GlobalDCE.cpp revision 794fd75c67a2cdc128d67342c6d88a504d186896
1//===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file was developed by the LLVM research group and is distributed under 6// the University of Illinois Open Source 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 const int ID; // Pass identifcation, replacement for typeid 34 GlobalDCE() : ModulePass((intptr_t)&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 const int GlobalDCE::ID = 0; 53 RegisterPass<GlobalDCE> X("globaldce", "Dead Global Elimination"); 54} 55 56ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); } 57 58bool GlobalDCE::runOnModule(Module &M) { 59 bool Changed = false; 60 // Loop over the module, adding globals which are obviously necessary. 61 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) { 62 Changed |= RemoveUnusedGlobalValue(*I); 63 // Functions with external linkage are needed if they have a body 64 if ((!I->hasInternalLinkage() && !I->hasLinkOnceLinkage()) && 65 !I->isDeclaration()) 66 GlobalIsNeeded(I); 67 } 68 69 for (Module::global_iterator I = M.global_begin(), E = M.global_end(); 70 I != E; ++I) { 71 Changed |= RemoveUnusedGlobalValue(*I); 72 // Externally visible & appending globals are needed, if they have an 73 // initializer. 74 if ((!I->hasInternalLinkage() && !I->hasLinkOnceLinkage()) && 75 !I->isDeclaration()) 76 GlobalIsNeeded(I); 77 } 78 79 80 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); 81 I != E; ++I) { 82 // Aliases are always needed even if they are not used. 83 MarkUsedGlobalsAsNeeded(I->getAliasee()); 84 } 85 86 // Now that all globals which are needed are in the AliveGlobals set, we loop 87 // through the program, deleting those which are not alive. 88 // 89 90 // The first pass is to drop initializers of global variables which are dead. 91 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals 92 for (Module::global_iterator I = M.global_begin(), E = M.global_end(); I != E; ++I) 93 if (!AliveGlobals.count(I)) { 94 DeadGlobalVars.push_back(I); // Keep track of dead globals 95 I->setInitializer(0); 96 } 97 98 99 // The second pass drops the bodies of functions which are dead... 100 std::vector<Function*> DeadFunctions; 101 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) 102 if (!AliveGlobals.count(I)) { 103 DeadFunctions.push_back(I); // Keep track of dead globals 104 if (!I->isDeclaration()) 105 I->deleteBody(); 106 } 107 108 if (!DeadFunctions.empty()) { 109 // Now that all interreferences have been dropped, delete the actual objects 110 // themselves. 111 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) { 112 RemoveUnusedGlobalValue(*DeadFunctions[i]); 113 M.getFunctionList().erase(DeadFunctions[i]); 114 } 115 NumFunctions += DeadFunctions.size(); 116 Changed = true; 117 } 118 119 if (!DeadGlobalVars.empty()) { 120 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) { 121 RemoveUnusedGlobalValue(*DeadGlobalVars[i]); 122 M.getGlobalList().erase(DeadGlobalVars[i]); 123 } 124 NumVariables += DeadGlobalVars.size(); 125 Changed = true; 126 } 127 128 // Make sure that all memory is released 129 AliveGlobals.clear(); 130 return Changed; 131} 132 133/// MarkGlobalIsNeeded - the specific global value as needed, and 134/// recursively mark anything that it uses as also needed. 135void GlobalDCE::GlobalIsNeeded(GlobalValue *G) { 136 std::set<GlobalValue*>::iterator I = AliveGlobals.lower_bound(G); 137 138 // If the global is already in the set, no need to reprocess it. 139 if (I != AliveGlobals.end() && *I == G) return; 140 141 // Otherwise insert it now, so we do not infinitely recurse 142 AliveGlobals.insert(I, G); 143 144 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) { 145 // If this is a global variable, we must make sure to add any global values 146 // referenced by the initializer to the alive set. 147 if (GV->hasInitializer()) 148 MarkUsedGlobalsAsNeeded(GV->getInitializer()); 149 } else if (!isa<GlobalAlias>(G)) { 150 // Otherwise this must be a function object. We have to scan the body of 151 // the function looking for constants and global values which are used as 152 // operands. Any operands of these types must be processed to ensure that 153 // any globals used will be marked as needed. 154 Function *F = cast<Function>(G); 155 // For all basic blocks... 156 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) 157 // For all instructions... 158 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 159 // For all operands... 160 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U) 161 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U)) 162 GlobalIsNeeded(GV); 163 else if (Constant *C = dyn_cast<Constant>(*U)) 164 MarkUsedGlobalsAsNeeded(C); 165 } 166} 167 168void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) { 169 if (GlobalValue *GV = dyn_cast<GlobalValue>(C)) 170 GlobalIsNeeded(GV); 171 else { 172 // Loop over all of the operands of the constant, adding any globals they 173 // use to the list of needed globals. 174 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) 175 MarkUsedGlobalsAsNeeded(cast<Constant>(*I)); 176 } 177} 178 179// RemoveUnusedGlobalValue - Loop over all of the uses of the specified 180// GlobalValue, looking for the constant pointer ref that may be pointing to it. 181// If found, check to see if the constant pointer ref is safe to destroy, and if 182// so, nuke it. This will reduce the reference count on the global value, which 183// might make it deader. 184// 185bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) { 186 if (GV.use_empty()) return false; 187 GV.removeDeadConstantUsers(); 188 return GV.use_empty(); 189} 190 191// SafeToDestroyConstant - It is safe to destroy a constant iff it is only used 192// by constants itself. Note that constants cannot be cyclic, so this test is 193// pretty easy to implement recursively. 194// 195bool GlobalDCE::SafeToDestroyConstant(Constant *C) { 196 for (Value::use_iterator I = C->use_begin(), E = C->use_end(); I != E; ++I) 197 if (Constant *User = dyn_cast<Constant>(*I)) { 198 if (!SafeToDestroyConstant(User)) return false; 199 } else { 200 return false; 201 } 202 return true; 203} 204