ScalarReplAggregates.cpp revision b576c94c15af9a440f69d9d03c2afead7971118c
1//===- ScalarReplAggregates.cpp - Scalar Replacement of Aggregates --------===// 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 transformation implements the well known scalar replacement of 11// aggregates transformation. This xform breaks up alloca instructions of 12// aggregate type (structure or array) into individual alloca instructions for 13// each member (if possible). Then, if possible, it transforms the individual 14// alloca instructions into nice clean scalar SSA form. 15// 16// This combines a simple SRoA algorithm with the Mem2Reg algorithm because 17// often interact, especially for C++ programs. As such, iterating between 18// SRoA, then Mem2Reg until we run out of things to promote works well. 19// 20//===----------------------------------------------------------------------===// 21 22#include "llvm/Transforms/Scalar.h" 23#include "llvm/Constants.h" 24#include "llvm/DerivedTypes.h" 25#include "llvm/Function.h" 26#include "llvm/Pass.h" 27#include "llvm/iMemory.h" 28#include "llvm/Analysis/Dominators.h" 29#include "llvm/Target/TargetData.h" 30#include "llvm/Transforms/Utils/PromoteMemToReg.h" 31#include "Support/Debug.h" 32#include "Support/Statistic.h" 33#include "Support/StringExtras.h" 34 35namespace { 36 Statistic<> NumReplaced("scalarrepl", "Number of allocas broken up"); 37 Statistic<> NumPromoted("scalarrepl", "Number of allocas promoted"); 38 39 struct SROA : public FunctionPass { 40 bool runOnFunction(Function &F); 41 42 bool performScalarRepl(Function &F); 43 bool performPromotion(Function &F); 44 45 // getAnalysisUsage - This pass does not require any passes, but we know it 46 // will not alter the CFG, so say so. 47 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 48 AU.addRequired<DominatorTree>(); 49 AU.addRequired<DominanceFrontier>(); 50 AU.addRequired<TargetData>(); 51 AU.setPreservesCFG(); 52 } 53 54 private: 55 bool isSafeElementUse(Value *Ptr); 56 bool isSafeUseOfAllocation(Instruction *User); 57 bool isSafeStructAllocaToPromote(AllocationInst *AI); 58 bool isSafeArrayAllocaToPromote(AllocationInst *AI); 59 AllocaInst *AddNewAlloca(Function &F, const Type *Ty, AllocationInst *Base); 60 }; 61 62 RegisterOpt<SROA> X("scalarrepl", "Scalar Replacement of Aggregates"); 63} 64 65Pass *createScalarReplAggregatesPass() { return new SROA(); } 66 67 68bool SROA::runOnFunction(Function &F) { 69 bool Changed = performPromotion(F); 70 while (1) { 71 bool LocalChange = performScalarRepl(F); 72 if (!LocalChange) break; // No need to repromote if no scalarrepl 73 Changed = true; 74 LocalChange = performPromotion(F); 75 if (!LocalChange) break; // No need to re-scalarrepl if no promotion 76 } 77 78 return Changed; 79} 80 81 82bool SROA::performPromotion(Function &F) { 83 std::vector<AllocaInst*> Allocas; 84 const TargetData &TD = getAnalysis<TargetData>(); 85 DominatorTree &DT = getAnalysis<DominatorTree>(); 86 DominanceFrontier &DF = getAnalysis<DominanceFrontier>(); 87 88 BasicBlock &BB = F.getEntryBlock(); // Get the entry node for the function 89 90 bool Changed = false; 91 92 while (1) { 93 Allocas.clear(); 94 95 // Find allocas that are safe to promote, by looking at all instructions in 96 // the entry node 97 for (BasicBlock::iterator I = BB.begin(), E = --BB.end(); I != E; ++I) 98 if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) // Is it an alloca? 99 if (isAllocaPromotable(AI, TD)) 100 Allocas.push_back(AI); 101 102 if (Allocas.empty()) break; 103 104 PromoteMemToReg(Allocas, DT, DF, TD); 105 NumPromoted += Allocas.size(); 106 Changed = true; 107 } 108 109 return Changed; 110} 111 112 113// performScalarRepl - This algorithm is a simple worklist driven algorithm, 114// which runs on all of the malloc/alloca instructions in the function, removing 115// them if they are only used by getelementptr instructions. 