1 //===- Core/SymbolTable.cpp - Main Symbol Table ---------------------------===//
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 "lld/Core/SymbolTable.h"
10 #include "lld/Common/LLVM.h"
11 #include "lld/Core/AbsoluteAtom.h"
12 #include "lld/Core/Atom.h"
13 #include "lld/Core/DefinedAtom.h"
14 #include "lld/Core/File.h"
15 #include "lld/Core/LinkingContext.h"
16 #include "lld/Core/Resolver.h"
17 #include "lld/Core/SharedLibraryAtom.h"
18 #include "lld/Core/UndefinedAtom.h"
19 #include "llvm/ADT/ArrayRef.h"
20 #include "llvm/ADT/DenseMapInfo.h"
21 #include "llvm/ADT/Hashing.h"
22 #include "llvm/Support/ErrorHandling.h"
23 #include "llvm/Support/raw_ostream.h"
24 #include <algorithm>
25 #include <cassert>
26 #include <cstdlib>
27 #include <vector>
28 
29 namespace lld {
30 bool SymbolTable::add(const UndefinedAtom &atom) { return addByName(atom); }
31 
32 bool SymbolTable::add(const SharedLibraryAtom &atom) { return addByName(atom); }
33 
34 bool SymbolTable::add(const AbsoluteAtom &atom) { return addByName(atom); }
35 
36 bool SymbolTable::add(const DefinedAtom &atom) {
37   if (!atom.name().empty() &&
38       atom.scope() != DefinedAtom::scopeTranslationUnit) {
39     // Named atoms cannot be merged by content.
40     assert(atom.merge() != DefinedAtom::mergeByContent);
41     // Track named atoms that are not scoped to file (static).
42     return addByName(atom);
43   }
44   if (atom.merge() == DefinedAtom::mergeByContent) {
45     // Named atoms cannot be merged by content.
46     assert(atom.name().empty());
47     // Currently only read-only constants can be merged.
48     if (atom.permissions() == DefinedAtom::permR__)
49       return addByContent(atom);
50     // TODO: support mergeByContent of data atoms by comparing content & fixups.
51   }
52   return false;
53 }
54 
55 enum NameCollisionResolution {
56   NCR_First,
57   NCR_Second,
58   NCR_DupDef,
59   NCR_DupUndef,
60   NCR_DupShLib,
61   NCR_Error
62 };
63 
64 static NameCollisionResolution cases[4][4] = {
65   //regular     absolute    undef      sharedLib
66   {
67     // first is regular
68     NCR_DupDef, NCR_Error,   NCR_First, NCR_First
69   },
70   {
71     // first is absolute
72     NCR_Error,  NCR_Error,  NCR_First, NCR_First
73   },
74   {
75     // first is undef
76     NCR_Second, NCR_Second, NCR_DupUndef, NCR_Second
77   },
78   {
79     // first is sharedLib
80     NCR_Second, NCR_Second, NCR_First, NCR_DupShLib
81   }
82 };
83 
84 static NameCollisionResolution collide(Atom::Definition first,
85                                        Atom::Definition second) {
86   return cases[first][second];
87 }
88 
89 enum MergeResolution {
90   MCR_First,
91   MCR_Second,
92   MCR_Largest,
93   MCR_SameSize,
94   MCR_Error
95 };
96 
97 static MergeResolution mergeCases[][6] = {
98   // no          tentative      weak          weakAddress   sameNameAndSize largest
99   {MCR_Error,    MCR_First,     MCR_First,    MCR_First,    MCR_SameSize,   MCR_Largest},  // no
100   {MCR_Second,   MCR_Largest,   MCR_Second,   MCR_Second,   MCR_SameSize,   MCR_Largest},  // tentative
101   {MCR_Second,   MCR_First,     MCR_First,    MCR_Second,   MCR_SameSize,   MCR_Largest},  // weak
102   {MCR_Second,   MCR_First,     MCR_First,    MCR_First,    MCR_SameSize,   MCR_Largest},  // weakAddress
103   {MCR_SameSize, MCR_SameSize,  MCR_SameSize, MCR_SameSize, MCR_SameSize,   MCR_SameSize}, // sameSize
104   {MCR_Largest,  MCR_Largest,   MCR_Largest,  MCR_Largest,  MCR_SameSize,   MCR_Largest},  // largest
105 };
106 
107 static MergeResolution mergeSelect(DefinedAtom::Merge first,
108                                    DefinedAtom::Merge second) {
109   assert(first != DefinedAtom::mergeByContent);
110   assert(second != DefinedAtom::mergeByContent);
111   return mergeCases[first][second];
112 }
113 
114 bool SymbolTable::addByName(const Atom &newAtom) {
115   StringRef name = newAtom.name();
116   assert(!name.empty());
117   const Atom *existing = findByName(name);
118   if (existing == nullptr) {
119     // Name is not in symbol table yet, add it associate with this atom.
120     _nameTable[name] = &newAtom;
121     return true;
122   }
123 
124   // Do nothing if the same object is added more than once.
125   if (existing == &newAtom)
126     return false;
127 
128   // Name is already in symbol table and associated with another atom.
