ScalarReplAggregates.cpp revision 02a3be020a6b4eedb4b489959997d23a22cdf22e
1//===- ScalarReplAggregates.cpp - Scalar Replacement of Aggregates --------===// 2// 3// This transformation implements the well known scalar replacement of 4// aggregates transformation. This xform breaks up alloca instructions of 5// aggregate type (structure or array) into individual alloca instructions for 6// each member (if possible). Then, if possible, it transforms the individual 7// alloca instructions into nice clean scalar SSA form. 8// 9// This combines a simple SRoA algorithm with the Mem2Reg algorithm because 10// often interact, especially for C++ programs. As such, iterating between 11// SRoA, then Mem2Reg until we run out of things to promote works well. 12// 13//===----------------------------------------------------------------------===// 14 15#include "llvm/Transforms/Scalar.h" 16#include "llvm/Constants.h" 17#include "llvm/DerivedTypes.h" 18#include "llvm/Function.h" 19#include "llvm/Pass.h" 20#include "llvm/iMemory.h" 21#include "llvm/Analysis/Dominators.h" 22#include "llvm/Target/TargetData.h" 23#include "llvm/Transforms/Utils/PromoteMemToReg.h" 24#include "Support/Debug.h" 25#include "Support/Statistic.h" 26#include "Support/StringExtras.h" 27 28namespace { 29 Statistic<> NumReplaced("scalarrepl", "Number of allocas broken up"); 30 Statistic<> NumPromoted("scalarrepl", "Number of allocas promoted"); 31 32 struct SROA : public FunctionPass { 33 bool runOnFunction(Function &F); 34 35 bool performScalarRepl(Function &F); 36 bool performPromotion(Function &F); 37 38 // getAnalysisUsage - This pass does not require any passes, but we know it 39 // will not alter the CFG, so say so. 40 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 41 AU.addRequired<DominanceFrontier>(); 42 AU.addRequired<TargetData>(); 43 AU.setPreservesCFG(); 44 } 45 46 private: 47 bool isSafeElementUse(Value *Ptr); 48 bool isSafeUseOfAllocation(Instruction *User); 49 bool isSafeStructAllocaToPromote(AllocationInst *AI); 50 bool isSafeArrayAllocaToPromote(AllocationInst *AI); 51 AllocaInst *AddNewAlloca(Function &F, const Type *Ty, AllocationInst *Base); 52 }; 53 54 RegisterOpt<SROA> X("scalarrepl", "Scalar Replacement of Aggregates"); 55} 56 57Pass *createScalarReplAggregatesPass() { return new SROA(); } 58 59 60bool SROA::runOnFunction(Function &F) { 61 bool Changed = performPromotion(F); 62 while (1) { 63 bool LocalChange = performScalarRepl(F); 64 if (!LocalChange) break; // No need to repromote if no scalarrepl 65 Changed = true; 66 LocalChange = performPromotion(F); 67 if (!LocalChange) break; // No need to re-scalarrepl if no promotion 68 } 69 70 return Changed; 71} 72 73 74bool SROA::performPromotion(Function &F) { 75 std::vector<AllocaInst*> Allocas; 76 const TargetData &TD = getAnalysis<TargetData>(); 77 78 BasicBlock &BB = F.getEntryBlock(); // Get the entry node for the function 79 80 bool Changed = false; 81 82 while (1) { 83 Allocas.clear(); 84 85 // Find allocas that are safe to promote, by looking at all instructions in 86 // the entry node 87 for (BasicBlock::iterator I = BB.begin(), E = --BB.end(); I != E; ++I) 88 if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) // Is it an alloca? 89 if (isAllocaPromotable(AI, TD)) 90 Allocas.push_back(AI); 91 92 if (Allocas.empty()) break; 93 94 PromoteMemToReg(Allocas, getAnalysis<DominanceFrontier>(), TD); 95 NumPromoted += Allocas.size(); 96 Changed = true; 97 } 98 99 return Changed; 100} 101 102 103// performScalarRepl - This algorithm is a simple worklist driven algorithm, 104// which runs on all of the malloc/alloca instructions in the function, removing 105// them if they are only used by getelementptr instructions. 