1 //===- lib/ReaderWriter/MachO/GOTPass.cpp -----------------------*- C++ -*-===//
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 /// \file
10 /// This linker pass transforms all GOT kind references to real references.
11 /// That is, in assembly you can write something like:
12 /// movq foo@GOTPCREL(%rip), %rax
13 /// which means you want to load a pointer to "foo" out of the GOT (global
14 /// Offsets Table). In the object file, the Atom containing this instruction
15 /// has a Reference whose target is an Atom named "foo" and the Reference
16 /// kind is a GOT load. The linker needs to instantiate a pointer sized
17 /// GOT entry. This is done be creating a GOT Atom to represent that pointer
18 /// sized data in this pass, and altering the Atom graph so the Reference now
19 /// points to the GOT Atom entry (corresponding to "foo") and changing the
20 /// Reference Kind to reflect it is now pointing to a GOT entry (rather
21 /// then needing a GOT entry).
22 ///
23 /// There is one optimization the linker can do here. If the target of the GOT
24 /// is in the same linkage unit and does not need to be interposable, and
25 /// the GOT use is just a load (not some other operation), this pass can
26 /// transform that load into an LEA (add). This optimizes away one memory load
27 /// which at runtime that could stall the pipeline. This optimization only
28 /// works for architectures in which a (GOT) load instruction can be change to
29 /// an LEA instruction that is the same size. The method isGOTAccess() should
30 /// only return true for "canBypassGOT" if this optimization is supported.
31 ///
32 //===----------------------------------------------------------------------===//
33
34 #include "ArchHandler.h"
35 #include "File.h"
36 #include "MachOPasses.h"
37 #include "lld/Common/LLVM.h"
38 #include "lld/Core/DefinedAtom.h"
39 #include "lld/Core/File.h"
40 #include "lld/Core/Reference.h"
41 #include "lld/Core/Simple.h"
42 #include "llvm/ADT/DenseMap.h"
43 #include "llvm/ADT/STLExtras.h"
44
45 namespace lld {
46 namespace mach_o {
47
48 //
49 // GOT Entry Atom created by the GOT pass.
50 //
51 class GOTEntryAtom : public SimpleDefinedAtom {
52 public:
GOTEntryAtom(const File & file,bool is64,StringRef name)53 GOTEntryAtom(const File &file, bool is64, StringRef name)
54 : SimpleDefinedAtom(file), _is64(is64), _name(name) { }
55
56 ~GOTEntryAtom() override = default;
57
contentType() const58 ContentType contentType() const override {
59 return DefinedAtom::typeGOT;
60 }
61
alignment() const62 Alignment alignment() const override {
63 return _is64 ? 8 : 4;
64 }
65
size() const66 uint64_t size() const override {
67 return _is64 ? 8 : 4;
68 }
69
permissions() const70 ContentPermissions permissions() const override {
71 return DefinedAtom::permRW_;
72 }
73
rawContent() const74 ArrayRef<uint8_t> rawContent() const override {
75 static const uint8_t zeros[] =
76 { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
77 return llvm::makeArrayRef(zeros, size());
78 }
79
slotName() const80 StringRef slotName() const {
81 return _name;
82 }
83
84 private:
85 const bool _is64;
86 StringRef _name;
87 };
88
89 /// Pass for instantiating and optimizing GOT slots.
90 ///
91 class GOTPass : public Pass {
92 public:
GOTPass(const MachOLinkingContext & context)93 GOTPass(const MachOLinkingContext &context)
94 : _ctx(context), _archHandler(_ctx.archHandler()),
95 _file(*_ctx.make_file<MachOFile>("<mach-o GOT Pass>")) {
96 _file.setOrdinal(_ctx.getNextOrdinalAndIncrement());
97 }
98
99 private:
perform(SimpleFile & mergedFile)100 llvm::Error perform(SimpleFile &mergedFile) override {
101 // Scan all references in all atoms.
102 for (const DefinedAtom *atom : mergedFile.defined()) {
103 for (const Reference *ref : *atom) {
104 // Look at instructions accessing the GOT.
105 bool canBypassGOT;
106 if (!_archHandler.isGOTAccess(*ref, canBypassGOT))
107 continue;
108 const Atom *target = ref->target();
109 assert(target != nullptr);
110
111 if (!shouldReplaceTargetWithGOTAtom(target, canBypassGOT)) {
112 // Update reference kind to reflect that target is a direct access.
113 _archHandler.updateReferenceToGOT(ref, false);
114 } else {
115 // Replace the target with a reference to a GOT entry.
116 const DefinedAtom *gotEntry = makeGOTEntry(target);
117 const_cast<Reference *>(ref)->setTarget(gotEntry);
118 // Update reference kind to reflect that target is now a GOT entry.
119 _archHandler.updateReferenceToGOT(ref, true);
120 }
121 }
122 }
123
124 // Sort and add all created GOT Atoms to master file
125 std::vector<const GOTEntryAtom *> entries;
126 entries.reserve(_targetToGOT.size());
127 for (auto &it : _targetToGOT)
128 entries.push_back(it.second);
129 std::sort(entries.begin(), entries.end(),
130 [](const GOTEntryAtom *left, const GOTEntryAtom *right) {
131 return (left->slotName().compare(right->slotName()) < 0);
132 });
133 for (const GOTEntryAtom *slot : entries)
134 mergedFile.addAtom(*slot);
135
136 return llvm::Error::success();
137 }
138
shouldReplaceTargetWithGOTAtom(const Atom * target,bool canBypassGOT)139 bool shouldReplaceTargetWithGOTAtom(const Atom *target, bool canBypassGOT) {
140 // Accesses to shared library symbols must go through GOT.
141 if (isa<SharedLibraryAtom>(target))
142 return true;
143 // Accesses to interposable symbols in same linkage unit must also go
144 // through GOT.
145 const DefinedAtom *defTarget = dyn_cast<DefinedAtom>(target);
146 if (defTarget != nullptr &&
147 defTarget->interposable() != DefinedAtom::interposeNo) {
148 assert(defTarget->scope() != DefinedAtom::scopeTranslationUnit);
149 return true;
150 }
151 // Target does not require indirection. So, if instruction allows GOT to be
152 // by-passed, do that optimization and don't create GOT entry.
153 return !canBypassGOT;
154 }
155
makeGOTEntry(const Atom * target)156 const DefinedAtom *makeGOTEntry(const Atom *target) {
157 auto pos = _targetToGOT.find(target);
158 if (pos == _targetToGOT.end()) {
159 auto *gotEntry = new (_file.allocator())
160 GOTEntryAtom(_file, _ctx.is64Bit(), target->name());
161 _targetToGOT[target] = gotEntry;
162 const ArchHandler::ReferenceInfo &nlInfo = _archHandler.stubInfo().
163 nonLazyPointerReferenceToBinder;
164 gotEntry->addReference(Reference::KindNamespace::mach_o, nlInfo.arch,
165 nlInfo.kind, 0, target, 0);
166 return gotEntry;
167 }
168 return pos->second;
169 }
170
171 const MachOLinkingContext &_ctx;
172 mach_o::ArchHandler &_archHandler;
173 MachOFile &_file;
174 llvm::DenseMap<const Atom*, const GOTEntryAtom*> _targetToGOT;
175 };
176
addGOTPass(PassManager & pm,const MachOLinkingContext & ctx)177 void addGOTPass(PassManager &pm, const MachOLinkingContext &ctx) {
178 assert(ctx.needsGOTPass());
179 pm.add(std::make_unique<GOTPass>(ctx));
180 }
181
182 } // end namespace mach_o
183 } // end namespace lld
184