1 //===- LoadCombine.cpp - Combine Adjacent Loads ---------------------------===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 /// \file 10 /// This transformation combines adjacent loads. 11 /// 12 //===----------------------------------------------------------------------===// 13 14 #include "llvm/Transforms/Scalar.h" 15 16 #include "llvm/ADT/DenseMap.h" 17 #include "llvm/ADT/Statistic.h" 18 #include "llvm/Analysis/TargetFolder.h" 19 #include "llvm/Pass.h" 20 #include "llvm/IR/DataLayout.h" 21 #include "llvm/IR/Function.h" 22 #include "llvm/IR/Instructions.h" 23 #include "llvm/IR/IRBuilder.h" 24 #include "llvm/Support/Debug.h" 25 #include "llvm/Support/MathExtras.h" 26 #include "llvm/Support/raw_ostream.h" 27 28 using namespace llvm; 29 30 #define DEBUG_TYPE "load-combine" 31 32 STATISTIC(NumLoadsAnalyzed, "Number of loads analyzed for combining"); 33 STATISTIC(NumLoadsCombined, "Number of loads combined"); 34 35 namespace { 36 struct PointerOffsetPair { 37 Value *Pointer; 38 uint64_t Offset; 39 }; 40 41 struct LoadPOPPair { 42 LoadPOPPair(LoadInst *L, PointerOffsetPair P, unsigned O) 43 : Load(L), POP(P), InsertOrder(O) {} 44 LoadPOPPair() {} 45 LoadInst *Load; 46 PointerOffsetPair POP; 47 /// \brief The new load needs to be created before the first load in IR order. 48 unsigned InsertOrder; 49 }; 50 51 class LoadCombine : public BasicBlockPass { 52 LLVMContext *C; 53 const DataLayout *DL; 54 55 public: 56 LoadCombine() 57 : BasicBlockPass(ID), 58 C(nullptr), DL(nullptr) { 59 initializeSROAPass(*PassRegistry::getPassRegistry()); 60 } 61 bool doInitialization(Function &) override; 62 bool runOnBasicBlock(BasicBlock &BB) override; 63 void getAnalysisUsage(AnalysisUsage &AU) const override; 64 65 const char *getPassName() const override { return "LoadCombine"; } 66 static char ID; 67 68 typedef IRBuilder<true, TargetFolder> BuilderTy; 69 70 private: 71 BuilderTy *Builder; 72 73 PointerOffsetPair getPointerOffsetPair(LoadInst &); 74 bool combineLoads(DenseMap<const Value *, SmallVector<LoadPOPPair, 8>> &); 75 bool aggregateLoads(SmallVectorImpl<LoadPOPPair> &); 76 bool combineLoads(SmallVectorImpl<LoadPOPPair> &); 77 }; 78 } 79 80 bool LoadCombine::doInitialization(Function &F) { 81 DEBUG(dbgs() << "LoadCombine function: " << F.getName() << "\n"); 82 C = &F.getContext(); 83 DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>(); 84 if (!DLP) { 85 DEBUG(dbgs() << " Skipping LoadCombine -- no target data!\n"); 86 return false; 87 } 88 DL = &DLP->getDataLayout(); 89 return true; 90 } 91 92 PointerOffsetPair LoadCombine::getPointerOffsetPair(LoadInst &LI) { 93 PointerOffsetPair POP; 94 POP.Pointer = LI.getPointerOperand(); 95 POP.Offset = 0; 96 while (isa<BitCastInst>(POP.Pointer) || isa<GetElementPtrInst>(POP.Pointer)) { 97 if (auto *GEP = dyn_cast<GetElementPtrInst>(POP.Pointer)) { 98 unsigned BitWidth = DL->getPointerTypeSizeInBits(GEP->getType()); 99 APInt Offset(BitWidth, 0); 100 if (GEP->accumulateConstantOffset(*DL, Offset)) 101 POP.Offset += Offset.getZExtValue(); 102 else 103 // Can't handle GEPs with variable indices. 104 return POP; 105 POP.Pointer = GEP->getPointerOperand(); 106 } else if (auto *BC = dyn_cast<BitCastInst>(POP.Pointer)) 107 POP.Pointer = BC->getOperand(0); 108 } 109 return POP; 110 } 111 112 bool LoadCombine::combineLoads( 113 DenseMap<const Value *, SmallVector<LoadPOPPair, 8>> &LoadMap) { 114 bool Combined = false; 115 for (auto &Loads : LoadMap) { 116 if (Loads.second.size() < 2) 117 continue; 118 std::sort(Loads.second.begin(), Loads.second.end(), 119 [](const LoadPOPPair &A, const LoadPOPPair &B) { 120 return A.POP.Offset < B.POP.Offset; 121 }); 122 if (aggregateLoads(Loads.second)) 123 Combined = true; 124 } 125 return Combined; 126 } 127 128 /// \brief Try to aggregate loads from a sorted list of loads to be combined. 129 /// 130 /// It is guaranteed that no writes occur between any of the loads. All loads 131 /// have the same base pointer. There are at least two loads. 