lib/Analysis/LoopDependenceAnalysis.cpp

   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   return isa<LoadInst>(I) || isa<StoreInst>(I);
  80 }
  81
  82 static Value *GetPointerOperand(Value *I) {
  83   if (LoadInst *i = dyn_cast<LoadInst>(I))
  84     return i->getPointerOperand();
  85   if (StoreInst *i = dyn_cast<StoreInst>(I))
  86     return i->getPointerOperand();
  87   llvm_unreachable("Value is no load or store instruction!");
  88   // Never reached.
  89   return 0;
  90 }
  91
  92 static AliasAnalysis::AliasResult UnderlyingObjectsAlias(AliasAnalysis *AA,
  93                                                          const Value *A,
  94                                                          const Value *B) {
  95   const Value *aObj = GetUnderlyingObject(A);
  96   const Value *bObj = GetUnderlyingObject(B);
  97   return AA->alias(aObj, AA->getTypeStoreSize(aObj->getType()),
  98                    bObj, AA->getTypeStoreSize(bObj->getType()));
  99 }
 100
 101 static inline const SCEV *GetZeroSCEV(ScalarEvolution *SE) {
 102   return SE->getConstant(Type::getInt32Ty(SE->getContext()), 0L);
 103 }
 104
 105 //===----------------------------------------------------------------------===//
 106 //                             Dependence Testing
 107 //===----------------------------------------------------------------------===//
 108
 109 bool LoopDependenceAnalysis::isDependencePair(const Value *A,
 110                                               const Value *B) const {
 111   return IsMemRefInstr(A) &&
 112          IsMemRefInstr(B) &&
 113          (cast<const Instruction>(A)->mayWriteToMemory() ||
 114           cast<const Instruction>(B)->mayWriteToMemory());
 115 }
 116
 117 bool LoopDependenceAnalysis::findOrInsertDependencePair(Value *A,
 118                                                         Value *B,
 119                                                         DependencePair *&P) {
 120   void *insertPos = 0;
 121   FoldingSetNodeID id;
 122   id.AddPointer(A);
 123   id.AddPointer(B);
 124
 125   P = Pairs.FindNodeOrInsertPos(id, insertPos);
 126   if (P) return true;
 127
 128   P = new (PairAllocator) DependencePair(id, A, B);
 129   Pairs.InsertNode(P, insertPos);
 130   return false;
 131 }
 132
 133 void LoopDependenceAnalysis::getLoops(const SCEV *S,
 134                                       DenseSet<const Loop*>* Loops) const {
 135   // Refactor this into an SCEVVisitor, if efficiency becomes a concern.
 136   for (const Loop *L = this->L; L != 0; L = L->getParentLoop())
 137     if (!SE->isLoopInvariant(S, L))
 138       Loops->insert(L);
 139 }
 140
 141 bool LoopDependenceAnalysis::isLoopInvariant(const SCEV *S) const {
 142   DenseSet<const Loop*> loops;
 143   getLoops(S, &loops);
 144   return loops.empty();
 145 }
 146
 147 bool LoopDependenceAnalysis::isAffine(const SCEV *S) const {
 148   const SCEVAddRecExpr *rec = dyn_cast<SCEVAddRecExpr>(S);
 149   return isLoopInvariant(S) || (rec && rec->isAffine());
 150 }
 151
 152 bool LoopDependenceAnalysis::isZIVPair(const SCEV *A, const SCEV *B) const {
 153   return isLoopInvariant(A) && isLoopInvariant(B);
 154 }
 155
 156 bool LoopDependenceAnalysis::isSIVPair(const SCEV *A, const SCEV *B) const {
 157   DenseSet<const Loop*> loops;
 158   getLoops(A, &loops);
 159   getLoops(B, &loops);
 160   return loops.size() == 1;
 161 }
 162
 163 LoopDependenceAnalysis::DependenceResult
 164 LoopDependenceAnalysis::analyseZIV(const SCEV *A,
 165                                    const SCEV *B,
 166                                    Subscript *S) const {
 167   assert(isZIVPair(A, B) && "Attempted to ZIV-test non-ZIV SCEVs!");
 168   return A == B ? Dependent : Independent;
 169 }
 170
 171 LoopDependenceAnalysis::DependenceResult
 172 LoopDependenceAnalysis::analyseSIV(const SCEV *A,
 173                                    const SCEV *B,
 174                                    Subscript *S) const {
 175   return Unknown; // TODO: Implement.
