PromoteMemoryToRegister.cpp revision c8789cb40b81d032b79e02023e025d3ca7711365
1//===- PromoteMemoryToRegister.cpp - Convert memory refs to regs ----------===// 2// 3// This file is used to promote memory references to be register references. A 4// simple example of the transformation performed by this function 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// The code is transformed by looping over all of the alloca instruction, 13// calculating dominator frontiers, then inserting phi-nodes following the usual 14// SSA construction algorithm. This code does not modify the CFG of the 15// function. 16// 17//===----------------------------------------------------------------------===// 18 19#include "llvm/Transforms/Utils/PromoteMemToReg.h" 20#include "llvm/Analysis/Dominators.h" 21#include "llvm/iMemory.h" 22#include "llvm/iPHINode.h" 23#include "llvm/iTerminators.h" 24#include "llvm/Function.h" 25#include "llvm/Constant.h" 26#include "llvm/Type.h" 27#include "llvm/Support/CFG.h" 28#include "Support/StringExtras.h" 29 30/// isAllocaPromotable - Return true if this alloca is legal for promotion. 31/// This is true if there are only loads and stores to the alloca... 32/// 33bool isAllocaPromotable(const AllocaInst *AI, const TargetData &TD) { 34 // FIXME: If the memory unit is of pointer or integer type, we can permit 35 // assignments to subsections of the memory unit. 36 37 // Only allow direct loads and stores... 38 for (Value::use_const_iterator UI = AI->use_begin(), UE = AI->use_end(); 39 UI != UE; ++UI) // Loop over all of the uses of the alloca 40 if (!isa<LoadInst>(*UI)) 41 if (const StoreInst *SI = dyn_cast<StoreInst>(*UI)) { 42 if (SI->getOperand(0) == AI) 43 return false; // Don't allow a store of the AI, only INTO the AI. 44 } else { 45 return false; // Not a load or store? 46 } 47 48 return true; 49} 50 51 52namespace { 53 struct PromoteMem2Reg { 54 const std::vector<AllocaInst*> &Allocas; // the alloca instructions.. 55 std::vector<unsigned> VersionNumbers; // Current version counters 56 DominanceFrontier &DF; 57 const TargetData &TD; 58 59 std::map<Instruction*, unsigned> AllocaLookup; // reverse mapping of above 60 61 std::vector<std::vector<BasicBlock*> > PhiNodes;// Idx corresponds 2 Allocas 62 63 // List of instructions to remove at end of pass 64 std::vector<Instruction *> KillList; 65 66 std::map<BasicBlock*, 67 std::vector<PHINode*> > NewPhiNodes; // the PhiNodes we're adding 68 69 public: 70 PromoteMem2Reg(const std::vector<AllocaInst*> &A, DominanceFrontier &df, 71 const TargetData &td) 72 : Allocas(A), DF(df), TD(td) {} 73 74 void run(); 75 76 private: 77 void RenamePass(BasicBlock *BB, BasicBlock *Pred, 78 std::vector<Value*> &IncVals, 79 std::set<BasicBlock*> &Visited); 80 bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx); 81 }; 82} // end of anonymous namespace 83 84 85void PromoteMem2Reg::run() { 86 // If there is nothing to do, bail out... 87 if (Allocas.empty()) return; 88 89 Function &F = *DF.getRoot()->getParent(); 90 VersionNumbers.resize(Allocas.size()); 91 92 for (unsigned i = 0, e = Allocas.size(); i != e; ++i) { 93 assert(isAllocaPromotable(Allocas[i], TD) && 94 "Cannot promote non-promotable alloca!"); 95 assert(Allocas[i]->getParent()->getParent() == &F && 96 "All allocas should be in the same function, which is same as DF!"); 97 AllocaLookup[Allocas[i]] = i; 98 } 99 100 101 // Add each alloca to the KillList. Note: KillList is destroyed MOST recently 102 // added to least recently. 103 KillList.assign(Allocas.begin(), Allocas.end()); 104 105 // Calculate the set of write-locations for each alloca. This is analogous to 106 // counting the number of 'redefinitions' of each variable. 