106// 107bool SROA::performScalarRepl(Function &F) { 108 std::vector<AllocationInst*> WorkList; 109 110 // Scan the entry basic block, adding any alloca's and mallocs to the worklist 111 BasicBlock &BB = F.getEntryBlock(); 112 for (BasicBlock::iterator I = BB.begin(), E = BB.end(); I != E; ++I) 113 if (AllocationInst *A = dyn_cast<AllocationInst>(I)) 114 WorkList.push_back(A); 115 116 // Process the worklist 117 bool Changed = false; 118 while (!WorkList.empty()) { 119 AllocationInst *AI = WorkList.back(); 120 WorkList.pop_back(); 121 122 // We cannot transform the allocation instruction if it is an array 123 // allocation (allocations OF arrays are ok though), and an allocation of a 124 // scalar value cannot be decomposed at all. 125 // 126 if (AI->isArrayAllocation() || 127 (!isa<StructType>(AI->getAllocatedType()) && 128 !isa<ArrayType>(AI->getAllocatedType()))) continue; 129 130 // Check that all of the users of the allocation are capable of being 131 // transformed. 132 if (isa<StructType>(AI->getAllocatedType())) { 133 if (!isSafeStructAllocaToPromote(AI)) 134 continue; 135 } else if (!isSafeArrayAllocaToPromote(AI)) 136 continue; 137 138 DEBUG(std::cerr << "Found inst to xform: " << *AI); 139 Changed = true; 140 141 std::vector<AllocaInst*> ElementAllocas; 142 if (const StructType *ST = dyn_cast<StructType>(AI->getAllocatedType())) { 143 ElementAllocas.reserve(ST->getNumContainedTypes()); 144 for (unsigned i = 0, e = ST->getNumContainedTypes(); i != e; ++i) { 145 AllocaInst *NA = new AllocaInst(ST->getContainedType(i), 0, 146 AI->getName() + "." + utostr(i), AI); 147 ElementAllocas.push_back(NA); 148 WorkList.push_back(NA); // Add to worklist for recursive processing 149 } 150 } else { 151 const ArrayType *AT = cast<ArrayType>(AI->getAllocatedType()); 152 ElementAllocas.reserve(AT->getNumElements()); 153 const Type *ElTy = AT->getElementType(); 154 for (unsigned i = 0, e = AT->getNumElements(); i != e; ++i) { 155 AllocaInst *NA = new AllocaInst(ElTy, 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 } 161 162 // Now that we have created the alloca instructions that we want to use, 163 // expand the getelementptr instructions to use them. 164 // 165 for (Value::use_iterator I = AI->use_begin(), E = AI->use_end(); 166 I != E; ++I) { 167 Instruction *User = cast<Instruction>(*I); 168 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) { 169 // We now know that the GEP is of the form: GEP <ptr>, 0, <cst> 170 uint64_t Idx = cast<ConstantInt>(GEPI->getOperand(2))->getRawValue(); 171 172 assert(Idx < ElementAllocas.size() && "Index out of range?"); 173 AllocaInst *AllocaToUse = ElementAllocas[Idx]; 174 175 Value *RepValue; 176 if (GEPI->getNumOperands() == 3) { 177 // Do not insert a new getelementptr instruction with zero indices, 178 // only to have it optimized out later. 179 RepValue = AllocaToUse; 180 } else { 181 // We are indexing deeply into the structure, so we still need a 182 // getelement ptr instruction to finish the indexing. This may be 183 // expanded itself once the worklist is rerun. 184 // 185 std::string OldName = GEPI->getName(); // Steal the old name... 186 std::vector<Value*> NewArgs; 187 NewArgs.push_back(Constant::getNullValue(Type::LongTy)); 188 NewArgs.insert(NewArgs.end(), GEPI->op_begin()+3, GEPI->op_end()); 189 GEPI->setName(""); 190 RepValue = 191 new GetElementPtrInst(AllocaToUse, NewArgs, OldName, GEPI); 192 } 193 194 // Move all of the users over to the new GEP. 195 GEPI->replaceAllUsesWith(RepValue); 196 // Delete the old GEP 197 GEPI->getParent()->getInstList().erase(GEPI); 198 } else { 199 assert(0 && "Unexpected instruction type!"); 200 } 201 } 202 203 // Finally, delete the Alloca instruction 204 AI->getParent()->getInstList().erase(AI); 205 NumReplaced++; 206 } 207 208 return Changed; 209} 210 211 212/// isSafeUseOfAllocation - Check to see if this user is an allowed use for an 213/// aggregate allocation. 