 176 }
 177
 178 LoopDependenceAnalysis::DependenceResult
 179 LoopDependenceAnalysis::analyseMIV(const SCEV *A,
 180                                    const SCEV *B,
 181                                    Subscript *S) const {
 182   return Unknown; // TODO: Implement.
 183 }
 184
 185 LoopDependenceAnalysis::DependenceResult
 186 LoopDependenceAnalysis::analyseSubscript(const SCEV *A,
 187                                          const SCEV *B,
 188                                          Subscript *S) const {
 189   DEBUG(dbgs() << "  Testing subscript: " << *A << ", " << *B << "\n");
 190
 191   if (A == B) {
 192     DEBUG(dbgs() << "  -> [D] same SCEV\n");
 193     return Dependent;
 194   }
 195
 196   if (!isAffine(A) || !isAffine(B)) {
 197     DEBUG(dbgs() << "  -> [?] not affine\n");
 198     return Unknown;
 199   }
 200
 201   if (isZIVPair(A, B))
 202     return analyseZIV(A, B, S);
 203
 204   if (isSIVPair(A, B))
 205     return analyseSIV(A, B, S);
 206
 207   return analyseMIV(A, B, S);
 208 }
 209
 210 LoopDependenceAnalysis::DependenceResult
 211 LoopDependenceAnalysis::analysePair(DependencePair *P) const {
 212   DEBUG(dbgs() << "Analysing:\n" << *P->A << "\n" << *P->B << "\n");
 213
 214   // We only analyse loads and stores but no possible memory accesses by e.g.
 215   // free, call, or invoke instructions.
 216   if (!IsLoadOrStoreInst(P->A) || !IsLoadOrStoreInst(P->B)) {
 217     DEBUG(dbgs() << "--> [?] no load/store\n");
 218     return Unknown;
 219   }
 220
 221   Value *aPtr = GetPointerOperand(P->A);
 222   Value *bPtr = GetPointerOperand(P->B);
 223
 224   switch (UnderlyingObjectsAlias(AA, aPtr, bPtr)) {
 225   case AliasAnalysis::MayAlias:
 226   case AliasAnalysis::PartialAlias:
 227     // We can not analyse objects if we do not know about their aliasing.
 228     DEBUG(dbgs() << "---> [?] may alias\n");
 229     return Unknown;
 230
 231   case AliasAnalysis::NoAlias:
 232     // If the objects noalias, they are distinct, accesses are independent.
 233     DEBUG(dbgs() << "---> [I] no alias\n");
 234     return Independent;
 235
 236   case AliasAnalysis::MustAlias:
 237     break; // The underlying objects alias, test accesses for dependence.
 238   }
 239
 240   const GEPOperator *aGEP = dyn_cast<GEPOperator>(aPtr);
 241   const GEPOperator *bGEP = dyn_cast<GEPOperator>(bPtr);
 242
 243   if (!aGEP || !bGEP)
 244     return Unknown;
 245
 246   // FIXME: Is filtering coupled subscripts necessary?
 247
 248   // Collect GEP operand pairs (FIXME: use GetGEPOperands from BasicAA), adding
 249   // trailing zeroes to the smaller GEP, if needed.
 250   typedef SmallVector<std::pair<const SCEV*, const SCEV*>, 4> GEPOpdPairsTy;
 251   GEPOpdPairsTy opds;
 252   for(GEPOperator::const_op_iterator aIdx = aGEP->idx_begin(),
 253                                      aEnd = aGEP->idx_end(),
 254                                      bIdx = bGEP->idx_begin(),
 255                                      bEnd = bGEP->idx_end();
 256       aIdx != aEnd && bIdx != bEnd;
 257       aIdx += (aIdx != aEnd), bIdx += (bIdx != bEnd)) {
 258     const SCEV* aSCEV = (aIdx != aEnd) ? SE->getSCEV(*aIdx) : GetZeroSCEV(SE);
 259     const SCEV* bSCEV = (bIdx != bEnd) ? SE->getSCEV(*bIdx) : GetZeroSCEV(SE);
 260     opds.push_back(std::make_pair(aSCEV, bSCEV));
 261   }
 262
 263   if (!opds.empty() && opds[0].first != opds[0].second) {
 264     // We cannot (yet) handle arbitrary GEP pointer offsets. By limiting
 265     //
 266     // TODO: this could be relaxed by adding the size of the underlying object
 267     // to the first subscript. If we have e.g. (GEP x,0,i; GEP x,2,-i) and we
 268     // know that x is a [100 x i8]*, we could modify the first subscript to be
 269     // (i, 200-i) instead of (i, -i).
