1//===- MarkLive.cpp -------------------------------------------------------===//
2//
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
6//
7//===----------------------------------------------------------------------===//
8
9#include "MarkLive.h"
10#include "Config.h"
11#include "OutputSegment.h"
12#include "SymbolTable.h"
13#include "Symbols.h"
14#include "UnwindInfoSection.h"
15
16#include "lld/Common/ErrorHandler.h"
17#include "llvm/Support/TimeProfiler.h"
18
19#include "mach-o/compact_unwind_encoding.h"
20
21namespace lld::macho {
22
23using namespace llvm;
24using namespace llvm::MachO;
25
26struct WhyLiveEntry {
27 InputSection *isec;
28 // Keep track of the entry that caused us to mark `isec` as live.
29 const WhyLiveEntry *prev;
30
31 WhyLiveEntry(InputSection *isec, const WhyLiveEntry *prev)
32 : isec(isec), prev(prev) {}
33};
34
35// Type-erased interface to MarkLiveImpl. Used for adding roots to the liveness
36// graph.
37class MarkLive {
38public:
39 virtual void enqueue(InputSection *isec, uint64_t off) = 0;
40 virtual void addSym(Symbol *s) = 0;
41 virtual void markTransitively() = 0;
42 virtual ~MarkLive() = default;
43};
44
45template <bool RecordWhyLive> class MarkLiveImpl : public MarkLive {
46public:
47 // -why_live is a rarely used option, so we don't want support for that flag
48 // to slow down the main -dead_strip code path. As such, we employ templates
49 // to avoid the usage of WhyLiveEntry in the main code path. This saves us
50 // from needless allocations and pointer indirections.
51 using WorklistEntry =
52 std::conditional_t<RecordWhyLive, WhyLiveEntry, InputSection>;
53
54 void enqueue(InputSection *isec, uint64_t off) override {
55 enqueue(isec, off, nullptr);
56 }
57 void addSym(Symbol *s) override { addSym(s, nullptr); }
58 void markTransitively() override;
59
60private:
61 void enqueue(InputSection *isec, uint64_t off, const WorklistEntry *prev);
62 void addSym(Symbol *s, const WorklistEntry *prev);
63 const InputSection *getInputSection(const WorklistEntry *) const;
64 WorklistEntry *makeEntry(InputSection *, const WorklistEntry *prev) const;
65
66 // We build up a worklist of sections which have been marked as live. We
67 // only push into the worklist when we discover an unmarked section, and we
68 // mark as we push, so sections never appear twice in the list. Literal
69 // sections cannot contain references to other sections, so we only store
70 // ConcatInputSections in our worklist.
71 SmallVector<WorklistEntry *, 256> worklist;
72};
73
74template <bool RecordWhyLive>
75void MarkLiveImpl<RecordWhyLive>::enqueue(
76 InputSection *isec, uint64_t off,
77 const typename MarkLiveImpl<RecordWhyLive>::WorklistEntry *prev) {
78 if (isec->isLive(off))
79 return;
80 isec->markLive(off);
81 if (auto s = dyn_cast<ConcatInputSection>(Val: isec)) {
82 assert(!s->isCoalescedWeak());
83 worklist.push_back(makeEntry(s, prev));
84 }
85}
86
87static void printWhyLive(const Symbol *s, const WhyLiveEntry *prev) {
88 std::string out = toString(*s) + " from " + toString(file: s->getFile());
89 int indent = 2;
90 for (const WhyLiveEntry *entry = prev; entry;
91 entry = entry->prev, indent += 2) {
92 const TinyPtrVector<Defined *> &symbols = entry->isec->symbols;
93 // With .subsections_with_symbols set, most isecs will have exactly one
94 // entry in their symbols vector, so we just print the first one.
