1 //===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This transform is designed to eliminate unreachable internal globals from the
11 // program. It uses an aggressive algorithm, searching out globals that are
12 // known to be alive. After it finds all of the globals which are needed, it
13 // deletes whatever is left over. This allows it to delete recursive chunks of
14 // the program which are unreachable.
16 //===----------------------------------------------------------------------===//
18 #include "llvm/Transforms/IPO.h"
19 #include "llvm/ADT/SmallPtrSet.h"
20 #include "llvm/ADT/Statistic.h"
21 #include "llvm/IR/Constants.h"
22 #include "llvm/IR/Instructions.h"
23 #include "llvm/IR/Module.h"
24 #include "llvm/Transforms/Utils/CtorUtils.h"
25 #include "llvm/Pass.h"
28 #define DEBUG_TYPE "globaldce"
30 STATISTIC(NumAliases , "Number of global aliases removed");
31 STATISTIC(NumFunctions, "Number of functions removed");
32 STATISTIC(NumVariables, "Number of global variables removed");
35 struct GlobalDCE : public ModulePass {
36 static char ID; // Pass identification, replacement for typeid
37 GlobalDCE() : ModulePass(ID) {
38 initializeGlobalDCEPass(*PassRegistry::getPassRegistry());
41 // run - Do the GlobalDCE pass on the specified module, optionally updating
42 // the specified callgraph to reflect the changes.
44 bool runOnModule(Module &M) override;
47 SmallPtrSet<Constant *, 32> AliveGlobals;
48 SmallPtrSet<Constant *, 8> SeenConstants;
49 SmallPtrSet<GlobalVariable *, 8> DiscardableGlobalInitializers;
51 /// GlobalIsNeeded - mark the specific global value as needed, and
52 /// recursively mark anything that it uses as also needed.
53 void GlobalIsNeeded(GlobalValue *GV);
54 void MarkUsedGlobalsAsNeeded(Constant *C);
56 /// \brief Checks if C is alive or is a ConstantExpr that refers to an alive
58 bool ContainsUsedGlobal(Constant *C);
60 bool RemoveUnusedGlobalValue(GlobalValue &GV);
64 /// Returns true if F contains only a single "ret" instruction.
65 static bool isEmptyFunction(Function *F) {
66 BasicBlock &Entry = F->getEntryBlock();
67 if (Entry.size() != 1 || !isa<ReturnInst>(Entry.front()))
69 ReturnInst &RI = cast<ReturnInst>(Entry.front());
70 return RI.getReturnValue() == nullptr;
73 char GlobalDCE::ID = 0;
74 INITIALIZE_PASS(GlobalDCE, "globaldce",
75 "Dead Global Elimination", false, false)
77 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
79 bool GlobalDCE::runOnModule(Module &M) {
82 // Remove empty functions from the global ctors list.
83 Changed |= optimizeGlobalCtorsList(M, isEmptyFunction);
85 typedef std::multimap<const Comdat *, GlobalValue *> ComdatGVPairsTy;
86 ComdatGVPairsTy ComdatGVPairs;
88 // Loop over the module, adding globals which are obviously necessary.
89 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
90 Changed |= RemoveUnusedGlobalValue(*I);
91 // Functions with external linkage are needed if they have a body
92 if (!I->isDeclaration() && !I->hasAvailableExternallyLinkage()) {
93 if (!I->isDiscardableIfUnused())
95 else if (const Comdat *C = I->getComdat())
96 ComdatGVPairs.insert(std::make_pair(C, I));
100 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
102 Changed |= RemoveUnusedGlobalValue(*I);
103 // Externally visible & appending globals are needed, if they have an
105 if (!I->isDeclaration() && !I->hasAvailableExternallyLinkage()) {
106 if (!I->isDiscardableIfUnused())
108 else if (const Comdat *C = I->getComdat())
109 ComdatGVPairs.insert(std::make_pair(C, I));
113 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
115 Changed |= RemoveUnusedGlobalValue(*I);
116 // Externally visible aliases are needed.
117 if (!I->isDiscardableIfUnused()) {
119 } else if (const Comdat *C = I->getComdat()) {
120 ComdatGVPairs.insert(std::make_pair(C, I));
124 for (ComdatGVPairsTy::iterator I = ComdatGVPairs.begin(),
125 E = ComdatGVPairs.end();
127 ComdatGVPairsTy::iterator UB = ComdatGVPairs.upper_bound(I->first);
128 bool CanDiscard = std::all_of(I, UB, [](ComdatGVPairsTy::value_type Pair) {
129 return Pair.second->isDiscardableIfUnused();
132 std::for_each(I, UB, [this](ComdatGVPairsTy::value_type Pair) {
133 GlobalIsNeeded(Pair.second);
139 // Now that all globals which are needed are in the AliveGlobals set, we loop
140 // through the program, deleting those which are not alive.
