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/GlobalDCE.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/Pass.h"
25 #include "llvm/Transforms/IPO.h"
26 #include "llvm/Transforms/Utils/CtorUtils.h"
27 #include "llvm/Transforms/Utils/GlobalStatus.h"
31 #define DEBUG_TYPE "globaldce"
33 STATISTIC(NumAliases , "Number of global aliases removed");
34 STATISTIC(NumFunctions, "Number of functions removed");
35 STATISTIC(NumIFuncs, "Number of indirect functions removed");
36 STATISTIC(NumVariables, "Number of global variables removed");
39 class GlobalDCELegacyPass : public ModulePass {
41 static char ID; // Pass identification, replacement for typeid
42 GlobalDCELegacyPass() : ModulePass(ID) {
43 initializeGlobalDCELegacyPassPass(*PassRegistry::getPassRegistry());
46 // run - Do the GlobalDCE pass on the specified module, optionally updating
47 // the specified callgraph to reflect the changes.
49 bool runOnModule(Module &M) override {
53 // We need a minimally functional dummy module analysis manager. It needs
54 // to at least know about the possibility of proxying a function analysis
56 FunctionAnalysisManager DummyFAM;
57 ModuleAnalysisManager DummyMAM;
58 DummyMAM.registerPass(
59 [&] { return FunctionAnalysisManagerModuleProxy(DummyFAM); });
61 auto PA = Impl.run(M, DummyMAM);
62 return !PA.areAllPreserved();
70 char GlobalDCELegacyPass::ID = 0;
71 INITIALIZE_PASS(GlobalDCELegacyPass, "globaldce",
72 "Dead Global Elimination", false, false)
74 // Public interface to the GlobalDCEPass.
75 ModulePass *llvm::createGlobalDCEPass() {
76 return new GlobalDCELegacyPass();
79 /// Returns true if F contains only a single "ret" instruction.
80 static bool isEmptyFunction(Function *F) {
81 BasicBlock &Entry = F->getEntryBlock();
82 if (Entry.size() != 1 || !isa<ReturnInst>(Entry.front()))
84 ReturnInst &RI = cast<ReturnInst>(Entry.front());
85 return RI.getReturnValue() == nullptr;
88 /// Compute the set of GlobalValue that depends from V.
89 /// The recursion stops as soon as a GlobalValue is met.
90 void GlobalDCEPass::ComputeDependencies(Value *V,
91 SmallPtrSetImpl<GlobalValue *> &Deps) {
92 if (auto *I = dyn_cast<Instruction>(V)) {
93 Function *Parent = I->getParent()->getParent();
95 } else if (auto *GV = dyn_cast<GlobalValue>(V)) {
97 } else if (auto *CE = dyn_cast<Constant>(V)) {
98 // Avoid walking the whole tree of a big ConstantExprs multiple times.
99 auto Where = ConstantDependenciesCache.find(CE);
100 if (Where != ConstantDependenciesCache.end()) {
101 auto const &K = Where->second;
102 Deps.insert(K.begin(), K.end());
104 SmallPtrSetImpl<GlobalValue *> &LocalDeps = ConstantDependenciesCache[CE];
105 for (User *CEUser : CE->users())
106 ComputeDependencies(CEUser, LocalDeps);
107 Deps.insert(LocalDeps.begin(), LocalDeps.end());
112 void GlobalDCEPass::UpdateGVDependencies(GlobalValue &GV) {
113 SmallPtrSet<GlobalValue *, 8> Deps;
114 for (User *User : GV.users())
115 ComputeDependencies(User, Deps);
116 Deps.erase(&GV); // Remove self-reference.
117 for (GlobalValue *GVU : Deps) {
118 GVDependencies.insert(std::make_pair(GVU, &GV));
122 /// Mark Global value as Live
123 void GlobalDCEPass::MarkLive(GlobalValue &GV,
124 SmallVectorImpl<GlobalValue *> *Updates) {
125 auto const Ret = AliveGlobals.insert(&GV);
130 Updates->push_back(&GV);
131 if (Comdat *C = GV.getComdat()) {
132 for (auto &&CM : make_range(ComdatMembers.equal_range(C)))
133 MarkLive(*CM.second, Updates); // Recursion depth is only two because only
134 // globals in the same comdat are visited.
138 PreservedAnalyses GlobalDCEPass::run(Module &M, ModuleAnalysisManager &MAM) {
139 bool Changed = false;
141 // The algorithm first computes the set L of global variables that are
142 // trivially live. Then it walks the initialization of these variables to
143 // compute the globals used to initialize them, which effectively builds a
144 // directed graph where nodes are global variables, and an edge from A to B
145 // means B is used to initialize A. Finally, it propagates the liveness
146 // information through the graph starting from the nodes in L. Nodes note
147 // marked as alive are discarded.
149 // Remove empty functions from the global ctors list.
150 Changed |= optimizeGlobalCtorsList(M, isEmptyFunction);
152 // Collect the set of members for each comdat.