107 std::vector<std::vector<BasicBlock*> > WriteSets;// Idx corresponds to Allocas 108 WriteSets.resize(Allocas.size()); 109 for (unsigned i = 0; i != Allocas.size(); ++i) { 110 AllocaInst *AI = Allocas[i]; 111 for (Value::use_iterator U =AI->use_begin(), E = AI->use_end(); U != E; ++U) 112 if (StoreInst *SI = dyn_cast<StoreInst>(*U)) 113 // jot down the basic-block it came from 114 WriteSets[i].push_back(SI->getParent()); 115 } 116 117 // Compute the locations where PhiNodes need to be inserted. Look at the 118 // dominance frontier of EACH basic-block we have a write in 119 // 120 PhiNodes.resize(Allocas.size()); 121 for (unsigned i = 0; i != Allocas.size(); ++i) { 122 for (unsigned j = 0; j != WriteSets[i].size(); j++) { 123 // Look up the DF for this write, add it to PhiNodes 124 DominanceFrontier::const_iterator it = DF.find(WriteSets[i][j]); 125 if (it != DF.end()) { 126 const DominanceFrontier::DomSetType &S = it->second; 127 for (DominanceFrontier::DomSetType::iterator P = S.begin(),PE = S.end(); 128 P != PE; ++P) 129 QueuePhiNode(*P, i); 130 } 131 } 132 133 // Perform iterative step 134 for (unsigned k = 0; k != PhiNodes[i].size(); k++) { 135 DominanceFrontier::const_iterator it = DF.find(PhiNodes[i][k]); 136 if (it != DF.end()) { 137 const DominanceFrontier::DomSetType &S = it->second; 138 for (DominanceFrontier::DomSetType::iterator P = S.begin(),PE = S.end(); 139 P != PE; ++P) 140 QueuePhiNode(*P, i); 141 } 142 } 143 } 144 145 // Set the incoming values for the basic block to be null values for all of 146 // the alloca's. We do this in case there is a load of a value that has not 147 // been stored yet. In this case, it will get this null value. 148 // 149 std::vector<Value *> Values(Allocas.size()); 150 for (unsigned i = 0, e = Allocas.size(); i != e; ++i) 151 Values[i] = Constant::getNullValue(Allocas[i]->getAllocatedType()); 152 153 // Walks all basic blocks in the function performing the SSA rename algorithm 154 // and inserting the phi nodes we marked as necessary 155 // 156 std::set<BasicBlock*> Visited; // The basic blocks we've already visited 157 RenamePass(F.begin(), 0, Values, Visited); 158 159 // Remove all instructions marked by being placed in the KillList... 160 // 161 while (!KillList.empty()) { 162 Instruction *I = KillList.back(); 163 KillList.pop_back(); 164 165 // If there are any uses of these instructions left, they must be in 166 // sections of dead code that were not processed on the dominance frontier. 167 // Just delete the users now. 168 // 169 while (!I->use_empty()) { 170 Instruction *U = cast<Instruction>(I->use_back()); 171 if (!U->use_empty()) // If uses remain in dead code segment... 172 U->replaceAllUsesWith(Constant::getNullValue(U->getType())); 173 U->getParent()->getInstList().erase(U); 174 } 175 176 I->getParent()->getInstList().erase(I); 177 } 178} 179 180 181// QueuePhiNode - queues a phi-node to be added to a basic-block for a specific 182// Alloca returns true if there wasn't already a phi-node for that variable 183// 184bool PromoteMem2Reg::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo) { 185 // Look up the basic-block in question 186 std::vector<PHINode*> &BBPNs = NewPhiNodes[BB]; 187 if (BBPNs.empty()) BBPNs.resize(Allocas.size()); 188 189 // If the BB already has a phi node added for the i'th alloca then we're done! 190 if (BBPNs[AllocaNo]) return false; 191 192 // Create a PhiNode using the dereferenced type... and add the phi-node to the 193 // BasicBlock. 