1 //===- LoopDependenceAnalysis.cpp - LDA Implementation ----------*- C++ -*-===// 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 // 10 // This is the (beginning) of an implementation of a loop dependence analysis 11 // framework, which is used to detect dependences in memory accesses in loops. 12 // 13 // Please note that this is work in progress and the interface is subject to 14 // change. 15 // 16 // TODO: adapt as implementation progresses. 17 // 18 // TODO: document lingo (pair, subscript, index) 19 // 20 //===----------------------------------------------------------------------===// 21 22 #define DEBUG_TYPE "lda" 23 #include "llvm/ADT/DenseSet.h" 24 #include "llvm/ADT/Statistic.h" 25 #include "llvm/Analysis/AliasAnalysis.h" 26 #include "llvm/Analysis/LoopDependenceAnalysis.h" 27 #include "llvm/Analysis/LoopPass.h" 28 #include "llvm/Analysis/ScalarEvolution.h" 29 #include "llvm/Analysis/ScalarEvolutionExpressions.h" 30 #include "llvm/Analysis/ValueTracking.h" 31 #include "llvm/Assembly/Writer.h" 32 #include "llvm/Instructions.h" 33 #include "llvm/Operator.h" 34 #include "llvm/Support/Allocator.h" 35 #include "llvm/Support/Debug.h" 36 #include "llvm/Support/ErrorHandling.h" 37 #include "llvm/Support/raw_ostream.h" 38 #include "llvm/Target/TargetData.h" 39 using namespace llvm; 40 41 STATISTIC(NumAnswered, "Number of dependence queries answered"); 42 STATISTIC(NumAnalysed, "Number of distinct dependence pairs analysed"); 43 STATISTIC(NumDependent, "Number of pairs with dependent accesses"); 44 STATISTIC(NumIndependent, "Number of pairs with independent accesses"); 45 STATISTIC(NumUnknown, "Number of pairs with unknown accesses"); 46 47 LoopPass *llvm::createLoopDependenceAnalysisPass() { 48 return new LoopDependenceAnalysis(); 49 } 50 51 INITIALIZE_PASS_BEGIN(LoopDependenceAnalysis, "lda", 52 "Loop Dependence Analysis", false, true) 53 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution) 54 INITIALIZE_AG_DEPENDENCY(AliasAnalysis) 55 INITIALIZE_PASS_END(LoopDependenceAnalysis, "lda", 56 "Loop Dependence Analysis", false, true) 57 char LoopDependenceAnalysis::ID = 0; 58 59 //===----------------------------------------------------------------------===// 60 // Utility Functions 61 //===----------------------------------------------------------------------===// 62 63 static inline bool IsMemRefInstr(const Value *V) { 64 const Instruction *I = dyn_cast<const Instruction>(V); 65 return I && (I->mayReadFromMemory() || I->mayWriteToMemory()); 66 } 67 68 static void GetMemRefInstrs(const Loop *L, 69 SmallVectorImpl<Instruction*> &Memrefs) { 70 for (Loop::block_iterator b = L->block_begin(), be = L->block_end(); 71 b != be; ++b) 72 for (BasicBlock::iterator i = (*b)->begin(), ie = (*b)->end(); 73 i != ie; ++i) 74 if (IsMemRefInstr(i)) 75 Memrefs.push_back(i); 76 } 77 78 static bool IsLoadOrStoreInst(Value *I) { 79 // Returns true if the load or store can be analyzed. Atomic and volatile 80 // operations have properties which this analysis does not understand. 