 270     return Unknown;
 271   }
 272
 273   // Now analyse the collected operand pairs (skipping the GEP ptr offsets).
 274   for (GEPOpdPairsTy::const_iterator i = opds.begin() + 1, end = opds.end();
 275        i != end; ++i) {
 276     Subscript subscript;
 277     DependenceResult result = analyseSubscript(i->first, i->second, &subscript);
 278     if (result != Dependent) {
 279       // We either proved independence or failed to analyse this subscript.
 280       // Further subscripts will not improve the situation, so abort early.
 281       return result;
 282     }
 283     P->Subscripts.push_back(subscript);
 284   }
 285   // We successfully analysed all subscripts but failed to prove independence.
 286   return Dependent;
 287 }
 288
 289 bool LoopDependenceAnalysis::depends(Value *A, Value *B) {
 290   assert(isDependencePair(A, B) && "Values form no dependence pair!");
 291   ++NumAnswered;
 292
 293   DependencePair *p;
 294   if (!findOrInsertDependencePair(A, B, p)) {
 295     // The pair is not cached, so analyse it.
 296     ++NumAnalysed;
 297     switch (p->Result = analysePair(p)) {
 298     case Dependent:   ++NumDependent;   break;
 299     case Independent: ++NumIndependent; break;
 300     case Unknown:     ++NumUnknown;     break;
 301     }
 302   }
 303   return p->Result != Independent;
 304 }
 305
 306 //===----------------------------------------------------------------------===//
 307 //                   LoopDependenceAnalysis Implementation
 308 //===----------------------------------------------------------------------===//
 309
 310 bool LoopDependenceAnalysis::runOnLoop(Loop *L, LPPassManager &) {
 311   this->L = L;
 312   AA = &getAnalysis<AliasAnalysis>();
 313   SE = &getAnalysis<ScalarEvolution>();
 314   return false;
 315 }
 316
 317 void LoopDependenceAnalysis::releaseMemory() {
 318   Pairs.clear();
 319   PairAllocator.Reset();
 320 }
 321
 322 void LoopDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
 323   AU.setPreservesAll();
 324   AU.addRequiredTransitive<AliasAnalysis>();
 325   AU.addRequiredTransitive<ScalarEvolution>();
 326 }
 327
 328 static void PrintLoopInfo(raw_ostream &OS,
 329                           LoopDependenceAnalysis *LDA, const Loop *L) {
 330   if (!L->empty()) return; // ignore non-innermost loops
 331
 332   SmallVector<Instruction*, 8> memrefs;
 333   GetMemRefInstrs(L, memrefs);
 334
 335   OS << "Loop at depth " << L->getLoopDepth() << ", header block: ";
 336   WriteAsOperand(OS, L->getHeader(), false);
 337   OS << "\n";
 338
 339   OS << "  Load/store instructions: " << memrefs.size() << "\n";
 340   for (SmallVector<Instruction*, 8>::const_iterator x = memrefs.begin(),
 341        end = memrefs.end(); x != end; ++x)
 342     OS << "\t" << (x - memrefs.begin()) << ": " << **x << "\n";
 343
 344   OS << "  Pairwise dependence results:\n";
 345   for (SmallVector<Instruction*, 8>::const_iterator x = memrefs.begin(),
 346        end = memrefs.end(); x != end; ++x)
 347     for (SmallVector<Instruction*, 8>::const_iterator y = x + 1;
 348          y != end; ++y)
 349       if (LDA->isDependencePair(*x, *y))
 350         OS << "\t" << (x - memrefs.begin()) << "," << (y - memrefs.begin())
 351            << ": " << (LDA->depends(*x, *y) ? "dependent" : "independent")
 352            << "\n";
 353 }
 354
 355 void LoopDependenceAnalysis::print(raw_ostream &OS, const Module*) const {
 356   // TODO: doc why const_cast is safe
 357   PrintLoopInfo(OS, const_cast<LoopDependenceAnalysis*>(this), this->L);
 358 }