1 //===- MarkLive.cpp -------------------------------------------------------===//
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7 //===----------------------------------------------------------------------===//
9 // This file implements --gc-sections, which is a feature to remove unused
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
20 //===----------------------------------------------------------------------===//
66   // A list of sections to visit.
70   // identifiers, so we just store a SmallVector instead of a multimap.
78   return target->getImplicitAddend(sec.content().begin() + rel.r_offset,  in getAddend()
79                                    rel.getType(config->isMips64EL));  in getAddend()
99   // If a symbol is referenced in a live section, it is used.  in resolveReloc()
100   Symbol &sym = sec.file->getRelocTargetSym(rel);  in resolveReloc()
104     auto *relSec = dyn_cast_or_null<InputSectionBase>(d->section);  in resolveReloc()
108     uint64_t offset = d->value;  in resolveReloc()
109     if (d->isSection())  in resolveReloc()
112     // fromFDE being true means this is referenced by a FDE in a .eh_frame  in resolveReloc()
113     // piece. The relocation points to the described function or to a LSDA. We  in resolveReloc()
114     // only need to keep the LSDA live, so ignore anything that points to  in resolveReloc()
115     // executable sections. If the LSDA is in a section group or has the  in resolveReloc()
116     // SHF_LINK_ORDER flag, we ignore the relocation as well because (a) if the  in resolveReloc()
117     // associated text section is live, the LSDA will be retained due to section  in resolveReloc()
120     if (!(fromFDE && ((relSec->flags & (SHF_EXECINSTR | SHF_LINK_ORDER)) ||  in resolveReloc()
121                       relSec->nextInSectionGroup)))  in resolveReloc()
127     if (!ss->isWeak())  in resolveReloc()
128       cast<SharedFile>(ss->file)->isNeeded = true;  in resolveReloc()
136 // * CIEs can refer to a personality function.
137 // * FDEs can refer to a LSDA
139 // The last kind of relocation cannot keep the referred section alive, or they
140 // would keep everything alive in a common object file. In fact, each FDE is
142 // To keep things simple, in here we just ignore the last relocation kind. The
143 // other two keep the referred section alive.
145 // A possible improvement would be to fully process .eh_frame in the middle of
153     if (cie.firstRelocation != unsigned(-1))  in scanEhFrameSection()
157     if (firstRelI == (unsigned)-1)  in scanEhFrameSection()
167 // garbage-collected. This function returns true if a given section is such
170   switch (sec->type) {  in isReserved()
176     // SHT_NOTE sections in a group are subject to garbage collection.  in isReserved()
177     return !sec->nextInSectionGroup;  in isReserved()
180     // .init_array.N (https://github.com/rust-lang/rust/issues/92181) for a  in isReserved()
182     StringRef s = sec->name;  in isReserved()
190   // Usually, a whole section is marked as live or dead, but in mergeable  in enqueue()
194     ms->getSectionPiece(offset).live = true;  in enqueue()
196   // Set Sec->Partition to the meet (i.e. the "minimum") of Partition and  in enqueue()
197   // Sec->Partition in the following lattice: 1 < other < 0. If Sec->Partition  in enqueue()
199   if (sec->partition == 1 || sec->partition == partition)  in enqueue()
201   sec->partition = sec->partition ? 1 : partition;  in enqueue()
210     if (auto *isec = dyn_cast_or_null<InputSectionBase>(d->section))  in markSymbol()
211       enqueue(isec, d->value);  in markSymbol()
215 // Starting from GC-root sections, this function visits all reachable
216 // sections to set their "Live" bits.
220   // Preserve externally-visible symbols if the symbols defined by this  in run()
223     if (sym->includeInDynsym() && sym->partition == partition)  in run()
232   markSymbol(symtab.find(config->entry));  in run()
233   markSymbol(symtab.find(config->init));  in run()
234   markSymbol(symtab.find(config->fini));  in run()
235   for (StringRef s : config->undefined)  in run()
237   for (StringRef s : script->referencedSymbols)  in run()
244   // Mark .eh_frame sections as live because there are usually no relocations  in run()
250         eh->template relsOrRelas<ELFT>(/*supportsCrel=*/false);  in run()
257     if (sec->flags & SHF_GNU_RETAIN) {  in run()
261     if (sec->flags & SHF_LINK_ORDER)  in run()
264     // Usually, non-SHF_ALLOC sections are not removed even if they are  in run()
265     // unreachable through relocations because reachability is not a good signal  in run()
267     // referring to a .comment section, but we want to keep it.) When a  in run()
268     // non-SHF_ALLOC section is retained, we also retain sections dependent on  in run()
272     // have a reverse dependency on the InputSection they are linked with.  in run()
275     // Note on SHF_REL{,A}: Such sections reach here only when -r  in run()
276     // or --emit-reloc were given. And they are subject of garbage  in run()
277     // collection because, if we remove a text section, we also  in run()
281     // included or omitted as a unit. We take the interpretation that:  in run()
283     // - Group members (nextInSectionGroup != nullptr) are subject to garbage  in run()
285     // - Groups members are retained or discarded as a unit.  in run()
286     if (!(sec->flags & SHF_ALLOC)) {  in run()
287       if (!isStaticRelSecType(sec->type) && !sec->nextInSectionGroup) {  in run()
288         sec->markLive();  in run()
289         for (InputSection *isec : sec->dependentSections)  in run()
290           isec->markLive();  in run()
295     // script KEEP command.  in run()
296     if (isReserved(sec) || script->shouldKeep(sec)) {  in run()
298     } else if ((!config->zStartStopGC || sec->name.starts_with("__libc_")) &&  in run()
299                isValidCIdentifier(sec->name)) {  in run()
300       // As a workaround for glibc libc.a before 2.34  in run()
303       cNamedSections[saver().save("__start_" + sec->name)].push_back(sec);  in run()
304       cNamedSections[saver().save("__stop_" + sec->name)].push_back(sec);  in run()
344     for (Symbol *s : file->getSymbols())  in moveToMain()
346         if ((d->type == STT_GNU_IFUNC || d->type == STT_TLS) && d->section &&  in moveToMain()
347             d->section->isLive())  in moveToMain()
351     if (!sec->isLive() || !isValidCIdentifier(sec->name))  in moveToMain()
353     if (symtab.find(("__start_" + sec->name).str()) ||  in moveToMain()
354         symtab.find(("__stop_" + sec->name).str()))  in moveToMain()
361 // Before calling this function, Live bits are off for all
366   // If --gc-sections is not given, retain all input sections.  in markLive()
367   if (!config->gcSections) {  in markLive()
368     // If a DSO defines a symbol referenced in a regular object, it is needed.  in markLive()
371         if (s->isUsedInRegularObj && !s->isWeak())  in markLive()
372           cast<SharedFile>(s->file)->isNeeded = true;  in markLive()
377     sec->markDead();  in markLive()
379   // Follow the graph to mark all live sections.  in markLive()
383   // If we have multiple partitions, some sections need to live in the main  in markLive()
384   // partition even if they were allocated to a loadable partition. Move them  in markLive()
389   // Report garbage-collected sections.  in markLive()
390   if (config->printGcSections)  in markLive()
392       if (!sec->isLive())  in markLive()