116// 117bool SROA::performScalarRepl(Function &F) { 118 std::vector<AllocationInst*> WorkList; 119 120 // Scan the entry basic block, adding any alloca's and mallocs to the worklist 121 BasicBlock &BB = F.getEntryBlock(); 122 for (BasicBlock::iterator I = BB.begin(), E = BB.end(); I != E; ++I) 123 if (AllocationInst *A = dyn_cast<AllocationInst>(I)) 124 WorkList.push_back(A); 125 126 // Process the worklist 127 bool Changed = false; 128 while (!WorkList.empty()) { 129 AllocationInst *AI = WorkList.back(); 130 WorkList.pop_back(); 131 132 // We cannot transform the allocation instruction if it is an array 133 // allocation (allocations OF arrays are ok though), and an allocation of a 134 // scalar value cannot be decomposed at all. 135 // 136 if (AI->isArrayAllocation() || 137 (!isa<StructType>(AI->getAllocatedType()) && 138 !isa<ArrayType>(AI->getAllocatedType()))) continue; 139 140 // Check that all of the users of the allocation are capable of being 141 // transformed. 142 if (isa<StructType>(AI->getAllocatedType())) { 143 if (!isSafeStructAllocaToPromote(AI)) 144 continue; 145 } else if (!isSafeArrayAllocaToPromote(AI)) 146 continue; 147 148 DEBUG(std::cerr << "Found inst to xform: " << *AI); 149 Changed = true; 150 151 std::vector<AllocaInst*> ElementAllocas; 152 if (const StructType *ST = dyn_cast<StructType>(AI->getAllocatedType())) { 153 ElementAllocas.reserve(ST->getNumContainedTypes()); 154 for (unsigned i = 0, e = ST->getNumContainedTypes(); i != e; ++i) { 155 AllocaInst *NA = new AllocaInst(ST->getContainedType(i), 0, 156 AI->getName() + "." + utostr(i), AI); 157 ElementAllocas.push_back(NA); 158 WorkList.push_back(NA); // Add to worklist for recursive processing 159 } 160 } else { 161 const ArrayType *AT = cast<ArrayType>(AI->getAllocatedType()); 162 ElementAllocas.reserve(AT->getNumElements()); 163 const Type *ElTy = AT->getElementType(); 164 for (unsigned i = 0, e = AT->getNumElements(); i != e; ++i) { 165 AllocaInst *NA = new AllocaInst(ElTy, 0, 166 AI->getName() + "." + utostr(i), AI); 167 ElementAllocas.push_back(NA); 168 WorkList.push_back(NA); // Add to worklist for recursive processing 169 } 170 } 171 172 // Now that we have created the alloca instructions that we want to use, 173 // expand the getelementptr instructions to use them. 174 // 175 for (Value::use_iterator I = AI->use_begin(), E = AI->use_end(); 176 I != E; ++I) { 177 Instruction *User = cast<Instruction>(*I); 178 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) { 179 // We now know that the GEP is of the form: GEP <ptr>, 0, <cst> 180 uint64_t Idx = cast<ConstantInt>(GEPI->getOperand(2))->getRawValue(); 181 182 assert(Idx < ElementAllocas.size() && "Index out of range?"); 183 AllocaInst *AllocaToUse = ElementAllocas[Idx]; 184 185 Value *RepValue; 186 if (GEPI->getNumOperands() == 3) { 187 // Do not insert a new getelementptr instruction with zero indices, 188 // only to have it optimized out later. 189 RepValue = AllocaToUse; 190 } else { 191 // We are indexing deeply into the structure, so we still need a 192 // getelement ptr instruction to finish the indexing. This may be 193 // expanded itself once the worklist is rerun. 194 // 195 std::string OldName = GEPI->getName(); // Steal the old name... 196 std::vector<Value*> NewArgs; 197 NewArgs.push_back(Constant::getNullValue(Type::LongTy)); 198 NewArgs.insert(NewArgs.end(), GEPI->op_begin()+3, GEPI->op_end()); 199 GEPI->setName(""); 200 RepValue = 201 new GetElementPtrInst(AllocaToUse, NewArgs, OldName, GEPI); 202 } 203 204 // Move all of the users over to the new GEP. 205 GEPI->replaceAllUsesWith(RepValue); 206 // Delete the old GEP 207 GEPI->getParent()->getInstList().erase(GEPI); 208 } else { 209 assert(0 && "Unexpected instruction type!"); 210 } 211 } 212 213 // Finally, delete the Alloca instruction 214 AI->getParent()->getInstList().erase(AI); 215 NumReplaced++; 216 } 217 218 return Changed; 219} 220 221 222/// isSafeUseOfAllocation - Check to see if this user is an allowed use for an 223/// aggregate allocation. 