1 //===-- Internalize.cpp - Mark functions internal -------------------------===//
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 // This pass loops over all of the functions and variables in the input module.
10 // If the function or variable does not need to be preserved according to the
11 // client supplied callback, it is marked as internal.
12 //
13 // This transformation would not be legal in a regular compilation, but it gets
14 // extra information from the linker about what is safe.
15 //
16 // For example: Internalizing a function with external linkage. Only if we are
17 // told it is only used from within this module, it is safe to do it.
18 //
19 //===----------------------------------------------------------------------===//
20
21 #include "llvm/Transforms/IPO/Internalize.h"
22 #include "llvm/ADT/SmallPtrSet.h"
23 #include "llvm/ADT/Statistic.h"
24 #include "llvm/ADT/StringSet.h"
25 #include "llvm/Analysis/CallGraph.h"
26 #include "llvm/IR/Module.h"
27 #include "llvm/InitializePasses.h"
28 #include "llvm/Pass.h"
29 #include "llvm/Support/CommandLine.h"
30 #include "llvm/Support/Debug.h"
31 #include "llvm/Support/LineIterator.h"
32 #include "llvm/Support/MemoryBuffer.h"
33 #include "llvm/Support/raw_ostream.h"
34 #include "llvm/Transforms/IPO.h"
35 #include "llvm/Transforms/Utils/GlobalStatus.h"
36 using namespace llvm;
37
38 #define DEBUG_TYPE "internalize"
39
40 STATISTIC(NumAliases, "Number of aliases internalized");
41 STATISTIC(NumFunctions, "Number of functions internalized");
42 STATISTIC(NumGlobals, "Number of global vars internalized");
43
44 // APIFile - A file which contains a list of symbols that should not be marked
45 // external.
46 static cl::opt<std::string>
47 APIFile("internalize-public-api-file", cl::value_desc("filename"),
48 cl::desc("A file containing list of symbol names to preserve"));
49
50 // APIList - A list of symbols that should not be marked internal.
51 static cl::list<std::string>
52 APIList("internalize-public-api-list", cl::value_desc("list"),
53 cl::desc("A list of symbol names to preserve"), cl::CommaSeparated);
54
55 namespace {
56 // Helper to load an API list to preserve from file and expose it as a functor
57 // for internalization.
58 class PreserveAPIList {
59 public:
PreserveAPIList()60 PreserveAPIList() {
61 if (!APIFile.empty())
62 LoadFile(APIFile);
63 ExternalNames.insert(APIList.begin(), APIList.end());
64 }
65
operator ()(const GlobalValue & GV)66 bool operator()(const GlobalValue &GV) {
67 return ExternalNames.count(GV.getName());
68 }
69
70 private:
71 // Contains the set of symbols loaded from file
72 StringSet<> ExternalNames;
73
LoadFile(StringRef Filename)74 void LoadFile(StringRef Filename) {
75 // Load the APIFile...
76 ErrorOr<std::unique_ptr<MemoryBuffer>> Buf =
77 MemoryBuffer::getFile(Filename);
78 if (!Buf) {
79 errs() << "WARNING: Internalize couldn't load file '" << Filename
80 << "'! Continuing as if it's empty.\n";
81 return; // Just continue as if the file were empty
82 }
83 for (line_iterator I(*Buf->get(), true), E; I != E; ++I)
84 ExternalNames.insert(*I);
85 }
86 };
87 } // end anonymous namespace
88
shouldPreserveGV(const GlobalValue & GV)89 bool InternalizePass::shouldPreserveGV(const GlobalValue &GV) {
90 // Function must be defined here
91 if (GV.isDeclaration())
92 return true;
93
94 // Available externally is really just a "declaration with a body".
95 if (GV.hasAvailableExternallyLinkage())
96 return true;
97
98 // Assume that dllexported symbols are referenced elsewhere
99 if (GV.hasDLLExportStorageClass())
100 return true;
101
102 // Already local, has nothing to do.
