GlobalDCE.cpp revision 1434dfa8cead98bd1e63411fcb9424e1d37f61ac
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" 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 SmallPtrSet<GlobalValue*, 32> 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; 54INITIALIZE_PASS(GlobalDCE, "globaldce", 55 "Dead Global Elimination", false, false) 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(); 96 I != E; ++I) 97 if (!AliveGlobals.count(I)) { 98 DeadGlobalVars.push_back(I); // Keep track of dead globals 99 I->setInitializer(0); 100 } 101 102 // The second pass drops the bodies of functions which are dead... 103 std::vector<Function*> DeadFunctions; 104 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) 105 if (!AliveGlobals.count(I)) { 106 DeadFunctions.push_back(I); // Keep track of dead globals 107 if (!I->isDeclaration()) 108 I->deleteBody(); 109 } 110 111 // The third pass drops targets of aliases which are dead... 112 std::vector<GlobalAlias*> DeadAliases; 113 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E; 114 ++I) 115 if (!AliveGlobals.count(I)) { 116 DeadAliases.push_back(I); 117 I->setAliasee(0); 118 } 119 120 if (!DeadFunctions.empty()) { 121 // Now that all interferences have been dropped, delete the actual objects 122 // themselves. 123 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) { 124 RemoveUnusedGlobalValue(*DeadFunctions[i]); 125 M.getFunctionList().erase(DeadFunctions[i]); 126 } 127 NumFunctions += DeadFunctions.size(); 128 Changed = true; 129 } 130 131 if (!DeadGlobalVars.empty()) { 132 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) { 133 RemoveUnusedGlobalValue(*DeadGlobalVars[i]); 134 M.getGlobalList().erase(DeadGlobalVars[i]); 135 } 136 NumVariables += DeadGlobalVars.size(); 137 Changed = true; 138 } 139 140 // Now delete any dead aliases. 141 if (!DeadAliases.empty()) { 142 for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) { 143 RemoveUnusedGlobalValue(*DeadAliases[i]); 144 M.getAliasList().erase(DeadAliases[i]); 145 } 146 NumAliases += DeadAliases.size(); 147 Changed = true; 148 } 149 150 // Make sure that all memory is released 151 AliveGlobals.clear(); 152 153 return Changed; 154} 155 156/// GlobalIsNeeded - the specific global value as needed, and 157/// recursively mark anything that it uses as also needed. 158void GlobalDCE::GlobalIsNeeded(GlobalValue *G) { 159 // If the global is already in the set, no need to reprocess it. 160 if (!AliveGlobals.insert(G)) 161 return; 162 163 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) { 164 // If this is a global variable, we must make sure to add any global values 165 // referenced by the initializer to the alive set. 166 if (GV->hasInitializer()) 167 MarkUsedGlobalsAsNeeded(GV->getInitializer()); 168 } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) { 169 // The target of a global alias is needed. 170 MarkUsedGlobalsAsNeeded(GA->getAliasee()); 171 } else { 172 // Otherwise this must be a function object. We have to scan the body of 173 // the function looking for constants and global values which are used as 174 // operands. Any operands of these types must be processed to ensure that 175 // any globals used will be marked as needed. 176 Function *F = cast<Function>(G); 177 178 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) 179 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 180 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U) 181 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U)) 182 GlobalIsNeeded(GV); 183 else if (Constant *C = dyn_cast<Constant>(*U)) 184 MarkUsedGlobalsAsNeeded(C); 185 } 186} 187 188void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) { 189 if (GlobalValue *GV = dyn_cast<GlobalValue>(C)) 190 return GlobalIsNeeded(GV); 191 192 // Loop over all of the operands of the constant, adding any globals they 193 // use to the list of needed globals. 194 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) 195 if (Constant *OpC = dyn_cast<Constant>(*I)) 196 MarkUsedGlobalsAsNeeded(OpC); 197} 198 199// RemoveUnusedGlobalValue - Loop over all of the uses of the specified 200// GlobalValue, looking for the constant pointer ref that may be pointing to it. 201// If found, check to see if the constant pointer ref is safe to destroy, and if 202// so, nuke it. This will reduce the reference count on the global value, which 203// might make it deader. 204// 205bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) { 206 if (GV.use_empty()) return false; 207 GV.removeDeadConstantUsers(); 208 return GV.use_empty(); 209} 210