194 PHINode *PN = new PHINode(Allocas[AllocaNo]->getAllocatedType(), 195 Allocas[AllocaNo]->getName() + "." + 196 utostr(VersionNumbers[AllocaNo]++), 197 BB->begin()); 198 199 // Add null incoming values for all predecessors. This ensures that if one of 200 // the predecessors is not found in the depth-first traversal of the CFG (ie, 201 // because it is an unreachable predecessor), that all PHI nodes will have the 202 // correct number of entries for their predecessors. 203 Value *NullVal = Constant::getNullValue(PN->getType()); 204 205 // This is neccesary because adding incoming values to the PHI node adds uses 206 // to the basic blocks being used, which can invalidate the predecessor 207 // iterator! 208 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB)); 209 for (unsigned i = 0, e = Preds.size(); i != e; ++i) 210 PN->addIncoming(NullVal, Preds[i]); 211 212 BBPNs[AllocaNo] = PN; 213 PhiNodes[AllocaNo].push_back(BB); 214 return true; 215} 216 217void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred, 218 std::vector<Value*> &IncomingVals, 219 std::set<BasicBlock*> &Visited) { 220 // If this is a BB needing a phi node, lookup/create the phinode for each 221 // variable we need phinodes for. 222 std::vector<PHINode *> &BBPNs = NewPhiNodes[BB]; 223 for (unsigned k = 0; k != BBPNs.size(); ++k) 224 if (PHINode *PN = BBPNs[k]) { 225 // The PHI node may have multiple entries for this predecessor. We must 226 // make sure we update all of them. 227 for (unsigned i = 0, e = PN->getNumOperands(); i != e; i += 2) { 228 if (PN->getOperand(i+1) == Pred) 229 // At this point we can assume that the array has phi nodes.. let's 230 // update the incoming data. 231 PN->setOperand(i, IncomingVals[k]); 232 } 233 // also note that the active variable IS designated by the phi node 234 IncomingVals[k] = PN; 235 } 236 237 // don't revisit nodes 238 if (Visited.count(BB)) return; 239 240 // mark as visited 241 Visited.insert(BB); 242 243 // keep track of the value of each variable we're watching.. how? 244 for (BasicBlock::iterator II = BB->begin(); II != BB->end(); ++II) { 245 Instruction *I = II; // get the instruction 246 247 if (LoadInst *LI = dyn_cast<LoadInst>(I)) { 248 if (AllocaInst *Src = dyn_cast<AllocaInst>(LI->getPointerOperand())) { 249 std::map<Instruction*, unsigned>::iterator AI = AllocaLookup.find(Src); 250 if (AI != AllocaLookup.end()) { 251 Value *V = IncomingVals[AI->second]; 252 253 // walk the use list of this load and replace all uses with r 254 LI->replaceAllUsesWith(V); 255 KillList.push_back(LI); // Mark the load to be deleted 256 } 257 } 258 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) { 259 // Delete this instruction and mark the name as the current holder of the 260 // value 261 if (AllocaInst *Dest = dyn_cast<AllocaInst>(SI->getPointerOperand())) { 262 std::map<Instruction *, unsigned>::iterator ai =AllocaLookup.find(Dest); 263 if (ai != AllocaLookup.end()) { 264 // what value were we writing? 265 IncomingVals[ai->second] = SI->getOperand(0); 266 KillList.push_back(SI); // Mark the store to be deleted 267 } 268 } 269 270 } else if (TerminatorInst *TI = dyn_cast<TerminatorInst>(I)) { 271 // Recurse across our successors 272 for (unsigned i = 0; i != TI->getNumSuccessors(); i++) { 273 std::vector<Value*> OutgoingVals(IncomingVals); 274 RenamePass(TI->getSuccessor(i), BB, OutgoingVals, Visited); 275 } 276 } 277 } 278} 279 280/// PromoteMemToReg - Promote the specified list of alloca instructions into 281/// scalar registers, inserting PHI nodes as appropriate. This function makes 282/// use of DominanceFrontier information. This function does not modify the CFG 283/// of the function at all. All allocas must be from the same function. 284/// 285void PromoteMemToReg(const std::vector<AllocaInst*> &Allocas, 286 DominanceFrontier &DF, const TargetData &TD) { 287 PromoteMem2Reg(Allocas, DF, TD).run(); 288} 289