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