132 bool LoadCombine::aggregateLoads(SmallVectorImpl<LoadPOPPair> &Loads) { 133 assert(Loads.size() >= 2 && "Insufficient loads!"); 134 LoadInst *BaseLoad = nullptr; 135 SmallVector<LoadPOPPair, 8> AggregateLoads; 136 bool Combined = false; 137 uint64_t PrevOffset = -1ull; 138 uint64_t PrevSize = 0; 139 for (auto &L : Loads) { 140 if (PrevOffset == -1ull) { 141 BaseLoad = L.Load; 142 PrevOffset = L.POP.Offset; 143 PrevSize = DL->getTypeStoreSize(L.Load->getType()); 144 AggregateLoads.push_back(L); 145 continue; 146 } 147 if (L.Load->getAlignment() > BaseLoad->getAlignment()) 148 continue; 149 if (L.POP.Offset > PrevOffset + PrevSize) { 150 // No other load will be combinable 151 if (combineLoads(AggregateLoads)) 152 Combined = true; 153 AggregateLoads.clear(); 154 PrevOffset = -1; 155 continue; 156 } 157 if (L.POP.Offset != PrevOffset + PrevSize) 158 // This load is offset less than the size of the last load. 159 // FIXME: We may want to handle this case. 160 continue; 161 PrevOffset = L.POP.Offset; 162 PrevSize = DL->getTypeStoreSize(L.Load->getType()); 163 AggregateLoads.push_back(L); 164 } 165 if (combineLoads(AggregateLoads)) 166 Combined = true; 167 return Combined; 168 } 169 170 /// \brief Given a list of combinable load. Combine the maximum number of them. 171 bool LoadCombine::combineLoads(SmallVectorImpl<LoadPOPPair> &Loads) { 172 // Remove loads from the end while the size is not a power of 2. 173 unsigned TotalSize = 0; 174 for (const auto &L : Loads) 175 TotalSize += L.Load->getType()->getPrimitiveSizeInBits(); 176 while (TotalSize != 0 && !isPowerOf2_32(TotalSize)) 177 TotalSize -= Loads.pop_back_val().Load->getType()->getPrimitiveSizeInBits(); 178 if (Loads.size() < 2) 179 return false; 180 181 DEBUG({ 182 dbgs() << "***** Combining Loads ******\n"; 183 for (const auto &L : Loads) { 184 dbgs() << L.POP.Offset << ": " << *L.Load << "\n"; 185 } 186 }); 187 188 // Find first load. This is where we put the new load. 189 LoadPOPPair FirstLP; 190 FirstLP.InsertOrder = -1u; 191 for (const auto &L : Loads) 192 if (L.InsertOrder < FirstLP.InsertOrder) 193 FirstLP = L; 194 195 unsigned AddressSpace = 196 FirstLP.POP.Pointer->getType()->getPointerAddressSpace(); 197 198 Builder->SetInsertPoint(FirstLP.Load); 199 Value *Ptr = Builder->CreateConstGEP1_64( 200 Builder->CreatePointerCast(Loads[0].POP.Pointer, 201 Builder->getInt8PtrTy(AddressSpace)), 202 Loads[0].POP.Offset); 203 LoadInst *NewLoad = new LoadInst( 204 Builder->CreatePointerCast( 205 Ptr, PointerType::get(IntegerType::get(Ptr->getContext(), TotalSize), 206 Ptr->getType()->getPointerAddressSpace())), 207 Twine(Loads[0].Load->getName()) + ".combined", false, 208 Loads[0].Load->getAlignment(), FirstLP.Load); 209 210 for (const auto &L : Loads) { 211 Builder->SetInsertPoint(L.Load); 212 Value *V = Builder->CreateExtractInteger( 213 *DL, NewLoad, cast<IntegerType>(L.Load->getType()), 214 L.POP.Offset - Loads[0].POP.Offset, "combine.extract"); 215 L.Load->replaceAllUsesWith(V); 216 } 217 218 NumLoadsCombined = NumLoadsCombined + Loads.size(); 219 return true; 220 } 221 222 bool LoadCombine::runOnBasicBlock(BasicBlock &BB) { 223 if (skipOptnoneFunction(BB) || !DL) 224 return false; 225 226 IRBuilder<true, TargetFolder> 227 TheBuilder(BB.getContext(), TargetFolder(DL)); 228 Builder = &TheBuilder; 229 230 DenseMap<const Value *, SmallVector<LoadPOPPair, 8>> LoadMap; 231 232 bool Combined = false; 233 unsigned Index = 0; 234 for (auto &I : BB) { 235 if (I.mayWriteToMemory() || I.mayThrow()) { 236 if (combineLoads(LoadMap)) 237 Combined = true; 238 LoadMap.clear(); 239 continue; 240 } 241 LoadInst *LI = dyn_cast<LoadInst>(&I); 242 if (!LI) 243 continue; 244 ++NumLoadsAnalyzed; 245 if (!LI->isSimple() || !LI->getType()->isIntegerTy()) 246 continue; 247 auto POP = getPointerOffsetPair(*LI); 248 if (!POP.Pointer) 249 continue; 250 LoadMap[POP.Pointer].push_back(LoadPOPPair(LI, POP, Index++)); 251 } 252 if (combineLoads(LoadMap)) 253 Combined = true; 254 return Combined; 255 } 256 257 void LoadCombine::getAnalysisUsage(AnalysisUsage &AU) const { 258 AU.setPreservesCFG(); 259 } 260 261 char LoadCombine::ID = 0; 262 263 BasicBlockPass *llvm::createLoadCombinePass() { 264 return new LoadCombine(); 265 } 266 267 INITIALIZE_PASS(LoadCombine, "load-combine", "Combine Adjacent Loads", false, 268 false) 269