81 if (LoadInst *LI = dyn_cast<LoadInst>(I)) 82 return LI->isUnordered(); 83 else if (StoreInst *SI = dyn_cast<StoreInst>(I)) 84 return SI->isUnordered(); 85 return false; 86 } 87 88 static Value *GetPointerOperand(Value *I) { 89 if (LoadInst *i = dyn_cast<LoadInst>(I)) 90 return i->getPointerOperand(); 91 if (StoreInst *i = dyn_cast<StoreInst>(I)) 92 return i->getPointerOperand(); 93 llvm_unreachable("Value is no load or store instruction!"); 94 // Never reached. 95 return 0; 96 } 97 98 static AliasAnalysis::AliasResult UnderlyingObjectsAlias(AliasAnalysis *AA, 99 const Value *A, 100 const Value *B) { 101 const Value *aObj = GetUnderlyingObject(A); 102 const Value *bObj = GetUnderlyingObject(B); 103 return AA->alias(aObj, AA->getTypeStoreSize(aObj->getType()), 104 bObj, AA->getTypeStoreSize(bObj->getType())); 105 } 106 107 static inline const SCEV *GetZeroSCEV(ScalarEvolution *SE) { 108 return SE->getConstant(Type::getInt32Ty(SE->getContext()), 0L); 109 } 110 111 //===----------------------------------------------------------------------===// 112 // Dependence Testing 113 //===----------------------------------------------------------------------===// 114 115 bool LoopDependenceAnalysis::isDependencePair(const Value *A, 116 const Value *B) const { 117 return IsMemRefInstr(A) && 118 IsMemRefInstr(B) && 119 (cast<const Instruction>(A)->mayWriteToMemory() || 120 cast<const Instruction>(B)->mayWriteToMemory()); 121 } 122 123 bool LoopDependenceAnalysis::findOrInsertDependencePair(Value *A, 124 Value *B, 125 DependencePair *&P) { 126 void *insertPos = 0; 127 FoldingSetNodeID id; 128 id.AddPointer(A); 129 id.AddPointer(B); 130 131 P = Pairs.FindNodeOrInsertPos(id, insertPos); 132 if (P) return true; 133 134 P = new (PairAllocator) DependencePair(id, A, B); 135 Pairs.InsertNode(P, insertPos); 136 return false; 137 } 138 139 void LoopDependenceAnalysis::getLoops(const SCEV *S, 140 DenseSet<const Loop*>* Loops) const { 141 // Refactor this into an SCEVVisitor, if efficiency becomes a concern. 142 for (const Loop *L = this->L; L != 0; L = L->getParentLoop()) 143 if (!SE->isLoopInvariant(S, L)) 144 Loops->insert(L); 145 } 146 147 bool LoopDependenceAnalysis::isLoopInvariant(const SCEV *S) const { 148 DenseSet<const Loop*> loops; 149 getLoops(S, &loops); 150 return loops.empty(); 151 } 152 153 bool LoopDependenceAnalysis::isAffine(const SCEV *S) const { 154 const SCEVAddRecExpr *rec = dyn_cast<SCEVAddRecExpr>(S); 155 return isLoopInvariant(S) || (rec && rec->isAffine()); 156 } 157 158 bool LoopDependenceAnalysis::isZIVPair(const SCEV *A, const SCEV *B) const { 159 return isLoopInvariant(A) && isLoopInvariant(B); 160 } 161 162 bool LoopDependenceAnalysis::isSIVPair(const SCEV *A, const SCEV *B) const { 163 DenseSet<const Loop*> loops; 164 getLoops(A, &loops); 165 getLoops(B, &loops); 166 return loops.size() == 1; 167 } 168 169 LoopDependenceAnalysis::DependenceResult 170 LoopDependenceAnalysis::analyseZIV(const SCEV *A, 171 const SCEV *B, 172 Subscript *S) const { 173 assert(isZIVPair(A, B) && "Attempted to ZIV-test non-ZIV SCEVs!"); 174 return A == B ? Dependent : Independent; 175 } 176 177 LoopDependenceAnalysis::DependenceResult 178 LoopDependenceAnalysis::analyseSIV(const SCEV *A, 179 const SCEV *B, 180 Subscript *S) const { 181 return Unknown; // TODO: Implement. 