103 if (GV.hasLocalLinkage())
104 return false;
105
106 // Check some special cases
107 if (AlwaysPreserved.count(GV.getName()))
108 return true;
109
110 return MustPreserveGV(GV);
111 }
112
maybeInternalize(GlobalValue & GV,const DenseSet<const Comdat * > & ExternalComdats)113 bool InternalizePass::maybeInternalize(
114 GlobalValue &GV, const DenseSet<const Comdat *> &ExternalComdats) {
115 if (Comdat *C = GV.getComdat()) {
116 if (ExternalComdats.count(C))
117 return false;
118
119 // If a comdat is not externally visible we can drop it.
120 if (auto GO = dyn_cast<GlobalObject>(&GV))
121 GO->setComdat(nullptr);
122
123 if (GV.hasLocalLinkage())
124 return false;
125 } else {
126 if (GV.hasLocalLinkage())
127 return false;
128
129 if (shouldPreserveGV(GV))
130 return false;
131 }
132
133 GV.setVisibility(GlobalValue::DefaultVisibility);
134 GV.setLinkage(GlobalValue::InternalLinkage);
135 return true;
136 }
137
138 // If GV is part of a comdat and is externally visible, keep track of its
139 // comdat so that we don't internalize any of its members.
checkComdatVisibility(GlobalValue & GV,DenseSet<const Comdat * > & ExternalComdats)140 void InternalizePass::checkComdatVisibility(
141 GlobalValue &GV, DenseSet<const Comdat *> &ExternalComdats) {
142 Comdat *C = GV.getComdat();
143 if (!C)
144 return;
145
146 if (shouldPreserveGV(GV))
147 ExternalComdats.insert(C);
148 }
149
internalizeModule(Module & M,CallGraph * CG)150 bool InternalizePass::internalizeModule(Module &M, CallGraph *CG) {
151 bool Changed = false;
152 CallGraphNode *ExternalNode = CG ? CG->getExternalCallingNode() : nullptr;
153
154 SmallPtrSet<GlobalValue *, 8> Used;
155 collectUsedGlobalVariables(M, Used, false);
156
157 // Collect comdat visiblity information for the module.
158 DenseSet<const Comdat *> ExternalComdats;
159 if (!M.getComdatSymbolTable().empty()) {
160 for (Function &F : M)
161 checkComdatVisibility(F, ExternalComdats);
162 for (GlobalVariable &GV : M.globals())
163 checkComdatVisibility(GV, ExternalComdats);
164 for (GlobalAlias &GA : M.aliases())
165 checkComdatVisibility(GA, ExternalComdats);
166 }
167
168 // We must assume that globals in llvm.used have a reference that not even
169 // the linker can see, so we don't internalize them.
170 // For llvm.compiler.used the situation is a bit fuzzy. The assembler and
171 // linker can drop those symbols. If this pass is running as part of LTO,
172 // one might think that it could just drop llvm.compiler.used. The problem
173 // is that even in LTO llvm doesn't see every reference. For example,
174 // we don't see references from function local inline assembly. To be
175 // conservative, we internalize symbols in llvm.compiler.used, but we
176 // keep llvm.compiler.used so that the symbol is not deleted by llvm.
177 for (GlobalValue *V : Used) {
178 AlwaysPreserved.insert(V->getName());
179 }
180
181 // Mark all functions not in the api as internal.
182 for (Function &I : M) {
183 if (!maybeInternalize(I, ExternalComdats))
184 continue;
185 Changed = true;
186
187 if (ExternalNode)
188 // Remove a callgraph edge from the external node to this function.
189 ExternalNode->removeOneAbstractEdgeTo((*CG)[&I]);
190
191 ++NumFunctions;
192 LLVM_DEBUG(dbgs() << "Internalizing func " << I.getName() << "\n");
193 }
194
195 // Never internalize the llvm.used symbol. It is used to implement
196 // attribute((used)).
197 // FIXME: Shouldn't this just filter on llvm.metadata section??
198 AlwaysPreserved.insert("llvm.used");
199 AlwaysPreserved.insert("llvm.compiler.used");
200
201 // Never internalize anchors used by the machine module info, else the info
202 // won't find them. (see MachineModuleInfo.)
