GlobalDCE.cpp revision 5a44ef9fd5f7c3964ad79b94778261175dea5c33
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/Statistic.h" 24#include <set> 25using namespace llvm; 26 27STATISTIC(NumAliases , "Number of global aliases removed"); 28STATISTIC(NumFunctions, "Number of functions removed"); 29STATISTIC(NumVariables, "Number of global variables removed"); 30 31namespace { 32 struct GlobalDCE : public ModulePass { 33 static char ID; // Pass identification, replacement for typeid 34 GlobalDCE() : ModulePass(&ID) {} 35 36 // run - Do the GlobalDCE pass on the specified module, optionally updating 37 // the specified callgraph to reflect the changes. 38 // 39 bool runOnModule(Module &M); 40 41 private: 42 std::set<GlobalValue*> AliveGlobals; 43 44 /// GlobalIsNeeded - mark the specific global value as needed, and 45 /// recursively mark anything that it uses as also needed. 46 void GlobalIsNeeded(GlobalValue *GV); 47 void MarkUsedGlobalsAsNeeded(Constant *C); 48 49 bool RemoveUnusedGlobalValue(GlobalValue &GV); 50 }; 51} 52 53char GlobalDCE::ID = 0; 54static RegisterPass<GlobalDCE> X("globaldce", "Dead Global Elimination"); 55 56ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); } 57 58bool GlobalDCE::runOnModule(Module &M) { 59 bool Changed = false; 60 61 // Loop over the module, adding globals which are obviously necessary. 62 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) { 63 Changed |= RemoveUnusedGlobalValue(*I); 64 // Functions with external linkage are needed if they have a body 65 if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage() && 66 !I->isDeclaration() && !I->hasAvailableExternallyLinkage()) 67 GlobalIsNeeded(I); 68 } 69 70 for (Module::global_iterator I = M.global_begin(), E = M.global_end(); 71 I != E; ++I) { 72 Changed |= RemoveUnusedGlobalValue(*I); 73 // Externally visible & appending globals are needed, if they have an 74 // initializer. 75 if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage() && 76 !I->isDeclaration() && !I->hasAvailableExternallyLinkage()) 77 GlobalIsNeeded(I); 78 } 79 80 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); 81 I != E; ++I) { 82 Changed |= RemoveUnusedGlobalValue(*I); 83 // Externally visible aliases are needed. 84 if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage()) 85 GlobalIsNeeded(I); 86 } 87 88 // Now that all globals which are needed are in the AliveGlobals set, we loop 89 // through the program, deleting those which are not alive. 90 // 91 92 // The first pass is to drop initializers of global variables which are dead. 93 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals 94 for (Module::global_iterator I = M.global_begin(), E = M.global_end(); I != E; ++I) 95 if (!AliveGlobals.count(I)) { 96 DeadGlobalVars.push_back(I); // Keep track of dead globals 97 I->setInitializer(0); 98 } 99 100 // The second pass drops the bodies of functions which are dead... 101 std::vector<Function*> DeadFunctions; 102 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) 103 if (!AliveGlobals.count(I)) { 104 DeadFunctions.push_back(I); // Keep track of dead globals 105 if (!I->isDeclaration()) 106 I->deleteBody(); 107 } 108 109 // The third pass drops targets of aliases which are dead... 110 std::vector<GlobalAlias*> DeadAliases; 111 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E; 112 ++I) 113 if (!AliveGlobals.count(I)) { 114 DeadAliases.push_back(I); 115 I->setAliasee(0); 116 } 117 118 if (!DeadFunctions.empty()) { 119 // Now that all interferences have been dropped, delete the actual objects 120 // themselves. 121 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) { 122 RemoveUnusedGlobalValue(*DeadFunctions[i]); 123 M.getFunctionList().erase(DeadFunctions[i]); 124 } 125 NumFunctions += DeadFunctions.size(); 126 Changed = true; 127 } 128 129 if (!DeadGlobalVars.empty()) { 130 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) { 131 RemoveUnusedGlobalValue(*DeadGlobalVars[i]); 132 M.getGlobalList().erase(DeadGlobalVars[i]); 133 } 134 NumVariables += DeadGlobalVars.size(); 135 Changed = true; 136 } 137 138 // Now delete any dead aliases. 139 if (!DeadAliases.empty()) { 140 for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) { 141 RemoveUnusedGlobalValue(*DeadAliases[i]); 142 M.getAliasList().erase(DeadAliases[i]); 143 } 144 NumAliases += DeadAliases.size(); 145 Changed = true; 146 } 147 148 // Make sure that all memory is released 149 AliveGlobals.clear(); 150 151 return Changed; 152} 153 154/// GlobalIsNeeded - the specific global value as needed, and 155/// recursively mark anything that it uses as also needed. 156void GlobalDCE::GlobalIsNeeded(GlobalValue *G) { 157 std::set<GlobalValue*>::iterator I = AliveGlobals.find(G); 158 159 // If the global is already in the set, no need to reprocess it. 160 if (I != AliveGlobals.end()) return; 161 162 // Otherwise insert it now, so we do not infinitely recurse 163 AliveGlobals.insert(I, G); 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 // For all basic blocks... 180 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) 181 // For all instructions... 182 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 183 // For all operands... 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 192void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) { 193 if (GlobalValue *GV = dyn_cast<GlobalValue>(C)) 194 GlobalIsNeeded(GV); 195 else { 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 MarkUsedGlobalsAsNeeded(cast<Constant>(*I)); 200 } 201} 202 203// RemoveUnusedGlobalValue - Loop over all of the uses of the specified 204// GlobalValue, looking for the constant pointer ref that may be pointing to it. 205// If found, check to see if the constant pointer ref is safe to destroy, and if 206// so, nuke it. This will reduce the reference count on the global value, which 207// might make it deader. 208// 209bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) { 210 if (GV.use_empty()) return false; 211 GV.removeDeadConstantUsers(); 212 return GV.use_empty(); 213} 214