PromoteMemoryToRegister.cpp revision 5f0eb8da62308126d5b61e3eee5bee75b9dc5194
1//===- PromoteMemoryToRegister.cpp - Convert memory refs to regs ----------===// 2// 3// This pass is used to promote memory references to be register references. A 4// simple example of the transformation performed by this pass is: 5// 6// FROM CODE TO CODE 7// %X = alloca int, uint 1 ret int 42 8// store int 42, int *%X 9// %Y = load int* %X 10// ret int %Y 11// 12// To do this transformation, a simple analysis is done to ensure it is safe. 13// Currently this just loops over all alloca instructions, looking for 14// instructions that are only used in simple load and stores. 15// 16// After this, the code is transformed by...something magical :) 17// 18//===----------------------------------------------------------------------===// 19 20#include "llvm/Transforms/Scalar.h" 21#include "llvm/Analysis/Dominators.h" 22#include "llvm/iMemory.h" 23#include "llvm/iPHINode.h" 24#include "llvm/iTerminators.h" 25#include "llvm/Function.h" 26#include "llvm/BasicBlock.h" 27#include "llvm/Constant.h" 28#include "llvm/Type.h" 29#include "Support/StatisticReporter.h" 30 31static Statistic<> NumPromoted("mem2reg\t\t- Number of alloca's promoted"); 32 33using std::vector; 34using std::map; 35using std::set; 36 37namespace { 38 struct PromotePass : public FunctionPass { 39 vector<AllocaInst*> Allocas; // the alloca instruction.. 40 map<Instruction*, unsigned> AllocaLookup; // reverse mapping of above 41 42 vector<vector<BasicBlock*> > PhiNodes; // index corresponds to Allocas 43 44 // List of instructions to remove at end of pass 45 vector<Instruction *> KillList; 46 47 map<BasicBlock*,vector<PHINode*> > NewPhiNodes; // the PhiNodes we're adding 48 49 public: 50 // runOnFunction - To run this pass, first we calculate the alloca 51 // instructions that are safe for promotion, then we promote each one. 52 // 53 virtual bool runOnFunction(Function &F); 54 55 // getAnalysisUsage - We need dominance frontiers 56 // 57 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 58 AU.addRequired<DominanceFrontier>(); 59 AU.preservesCFG(); 60 } 61 62 private: 63 void Traverse(BasicBlock *BB, BasicBlock *Pred, vector<Value*> &IncVals, 64 set<BasicBlock*> &Visited); 65 bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx); 66 void FindSafeAllocas(Function &F); 67 }; 68 69 RegisterOpt<PromotePass> X("mem2reg", "Promote Memory to Register"); 70} // end of anonymous namespace 71 72 73// isSafeAlloca - This predicate controls what types of alloca instructions are 74// allowed to be promoted... 75// 76static inline bool isSafeAlloca(const AllocaInst *AI) { 77 if (AI->isArrayAllocation()) return false; 78 79 for (Value::use_const_iterator UI = AI->use_begin(), UE = AI->use_end(); 80 UI != UE; ++UI) { // Loop over all of the uses of the alloca 81 82 // Only allow nonindexed memory access instructions... 83 if (MemAccessInst *MAI = dyn_cast<MemAccessInst>(*UI)) { 84 if (MAI->getPointerOperand() != (Value*)AI) 85 return false; // Reject stores of alloca pointer into some other loc. 86 87 if (MAI->hasIndices()) { // indexed? 88 // Allow the access if there is only one index and the index is 89 // zero. 90 if (*MAI->idx_begin() != Constant::getNullValue(Type::UIntTy) || 91 MAI->idx_begin()+1 != MAI->idx_end()) 92 return false; 93 } 94 } else { 95 return false; // Not a load or store? 