182 } 183 184 LoopDependenceAnalysis::DependenceResult 185 LoopDependenceAnalysis::analyseMIV(const SCEV *A, 186 const SCEV *B, 187 Subscript *S) const { 188 return Unknown; // TODO: Implement. 189 } 190 191 LoopDependenceAnalysis::DependenceResult 192 LoopDependenceAnalysis::analyseSubscript(const SCEV *A, 193 const SCEV *B, 194 Subscript *S) const { 195 DEBUG(dbgs() << " Testing subscript: " << *A << ", " << *B << "\n"); 196 197 if (A == B) { 198 DEBUG(dbgs() << " -> [D] same SCEV\n"); 199 return Dependent; 200 } 201 202 if (!isAffine(A) || !isAffine(B)) { 203 DEBUG(dbgs() << " -> [?] not affine\n"); 204 return Unknown; 205 } 206 207 if (isZIVPair(A, B)) 208 return analyseZIV(A, B, S); 209 210 if (isSIVPair(A, B)) 211 return analyseSIV(A, B, S); 212 213 return analyseMIV(A, B, S); 214 } 215 216 LoopDependenceAnalysis::DependenceResult 217 LoopDependenceAnalysis::analysePair(DependencePair *P) const { 218 DEBUG(dbgs() << "Analysing:\n" << *P->A << "\n" << *P->B << "\n"); 219 220 // We only analyse loads and stores but no possible memory accesses by e.g. 221 // free, call, or invoke instructions. 222 if (!IsLoadOrStoreInst(P->A) || !IsLoadOrStoreInst(P->B)) { 223 DEBUG(dbgs() << "--> [?] no load/store\n"); 224 return Unknown; 225 } 226 227 Value *aPtr = GetPointerOperand(P->A); 228 Value *bPtr = GetPointerOperand(P->B); 229 230 switch (UnderlyingObjectsAlias(AA, aPtr, bPtr)) { 231 case AliasAnalysis::MayAlias: 232 case AliasAnalysis::PartialAlias: 233 // We can not analyse objects if we do not know about their aliasing. 234 DEBUG(dbgs() << "---> [?] may alias\n"); 235 return Unknown; 236 237 case AliasAnalysis::NoAlias: 238 // If the objects noalias, they are distinct, accesses are independent. 239 DEBUG(dbgs() << "---> [I] no alias\n"); 240 return Independent; 241 242 case AliasAnalysis::MustAlias: 243 break; // The underlying objects alias, test accesses for dependence. 244 } 245 246 const GEPOperator *aGEP = dyn_cast<GEPOperator>(aPtr); 247 const GEPOperator *bGEP = dyn_cast<GEPOperator>(bPtr); 248 249 if (!aGEP || !bGEP) 250 return Unknown; 251 252 // FIXME: Is filtering coupled subscripts necessary? 253 254 // Collect GEP operand pairs (FIXME: use GetGEPOperands from BasicAA), adding 255 // trailing zeroes to the smaller GEP, if needed. 256 typedef SmallVector<std::pair<const SCEV*, const SCEV*>, 4> GEPOpdPairsTy; 257 GEPOpdPairsTy opds; 258 for(GEPOperator::const_op_iterator aIdx = aGEP->idx_begin(), 259 aEnd = aGEP->idx_end(), 260 bIdx = bGEP->idx_begin(), 261 bEnd = bGEP->idx_end(); 262 aIdx != aEnd && bIdx != bEnd; 263 aIdx += (aIdx != aEnd), bIdx += (bIdx != bEnd)) { 264 const SCEV* aSCEV = (aIdx != aEnd) ? SE->getSCEV(*aIdx) : GetZeroSCEV(SE); 265 const SCEV* bSCEV = (bIdx != bEnd) ? SE->getSCEV(*bIdx) : GetZeroSCEV(SE); 266 opds.push_back(std::make_pair(aSCEV, bSCEV)); 267 } 268 269 if (!opds.empty() && opds[0].first != opds[0].second) { 270 // We cannot (yet) handle arbitrary GEP pointer offsets. By limiting 271 // 272 // TODO: this could be relaxed by adding the size of the underlying object 273 // to the first subscript. If we have e.g. (GEP x,0,i; GEP x,2,-i) and we 274 // know that x is a [100 x i8]*, we could modify the first subscript to be 275 // (i, 200-i) instead of (i, -i). 