224/// 225bool SROA::isSafeUseOfAllocation(Instruction *User) { 226 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) { 227 // The GEP is safe to transform if it is of the form GEP <ptr>, 0, <cst> 228 if (GEPI->getNumOperands() <= 2 || 229 GEPI->getOperand(1) != Constant::getNullValue(Type::LongTy) || 230 !isa<Constant>(GEPI->getOperand(2)) || 231 isa<ConstantExpr>(GEPI->getOperand(2))) 232 return false; 233 } else { 234 return false; 235 } 236 return true; 237} 238 239/// isSafeElementUse - Check to see if this use is an allowed use for a 240/// getelementptr instruction of an array aggregate allocation. 241/// 242bool SROA::isSafeElementUse(Value *Ptr) { 243 for (Value::use_iterator I = Ptr->use_begin(), E = Ptr->use_end(); 244 I != E; ++I) { 245 Instruction *User = cast<Instruction>(*I); 246 switch (User->getOpcode()) { 247 case Instruction::Load: break; 248 case Instruction::Store: 249 // Store is ok if storing INTO the pointer, not storing the pointer 250 if (User->getOperand(0) == Ptr) return false; 251 break; 252 case Instruction::GetElementPtr: { 253 GetElementPtrInst *GEP = cast<GetElementPtrInst>(User); 254 if (GEP->getNumOperands() > 1) { 255 if (!isa<Constant>(GEP->getOperand(1)) || 256 !cast<Constant>(GEP->getOperand(1))->isNullValue()) 257 return false; // Using pointer arithmetic to navigate the array... 258 } 259 if (!isSafeElementUse(GEP)) return false; 260 break; 261 } 262 default: 263 DEBUG(std::cerr << " Transformation preventing inst: " << *User); 264 return false; 265 } 266 } 267 return true; // All users look ok :) 268} 269 270 271/// isSafeStructAllocaToPromote - Check to see if the specified allocation of a 272/// structure can be broken down into elements. 273/// 274bool SROA::isSafeStructAllocaToPromote(AllocationInst *AI) { 275 // Loop over the use list of the alloca. We can only transform it if all of 276 // the users are safe to transform. 277 // 278 for (Value::use_iterator I = AI->use_begin(), E = AI->use_end(); 279 I != E; ++I) { 280 if (!isSafeUseOfAllocation(cast<Instruction>(*I))) { 281 DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: " 282 << *I); 283 return false; 284 } 285 286 // Pedantic check to avoid breaking broken programs... 287 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(*I)) 288 if (GEPI->getNumOperands() == 3 && !isSafeElementUse(GEPI)) 289 return false; 290 } 291 return true; 292} 293 294 295/// isSafeArrayAllocaToPromote - Check to see if the specified allocation of a 296/// structure can be broken down into elements. 297/// 298bool SROA::isSafeArrayAllocaToPromote(AllocationInst *AI) { 299 const ArrayType *AT = cast<ArrayType>(AI->getAllocatedType()); 300 int64_t NumElements = AT->getNumElements(); 301 302 // Loop over the use list of the alloca. We can only transform it if all of 303 // the users are safe to transform. Array allocas have extra constraints to 304 // meet though. 305 // 306 for (Value::use_iterator I = AI->use_begin(), E = AI->use_end(); 307 I != E; ++I) { 308 Instruction *User = cast<Instruction>(*I); 309 if (!isSafeUseOfAllocation(User)) { 310 DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: " 311 << User); 312 return false; 313 } 314 315 // Check to make sure that getelementptr follow the extra rules for arrays: 316 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) { 317 // Check to make sure that index falls within the array. If not, 318 // something funny is going on, so we won't do the optimization. 319 // 320 if (cast<ConstantSInt>(GEPI->getOperand(2))->getValue() >= NumElements) 321 return false; 322 323 // Check to make sure that the only thing that uses the resultant pointer 324 // is safe for an array access. For example, code that looks like: 325 // P = &A[0]; P = P + 1 326 // is legal, and should prevent promotion. 327 // 328 if (!isSafeElementUse(GEPI)) { 329 DEBUG(std::cerr << "Cannot transform: " << *AI 330 << " due to uses of user: " << *GEPI); 331 return false; 332 } 333 } 334 } 335 return true; 336} 337 338