ScalarReplAggregates.cpp revision 5e062a1eda2c4adffd428a35e737a431fc37f4e0
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). 7// 8//===----------------------------------------------------------------------===// 9 10#include "llvm/Transforms/Scalar.h" 11#include "llvm/Function.h" 12#include "llvm/Pass.h" 13#include "llvm/iMemory.h" 14#include "llvm/DerivedTypes.h" 15#include "llvm/Constants.h" 16#include "Support/StringExtras.h" 17#include "Support/Statistic.h" 18 19namespace { 20 Statistic<> NumReplaced("scalarrepl", "Number of alloca's broken up"); 21 22 struct SROA : public FunctionPass { 23 bool runOnFunction(Function &F); 24 25 private: 26 bool isSafeArrayElementUse(Value *Ptr); 27 bool isSafeUseOfAllocation(Instruction *User); 28 bool isSafeStructAllocaToPromote(AllocationInst *AI); 29 bool isSafeArrayAllocaToPromote(AllocationInst *AI); 30 AllocaInst *AddNewAlloca(Function &F, const Type *Ty, AllocationInst *Base); 31 }; 32 33 RegisterOpt<SROA> X("scalarrepl", "Scalar Replacement of Aggregates"); 34} 35 36Pass *createScalarReplAggregatesPass() { return new SROA(); } 37 38 39// runOnFunction - This algorithm is a simple worklist driven algorithm, which 40// runs on all of the malloc/alloca instructions in the function, removing them 41// if they are only used by getelementptr instructions. 42// 43bool SROA::runOnFunction(Function &F) { 44 std::vector<AllocationInst*> WorkList; 45 46 // Scan the entry basic block, adding any alloca's and mallocs to the worklist 47 BasicBlock &BB = F.getEntryNode(); 48 for (BasicBlock::iterator I = BB.begin(), E = BB.end(); I != E; ++I) 49 if (AllocationInst *A = dyn_cast<AllocationInst>(I)) 50 WorkList.push_back(A); 51 52 // Process the worklist 53 bool Changed = false; 54 while (!WorkList.empty()) { 55 AllocationInst *AI = WorkList.back(); 56 WorkList.pop_back(); 57 58 // We cannot transform the allocation instruction if it is an array 59 // allocation (allocations OF arrays are ok though), and an allocation of a 60 // scalar value cannot be decomposed at all. 61 // 62 if (AI->isArrayAllocation() || 63 (!isa<StructType>(AI->getAllocatedType()) && 64 !isa<ArrayType>(AI->getAllocatedType()))) continue; 65 66 // Check that all of the users of the allocation are capable of being 67 // transformed. 68 if (isa<StructType>(AI->getAllocatedType())) { 69 if (!isSafeStructAllocaToPromote(AI)) 70 continue; 71 } else if (!isSafeArrayAllocaToPromote(AI)) 72 continue; 73 74 DEBUG(std::cerr << "Found inst to xform: " << *AI); 75 Changed = true; 76 77 std::vector<AllocaInst*> ElementAllocas; 78 if (const StructType *ST = dyn_cast<StructType>(AI->getAllocatedType())) { 79 ElementAllocas.reserve(ST->getNumContainedTypes()); 80 for (unsigned i = 0, e = ST->getNumContainedTypes(); i != e; ++i) { 81 AllocaInst *NA = new AllocaInst(ST->getContainedType(i), 0, 82 AI->getName() + "." + utostr(i), AI); 83 ElementAllocas.push_back(NA); 84 WorkList.push_back(NA); // Add to worklist for recursive processing 85 } 86 } else { 87 const ArrayType *AT = cast<ArrayType>(AI->getAllocatedType()); 88 ElementAllocas.reserve(AT->getNumElements()); 89 const Type *ElTy = AT->getElementType(); 90 for (unsigned i = 0, e = AT->getNumElements(); i != e; ++i) { 91 AllocaInst *NA = new AllocaInst(ElTy, 0, 92 AI->getName() + "." + utostr(i), AI); 93 ElementAllocas.push_back(NA); 94 WorkList.push_back(NA); // Add to worklist for recursive processing 95 } 96 } 97 98 // Now that we have created the alloca instructions that we want to use, 99 // expand the getelementptr instructions to use them. 100 // 101 for (Value::use_iterator I = AI->use_begin(), E = AI->use_end(); 102 I != E; ++I) { 103 Instruction *User = cast<Instruction>(*I); 104 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) { 105 // We now know that the GEP is of the form: GEP <ptr>, 0, <cst> 106 uint64_t Idx; 107 if (ConstantSInt *CSI = dyn_cast<ConstantSInt>(GEPI->getOperand(2))) 108 Idx = CSI->getValue(); 109 else 110 Idx = cast<ConstantUInt>(GEPI->getOperand(2))->getValue(); 111 112 assert(Idx < ElementAllocas.size() && "Index out of range?"); 113 AllocaInst *AllocaToUse = ElementAllocas[Idx]; 114 115 Value *RepValue; 116 if (GEPI->getNumOperands() == 3) { 117 // Do not insert a new getelementptr instruction with zero indices, 118 // only to have it optimized out later. 119 RepValue = AllocaToUse; 120 } else { 121 // We are indexing deeply into the structure, so we still need a 122 // getelement ptr instruction to finish the indexing. This may be 123 // expanded itself once the worklist is rerun. 