214/// 215bool SROA::isSafeUseOfAllocation(Instruction *User) { 216 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) { 217 // The GEP is safe to transform if it is of the form GEP <ptr>, 0, <cst> 218 if (GEPI->getNumOperands() <= 2 || 219 GEPI->getOperand(1) != Constant::getNullValue(Type::LongTy) || 220 !isa<Constant>(GEPI->getOperand(2)) || 221 isa<ConstantExpr>(GEPI->getOperand(2))) 222 return false; 223 } else { 224 return false; 225 } 226 return true; 227} 228 229/// isSafeElementUse - Check to see if this use is an allowed use for a 230/// getelementptr instruction of an array aggregate allocation. 231/// 232bool SROA::isSafeElementUse(Value *Ptr) { 233 for (Value::use_iterator I = Ptr->use_begin(), E = Ptr->use_end(); 234 I != E; ++I) { 235 Instruction *User = cast<Instruction>(*I); 236 switch (User->getOpcode()) { 237 case Instruction::Load: break; 238 case Instruction::Store: 239 // Store is ok if storing INTO the pointer, not storing the pointer 240 if (User->getOperand(0) == Ptr) return false; 241 break; 242 case Instruction::GetElementPtr: { 243 GetElementPtrInst *GEP = cast<GetElementPtrInst>(User); 244 if (GEP->getNumOperands() > 1) { 245 if (!isa<Constant>(GEP->getOperand(1)) || 246 !cast<Constant>(GEP->getOperand(1))->isNullValue()) 247 return false; // Using pointer arithmetic to navigate the array... 248 } 249 if (!isSafeElementUse(GEP)) return false; 250 break; 251 } 252 default: 253 DEBUG(std::cerr << " Transformation preventing inst: " << *User); 254 return false; 255 } 256 } 257 return true; // All users look ok :) 258} 259 260 261/// isSafeStructAllocaToPromote - Check to see if the specified allocation of a 262/// structure can be broken down into elements. 263/// 264bool SROA::isSafeStructAllocaToPromote(AllocationInst *AI) { 265 // Loop over the use list of the alloca. We can only transform it if all of 266 // the users are safe to transform. 267 // 268 for (Value::use_iterator I = AI->use_begin(), E = AI->use_end(); 269 I != E; ++I) { 270 if (!isSafeUseOfAllocation(cast<Instruction>(*I))) { 271 DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: " 272 << *I); 273 return false; 274 } 275 276 // Pedantic check to avoid breaking broken programs... 277 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(*I)) 278 if (GEPI->getNumOperands() == 3 && !isSafeElementUse(GEPI)) 279 return false; 280 } 281 return true; 282} 283 284 285/// isSafeArrayAllocaToPromote - Check to see if the specified allocation of a 286/// structure can be broken down into elements. 287/// 288bool SROA::isSafeArrayAllocaToPromote(AllocationInst *AI) { 289 const ArrayType *AT = cast<ArrayType>(AI->getAllocatedType()); 290 int64_t NumElements = AT->getNumElements(); 291 292 // Loop over the use list of the alloca. We can only transform it if all of 293 // the users are safe to transform. Array allocas have extra constraints to 294 // meet though. 295 // 296 for (Value::use_iterator I = AI->use_begin(), E = AI->use_end(); 297 I != E; ++I) { 298 Instruction *User = cast<Instruction>(*I); 299 if (!isSafeUseOfAllocation(User)) { 300 DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: " 301 << User); 302 return false; 303 } 304 305 // Check to make sure that getelementptr follow the extra rules for arrays: 306 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) { 307 // Check to make sure that index falls within the array. If not, 308 // something funny is going on, so we won't do the optimization. 309 // 310 if (cast<ConstantSInt>(GEPI->getOperand(2))->getValue() >= NumElements) 311 return false; 312 313 // Check to make sure that the only thing that uses the resultant pointer 314 // is safe for an array access. For example, code that looks like: 315 // P = &A[0]; P = P + 1 316 // is legal, and should prevent promotion. 317 // 318 if (!isSafeElementUse(GEPI)) { 319 DEBUG(std::cerr << "Cannot transform: " << *AI 320 << " due to uses of user: " << *GEPI); 321 return false; 322 } 323 } 324 } 325 return true; 326} 327 328