95 if (!symbols.empty())
96 out += "\n" + std::string(indent, ' ') + toString(*symbols.front()) +
97 " from " + toString(file: symbols.front()->getFile());
98 }
99 message(msg: out);
100}
101
102template <bool RecordWhyLive>
103void MarkLiveImpl<RecordWhyLive>::addSym(
104 Symbol *s,
105 const typename MarkLiveImpl<RecordWhyLive>::WorklistEntry *prev) {
106 if (s->used)
107 return;
108 s->used = true;
109 if constexpr (RecordWhyLive)
110 if (!config->whyLive.empty() && config->whyLive.match(symbolName: s->getName()))
111 printWhyLive(s, prev);
112 if (auto *d = dyn_cast<Defined>(Val: s)) {
113 if (d->isec())
114 enqueue(d->isec(), d->value, prev);
115 if (d->unwindEntry())
116 enqueue(d->unwindEntry(), 0, prev);
117 }
118}
119
120template <bool RecordWhyLive>
121const InputSection *MarkLiveImpl<RecordWhyLive>::getInputSection(
122 const MarkLiveImpl<RecordWhyLive>::WorklistEntry *entry) const {
123 if constexpr (RecordWhyLive)
124 return entry->isec;
125 else
126 return entry;
127}
128
129template <bool RecordWhyLive>
130typename MarkLiveImpl<RecordWhyLive>::WorklistEntry *
131MarkLiveImpl<RecordWhyLive>::makeEntry(
132 InputSection *isec,
133 const MarkLiveImpl<RecordWhyLive>::WorklistEntry *prev) const {
134 if constexpr (RecordWhyLive) {
135 if (!isec) {
136 assert(!prev);
137 return nullptr;
138 }
139 return make<WhyLiveEntry>(isec, prev);
140 } else {
141 return isec;
142 }
143}
144
145template <bool RecordWhyLive>
146void MarkLiveImpl<RecordWhyLive>::markTransitively() {
147 do {
148 // Mark things reachable from GC roots as live.
149 while (!worklist.empty()) {
150 WorklistEntry *entry = worklist.pop_back_val();
151 // Entries that get placed onto the worklist always contain
152 // ConcatInputSections. `WhyLiveEntry::prev` may point to entries that
153 // contain other types of InputSections (due to S_ATTR_LIVE_SUPPORT), but
154 // those entries should never be pushed onto the worklist.
155 auto *isec = cast<ConcatInputSection>(getInputSection(entry));
156 assert(isec->live && "We mark as live when pushing onto the worklist!");
157
158 // Mark all symbols listed in the relocation table for this section.
159 for (const Reloc &r : isec->relocs) {
160 if (auto *s = r.referent.dyn_cast<Symbol *>())
161 addSym(s, entry);
162 else
163 enqueue(r.referent.get<InputSection *>(), r.addend, entry);
164 }
165 for (Defined *d : getInputSection(entry)->symbols)
166 addSym(d, entry);
167 }
168
169 // S_ATTR_LIVE_SUPPORT sections are live if they point _to_ a live
170 // section. Process them in a second pass.
171 for (ConcatInputSection *isec : inputSections) {
172 // FIXME: Check if copying all S_ATTR_LIVE_SUPPORT sections into a
173 // separate vector and only walking that here is faster.
174 if (!(isec->getFlags() & S_ATTR_LIVE_SUPPORT) || isec->live)
175 continue;
176
177 for (const Reloc &r : isec->relocs) {
178 if (auto *s = r.referent.dyn_cast<Symbol *>()) {
179 if (s->isLive()) {
180 InputSection *referentIsec = nullptr;
181 if (auto *d = dyn_cast<Defined>(Val: s))
182 referentIsec = d->isec();
183 enqueue(isec, 0, makeEntry(isec: referentIsec, prev: nullptr));
184 }
185 } else {
186 auto *referentIsec = r.referent.get<InputSection *>();
187 if (referentIsec->isLive(off: r.addend))
188 enqueue(isec, 0, makeEntry(isec: referentIsec, prev: nullptr));
189 }
190 }
191 }
192
193 // S_ATTR_LIVE_SUPPORT could have marked additional sections live,
194 // which in turn could mark additional S_ATTR_LIVE_SUPPORT sections live.