96 } 97 } 98 99 return true; 100} 101 102// FindSafeAllocas - Find allocas that are safe to promote 103// 104void PromotePass::FindSafeAllocas(Function &F) { 105 BasicBlock &BB = F.getEntryNode(); // Get the entry node for the function 106 107 // Look at all instructions in the entry node 108 for (BasicBlock::iterator I = BB.begin(), E = BB.end(); I != E; ++I) 109 if (AllocaInst *AI = dyn_cast<AllocaInst>(&*I)) // Is it an alloca? 110 if (isSafeAlloca(AI)) { // If safe alloca, add alloca to safe list 111 AllocaLookup[AI] = Allocas.size(); // Keep reverse mapping 112 Allocas.push_back(AI); 113 } 114} 115 116 117 118bool PromotePass::runOnFunction(Function &F) { 119 // Calculate the set of safe allocas 120 FindSafeAllocas(F); 121 122 // If there is nothing to do, bail out... 123 if (Allocas.empty()) return false; 124 125 // Add each alloca to the KillList. Note: KillList is destroyed MOST recently 126 // added to least recently. 127 KillList.assign(Allocas.begin(), Allocas.end()); 128 129 // Calculate the set of write-locations for each alloca. This is analogous to 130 // counting the number of 'redefinitions' of each variable. 131 vector<vector<BasicBlock*> > WriteSets; // index corresponds to Allocas 132 WriteSets.resize(Allocas.size()); 133 for (unsigned i = 0; i != Allocas.size(); ++i) { 134 AllocaInst *AI = Allocas[i]; 135 for (Value::use_iterator U =AI->use_begin(), E = AI->use_end(); U != E; ++U) 136 if (StoreInst *SI = dyn_cast<StoreInst>(*U)) 137 // jot down the basic-block it came from 138 WriteSets[i].push_back(SI->getParent()); 139 } 140 141 // Get dominance frontier information... 142 DominanceFrontier &DF = getAnalysis<DominanceFrontier>(); 143 144 // Compute the locations where PhiNodes need to be inserted. Look at the 145 // dominance frontier of EACH basic-block we have a write in 146 // 147 PhiNodes.resize(Allocas.size()); 148 for (unsigned i = 0; i != Allocas.size(); ++i) { 149 for (unsigned j = 0; j != WriteSets[i].size(); j++) { 150 // Look up the DF for this write, add it to PhiNodes 151 DominanceFrontier::const_iterator it = DF.find(WriteSets[i][j]); 152 DominanceFrontier::DomSetType S = it->second; 153 for (DominanceFrontier::DomSetType::iterator P = S.begin(), PE = S.end(); 154 P != PE; ++P) 155 QueuePhiNode(*P, i); 156 } 157 158 // Perform iterative step 159 for (unsigned k = 0; k != PhiNodes[i].size(); k++) { 160 DominanceFrontier::const_iterator it = DF.find(PhiNodes[i][k]); 161 DominanceFrontier::DomSetType S = it->second; 162 for (DominanceFrontier::DomSetType::iterator P = S.begin(), PE = S.end(); 163 P != PE; ++P) 164 QueuePhiNode(*P, i); 165 } 166 } 167 168 // Set the incoming values for the basic block to be null values for all of 169 // the alloca's. We do this in case there is a load of a value that has not 170 // been stored yet. In this case, it will get this null value. 171 // 172 vector<Value *> Values(Allocas.size()); 173 for (unsigned i = 0, e = Allocas.size(); i != e; ++i) 174 Values[i] = Constant::getNullValue(Allocas[i]->getAllocatedType()); 175 176 // Walks all basic blocks in the function performing the SSA rename algorithm 177 // and inserting the phi nodes we marked as necessary 178 // 179 set<BasicBlock*> Visited; // The basic blocks we've already visited 180 Traverse(F.begin(), 0, Values, Visited); 181 182 // Remove all instructions marked by being placed in the KillList... 183 // 184 while (!KillList.empty()) { 185 Instruction *I = KillList.back(); 186 KillList.pop_back(); 187 188 I->getParent()->getInstList().erase(I); 189 } 190 191 NumPromoted += Allocas.size(); 192 193 // Purge data structurse so they are available the next iteration... 