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