ScalarReplAggregates.cpp revision 6806f5614d2ec260fda954c951d33f58e77ed610
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/Debug.h" 17#include "Support/Statistic.h" 18#include "Support/StringExtras.h" 19 20namespace { 21 Statistic<> NumReplaced("scalarrepl", "Number of alloca's broken up"); 22 23 struct SROA : public FunctionPass { 24 bool runOnFunction(Function &F); 25 26 private: 27 bool isSafeElementUse(Value *Ptr); 28 bool isSafeUseOfAllocation(Instruction *User); 29 bool isSafeStructAllocaToPromote(AllocationInst *AI); 30 bool isSafeArrayAllocaToPromote(AllocationInst *AI); 31 AllocaInst *AddNewAlloca(Function &F, const Type *Ty, AllocationInst *Base); 32 }; 33 34 RegisterOpt<SROA> X("scalarrepl", "Scalar Replacement of Aggregates"); 35} 36 37Pass *createScalarReplAggregatesPass() { return new SROA(); } 38 39 40// runOnFunction - This algorithm is a simple worklist driven algorithm, which 41// runs on all of the malloc/alloca instructions in the function, removing them 42// if they are only used by getelementptr instructions. 43// 44bool SROA::runOnFunction(Function &F) { 45 std::vector<AllocationInst*> WorkList; 46 47 // Scan the entry basic block, adding any alloca's and mallocs to the worklist 48 BasicBlock &BB = F.getEntryNode(); 49 for (BasicBlock::iterator I = BB.begin(), E = BB.end(); I != E; ++I) 50 if (AllocationInst *A = dyn_cast<AllocationInst>(I)) 51 WorkList.push_back(A); 52 53 // Process the worklist 54 bool Changed = false; 55 while (!WorkList.empty()) { 56 AllocationInst *AI = WorkList.back(); 57 WorkList.pop_back(); 58 59 // We cannot transform the allocation instruction if it is an array 60 // allocation (allocations OF arrays are ok though), and an allocation of a 61 // scalar value cannot be decomposed at all. 62 // 63 if (AI->isArrayAllocation() || 64 (!isa<StructType>(AI->getAllocatedType()) && 65 !isa<ArrayType>(AI->getAllocatedType()))) continue; 66 67 // Check that all of the users of the allocation are capable of being 68 // transformed. 69 if (isa<StructType>(AI->getAllocatedType())) { 70 if (!isSafeStructAllocaToPromote(AI)) 71 continue; 72 } else if (!isSafeArrayAllocaToPromote(AI)) 73 continue; 74 75 DEBUG(std::cerr << "Found inst to xform: " << *AI); 76 Changed = true; 77 78 std::vector<AllocaInst*> ElementAllocas; 79 if (const StructType *ST = dyn_cast<StructType>(AI->getAllocatedType())) { 80 ElementAllocas.reserve(ST->getNumContainedTypes()); 81 for (unsigned i = 0, e = ST->getNumContainedTypes(); i != e; ++i) { 82 AllocaInst *NA = new AllocaInst(ST->getContainedType(i), 0, 83 AI->getName() + "." + utostr(i), AI); 84 ElementAllocas.push_back(NA); 85 WorkList.push_back(NA); // Add to worklist for recursive processing 86 } 87 } else { 88 const ArrayType *AT = cast<ArrayType>(AI->getAllocatedType()); 89 ElementAllocas.reserve(AT->getNumElements()); 90 const Type *ElTy = AT->getElementType(); 91 for (unsigned i = 0, e = AT->getNumElements(); i != e; ++i) { 92 AllocaInst *NA = new AllocaInst(ElTy, 0, 93 AI->getName() + "." + utostr(i), AI); 94 ElementAllocas.push_back(NA); 95 WorkList.push_back(NA); // Add to worklist for recursive processing 96 } 97 } 98 99 // Now that we have created the alloca instructions that we want to use, 100 // expand the getelementptr instructions to use them. 101 // 102 for (Value::use_iterator I = AI->use_begin(), E = AI->use_end(); 103 I != E; ++I) { 104 Instruction *User = cast<Instruction>(*I); 105 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) { 106 // We now know that the GEP is of the form: GEP <ptr>, 0, <cst> 107 uint64_t Idx = cast<ConstantInt>(GEPI->getOperand(2))->getRawValue(); 108 109 assert(Idx < ElementAllocas.size() && "Index out of range?"); 110 AllocaInst *AllocaToUse = ElementAllocas[Idx]; 111 112 Value *RepValue; 113 if (GEPI->getNumOperands() == 3) { 114 // Do not insert a new getelementptr instruction with zero indices, 115 // only to have it optimized out later. 116 RepValue = AllocaToUse; 117 } else { 118 // We are indexing deeply into the structure, so we still need a 119 // getelement ptr instruction to finish the indexing. This may be 120 // expanded itself once the worklist is rerun. 