputSectionBase *sec) {
169 case SHT_PREINIT_ARRAY:
172 StringRef s = sec->name;
173 return s.startswith(".ctors") || s.startswith(".dtors") ||
174 s.startswith(".init") || s.startswith(".fini") ||
175 s.startswith(".jcr");
179 template <class ELFT>
180 void MarkLive<ELFT>::enqueue(InputSectionBase *sec, uint64_t offset) {
181 // Skip over discarded sections. This in theory shouldn't happen, because
182 // the ELF spec doesn't allow a relocation to point to a deduplicated
183 // COMDAT section directly. Unfortunately this happens in practice (e.g.
184 // .eh_frame) so we need to add a check.
185 if (sec == &InputSection::discarded)
188 // Usually, a whole section is marked as live or dead, but in mergeable
189 // (splittable) sections, each piece of data has independent liveness bit.
190 // So we explicitly tell it which offset is in use.
191 if (auto *ms = dyn_cast<MergeInputSection>(sec))
192 ms->getSectionPiece(offset)->live = true;
194 // Set Sec->Partition to the meet (i.e. the "minimum") of Partition and
195 // Sec->Partition in the following lattice: 1 < other < 0. If Sec->Partition
196 // doesn't change, we don't need to do anything.
197 if (sec->partition == 1 || sec->partition == partition)
199 sec->partition = sec->partition ? 1 : partition;
201 // Add input section to the queue.
202 if (InputSection *s = dyn_cast<InputSection>(sec))
206 template <class ELFT> void MarkLive<ELFT>::markSymbol(Symbol *sym) {
207 if (auto *d = dyn_cast_or_null<Defined>(sym))
208 if (auto *isec = dyn_cast_or_null<InputSectionBase>(d->section))
209 enqueue(isec, d->value);
212 // This is the main function of the garbage collector.
213 // Starting from GC-root sections, this function visits all reachable
214 // sections to set their "Live" bits.
215 template <class ELFT> void MarkLive<ELFT>::run() {
216 // Add GC root symbols.
218 // Preserve externally-visible symbols if the symbols defined by this
219 // file can interrupt other ELF file's symbols at runtime.
220 symtab->forEachSymbol([&](Symbol *sym) {
221 if (sym->includeInDynsym() && sym->partition == partition)
225 // If this isn't the main partition, that's all that we need to preserve.
226 if (partition != 1) {
231 markSymbol(symtab->find(config->entry));
232 markSymbol(symtab->find(config->init));
233 markSymbol(symtab->find(config->fini));
234 for (StringRef s : config->undefined)
235 markSymbol(symtab->find(s));
236 for (StringRef s : script->referencedSymbols)
237 markSymbol(symtab->find(s));
239 // Preserve special sections and those which are specified in linker
240 // script KEEP command.
241 for (InputSectionBase *sec : inputSections) {
242 // Mark .eh_frame sections as live because there are usually no relocations
243 // that point to .eh_frames. Otherwise, the garbage collector would drop
244 // all of them. We also want to preserve personality routines and LSDA
245 // referenced by .eh_frame sections, so we scan them for that here.
246 if (auto *eh = dyn_cast<EhInputSection>(sec)) {
248 if (!eh->numRelocations)
251 if (eh->areRelocsRela)
252 scanEhFrameSection(*eh, eh->template relas<ELFT>());
254 scanEhFrameSection(*eh, eh->template rels<ELFT>());
257 if (sec->flags & SHF_LINK_ORDER)
260 if (isReserved(sec) || script->shouldKeep(sec)) {
262 } else if (isValidCIdentifier(sec->name)) {
263 cNamedSections[saver.save("__start_" + sec->name)].push_back(sec);
264 cNamedSections[saver.save("__stop_" + sec->name)].push_back(sec);
271 template <class ELFT> void MarkLive<ELFT>::mark() {
272 // Mark all reachable sections.