153 for (Function &F : M)
154 if (Comdat *C = F.getComdat())
155 ComdatMembers.insert(std::make_pair(C, &F));
156 for (GlobalVariable &GV : M.globals())
157 if (Comdat *C = GV.getComdat())
158 ComdatMembers.insert(std::make_pair(C, &GV));
159 for (GlobalAlias &GA : M.aliases())
160 if (Comdat *C = GA.getComdat())
161 ComdatMembers.insert(std::make_pair(C, &GA));
163 // Loop over the module, adding globals which are obviously necessary.
164 for (GlobalObject &GO : M.global_objects()) {
165 Changed |= RemoveUnusedGlobalValue(GO);
166 // Functions with external linkage are needed if they have a body.
167 // Externally visible & appending globals are needed, if they have an
169 if (!GO.isDeclaration() && !GO.hasAvailableExternallyLinkage())
170 if (!GO.isDiscardableIfUnused())
173 UpdateGVDependencies(GO);
176 // Compute direct dependencies of aliases.
177 for (GlobalAlias &GA : M.aliases()) {
178 Changed |= RemoveUnusedGlobalValue(GA);
179 // Externally visible aliases are needed.
180 if (!GA.isDiscardableIfUnused())
183 UpdateGVDependencies(GA);
186 // Compute direct dependencies of ifuncs.
187 for (GlobalIFunc &GIF : M.ifuncs()) {
188 Changed |= RemoveUnusedGlobalValue(GIF);
189 // Externally visible ifuncs are needed.
190 if (!GIF.isDiscardableIfUnused())
193 UpdateGVDependencies(GIF);
196 // Propagate liveness from collected Global Values through the computed
198 SmallVector<GlobalValue *, 8> NewLiveGVs{AliveGlobals.begin(),
200 while (!NewLiveGVs.empty()) {
201 GlobalValue *LGV = NewLiveGVs.pop_back_val();
202 for (auto &&GVD : make_range(GVDependencies.equal_range(LGV)))
203 MarkLive(*GVD.second, &NewLiveGVs);
206 // Now that all globals which are needed are in the AliveGlobals set, we loop
207 // through the program, deleting those which are not alive.
210 // The first pass is to drop initializers of global variables which are dead.
211 std::vector<GlobalVariable *> DeadGlobalVars; // Keep track of dead globals
212 for (GlobalVariable &GV : M.globals())
213 if (!AliveGlobals.count(&GV)) {
214 DeadGlobalVars.push_back(&GV); // Keep track of dead globals
215 if (GV.hasInitializer()) {
216 Constant *Init = GV.getInitializer();
217 GV.setInitializer(nullptr);
218 if (isSafeToDestroyConstant(Init))
219 Init->destroyConstant();
223 // The second pass drops the bodies of functions which are dead...
224 std::vector<Function *> DeadFunctions;
225 for (Function &F : M)
226 if (!AliveGlobals.count(&F)) {
227 DeadFunctions.push_back(&F); // Keep track of dead globals
228 if (!F.isDeclaration())
232 // The third pass drops targets of aliases which are dead...
233 std::vector<GlobalAlias*> DeadAliases;
234 for (GlobalAlias &GA : M.aliases())
235 if (!AliveGlobals.count(&GA)) {
236 DeadAliases.push_back(&GA);
237 GA.setAliasee(nullptr);
240 // The fourth pass drops targets of ifuncs which are dead...
241 std::vector<GlobalIFunc*> DeadIFuncs;
242 for (GlobalIFunc &GIF : M.ifuncs())
243 if (!AliveGlobals.count(&GIF)) {
244 DeadIFuncs.push_back(&GIF);
245 GIF.setResolver(nullptr);
248 // Now that all interferences have been dropped, delete the actual objects
250 auto EraseUnusedGlobalValue = [&](GlobalValue *GV) {
251 RemoveUnusedGlobalValue(*GV);
252 GV->eraseFromParent();
256 NumFunctions += DeadFunctions.size();
257 for (Function *F : DeadFunctions)
258 EraseUnusedGlobalValue(F);
260 NumVariables += DeadGlobalVars.size();
261 for (GlobalVariable *GV : DeadGlobalVars)
262 EraseUnusedGlobalValue(GV);
264 NumAliases += DeadAliases.size();
265 for (GlobalAlias *GA : DeadAliases)
266 EraseUnusedGlobalValue(GA);
268 NumIFuncs += DeadIFuncs.size();
269 for (GlobalIFunc *GIF : DeadIFuncs)
270 EraseUnusedGlobalValue(GIF);
272 // Make sure that all memory is released
273 AliveGlobals.clear();
274 ConstantDependenciesCache.clear();
275 GVDependencies.clear();
276 ComdatMembers.clear();
279 return PreservedAnalyses::none();
280 return PreservedAnalyses::all();
283 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
284 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
285 // If found, check to see if the constant pointer ref is safe to destroy, and if
286 // so, nuke it. This will reduce the reference count on the global value, which
287 // might make it deader.
289 bool GlobalDCEPass::RemoveUnusedGlobalValue(GlobalValue &GV) {
292 GV.removeDeadConstantUsers();
293 return GV.use_empty();