ScalarReplAggregates.cpp revision 8430a4545c2743568aee94c39e3912795ce463ec
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/Support/GetElementPtrTypeIterator.h" 30#include "llvm/Target/TargetData.h" 31#include "llvm/Transforms/Utils/PromoteMemToReg.h" 32#include "Support/Debug.h" 33#include "Support/Statistic.h" 34#include "Support/StringExtras.h" 35using namespace llvm; 36 37namespace { 38 Statistic<> NumReplaced("scalarrepl", "Number of allocas broken up"); 39 Statistic<> NumPromoted("scalarrepl", "Number of allocas promoted"); 40 41 struct SROA : public FunctionPass { 42 bool runOnFunction(Function &F); 43 44 bool performScalarRepl(Function &F); 45 bool performPromotion(Function &F); 46 47 // getAnalysisUsage - This pass does not require any passes, but we know it 48 // will not alter the CFG, so say so. 49 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 50 AU.addRequired<DominatorTree>(); 51 AU.addRequired<DominanceFrontier>(); 52 AU.addRequired<TargetData>(); 53 AU.setPreservesCFG(); 54 } 55 56 private: 57 bool isSafeElementUse(Value *Ptr); 58 bool isSafeUseOfAllocation(Instruction *User); 59 bool isSafeAllocaToPromote(AllocationInst *AI); 60 AllocaInst *AddNewAlloca(Function &F, const Type *Ty, AllocationInst *Base); 61 }; 62 63 RegisterOpt<SROA> X("scalarrepl", "Scalar Replacement of Aggregates"); 64} 65 66// Public interface to the ScalarReplAggregates pass 67Pass *llvm::createScalarReplAggregatesPass() { return new SROA(); } 68 69 70bool SROA::runOnFunction(Function &F) { 71 bool Changed = performPromotion(F); 72 while (1) { 73 bool LocalChange = performScalarRepl(F); 74 if (!LocalChange) break; // No need to repromote if no scalarrepl 75 Changed = true; 76 LocalChange = performPromotion(F); 77 if (!LocalChange) break; // No need to re-scalarrepl if no promotion 78 } 79 80 return Changed; 81} 82 83 84bool SROA::performPromotion(Function &F) { 85 std::vector<AllocaInst*> Allocas; 86 const TargetData &TD = getAnalysis<TargetData>(); 87 DominatorTree &DT = getAnalysis<DominatorTree>(); 88 DominanceFrontier &DF = getAnalysis<DominanceFrontier>(); 89 90 BasicBlock &BB = F.getEntryBlock(); // Get the entry node for the function 91 92 bool Changed = false; 93 94 while (1) { 95 Allocas.clear(); 96 97 // Find allocas that are safe to promote, by looking at all instructions in 98 // the entry node 99 for (BasicBlock::iterator I = BB.begin(), E = --BB.end(); I != E; ++I) 100 if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) // Is it an alloca? 101 if (isAllocaPromotable(AI, TD)) 102 Allocas.push_back(AI); 103 104 if (Allocas.empty()) break; 105 106 PromoteMemToReg(Allocas, DT, DF, TD); 107 NumPromoted += Allocas.size(); 108 Changed = true; 109 } 110 111 return Changed; 112} 113 114 115// performScalarRepl - This algorithm is a simple worklist driven algorithm, 116// which runs on all of the malloc/alloca instructions in the function, removing 117// them if they are only used by getelementptr instructions. 118// 119bool SROA::performScalarRepl(Function &F) { 120 std::vector<AllocationInst*> WorkList; 121 122 // Scan the entry basic block, adding any alloca's and mallocs to the worklist 123 BasicBlock &BB = F.getEntryBlock(); 124 for (BasicBlock::iterator I = BB.begin(), E = BB.end(); I != E; ++I) 125 if (AllocationInst *A = dyn_cast<AllocationInst>(I)) 126 WorkList.push_back(A); 127 128 // Process the worklist 129 bool Changed = false; 130 while (!WorkList.empty()) { 131 AllocationInst *AI = WorkList.back(); 132 WorkList.pop_back(); 133 134 // We cannot transform the allocation instruction if it is an array 135 // allocation (allocations OF arrays are ok though), and an allocation of a 136 // scalar value cannot be decomposed at all. 137 // 138 if (AI->isArrayAllocation() || 139 (!isa<StructType>(AI->getAllocatedType()) && 140 !isa<ArrayType>(AI->getAllocatedType()))) continue; 141 142 // Check that all of the users of the allocation are capable of being 143 // transformed. 144 if (!isSafeAllocaToPromote(AI)) 145 continue; 146 147 DEBUG(std::cerr << "Found inst to xform: " << *AI); 148 Changed = true; 149 150 std::vector<AllocaInst*> ElementAllocas; 151 if (const StructType *ST = dyn_cast<StructType>(AI->getAllocatedType())) { 152 ElementAllocas.reserve(ST->getNumContainedTypes()); 153 for (unsigned i = 0, e = ST->getNumContainedTypes(); i != e; ++i) { 154 AllocaInst *NA = new AllocaInst(ST->getContainedType(i), 0, 155 AI->getName() + "." + utostr(i), AI); 156 ElementAllocas.push_back(NA); 157 WorkList.push_back(NA); // Add to worklist for recursive processing 158 } 159 } else { 160 const ArrayType *AT = cast<ArrayType>(AI->getAllocatedType()); 161 ElementAllocas.reserve(AT->getNumElements()); 162 const Type *ElTy = AT->getElementType(); 163 for (unsigned i = 0, e = AT->getNumElements(); i != e; ++i) { 164 AllocaInst *NA = new AllocaInst(ElTy, 0, 165 AI->getName() + "." + utostr(i), AI); 166 ElementAllocas.push_back(NA); 167 WorkList.push_back(NA); // Add to worklist for recursive processing 168 } 169 } 170 171 // Now that we have created the alloca instructions that we want to use, 172 // expand the getelementptr instructions to use them. 173 // 174 while (!AI->use_empty()) { 175 Instruction *User = cast<Instruction>(AI->use_back()); 176 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) { 177 // We now know that the GEP is of the form: GEP <ptr>, 0, <cst> 178 uint64_t Idx = cast<ConstantInt>(GEPI->getOperand(2))->getRawValue(); 179 180 assert(Idx < ElementAllocas.size() && "Index out of range?"); 181 AllocaInst *AllocaToUse = ElementAllocas[Idx]; 182 183 Value *RepValue; 184 if (GEPI->getNumOperands() == 3) { 185 // Do not insert a new getelementptr instruction with zero indices, 186 // only to have it optimized out later. 187 RepValue = AllocaToUse; 188 } else { 189 // We are indexing deeply into the structure, so we still need a 190 // getelement ptr instruction to finish the indexing. This may be 191 // expanded itself once the worklist is rerun. 192 // 193 std::string OldName = GEPI->getName(); // Steal the old name... 194 std::vector<Value*> NewArgs; 195 NewArgs.push_back(Constant::getNullValue(Type::IntTy)); 196 NewArgs.insert(NewArgs.end(), GEPI->op_begin()+3, GEPI->op_end()); 197 GEPI->setName(""); 198 RepValue = 199 new GetElementPtrInst(AllocaToUse, NewArgs, OldName, GEPI); 200 } 201 202 // Move all of the users over to the new GEP. 203 GEPI->replaceAllUsesWith(RepValue); 204 // Delete the old GEP 205 GEPI->getParent()->getInstList().erase(GEPI); 206 } else { 207 assert(0 && "Unexpected instruction type!"); 208 } 209 } 210 211 // Finally, delete the Alloca instruction 212 AI->getParent()->getInstList().erase(AI); 213 NumReplaced++; 214 } 215 216 return Changed; 217} 218 219 220/// isSafeUseOfAllocation - Check to see if this user is an allowed use for an 221/// aggregate allocation. 222/// 223bool SROA::isSafeUseOfAllocation(Instruction *User) { 224 if (!isa<GetElementPtrInst>(User)) return false; 225 226 GetElementPtrInst *GEPI = cast<GetElementPtrInst>(User); 227 gep_type_iterator I = gep_type_begin(GEPI), E = gep_type_end(GEPI); 228 229 // The GEP is safe to transform if it is of the form GEP <ptr>, 0, <cst> 230 if (I == E || 231 I.getOperand() != Constant::getNullValue(I.getOperand()->getType())) 232 return false; 233 234 ++I; 235 if (I == E || !isa<ConstantInt>(I.getOperand())) 236 return false; 237 238 // If this is a use of an array allocation, do a bit more checking for sanity. 239 if (const ArrayType *AT = dyn_cast<ArrayType>(*I)) { 240 uint64_t NumElements = AT->getNumElements(); 241 242 // Check to make sure that index falls within the array. If not, 243 // something funny is going on, so we won't do the optimization. 244 // 245 if (cast<ConstantInt>(GEPI->getOperand(2))->getRawValue() >= NumElements) 246 return false; 247 } 248 249 // If there are any non-simple uses of this getelementptr, make sure to reject 250 // them. 251 return isSafeElementUse(GEPI); 252} 253 254/// isSafeElementUse - Check to see if this use is an allowed use for a 255/// getelementptr instruction of an array aggregate allocation. 256/// 257bool SROA::isSafeElementUse(Value *Ptr) { 258 for (Value::use_iterator I = Ptr->use_begin(), E = Ptr->use_end(); 259 I != E; ++I) { 260 Instruction *User = cast<Instruction>(*I); 261 switch (User->getOpcode()) { 262 case Instruction::Load: break; 263 case Instruction::Store: 264 // Store is ok if storing INTO the pointer, not storing the pointer 265 if (User->getOperand(0) == Ptr) return false; 266 break; 267 case Instruction::GetElementPtr: { 268 GetElementPtrInst *GEP = cast<GetElementPtrInst>(User); 269 if (GEP->getNumOperands() > 1) { 270 if (!isa<Constant>(GEP->getOperand(1)) || 271 !cast<Constant>(GEP->getOperand(1))->isNullValue()) 272 return false; // Using pointer arithmetic to navigate the array... 273 } 274 if (!isSafeElementUse(GEP)) return false; 275 break; 276 } 277 default: 278 DEBUG(std::cerr << " Transformation preventing inst: " << *User); 279 return false; 280 } 281 } 282 return true; // All users look ok :) 283} 284 285 286/// isSafeStructAllocaToPromote - Check to see if the specified allocation of a 287/// structure can be broken down into elements. 288/// 289bool SROA::isSafeAllocaToPromote(AllocationInst *AI) { 290 // Loop over the use list of the alloca. We can only transform it if all of 291 // the users are safe to transform. 292 // 293 for (Value::use_iterator I = AI->use_begin(), E = AI->use_end(); 294 I != E; ++I) 295 if (!isSafeUseOfAllocation(cast<Instruction>(*I))) { 296 DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: " 297 << *I); 298 return false; 299 } 300 return true; 301} 302