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comparison lld/MachO/MarkLive.cpp @ 207:2e18cbf3894f

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LLVM12
author Shinji KONO <kono@ie.u-ryukyu.ac.jp>
date Tue, 08 Jun 2021 06:07:14 +0900
parents
children 5f17cb93ff66
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173:0572611fdcc8 207:2e18cbf3894f
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 #include "mach-o/compact_unwind_encoding.h"
16 #include "llvm/Support/TimeProfiler.h"
17
18 namespace lld {
19 namespace macho {
20
21 using namespace llvm;
22 using namespace llvm::MachO;
23
24 // Set live bit on for each reachable chunk. Unmarked (unreachable)
25 // InputSections will be ignored by Writer, so they will be excluded
26 // from the final output.
27 void markLive() {
28 TimeTraceScope timeScope("markLive");
29
30 // We build up a worklist of sections which have been marked as live. We only
31 // push into the worklist when we discover an unmarked section, and we mark
32 // as we push, so sections never appear twice in the list.
33 SmallVector<InputSection *, 256> worklist;
34
35 auto enqueue = [&](InputSection *s) {
36 if (s->live)
37 return;
38 s->live = true;
39 worklist.push_back(s);
40 };
41
42 auto addSym = [&](Symbol *s) {
43 s->used = true;
44 if (auto *d = dyn_cast<Defined>(s))
45 if (d->isec)
46 enqueue(d->isec);
47 };
48
49 // Add GC roots.
50 if (config->entry)
51 addSym(config->entry);
52 for (Symbol *sym : symtab->getSymbols()) {
53 if (auto *defined = dyn_cast<Defined>(sym)) {
54 // -exported_symbol(s_list)
55 if (!config->exportedSymbols.empty() &&
56 config->exportedSymbols.match(defined->getName())) {
57 // FIXME: Instead of doing this here, maybe the Driver code doing
58 // the matching should add them to explicitUndefineds? Then the
59 // explicitUndefineds code below would handle this automatically.
60 assert(!defined->privateExtern &&
61 "should have been rejected by driver");
62 addSym(defined);
63 continue;
64 }
65
66 // public symbols explicitly marked .no_dead_strip
67 if (defined->referencedDynamically || defined->noDeadStrip) {
68 addSym(defined);
69 continue;
70 }
71
72 // FIXME: When we implement these flags, make symbols from them GC roots:
73 // * -reexported_symbol(s_list)
74 // * -alias(-list)
75 // * -init
76
77 // In dylibs and bundles, all external functions are GC roots.
78 // FIXME: -export_dynamic should enable this for executables too.
79 if (config->outputType != MH_EXECUTE && !defined->privateExtern) {
80 addSym(defined);
81 continue;
82 }
83 }
84 }
85 // -u symbols
86 for (Symbol *sym : config->explicitUndefineds)
87 if (auto *defined = dyn_cast<Defined>(sym))
88 addSym(defined);
89 // local symbols explicitly marked .no_dead_strip
90 for (const InputFile *file : inputFiles)
91 if (auto *objFile = dyn_cast<ObjFile>(file))
92 for (Symbol *sym : objFile->symbols)
93 if (auto *defined = dyn_cast_or_null<Defined>(sym))
94 if (!defined->isExternal() && defined->noDeadStrip)
95 addSym(defined);
96 if (auto *stubBinder =
97 dyn_cast_or_null<DylibSymbol>(symtab->find("dyld_stub_binder")))
98 addSym(stubBinder);
99 for (InputSection *isec : inputSections) {
100 // Sections marked no_dead_strip
101 if (isec->flags & S_ATTR_NO_DEAD_STRIP) {
102 enqueue(isec);
103 continue;
104 }
105
106 // mod_init_funcs, mod_term_funcs sections
107 if (sectionType(isec->flags) == S_MOD_INIT_FUNC_POINTERS ||
108 sectionType(isec->flags) == S_MOD_TERM_FUNC_POINTERS) {
109 enqueue(isec);
110 continue;
111 }
112
113 // Dead strip runs before UnwindInfoSection handling so we need to keep
114 // __LD,__compact_unwind alive here.
115 // But that section contains absolute references to __TEXT,__text and
116 // keeps most code alive due to that. So we can't just enqueue() the
117 // section: We must skip the relocations for the functionAddress
118 // in each CompactUnwindEntry.
119 // See also scanEhFrameSection() in lld/ELF/MarkLive.cpp.
120 if (isec->segname == segment_names::ld &&
121 isec->name == section_names::compactUnwind) {
122 isec->live = true;
123 const int compactUnwindEntrySize =
124 target->wordSize == 8 ? sizeof(CompactUnwindEntry<uint64_t>)
125 : sizeof(CompactUnwindEntry<uint32_t>);
126 for (const Reloc &r : isec->relocs) {
127 // This is the relocation for the address of the function itself.
128 // Ignore it, else these would keep everything alive.
129 if (r.offset % compactUnwindEntrySize == 0)
130 continue;
131
132 if (auto *s = r.referent.dyn_cast<Symbol *>())
133 addSym(s);
134 else {
135 auto *referentIsec = r.referent.get<InputSection *>();
136 assert(!referentIsec->isCoalescedWeak());
137 enqueue(referentIsec);
138 }
139 }
140 continue;
141 }
142 }
143
144 do {
145 // Mark things reachable from GC roots as live.
146 while (!worklist.empty()) {
147 InputSection *s = worklist.pop_back_val();
148 assert(s->live && "We mark as live when pushing onto the worklist!");
149
150 // Mark all symbols listed in the relocation table for this section.
151 for (const Reloc &r : s->relocs) {
152 if (auto *s = r.referent.dyn_cast<Symbol *>()) {
153 addSym(s);
154 } else {
155 auto *referentIsec = r.referent.get<InputSection *>();
156 assert(!referentIsec->isCoalescedWeak());
157 enqueue(referentIsec);
158 }
159 }
160 }
161
162 // S_ATTR_LIVE_SUPPORT sections are live if they point _to_ a live section.
163 // Process them in a second pass.
164 for (InputSection *isec : inputSections) {
165 // FIXME: Check if copying all S_ATTR_LIVE_SUPPORT sections into a
166 // separate vector and only walking that here is faster.
167 if (!(isec->flags & S_ATTR_LIVE_SUPPORT) || isec->live)
168 continue;
169
170 for (const Reloc &r : isec->relocs) {
171 bool referentLive;
172 if (auto *s = r.referent.dyn_cast<Symbol *>())
173 referentLive = s->isLive();
174 else
175 referentLive = r.referent.get<InputSection *>()->live;
176 if (referentLive)
177 enqueue(isec);
178 }
179 }
180
181 // S_ATTR_LIVE_SUPPORT could have marked additional sections live,
182 // which in turn could mark additional S_ATTR_LIVE_SUPPORT sections live.
183 // Iterate. In practice, the second iteration won't mark additional
184 // S_ATTR_LIVE_SUPPORT sections live.
185 } while (!worklist.empty());
186 }
187
188 } // namespace macho
189 } // namespace lld