276 return Unknown; 277 } 278 279 // Now analyse the collected operand pairs (skipping the GEP ptr offsets). 280 for (GEPOpdPairsTy::const_iterator i = opds.begin() + 1, end = opds.end(); 281 i != end; ++i) { 282 Subscript subscript; 283 DependenceResult result = analyseSubscript(i->first, i->second, &subscript); 284 if (result != Dependent) { 285 // We either proved independence or failed to analyse this subscript. 286 // Further subscripts will not improve the situation, so abort early. 287 return result; 288 } 289 P->Subscripts.push_back(subscript); 290 } 291 // We successfully analysed all subscripts but failed to prove independence. 292 return Dependent; 293 } 294 295 bool LoopDependenceAnalysis::depends(Value *A, Value *B) { 296 assert(isDependencePair(A, B) && "Values form no dependence pair!"); 297 ++NumAnswered; 298 299 DependencePair *p; 300 if (!findOrInsertDependencePair(A, B, p)) { 301 // The pair is not cached, so analyse it. 302 ++NumAnalysed; 303 switch (p->Result = analysePair(p)) { 304 case Dependent: ++NumDependent; break; 305 case Independent: ++NumIndependent; break; 306 case Unknown: ++NumUnknown; break; 307 } 308 } 309 return p->Result != Independent; 310 } 311 312 //===----------------------------------------------------------------------===// 313 // LoopDependenceAnalysis Implementation 314 //===----------------------------------------------------------------------===// 315 316 bool LoopDependenceAnalysis::runOnLoop(Loop *L, LPPassManager &) { 317 this->L = L; 318 AA = &getAnalysis<AliasAnalysis>(); 319 SE = &getAnalysis<ScalarEvolution>(); 320 return false; 321 } 322 323 void LoopDependenceAnalysis::releaseMemory() { 324 Pairs.clear(); 325 PairAllocator.Reset(); 326 } 327 328 void LoopDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const { 329 AU.setPreservesAll(); 330 AU.addRequiredTransitive<AliasAnalysis>(); 331 AU.addRequiredTransitive<ScalarEvolution>(); 332 } 333 334 static void PrintLoopInfo(raw_ostream &OS, 335 LoopDependenceAnalysis *LDA, const Loop *L) { 336 if (!L->empty()) return; // ignore non-innermost loops 337 338 SmallVector<Instruction*, 8> memrefs; 339 GetMemRefInstrs(L, memrefs); 340 341 OS << "Loop at depth " << L->getLoopDepth() << ", header block: "; 342 WriteAsOperand(OS, L->getHeader(), false); 343 OS << "\n"; 344 345 OS << " Load/store instructions: " << memrefs.size() << "\n"; 346 for (SmallVector<Instruction*, 8>::const_iterator x = memrefs.begin(), 347 end = memrefs.end(); x != end; ++x) 348 OS << "\t" << (x - memrefs.begin()) << ": " << **x << "\n"; 349 350 OS << " Pairwise dependence results:\n"; 351 for (SmallVector<Instruction*, 8>::const_iterator x = memrefs.begin(), 352 end = memrefs.end(); x != end; ++x) 353 for (SmallVector<Instruction*, 8>::const_iterator y = x + 1; 354 y != end; ++y) 355 if (LDA->isDependencePair(*x, *y)) 356 OS << "\t" << (x - memrefs.begin()) << "," << (y - memrefs.begin()) 357 << ": " << (LDA->depends(*x, *y) ? "dependent" : "independent") 358 << "\n"; 359 } 360 361 void LoopDependenceAnalysis::print(raw_ostream &OS, const Module*) const { 362 // TODO: doc why const_cast is safe 363 PrintLoopInfo(OS, const_cast<LoopDependenceAnalysis*>(this), this->L); 364 } 365