1 //===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 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. 15 // 16 //===----------------------------------------------------------------------===// 17 18 #define DEBUG_TYPE "globaldce" 19 #include "llvm/Transforms/IPO.h" 20 #include "llvm/ADT/SmallPtrSet.h" 21 #include "llvm/ADT/Statistic.h" 22 #include "llvm/IR/Constants.h" 23 #include "llvm/IR/Module.h" 24 #include "llvm/Pass.h" 25 using namespace llvm; 26 27 STATISTIC(NumAliases , "Number of global aliases removed"); 28 STATISTIC(NumFunctions, "Number of functions removed"); 29 STATISTIC(NumVariables, "Number of global variables removed"); 30 31 namespace { 32 struct GlobalDCE : public ModulePass { 33 static char ID; // Pass identification, replacement for typeid 34 GlobalDCE() : ModulePass(ID) { 35 initializeGlobalDCEPass(*PassRegistry::getPassRegistry()); 36 } 37 38 // run - Do the GlobalDCE pass on the specified module, optionally updating 39 // the specified callgraph to reflect the changes. 40 // 41 bool runOnModule(Module &M); 42 43 private: 44 SmallPtrSet<GlobalValue*, 32> AliveGlobals; 45 SmallPtrSet<Constant *, 8> SeenConstants; 46 47 /// GlobalIsNeeded - mark the specific global value as needed, and 48 /// recursively mark anything that it uses as also needed. 49 void GlobalIsNeeded(GlobalValue *GV); 50 void MarkUsedGlobalsAsNeeded(Constant *C); 51 52 bool RemoveUnusedGlobalValue(GlobalValue &GV); 53 }; 54 } 55 56 char GlobalDCE::ID = 0; 57 INITIALIZE_PASS(GlobalDCE, "globaldce", 58 "Dead Global Elimination", false, false) 59 60 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); } 61 62 bool GlobalDCE::runOnModule(Module &M) { 63 bool Changed = false; 64 65 // Loop over the module, adding globals which are obviously necessary. 66 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) { 67 Changed |= RemoveUnusedGlobalValue(*I); 68 // Functions with external linkage are needed if they have a body 69 if (!I->isDiscardableIfUnused() && 70 !I->isDeclaration() && !I->hasAvailableExternallyLinkage()) 71 GlobalIsNeeded(I); 72 } 73 74 for (Module::global_iterator I = M.global_begin(), E = M.global_end(); 75 I != E; ++I) { 76 Changed |= RemoveUnusedGlobalValue(*I); 77 // Externally visible & appending globals are needed, if they have an 78 // initializer. 79 if (!I->isDiscardableIfUnused() && 80 !I->isDeclaration() && !I->hasAvailableExternallyLinkage()) 81 GlobalIsNeeded(I); 82 } 83 84 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); 85 I != E; ++I) { 86 Changed |= RemoveUnusedGlobalValue(*I); 87 // Externally visible aliases are needed. 88 if (!I->isDiscardableIfUnused()) 89 GlobalIsNeeded(I); 90 } 91 92 // Now that all globals which are needed are in the AliveGlobals set, we loop 93 // through the program, deleting those which are not alive. 94 // 95 96 // The first pass is to drop initializers of global variables which are dead. 97 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals 98 for (Module::global_iterator I = M.global_begin(), E = M.global_end(); 99 I != E; ++I) 100 if (!AliveGlobals.count(I)) { 101 DeadGlobalVars.push_back(I); // Keep track of dead globals 102 I->setInitializer(0); 103 } 104 105 // The second pass drops the bodies of functions which are dead... 106 std::vector<Function*> DeadFunctions; 107 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) 108 if (!AliveGlobals.count(I)) { 109 DeadFunctions.push_back(I); // Keep track of dead globals 110 if (!I->isDeclaration()) 111 I->deleteBody(); 112 } 113 114 // The third pass drops targets of aliases which are dead... 115 std::vector<GlobalAlias*> DeadAliases; 116 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E; 117 ++I) 118 if (!AliveGlobals.count(I)) { 119 DeadAliases.push_back(I); 120 I->setAliasee(0); 121 } 122 123 if (!DeadFunctions.empty()) { 124 // Now that all interferences have been dropped, delete the actual objects 125 // themselves. 126 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) { 127 RemoveUnusedGlobalValue(*DeadFunctions[i]); 128 M.getFunctionList().erase(DeadFunctions[i]); 129 } 130 NumFunctions += DeadFunctions.size(); 131 Changed = true; 132 } 133 134 if (!DeadGlobalVars.empty()) { 135 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) { 136 RemoveUnusedGlobalValue(*DeadGlobalVars[i]); 137 M.getGlobalList().erase(DeadGlobalVars[i]); 138 } 139 NumVariables += DeadGlobalVars.size(); 140 Changed = true; 141 } 142 143 // Now delete any dead aliases. 144 if (!DeadAliases.empty()) { 145 for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) { 146 RemoveUnusedGlobalValue(*DeadAliases[i]); 147 M.getAliasList().erase(DeadAliases[i]); 148 } 149 NumAliases += DeadAliases.size(); 150 Changed = true; 151 } 152 153 // Make sure that all memory is released 154 AliveGlobals.clear(); 155 SeenConstants.clear(); 156 157 return Changed; 158 } 159 160 /// GlobalIsNeeded - the specific global value as needed, and 161 /// recursively mark anything that it uses as also needed. 162 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) { 163 // If the global is already in the set, no need to reprocess it. 164 if (!AliveGlobals.insert(G)) 165 return; 166 167 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) { 168 // If this is a global variable, we must make sure to add any global values 169 // referenced by the initializer to the alive set. 170 if (GV->hasInitializer()) 171 MarkUsedGlobalsAsNeeded(GV->getInitializer()); 172 } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) { 173 // The target of a global alias is needed. 174 MarkUsedGlobalsAsNeeded(GA->getAliasee()); 175 } else { 176 // Otherwise this must be a function object. We have to scan the body of 177 // the function looking for constants and global values which are used as 178 // operands. Any operands of these types must be processed to ensure that 179 // any globals used will be marked as needed. 180 Function *F = cast<Function>(G); 181 182 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) 183 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 184 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U) 185 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U)) 186 GlobalIsNeeded(GV); 187 else if (Constant *C = dyn_cast<Constant>(*U)) 188 MarkUsedGlobalsAsNeeded(C); 189 } 190 } 191 192 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) { 193 if (GlobalValue *GV = dyn_cast<GlobalValue>(C)) 194 return GlobalIsNeeded(GV); 195 196 // Loop over all of the operands of the constant, adding any globals they 197 // use to the list of needed globals. 198 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) { 199 // If we've already processed this constant there's no need to do it again. 200 Constant *Op = dyn_cast<Constant>(*I); 201 if (Op && SeenConstants.insert(Op)) 202 MarkUsedGlobalsAsNeeded(Op); 203 } 204 } 205 206 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified 207 // GlobalValue, looking for the constant pointer ref that may be pointing to it. 208 // If found, check to see if the constant pointer ref is safe to destroy, and if 209 // so, nuke it. This will reduce the reference count on the global value, which 210 // might make it deader. 211 // 212 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) { 213 if (GV.use_empty()) return false; 214 GV.removeDeadConstantUsers(); 215 return GV.use_empty(); 216 } 217
