ScalarReplAggregates.cpp revision c07736a397012499e337c994f7f952b07c709544
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 isSafeElementUse(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 = cast<ConstantInt>(GEPI->getOperand(2))->getRawValue(); 107 108 assert(Idx < ElementAllocas.size() && "Index out of range?"); 109 AllocaInst *AllocaToUse = ElementAllocas[Idx]; 110 111 Value *RepValue; 112 if (GEPI->getNumOperands() == 3) { 113 // Do not insert a new getelementptr instruction with zero indices, 114 // only to have it optimized out later. 115 RepValue = AllocaToUse; 116 } else { 117 // We are indexing deeply into the structure, so we still need a 118 // getelement ptr instruction to finish the indexing. This may be 119 // expanded itself once the worklist is rerun. 120 // 121 std::string OldName = GEPI->getName(); // Steal the old name... 122 std::vector<Value*> NewArgs; 123 NewArgs.push_back(Constant::getNullValue(Type::LongTy)); 124 NewArgs.insert(NewArgs.end(), GEPI->op_begin()+3, GEPI->op_end()); 125 GEPI->setName(""); 126 RepValue = 127 new GetElementPtrInst(AllocaToUse, NewArgs, OldName, GEPI); 128 } 129 130 // Move all of the users over to the new GEP. 131 GEPI->replaceAllUsesWith(RepValue); 132 // Delete the old GEP 133 GEPI->getParent()->getInstList().erase(GEPI); 134 } else { 135 assert(0 && "Unexpected instruction type!"); 136 } 137 } 138 139 // Finally, delete the Alloca instruction 140 AI->getParent()->getInstList().erase(AI); 141 NumReplaced++; 142 } 143 144 return Changed; 145} 146 147 148/// isSafeUseOfAllocation - Check to see if this user is an allowed use for an 149/// aggregate allocation. 150/// 151bool SROA::isSafeUseOfAllocation(Instruction *User) { 152 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) { 153 // The GEP is safe to transform if it is of the form GEP <ptr>, 0, <cst> 154 if (GEPI->getNumOperands() <= 2 || 155 GEPI->getOperand(1) != Constant::getNullValue(Type::LongTy) || 156 !isa<Constant>(GEPI->getOperand(2)) || 157 isa<ConstantExpr>(GEPI->getOperand(2))) 158 return false; 159 } else { 160 return false; 161 } 162 return true; 163} 164 165/// isSafeElementUse - Check to see if this use is an allowed use for a 166/// getelementptr instruction of an array aggregate allocation. 167/// 168bool SROA::isSafeElementUse(Value *Ptr) { 169 for (Value::use_iterator I = Ptr->use_begin(), E = Ptr->use_end(); 170 I != E; ++I) { 171 Instruction *User = cast<Instruction>(*I); 172 switch (User->getOpcode()) { 173 case Instruction::Load: return true; 174 case Instruction::Store: return User->getOperand(0) != Ptr; 175 case Instruction::GetElementPtr: { 176 GetElementPtrInst *GEP = cast<GetElementPtrInst>(User); 177 if (GEP->getNumOperands() > 1) { 178 if (!isa<Constant>(GEP->getOperand(1)) || 179 !cast<Constant>(GEP->getOperand(1))->isNullValue()) 180 return false; // Using pointer arithmetic to navigate the array... 181 } 182 return isSafeElementUse(GEP); 183 } 184 default: 185 DEBUG(std::cerr << " Transformation preventing inst: " << *User); 186 return false; 187 } 188 } 189 return true; // All users look ok :) 190} 191 192 193/// isSafeStructAllocaToPromote - Check to see if the specified allocation of a 194/// structure can be broken down into elements. 195/// 196bool SROA::isSafeStructAllocaToPromote(AllocationInst *AI) { 197 // Loop over the use list of the alloca. We can only transform it if all of 198 // the users are safe to transform. 199 // 200 for (Value::use_iterator I = AI->use_begin(), E = AI->use_end(); 201 I != E; ++I) { 202 if (!isSafeUseOfAllocation(cast<Instruction>(*I))) { 203 DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: " 204 << *I); 205 return false; 206 } 207 208 // Pedantic check to avoid breaking broken programs... 209 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(*I)) 210 if (GEPI->getNumOperands() == 3 && !isSafeElementUse(GEPI)) 211 return false; 212 } 213 return true; 214} 215 216 217/// isSafeArrayAllocaToPromote - Check to see if the specified allocation of a 218/// structure can be broken down into elements. 219/// 220bool SROA::isSafeArrayAllocaToPromote(AllocationInst *AI) { 221 const ArrayType *AT = cast<ArrayType>(AI->getAllocatedType()); 222 int64_t NumElements = AT->getNumElements(); 223 224 // Loop over the use list of the alloca. We can only transform it if all of 225 // the users are safe to transform. Array allocas have extra constraints to 226 // meet though. 227 // 228 for (Value::use_iterator I = AI->use_begin(), E = AI->use_end(); 229 I != E; ++I) { 230 Instruction *User = cast<Instruction>(*I); 231 if (!isSafeUseOfAllocation(User)) { 232 DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: " 233 << User); 234 return false; 235 } 236 237 // Check to make sure that getelementptr follow the extra rules for arrays: 238 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) { 239 // Check to make sure that index falls within the array. If not, 240 // something funny is going on, so we won't do the optimization. 241 // 242 if (cast<ConstantSInt>(GEPI->getOperand(2))->getValue() >= NumElements) 243 return false; 244 245 // Check to make sure that the only thing that uses the resultant pointer 246 // is safe for an array access. For example, code that looks like: 247 // P = &A[0]; P = P + 1 248 // is legal, and should prevent promotion. 249 // 250 if (!isSafeElementUse(GEPI)) { 251 DEBUG(std::cerr << "Cannot transform: " << *AI 252 << " due to uses of user: " << *GEPI); 253 return false; 254 } 255 } 256 } 257 return true; 258} 259 260