195 // Iterate. In practice, the second iteration won't mark additional
196 // S_ATTR_LIVE_SUPPORT sections live.
197 } while (!worklist.empty());
198}
199
200// Set live bit on for each reachable chunk. Unmarked (unreachable)
201// InputSections will be ignored by Writer, so they will be excluded
202// from the final output.
203void markLive() {
204 TimeTraceScope timeScope("markLive");
205 MarkLive *marker;
206 if (config->whyLive.empty())
207 marker = make<MarkLiveImpl<false>>();
208 else
209 marker = make<MarkLiveImpl<true>>();
210 // Add GC roots.
211 if (config->entry)
212 marker->addSym(s: config->entry);
213 for (Symbol *sym : symtab->getSymbols()) {
214 if (auto *defined = dyn_cast<Defined>(Val: sym)) {
215 // -exported_symbol(s_list)
216 if (!config->exportedSymbols.empty() &&
217 config->exportedSymbols.match(symbolName: defined->getName())) {
218 // NOTE: Even though exporting private externs is an ill-defined
219 // operation, we are purposely not checking for privateExtern in
220 // order to follow ld64's behavior of treating all exported private
221 // extern symbols as live, irrespective of whether they are autohide.
222 marker->addSym(s: defined);
223 continue;
224 }
225
226 // public symbols explicitly marked .no_dead_strip
227 if (defined->referencedDynamically || defined->noDeadStrip) {
228 marker->addSym(s: defined);
229 continue;
230 }
231
232 // FIXME: When we implement these flags, make symbols from them GC
233 // roots:
234 // * -reexported_symbol(s_list)
235 // * -alias_list
236 // * -init
237
238 // In dylibs and bundles and in executables with -export_dynamic,
239 // all external functions are GC roots.
240 bool externsAreRoots =
241 config->outputType != MH_EXECUTE || config->exportDynamic;
242 if (externsAreRoots && !defined->privateExtern) {
243 marker->addSym(s: defined);
244 continue;
245 }
246 }
247 }
248 // -u symbols
249 for (Symbol *sym : config->explicitUndefineds)
250 marker->addSym(s: sym);
251 // local symbols explicitly marked .no_dead_strip
252 for (const InputFile *file : inputFiles)
253 if (auto *objFile = dyn_cast<ObjFile>(Val: file))
254 for (Symbol *sym : objFile->symbols)
255 if (auto *defined = dyn_cast_or_null<Defined>(Val: sym))
256 if (!defined->isExternal() && defined->noDeadStrip)
257 marker->addSym(s: defined);
258 if (auto *stubBinder =
259 dyn_cast_or_null<DylibSymbol>(Val: symtab->find(name: "dyld_stub_binder")))
260 marker->addSym(s: stubBinder);
261 for (ConcatInputSection *isec : inputSections) {
262 // Sections marked no_dead_strip
263 if (isec->getFlags() & S_ATTR_NO_DEAD_STRIP) {
264 marker->enqueue(isec, off: 0);
265 continue;
266 }
267
268 // mod_init_funcs, mod_term_funcs sections
269 if (sectionType(flags: isec->getFlags()) == S_MOD_INIT_FUNC_POINTERS ||
270 sectionType(flags: isec->getFlags()) == S_MOD_TERM_FUNC_POINTERS) {
271 assert(!config->emitInitOffsets ||
272 sectionType(isec->getFlags()) != S_MOD_INIT_FUNC_POINTERS);
273 marker->enqueue(isec, off: 0);
274 continue;
275 }
276 }
277
278 for (ConcatInputSection *isec : in.initOffsets->inputs())
279 marker->enqueue(isec, off: 0);
280
281 marker->markTransitively();
282}
283
284} // namespace lld::macho
285