GlobalDCE.cpp revision 26f8c27c34b44f7d87de74d1de2128c1a02855bf
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(NumFunctions, "Number of functions removed"); 29STATISTIC(NumVariables, "Number of global variables removed"); 30 31namespace { 32 struct VISIBILITY_HIDDEN 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 /// MarkGlobalIsNeeded - 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 SafeToDestroyConstant(Constant* C); 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 // 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->hasInternalLinkage() && !I->hasLinkOnceLinkage()) && 66 !I->isDeclaration()) 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->hasInternalLinkage() && !I->hasLinkOnceLinkage()) && 76 !I->isDeclaration()) 77 GlobalIsNeeded(I); 78 } 79 80 81 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); 82 I != E; ++I) { 83 // Aliases are always needed even if they are not used. 84 MarkUsedGlobalsAsNeeded(I->getAliasee()); 85 } 86 87 // Now that all globals which are needed are in the AliveGlobals set, we loop 88 // through the program, deleting those which are not alive. 89 // 90 91 // The first pass is to drop initializers of global variables which are dead. 92 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals 93 for (Module::global_iterator I = M.global_begin(), E = M.global_end(); I != E; ++I) 94 if (!AliveGlobals.count(I)) { 95 DeadGlobalVars.push_back(I); // Keep track of dead globals 96 I->setInitializer(0); 97 } 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 if (!DeadFunctions.empty()) { 110 // Now that all interreferences have been dropped, delete the actual objects 111 // themselves. 112 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) { 113 RemoveUnusedGlobalValue(*DeadFunctions[i]); 114 M.getFunctionList().erase(DeadFunctions[i]); 115 } 116 NumFunctions += DeadFunctions.size(); 117 Changed = true; 118 } 119 120 if (!DeadGlobalVars.empty()) { 121 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) { 122 RemoveUnusedGlobalValue(*DeadGlobalVars[i]); 123 M.getGlobalList().erase(DeadGlobalVars[i]); 124 } 125 NumVariables += DeadGlobalVars.size(); 126 Changed = true; 127 } 128 129 // Make sure that all memory is released 130 AliveGlobals.clear(); 131 return Changed; 132} 133 134/// MarkGlobalIsNeeded - the specific global value as needed, and 135/// recursively mark anything that it uses as also needed. 136void GlobalDCE::GlobalIsNeeded(GlobalValue *G) { 137 std::set<GlobalValue*>::iterator I = AliveGlobals.find(G); 138 139 // If the global is already in the set, no need to reprocess it. 140 if (I != AliveGlobals.end()) return; 141 142 // Otherwise insert it now, so we do not infinitely recurse 143 AliveGlobals.insert(I, G); 144 145 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) { 146 // If this is a global variable, we must make sure to add any global values 147 // referenced by the initializer to the alive set. 148 if (GV->hasInitializer()) 149 MarkUsedGlobalsAsNeeded(GV->getInitializer()); 150 } else if (!isa<GlobalAlias>(G)) { 151 // Otherwise this must be a function object. We have to scan the body of 152 // the function looking for constants and global values which are used as 153 // operands. Any operands of these types must be processed to ensure that 154 // any globals used will be marked as needed. 155 Function *F = cast<Function>(G); 156 // For all basic blocks... 157 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) 158 // For all instructions... 159 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 160 // For all operands... 161 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U) 162 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U)) 163 GlobalIsNeeded(GV); 164 else if (Constant *C = dyn_cast<Constant>(*U)) 165 MarkUsedGlobalsAsNeeded(C); 166 } 167} 168 169void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) { 170 if (GlobalValue *GV = dyn_cast<GlobalValue>(C)) 171 GlobalIsNeeded(GV); 172 else { 173 // Loop over all of the operands of the constant, adding any globals they 174 // use to the list of needed globals. 175 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) 176 MarkUsedGlobalsAsNeeded(cast<Constant>(*I)); 177 } 178} 179 180// RemoveUnusedGlobalValue - Loop over all of the uses of the specified 181// GlobalValue, looking for the constant pointer ref that may be pointing to it. 182// If found, check to see if the constant pointer ref is safe to destroy, and if 183// so, nuke it. This will reduce the reference count on the global value, which 184// might make it deader. 185// 186bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) { 187 if (GV.use_empty()) return false; 188 GV.removeDeadConstantUsers(); 189 return GV.use_empty(); 190} 191 192// SafeToDestroyConstant - It is safe to destroy a constant iff it is only used 193// by constants itself. Note that constants cannot be cyclic, so this test is 194// pretty easy to implement recursively. 195// 196bool GlobalDCE::SafeToDestroyConstant(Constant *C) { 197 for (Value::use_iterator I = C->use_begin(), E = C->use_end(); I != E; ++I) 198 if (Constant *User = dyn_cast<Constant>(*I)) { 199 if (!SafeToDestroyConstant(User)) return false; 200 } else { 201 return false; 202 } 203 return true; 204} 205