203 AlwaysPreserved.insert("llvm.global_ctors");
204 AlwaysPreserved.insert("llvm.global_dtors");
205 AlwaysPreserved.insert("llvm.global.annotations");
206
207 // Never internalize symbols code-gen inserts.
208 // FIXME: We should probably add this (and the __stack_chk_guard) via some
209 // type of call-back in CodeGen.
210 AlwaysPreserved.insert("__stack_chk_fail");
211 AlwaysPreserved.insert("__stack_chk_guard");
212
213 // Mark all global variables with initializers that are not in the api as
214 // internal as well.
215 for (auto &GV : M.globals()) {
216 if (!maybeInternalize(GV, ExternalComdats))
217 continue;
218 Changed = true;
219
220 ++NumGlobals;
221 LLVM_DEBUG(dbgs() << "Internalized gvar " << GV.getName() << "\n");
222 }
223
224 // Mark all aliases that are not in the api as internal as well.
225 for (auto &GA : M.aliases()) {
226 if (!maybeInternalize(GA, ExternalComdats))
227 continue;
228 Changed = true;
229
230 ++NumAliases;
231 LLVM_DEBUG(dbgs() << "Internalized alias " << GA.getName() << "\n");
232 }
233
234 return Changed;
235 }
236
InternalizePass()237 InternalizePass::InternalizePass() : MustPreserveGV(PreserveAPIList()) {}
238
run(Module & M,ModuleAnalysisManager & AM)239 PreservedAnalyses InternalizePass::run(Module &M, ModuleAnalysisManager &AM) {
240 if (!internalizeModule(M, AM.getCachedResult<CallGraphAnalysis>(M)))
241 return PreservedAnalyses::all();
242
243 PreservedAnalyses PA;
244 PA.preserve<CallGraphAnalysis>();
245 return PA;
246 }
247
248 namespace {
249 class InternalizeLegacyPass : public ModulePass {
250 // Client supplied callback to control wheter a symbol must be preserved.
251 std::function<bool(const GlobalValue &)> MustPreserveGV;
252
253 public:
254 static char ID; // Pass identification, replacement for typeid
255
InternalizeLegacyPass()256 InternalizeLegacyPass() : ModulePass(ID), MustPreserveGV(PreserveAPIList()) {}
257
InternalizeLegacyPass(std::function<bool (const GlobalValue &)> MustPreserveGV)258 InternalizeLegacyPass(std::function<bool(const GlobalValue &)> MustPreserveGV)
259 : ModulePass(ID), MustPreserveGV(std::move(MustPreserveGV)) {
260 initializeInternalizeLegacyPassPass(*PassRegistry::getPassRegistry());
261 }
262
runOnModule(Module & M)263 bool runOnModule(Module &M) override {
264 if (skipModule(M))
265 return false;
266
267 CallGraphWrapperPass *CGPass =
268 getAnalysisIfAvailable<CallGraphWrapperPass>();
269 CallGraph *CG = CGPass ? &CGPass->getCallGraph() : nullptr;
270 return internalizeModule(M, MustPreserveGV, CG);
271 }
272
getAnalysisUsage(AnalysisUsage & AU) const273 void getAnalysisUsage(AnalysisUsage &AU) const override {
274 AU.setPreservesCFG();
275 AU.addPreserved<CallGraphWrapperPass>();
276 }
277 };
278 }
279
280 char InternalizeLegacyPass::ID = 0;
281 INITIALIZE_PASS(InternalizeLegacyPass, "internalize",
282 "Internalize Global Symbols", false, false)
283
createInternalizePass()284 ModulePass *llvm::createInternalizePass() {
285 return new InternalizeLegacyPass();
286 }
287
createInternalizePass(std::function<bool (const GlobalValue &)> MustPreserveGV)288 ModulePass *llvm::createInternalizePass(
289 std::function<bool(const GlobalValue &)> MustPreserveGV) {
290 return new InternalizeLegacyPass(std::move(MustPreserveGV));
291 }
292