121 // 122 std::string OldName = GEPI->getName(); // Steal the old name... 123 std::vector<Value*> NewArgs; 124 NewArgs.push_back(Constant::getNullValue(Type::LongTy)); 125 NewArgs.insert(NewArgs.end(), GEPI->op_begin()+3, GEPI->op_end()); 126 GEPI->setName(""); 127 RepValue = 128 new GetElementPtrInst(AllocaToUse, NewArgs, OldName, GEPI); 129 } 130 131 // Move all of the users over to the new GEP. 132 GEPI->replaceAllUsesWith(RepValue); 133 // Delete the old GEP 134 GEPI->getParent()->getInstList().erase(GEPI); 135 } else { 136 assert(0 && "Unexpected instruction type!"); 137 } 138 } 139 140 // Finally, delete the Alloca instruction 141 AI->getParent()->getInstList().erase(AI); 142 NumReplaced++; 143 } 144 145 return Changed; 146} 147 148 149/// isSafeUseOfAllocation - Check to see if this user is an allowed use for an 150/// aggregate allocation. 151/// 152bool SROA::isSafeUseOfAllocation(Instruction *User) { 153 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) { 154 // The GEP is safe to transform if it is of the form GEP <ptr>, 0, <cst> 155 if (GEPI->getNumOperands() <= 2 || 156 GEPI->getOperand(1) != Constant::getNullValue(Type::LongTy) || 157 !isa<Constant>(GEPI->getOperand(2)) || 158 isa<ConstantExpr>(GEPI->getOperand(2))) 159 return false; 160 } else { 161 return false; 162 } 163 return true; 164} 165 166/// isSafeElementUse - Check to see if this use is an allowed use for a 167/// getelementptr instruction of an array aggregate allocation. 168/// 169bool SROA::isSafeElementUse(Value *Ptr) { 170 for (Value::use_iterator I = Ptr->use_begin(), E = Ptr->use_end(); 171 I != E; ++I) { 172 Instruction *User = cast<Instruction>(*I); 173 switch (User->getOpcode()) { 174 case Instruction::Load: return true; 175 case Instruction::Store: return User->getOperand(0) != Ptr; 176 case Instruction::GetElementPtr: { 177 GetElementPtrInst *GEP = cast<GetElementPtrInst>(User); 178 if (GEP->getNumOperands() > 1) { 179 if (!isa<Constant>(GEP->getOperand(1)) || 180 !cast<Constant>(GEP->getOperand(1))->isNullValue()) 181 return false; // Using pointer arithmetic to navigate the array... 182 } 183 return isSafeElementUse(GEP); 184 } 185 default: 186 DEBUG(std::cerr << " Transformation preventing inst: " << *User); 187 return false; 188 } 189 } 190 return true; // All users look ok :) 191} 192 193 194/// isSafeStructAllocaToPromote - Check to see if the specified allocation of a 195/// structure can be broken down into elements. 196/// 197bool SROA::isSafeStructAllocaToPromote(AllocationInst *AI) { 198 // Loop over the use list of the alloca. We can only transform it if all of 199 // the users are safe to transform. 200 // 201 for (Value::use_iterator I = AI->use_begin(), E = AI->use_end(); 202 I != E; ++I) { 203 if (!isSafeUseOfAllocation(cast<Instruction>(*I))) { 204 DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: " 205 << *I); 206 return false; 207 } 208 209 // Pedantic check to avoid breaking broken programs... 210 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(*I)) 211 if (GEPI->getNumOperands() == 3 && !isSafeElementUse(GEPI)) 212 return false; 213 } 214 return true; 215} 216 217 218/// isSafeArrayAllocaToPromote - Check to see if the specified allocation of a 219/// structure can be broken down into elements. 220/// 221bool SROA::isSafeArrayAllocaToPromote(AllocationInst *AI) { 222 const ArrayType *AT = cast<ArrayType>(AI->getAllocatedType()); 223 int64_t NumElements = AT->getNumElements(); 224 225 // Loop over the use list of the alloca. We can only transform it if all of 226 // the users are safe to transform. Array allocas have extra constraints to 227 // meet though. 228 // 229 for (Value::use_iterator I = AI->use_begin(), E = AI->use_end(); 230 I != E; ++I) { 231 Instruction *User = cast<Instruction>(*I); 232 if (!isSafeUseOfAllocation(User)) { 233 DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: " 234 << User); 235 return false; 236 } 237 238 // Check to make sure that getelementptr follow the extra rules for arrays: 239 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) { 240 // Check to make sure that index falls within the array. If not, 241 // something funny is going on, so we won't do the optimization. 242 // 243 if (cast<ConstantSInt>(GEPI->getOperand(2))->getValue() >= NumElements) 244 return false; 245 246 // Check to make sure that the only thing that uses the resultant pointer 247 // is safe for an array access. For example, code that looks like: 248 // P = &A[0]; P = P + 1 249 // is legal, and should prevent promotion. 250 // 251 if (!isSafeElementUse(GEPI)) { 252 DEBUG(std::cerr << "Cannot transform: " << *AI 253 << " due to uses of user: " << *GEPI); 254 return false; 255 } 256 } 257 } 258 return true; 259} 260 261