143 // The first pass is to drop initializers of global variables which are dead.
144 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals
145 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
147 if (!AliveGlobals.count(I)) {
148 DeadGlobalVars.push_back(I); // Keep track of dead globals
149 I->setInitializer(nullptr);
152 // The second pass drops the bodies of functions which are dead...
153 std::vector<Function*> DeadFunctions;
154 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
155 if (!AliveGlobals.count(I)) {
156 DeadFunctions.push_back(I); // Keep track of dead globals
157 if (!I->isDeclaration())
161 // The third pass drops targets of aliases which are dead...
162 std::vector<GlobalAlias*> DeadAliases;
163 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
165 if (!AliveGlobals.count(I)) {
166 DeadAliases.push_back(I);
167 I->setAliasee(nullptr);
170 // Look for available externally constants that we can turn into normal
171 // externals by deleting their initalizers. This allows us to remove other
172 // globals that are referenced by the initializer.
173 if (!DiscardableGlobalInitializers.empty()) {
174 for (GlobalVariable *GV : DiscardableGlobalInitializers) {
175 if (!ContainsUsedGlobal(GV->getInitializer())) {
176 GV->setInitializer(nullptr);
177 GV->setLinkage(GlobalValue::ExternalLinkage);
183 if (!DeadFunctions.empty()) {
184 // Now that all interferences have been dropped, delete the actual objects
186 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
187 RemoveUnusedGlobalValue(*DeadFunctions[i]);
188 M.getFunctionList().erase(DeadFunctions[i]);
190 NumFunctions += DeadFunctions.size();
194 if (!DeadGlobalVars.empty()) {
195 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
196 RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
197 M.getGlobalList().erase(DeadGlobalVars[i]);
199 NumVariables += DeadGlobalVars.size();
203 // Now delete any dead aliases.
204 if (!DeadAliases.empty()) {
205 for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) {
206 RemoveUnusedGlobalValue(*DeadAliases[i]);
207 M.getAliasList().erase(DeadAliases[i]);
209 NumAliases += DeadAliases.size();
213 // Make sure that all memory is released
214 AliveGlobals.clear();
215 SeenConstants.clear();
220 /// GlobalIsNeeded - the specific global value as needed, and
221 /// recursively mark anything that it uses as also needed.
222 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
223 // If the global is already in the set, no need to reprocess it.
224 if (!AliveGlobals.insert(G))
227 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
228 // If this is a global variable, we must make sure to add any global values
229 // referenced by the initializer to the alive set.
230 if (GV->hasInitializer()) {
231 if (GV->hasAvailableExternallyLinkage())
232 DiscardableGlobalInitializers.insert(GV);
234 MarkUsedGlobalsAsNeeded(GV->getInitializer());
236 } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
237 // The target of a global alias is needed.
238 MarkUsedGlobalsAsNeeded(GA->getAliasee());
240 // Otherwise this must be a function object. We have to scan the body of
241 // the function looking for constants and global values which are used as
242 // operands. Any operands of these types must be processed to ensure that
243 // any globals used will be marked as needed.
244 Function *F = cast<Function>(G);
246 if (F->hasPrefixData())
247 MarkUsedGlobalsAsNeeded(F->getPrefixData());
249 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
250 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
251 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
252 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
254 else if (Constant *C = dyn_cast<Constant>(*U))
255 MarkUsedGlobalsAsNeeded(C);
259 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
260 if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
261 return GlobalIsNeeded(GV);
263 // Loop over all of the operands of the constant, adding any globals they
264 // use to the list of needed globals.
265 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) {
266 // If we've already processed this constant there's no need to do it again.
267 Constant *Op = dyn_cast<Constant>(*I);
268 if (Op && SeenConstants.insert(Op))
269 MarkUsedGlobalsAsNeeded(Op);
273 bool GlobalDCE::ContainsUsedGlobal(Constant *C) {
274 // C contains a used global If C is alive or we visited it while marking
276 if (AliveGlobals.count(C) || SeenConstants.count(C))
279 // Now check all operands of a ConstantExpr.
280 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) {
281 Constant *Op = dyn_cast<Constant>(*I);
282 if (Op && ContainsUsedGlobal(Op))
288 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
289 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
290 // If found, check to see if the constant pointer ref is safe to destroy, and if
291 // so, nuke it. This will reduce the reference count on the global value, which
292 // might make it deader.
294 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
295 if (GV.use_empty()) return false;
296 GV.removeDeadConstantUsers();
297 return GV.use_empty();