129   bool useNew = true;
130   switch (collide(existing->definition(), newAtom.definition())) {
131   case NCR_First:
132     useNew = false;
133     break;
134   case NCR_Second:
135     useNew = true;
136     break;
137   case NCR_DupDef: {
138     const auto *existingDef = cast<DefinedAtom>(existing);
139     const auto *newDef = cast<DefinedAtom>(&newAtom);
140     switch (mergeSelect(existingDef->merge(), newDef->merge())) {
141     case MCR_First:
142       useNew = false;
143       break;
144     case MCR_Second:
145       useNew = true;
146       break;
147     case MCR_Largest: {
148       uint64_t existingSize = existingDef->sectionSize();
149       uint64_t newSize = newDef->sectionSize();
150       useNew = (newSize >= existingSize);
151       break;
152     }
153     case MCR_SameSize: {
154       uint64_t existingSize = existingDef->sectionSize();
155       uint64_t newSize = newDef->sectionSize();
156       if (existingSize == newSize) {
157         useNew = true;
158         break;
159       }
160       llvm::errs() << "Size mismatch: " << existing->name() << " ("
161                    << existingSize << ") " << newAtom.name() << " (" << newSize
162                    << ")\n";
163       LLVM_FALLTHROUGH;
164     }
165     case MCR_Error:
166       llvm::errs() << "Duplicate symbols: " << existing->name() << ":"
167                    << existing->file().path() << " and " << newAtom.name()
168                    << ":" << newAtom.file().path() << "\n";
169       llvm::report_fatal_error("duplicate symbol error");
170       break;
171     }
172     break;
173   }
174   case NCR_DupUndef: {
175     const UndefinedAtom* existingUndef = cast<UndefinedAtom>(existing);
176     const UndefinedAtom* newUndef = cast<UndefinedAtom>(&newAtom);
177 
178     bool sameCanBeNull = (existingUndef->canBeNull() == newUndef->canBeNull());
179     if (sameCanBeNull)
180       useNew = false;
181     else
182       useNew = (newUndef->canBeNull() < existingUndef->canBeNull());
183     break;
184   }
185   case NCR_DupShLib: {
186     useNew = false;
187     break;
188   }
189   case NCR_Error:
190     llvm::errs() << "SymbolTable: error while merging " << name << "\n";
191     llvm::report_fatal_error("duplicate symbol error");
192     break;
193   }
194 
195   if (useNew) {
196     // Update name table to use new atom.
197     _nameTable[name] = &newAtom;
198     // Add existing atom to replacement table.
199     _replacedAtoms[existing] = &newAtom;
200   } else {
201     // New atom is not being used.  Add it to replacement table.
202     _replacedAtoms[&newAtom] = existing;
203   }
204   return false;
205 }
206 
207 unsigned SymbolTable::AtomMappingInfo::getHashValue(const DefinedAtom *atom) {
208   auto content = atom->rawContent();
209   return llvm::hash_combine(atom->size(),
210                             atom->contentType(),
211                             llvm::hash_combine_range(content.begin(),
212                                                      content.end()));
213 }
214 
215 bool SymbolTable::AtomMappingInfo::isEqual(const DefinedAtom * const l,
216                                            const DefinedAtom * const r) {
217   if (l == r)
218     return true;
219   if (l == getEmptyKey() || r == getEmptyKey())
220     return false;
221   if (l == getTombstoneKey() || r == getTombstoneKey())
222     return false;
223   if (l->contentType() != r->contentType())
224     return false;
225   if (l->size() != r->size())
226     return false;
227   if (l->sectionChoice() != r->sectionChoice())
228     return false;
229   if (l->sectionChoice() == DefinedAtom::sectionCustomRequired) {
230     if (!l->customSectionName().equals(r->customSectionName()))
231       return false;
232   }
233   ArrayRef<uint8_t> lc = l->rawContent();
234   ArrayRef<uint8_t> rc = r->rawContent();
235   return memcmp(lc.data(), rc.data(), lc.size()) == 0;
236 }
237 
238 bool SymbolTable::addByContent(const DefinedAtom &newAtom) {
239   AtomContentSet::iterator pos = _contentTable.find(&newAtom);
240   if (pos == _contentTable.end()) {
241     _contentTable.insert(&newAtom);
242     return true;
243   }
244   const Atom* existing = *pos;
245   // New atom is not being used.  Add it to replacement table.
246   _replacedAtoms[&newAtom] = existing;
247   return false;
248 }
249 
250 const Atom *SymbolTable::findByName(StringRef sym) {
251   NameToAtom::iterator pos = _nameTable.find(sym);
252   if (pos == _nameTable.end())
253     return nullptr;
254   return pos->second;
255 }
256 
257 const Atom *SymbolTable::replacement(const Atom *atom) {
258   // Find the replacement for a given atom. Atoms in _replacedAtoms
259   // may be chained, so find the last one.
260   for (;;) {
261     AtomToAtom::iterator pos = _replacedAtoms.find(atom);
262     if (pos == _replacedAtoms.end())
263       return atom;
264     atom = pos->second;
265   }
266 }
267 
268 bool SymbolTable::isCoalescedAway(const Atom *atom) {
269   return _replacedAtoms.count(atom) > 0;
270 }
271 
272 std::vector<const UndefinedAtom *> SymbolTable::undefines() {
273   std::vector<const UndefinedAtom *> ret;
274   for (auto it : _nameTable) {
275     const Atom *atom = it.second;
276     assert(atom != nullptr);
277     if (const auto *undef = dyn_cast<const UndefinedAtom>(atom))
278       if (_replacedAtoms.count(undef) == 0)
279         ret.push_back(undef);
280   }
281   return ret;
282 }
283 
284 } // namespace lld
285