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