194 Allocas.clear(); 195 AllocaLookup.clear(); 196 PhiNodes.clear(); 197 NewPhiNodes.clear(); 198 return true; 199} 200 201 202// QueuePhiNode - queues a phi-node to be added to a basic-block for a specific 203// Alloca returns true if there wasn't already a phi-node for that variable 204// 205bool PromotePass::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo) { 206 // Look up the basic-block in question 207 vector<PHINode*> &BBPNs = NewPhiNodes[BB]; 208 if (BBPNs.empty()) BBPNs.resize(Allocas.size()); 209 210 // If the BB already has a phi node added for the i'th alloca then we're done! 211 if (BBPNs[AllocaNo]) return false; 212 213 // Create a PhiNode using the dereferenced type... 214 PHINode *PN = new PHINode(Allocas[AllocaNo]->getAllocatedType(), 215 Allocas[AllocaNo]->getName()+".mem2reg"); 216 BBPNs[AllocaNo] = PN; 217 218 // Add the phi-node to the basic-block 219 BB->getInstList().push_front(PN); 220 221 PhiNodes[AllocaNo].push_back(BB); 222 return true; 223} 224 225void PromotePass::Traverse(BasicBlock *BB, BasicBlock *Pred, 226 vector<Value*> &IncomingVals, 227 set<BasicBlock*> &Visited) { 228 // If this is a BB needing a phi node, lookup/create the phinode for each 229 // variable we need phinodes for. 230 vector<PHINode *> &BBPNs = NewPhiNodes[BB]; 231 for (unsigned k = 0; k != BBPNs.size(); ++k) 232 if (PHINode *PN = BBPNs[k]) { 233 // at this point we can assume that the array has phi nodes.. let's add 234 // the incoming data 235 PN->addIncoming(IncomingVals[k], Pred); 236 237 // also note that the active variable IS designated by the phi node 238 IncomingVals[k] = PN; 239 } 240 241 // don't revisit nodes 242 if (Visited.count(BB)) return; 243 244 // mark as visited 245 Visited.insert(BB); 246 247 // keep track of the value of each variable we're watching.. how? 248 for (BasicBlock::iterator II = BB->begin(); II != BB->end(); ++II) { 249 Instruction *I = II; // get the instruction 250 251 if (LoadInst *LI = dyn_cast<LoadInst>(I)) { 252 Value *Ptr = LI->getPointerOperand(); 253 254 if (AllocaInst *Src = dyn_cast<AllocaInst>(Ptr)) { 255 map<Instruction*, unsigned>::iterator AI = AllocaLookup.find(Src); 256 if (AI != AllocaLookup.end()) { 257 Value *V = IncomingVals[AI->second]; 258 259 // walk the use list of this load and replace all uses with r 260 LI->replaceAllUsesWith(V); 261 KillList.push_back(LI); // Mark the load to be deleted 262 } 263 } 264 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) { 265 // delete this instruction and mark the name as the current holder of the 266 // value 267 Value *Ptr = SI->getPointerOperand(); 268 if (AllocaInst *Dest = dyn_cast<AllocaInst>(Ptr)) { 269 map<Instruction *, unsigned>::iterator ai = AllocaLookup.find(Dest); 270 if (ai != AllocaLookup.end()) { 271 // what value were we writing? 272 IncomingVals[ai->second] = SI->getOperand(0); 273 KillList.push_back(SI); // Mark the store to be deleted 274 } 275 } 276 277 } else if (TerminatorInst *TI = dyn_cast<TerminatorInst>(I)) { 278 // Recurse across our successors 279 for (unsigned i = 0; i != TI->getNumSuccessors(); i++) { 280 vector<Value*> OutgoingVals(IncomingVals); 281 Traverse(TI->getSuccessor(i), BB, OutgoingVals, Visited); 282 } 283 } 284 } 285} 286 287 288// createPromoteMemoryToRegister - Provide an entry point to create this pass. 289// 290Pass *createPromoteMemoryToRegister() { 291 return new PromotePass(); 292} 293