273 while (!queue.empty()) {
274 InputSectionBase &sec = *queue.pop_back_val();
276 if (sec.areRelocsRela) {
277 for (const typename ELFT::Rela &rel : sec.template relas<ELFT>())
278 resolveReloc(sec, rel, false);
280 for (const typename ELFT::Rel &rel : sec.template rels<ELFT>())
281 resolveReloc(sec, rel, false);
284 for (InputSectionBase *isec : sec.dependentSections)
289 // Move the sections for some symbols to the main partition, specifically ifuncs
290 // (because they can result in an IRELATIVE being added to the main partition's
291 // GOT, which means that the ifunc must be available when the main partition is
292 // loaded) and TLS symbols (because we only know how to correctly process TLS
293 // relocations for the main partition).
294 template <class ELFT> void MarkLive<ELFT>::moveToMain() {
295 for (InputFile *file : objectFiles)
296 for (Symbol *s : file->getSymbols())
297 if (auto *d = dyn_cast<Defined>(s))
298 if ((d->type == STT_GNU_IFUNC || d->type == STT_TLS) && d->section &&
299 d->section->isLive())
305 // Before calling this function, Live bits are off for all
306 // input sections. This function make some or all of them on
307 // so that they are emitted to the output file.
308 template <class ELFT> void elf::markLive() {
309 // If -gc-sections is not given, no sections are removed.
310 if (!config->gcSections) {
311 for (InputSectionBase *sec : inputSections)
314 // If a DSO defines a symbol referenced in a regular object, it is needed.
315 symtab->forEachSymbol([](Symbol *sym) {
316 if (auto *s = dyn_cast<SharedSymbol>(sym))
317 if (s->isUsedInRegularObj && !s->isWeak())
318 s->getFile().isNeeded = true;
323 // Otheriwse, do mark-sweep GC.
325 // The -gc-sections option works only for SHF_ALLOC sections
326 // (sections that are memory-mapped at runtime). So we can
327 // unconditionally make non-SHF_ALLOC sections alive except
328 // SHF_LINK_ORDER and SHT_REL/SHT_RELA sections.
330 // Usually, non-SHF_ALLOC sections are not removed even if they are
331 // unreachable through relocations because reachability is not
332 // a good signal whether they are garbage or not (e.g. there is
333 // usually no section referring to a .comment section, but we
334 // want to keep it.).
336 // Note on SHF_LINK_ORDER: Such sections contain metadata and they
337 // have a reverse dependency on the InputSection they are linked with.
338 // We are able to garbage collect them.
340 // Note on SHF_REL{,A}: Such sections reach here only when -r
341 // or -emit-reloc were given. And they are subject of garbage
342 // collection because, if we remove a text section, we also
343 // remove its relocation section.
344 for (InputSectionBase *sec : inputSections) {
345 bool isAlloc = (sec->flags & SHF_ALLOC);
346 bool isLinkOrder = (sec->flags & SHF_LINK_ORDER);
347 bool isRel = (sec->type == SHT_REL || sec->type == SHT_RELA);
349 if (!isAlloc && !isLinkOrder && !isRel)
353 // Follow the graph to mark all live sections.
354 for (unsigned curPart = 1; curPart <= partitions.size(); ++curPart)
355 MarkLive<ELFT>(curPart).run();
357 // If we have multiple partitions, some sections need to live in the main
358 // partition even if they were allocated to a loadable partition. Move them
360 if (partitions.size() != 1)
361 MarkLive<ELFT>(1).moveToMain();
363 // Report garbage-collected sections.
364 if (config->printGcSections)
365 for (InputSectionBase *sec : inputSections)
367 message("removing unused section " + toString(sec));
370 template void elf::markLive<ELF32LE>();
371 template void elf::markLive<ELF32BE>();
372 template void elf::markLive<ELF64LE>();
373 template void elf::markLive<ELF64BE>();