GlobalDCE.cpp revision 492d06efde44a4e38a6ed321ada4af5a75494df6
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 "llvm/Support/Compiler.h" 25#include <set> 26using namespace llvm; 27 28STATISTIC(NumAliases , "Number of global aliases removed"); 29STATISTIC(NumFunctions, "Number of functions removed"); 30STATISTIC(NumVariables, "Number of global variables removed"); 31 32namespace { 33 struct GlobalDCE : public ModulePass { 34 static char ID; // Pass identification, replacement for typeid 35 GlobalDCE() : ModulePass(&ID) {} 36 37 // run - Do the GlobalDCE pass on the specified module, optionally updating 38 // the specified callgraph to reflect the changes. 39 // 40 bool runOnModule(Module &M); 41 42 private: 43 std::set<GlobalValue*> AliveGlobals; 44 45 /// GlobalIsNeeded - mark the specific global value as needed, and 46 /// recursively mark anything that it uses as also needed. 47 void GlobalIsNeeded(GlobalValue *GV); 48 void MarkUsedGlobalsAsNeeded(Constant *C); 49 50 bool RemoveUnusedGlobalValue(GlobalValue &GV); 51 }; 52} 53 54char GlobalDCE::ID = 0; 55static RegisterPass<GlobalDCE> X("globaldce", "Dead Global Elimination"); 56 57ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); } 58 59bool GlobalDCE::runOnModule(Module &M) { 60 bool Changed = false; 61 62 // Loop over the module, adding globals which are obviously necessary. 63 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) { 64 Changed |= RemoveUnusedGlobalValue(*I); 65 // Functions with external linkage are needed if they have a body 66 if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage() && 67 !I->isDeclaration() && !I->hasAvailableExternallyLinkage()) 68 GlobalIsNeeded(I); 69 } 70 71 for (Module::global_iterator I = M.global_begin(), E = M.global_end(); 72 I != E; ++I) { 73 Changed |= RemoveUnusedGlobalValue(*I); 74 // Externally visible & appending globals are needed, if they have an 75 // initializer. 76 if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage() && 77 !I->isDeclaration() && !I->hasAvailableExternallyLinkage()) 78 GlobalIsNeeded(I); 79 } 80 81 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); 82 I != E; ++I) { 83 Changed |= RemoveUnusedGlobalValue(*I); 84 // Externally visible aliases are needed. 85 if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage()) 86 GlobalIsNeeded(I); 87 } 88 89 // Now that all globals which are needed are in the AliveGlobals set, we loop 90 // through the program, deleting those which are not alive. 91 // 92 93 // The first pass is to drop initializers of global variables which are dead. 94 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals 95 for (Module::global_iterator I = M.global_begin(), E = M.global_end(); I != E; ++I) 96 if (!AliveGlobals.count(I)) { 97 DeadGlobalVars.push_back(I); // Keep track of dead globals 98 I->setInitializer(0); 99 } 100 101 // The second pass drops the bodies of functions which are dead... 102 std::vector<Function*> DeadFunctions; 103 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) 104 if (!AliveGlobals.count(I)) { 105 DeadFunctions.push_back(I); // Keep track of dead globals 106 if (!I->isDeclaration()) 107 I->deleteBody(); 108 } 109 110 // The third pass drops targets of aliases which are dead... 111 std::vector<GlobalAlias*> DeadAliases; 112 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E; 113 ++I) 114 if (!AliveGlobals.count(I)) { 115 DeadAliases.push_back(I); 116 I->setAliasee(0); 117 } 118 119 if (!DeadFunctions.empty()) { 120 // Now that all interferences have been dropped, delete the actual objects 121 // themselves. 122 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) { 123 RemoveUnusedGlobalValue(*DeadFunctions[i]); 124 M.getFunctionList().erase(DeadFunctions[i]); 125 } 126 NumFunctions += DeadFunctions.size(); 127 Changed = true; 128 } 129 130 if (!DeadGlobalVars.empty()) { 131 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) { 132 RemoveUnusedGlobalValue(*DeadGlobalVars[i]); 133 M.getGlobalList().erase(DeadGlobalVars[i]); 134 } 135 NumVariables += DeadGlobalVars.size(); 136 Changed = true; 137 } 138 139 // Now delete any dead aliases. 140 if (!DeadAliases.empty()) { 141 for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) { 142 RemoveUnusedGlobalValue(*DeadAliases[i]); 143 M.getAliasList().erase(DeadAliases[i]); 144 } 145 NumAliases += DeadAliases.size(); 146 Changed = true; 147 } 148 149 // Make sure that all memory is released 150 AliveGlobals.clear(); 151 152 return Changed; 153} 154 155/// GlobalIsNeeded - the specific global value as needed, and 156/// recursively mark anything that it uses as also needed. 157void GlobalDCE::GlobalIsNeeded(GlobalValue *G) { 158 std::set<GlobalValue*>::iterator I = AliveGlobals.find(G); 159 160 // If the global is already in the set, no need to reprocess it. 161 if (I != AliveGlobals.end()) return; 162 163 // Otherwise insert it now, so we do not infinitely recurse 164 AliveGlobals.insert(I, G); 165 166 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) { 167 // If this is a global variable, we must make sure to add any global values 168 // referenced by the initializer to the alive set. 169 if (GV->hasInitializer()) 170 MarkUsedGlobalsAsNeeded(GV->getInitializer()); 171 } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) { 172 // The target of a global alias is needed. 173 MarkUsedGlobalsAsNeeded(GA->getAliasee()); 174 } else { 175 // Otherwise this must be a function object. We have to scan the body of 176 // the function looking for constants and global values which are used as 177 // operands. Any operands of these types must be processed to ensure that 178 // any globals used will be marked as needed. 179 Function *F = cast<Function>(G); 180 // For all basic blocks... 181 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) 182 // For all instructions... 183 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 184 // For all operands... 185 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U) 186 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U)) 187 GlobalIsNeeded(GV); 188 else if (Constant *C = dyn_cast<Constant>(*U)) 189 MarkUsedGlobalsAsNeeded(C); 190 } 191} 192 193void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) { 194 if (GlobalValue *GV = dyn_cast<GlobalValue>(C)) 195 GlobalIsNeeded(GV); 196 else { 197 // Loop over all of the operands of the constant, adding any globals they 198 // use to the list of needed globals. 199 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) 200 MarkUsedGlobalsAsNeeded(cast<Constant>(*I)); 201 } 202} 203 204// RemoveUnusedGlobalValue - Loop over all of the uses of the specified 205// GlobalValue, looking for the constant pointer ref that may be pointing to it. 206// If found, check to see if the constant pointer ref is safe to destroy, and if 207// so, nuke it. This will reduce the reference count on the global value, which 208// might make it deader. 209// 210bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) { 211 if (GV.use_empty()) return false; 212 GV.removeDeadConstantUsers(); 213 return GV.use_empty(); 214} 215