124 // 125 std::string OldName = GEPI->getName(); // Steal the old name... 126 GEPI->setName(""); 127 RepValue = 128 new GetElementPtrInst(AllocaToUse, 129 std::vector<Value*>(GEPI->op_begin()+3, 130 GEPI->op_end()), 131 OldName, GEPI); 132 } 133 134 // Move all of the users over to the new GEP. 135 GEPI->replaceAllUsesWith(RepValue); 136 // Delete the old GEP 137 GEPI->getParent()->getInstList().erase(GEPI); 138 } else { 139 assert(0 && "Unexpected instruction type!"); 140 } 141 } 142 143 // Finally, delete the Alloca instruction 144 AI->getParent()->getInstList().erase(AI); 145 NumReplaced++; 146 } 147 148 return Changed; 149} 150 151 152/// isSafeUseOfAllocation - Check to see if this user is an allowed use for an 153/// aggregate allocation. 154/// 155bool SROA::isSafeUseOfAllocation(Instruction *User) { 156 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) { 157 // The GEP is safe to transform if it is of the form GEP <ptr>, 0, <cst> 158 if (GEPI->getNumOperands() <= 2 || 159 GEPI->getOperand(1) != Constant::getNullValue(Type::LongTy) || 160 !isa<Constant>(GEPI->getOperand(2)) || 161 isa<ConstantExpr>(GEPI->getOperand(2))) 162 return false; 163 } else { 164 return false; 165 } 166 return true; 167} 168 169 170/// isSafeArrayElementUse - Check to see if this use is an allowed use for a 171/// getelementptr instruction of an array aggregate allocation. 172/// 173bool SROA::isSafeArrayElementUse(Value *Ptr) { 174 for (Value::use_iterator I = Ptr->use_begin(), E = Ptr->use_end(); 175 I != E; ++I) { 176 Instruction *User = cast<Instruction>(*I); 177 switch (User->getOpcode()) { 178 case Instruction::Load: return true; 179 case Instruction::Store: return User->getOperand(0) != Ptr; 180 case Instruction::GetElementPtr: { 181 GetElementPtrInst *GEP = cast<GetElementPtrInst>(User); 182 if (GEP->getNumOperands() > 1) { 183 if (!isa<Constant>(GEP->getOperand(1)) || 184 !cast<Constant>(GEP->getOperand(1))->isNullValue()) 185 return false; // Using pointer arithmetic to navigate the array... 186 187 // Check to see if there are any structure indexes involved in this GEP. 188 // If so, then we can safely break the array up until at least the 189 // structure. 190 for (unsigned i = 2, e = GEP->getNumOperands(); i != e; ++i) 191 if (GEP->getOperand(i)->getType()->isUnsigned()) 192 break; 193 } 194 return isSafeArrayElementUse(GEP); 195 } 196 default: 197 DEBUG(std::cerr << " Transformation preventing inst: " << *User); 198 return false; 199 } 200 } 201 return true; // All users look ok :) 202} 203 204 205/// isSafeStructAllocaToPromote - Check to see if the specified allocation of a 206/// structure can be broken down into elements. 207/// 208bool SROA::isSafeStructAllocaToPromote(AllocationInst *AI) { 209 // Loop over the use list of the alloca. We can only transform it if all of 210 // the users are safe to transform. 211 // 212 for (Value::use_iterator I = AI->use_begin(), E = AI->use_end(); 213 I != E; ++I) 214 if (!isSafeUseOfAllocation(cast<Instruction>(*I))) { 215 DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: " 216 << *I); 217 return false; 218 } 219 return true; 220} 221 222 223/// isSafeArrayAllocaToPromote - Check to see if the specified allocation of a 224/// structure can be broken down into elements. 225/// 226bool SROA::isSafeArrayAllocaToPromote(AllocationInst *AI) { 227 const ArrayType *AT = cast<ArrayType>(AI->getAllocatedType()); 228 int64_t NumElements = AT->getNumElements(); 229 230 // Loop over the use list of the alloca. We can only transform it if all of 231 // the users are safe to transform. Array allocas have extra constraints to 232 // meet though. 233 // 234 for (Value::use_iterator I = AI->use_begin(), E = AI->use_end(); 235 I != E; ++I) { 236 Instruction *User = cast<Instruction>(*I); 237 if (!isSafeUseOfAllocation(User)) { 238 DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: " 239 << User); 240 return false; 241 } 242 243 // Check to make sure that getelementptr follow the extra rules for arrays: 244 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) { 245 // Check to make sure that index falls within the array. If not, 246 // something funny is going on, so we won't do the optimization. 247 // 248 if (cast<ConstantSInt>(GEPI->getOperand(2))->getValue() >= NumElements) 249 return false; 250 251 // Check to make sure that the only thing that uses the resultant pointer 252 // is safe for an array access. For example, code that looks like: 253 // P = &A[0]; P = P + 1 254 // is legal, and should prevent promotion. 255 // 256 if (!isSafeArrayElementUse(GEPI)) { 257 DEBUG(std::cerr << "Cannot transform: " << *AI 258 << " due to uses of user: " << *GEPI); 259 return false; 260 } 261 } 262 } 263 return true; 264} 265 266