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/Transforms/Utils/GlobalStatus.h"
26 #include "llvm/Pass.h"
29 #define DEBUG_TYPE "globaldce"
31 STATISTIC(NumAliases , "Number of global aliases removed");
32 STATISTIC(NumFunctions, "Number of functions removed");
33 STATISTIC(NumVariables, "Number of global variables removed");
36 struct GlobalDCE : public ModulePass {
37 static char ID; // Pass identification, replacement for typeid
38 GlobalDCE() : ModulePass(ID) {
39 initializeGlobalDCEPass(*PassRegistry::getPassRegistry());
42 // run - Do the GlobalDCE pass on the specified module, optionally updating
43 // the specified callgraph to reflect the changes.
45 bool runOnModule(Module &M) override;
48 SmallPtrSet<GlobalValue*, 32> AliveGlobals;
49 SmallPtrSet<Constant *, 8> SeenConstants;
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 bool RemoveUnusedGlobalValue(GlobalValue &GV);
60 /// Returns true if F contains only a single "ret" instruction.
61 static bool isEmptyFunction(Function *F) {
62 BasicBlock &Entry = F->getEntryBlock();
63 if (Entry.size() != 1 || !isa<ReturnInst>(Entry.front()))
65 ReturnInst &RI = cast<ReturnInst>(Entry.front());
66 return RI.getReturnValue() == nullptr;
69 char GlobalDCE::ID = 0;
70 INITIALIZE_PASS(GlobalDCE, "globaldce",
71 "Dead Global Elimination", false, false)
73 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
75 bool GlobalDCE::runOnModule(Module &M) {
78 // Remove empty functions from the global ctors list.
79 Changed |= optimizeGlobalCtorsList(M, isEmptyFunction);
81 typedef std::multimap<const Comdat *, GlobalValue *> ComdatGVPairsTy;
82 ComdatGVPairsTy ComdatGVPairs;
84 // Loop over the module, adding globals which are obviously necessary.
85 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
86 Changed |= RemoveUnusedGlobalValue(*I);
87 // Functions with external linkage are needed if they have a body
88 if (!I->isDeclaration() && !I->hasAvailableExternallyLinkage()) {
89 if (!I->isDiscardableIfUnused())
91 else if (const Comdat *C = I->getComdat())
92 ComdatGVPairs.insert(std::make_pair(C, I));
96 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
98 Changed |= RemoveUnusedGlobalValue(*I);
99 // Externally visible & appending globals are needed, if they have an
101 if (!I->isDeclaration() && !I->hasAvailableExternallyLinkage()) {
102 if (!I->isDiscardableIfUnused())
104 else if (const Comdat *C = I->getComdat())
105 ComdatGVPairs.insert(std::make_pair(C, I));
109 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
111 Changed |= RemoveUnusedGlobalValue(*I);
112 // Externally visible aliases are needed.
113 if (!I->isDiscardableIfUnused()) {
115 } else if (const Comdat *C = I->getComdat()) {
116 ComdatGVPairs.insert(std::make_pair(C, I));
120 for (ComdatGVPairsTy::iterator I = ComdatGVPairs.begin(),
121 E = ComdatGVPairs.end();
123 ComdatGVPairsTy::iterator UB = ComdatGVPairs.upper_bound(I->first);
124 bool CanDiscard = std::all_of(I, UB, [](ComdatGVPairsTy::value_type Pair) {
125 return Pair.second->isDiscardableIfUnused();
128 std::for_each(I, UB, [this](ComdatGVPairsTy::value_type Pair) {
129 GlobalIsNeeded(Pair.second);
135 // Now that all globals which are needed are in the AliveGlobals set, we loop
136 // through the program, deleting those which are not alive.
139 // The first pass is to drop initializers of global variables which are dead.
140 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals
141 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
143 if (!AliveGlobals.count(I)) {
144 DeadGlobalVars.push_back(I); // Keep track of dead globals
145 if (I->hasInitializer()) {
146 Constant *Init = I->getInitializer();
147 I->setInitializer(nullptr);
148 if (isSafeToDestroyConstant(Init))
149 Init->destroyConstant();
153 // The second pass drops the bodies of functions which are dead...
154 std::vector<Function*> DeadFunctions;
155 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
156 if (!AliveGlobals.count(I)) {
157 DeadFunctions.push_back(I); // Keep track of dead globals
158 if (!I->isDeclaration())
162 // The third pass drops targets of aliases which are dead...
163 std::vector<GlobalAlias*> DeadAliases;
164 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
166 if (!AliveGlobals.count(I)) {
167 DeadAliases.push_back(I);
168 I->setAliasee(nullptr);
171 if (!DeadFunctions.empty()) {
172 // Now that all interferences have been dropped, delete the actual objects
174 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
175 RemoveUnusedGlobalValue(*DeadFunctions[i]);
176 M.getFunctionList().erase(DeadFunctions[i]);
178 NumFunctions += DeadFunctions.size();
182 if (!DeadGlobalVars.empty()) {
183 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
184 RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
185 M.getGlobalList().erase(DeadGlobalVars[i]);
187 NumVariables += DeadGlobalVars.size();
191 // Now delete any dead aliases.
192 if (!DeadAliases.empty()) {
193 for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) {
194 RemoveUnusedGlobalValue(*DeadAliases[i]);
195 M.getAliasList().erase(DeadAliases[i]);
197 NumAliases += DeadAliases.size();
201 // Make sure that all memory is released
202 AliveGlobals.clear();
203 SeenConstants.clear();
208 /// GlobalIsNeeded - the specific global value as needed, and
209 /// recursively mark anything that it uses as also needed.
210 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
211 // If the global is already in the set, no need to reprocess it.
212 if (!AliveGlobals.insert(G))
215 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
216 // If this is a global variable, we must make sure to add any global values
217 // referenced by the initializer to the alive set.
218 if (GV->hasInitializer())
219 MarkUsedGlobalsAsNeeded(GV->getInitializer());
220 } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
221 // The target of a global alias is needed.
222 MarkUsedGlobalsAsNeeded(GA->getAliasee());
224 // Otherwise this must be a function object. We have to scan the body of
225 // the function looking for constants and global values which are used as
226 // operands. Any operands of these types must be processed to ensure that
227 // any globals used will be marked as needed.
228 Function *F = cast<Function>(G);
230 if (F->hasPrefixData())
231 MarkUsedGlobalsAsNeeded(F->getPrefixData());
233 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
234 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
235 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
236 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
238 else if (Constant *C = dyn_cast<Constant>(*U))
239 MarkUsedGlobalsAsNeeded(C);
243 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
244 if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
245 return GlobalIsNeeded(GV);
247 // Loop over all of the operands of the constant, adding any globals they
248 // use to the list of needed globals.
249 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) {
250 // If we've already processed this constant there's no need to do it again.
251 Constant *Op = dyn_cast<Constant>(*I);
252 if (Op && SeenConstants.insert(Op))
253 MarkUsedGlobalsAsNeeded(Op);
257 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
258 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
259 // If found, check to see if the constant pointer ref is safe to destroy, and if
260 // so, nuke it. This will reduce the reference count on the global value, which
261 // might make it deader.
263 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
264 if (GV.use_empty()) return false;
265 GV.removeDeadConstantUsers();
266 return GV.use_empty();