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/Statistic.h"
  24 #include "llvm/Analysis/AliasAnalysis.h"
  25 #include "llvm/Analysis/LoopDependenceAnalysis.h"
  26 #include "llvm/Analysis/LoopPass.h"
  27 #include "llvm/Analysis/ScalarEvolution.h"
  28 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
  29 #include "llvm/Instructions.h"
  30 #include "llvm/Operator.h"
  31 #include "llvm/Support/Allocator.h"
  32 #include "llvm/Support/Debug.h"
  33 #include "llvm/Support/ErrorHandling.h"
  34 #include "llvm/Support/raw_ostream.h"
  35 #include "llvm/Target/TargetData.h"
  36 using namespace llvm;
  37
  38 STATISTIC(NumAnswered,    "Number of dependence queries answered");
  39 STATISTIC(NumAnalysed,    "Number of distinct dependence pairs analysed");
  40 STATISTIC(NumDependent,   "Number of pairs with dependent accesses");
  41 STATISTIC(NumIndependent, "Number of pairs with independent accesses");
  42 STATISTIC(NumUnknown,     "Number of pairs with unknown accesses");
  43
  44 LoopPass *llvm::createLoopDependenceAnalysisPass() {
  45   return new LoopDependenceAnalysis();
  46 }
  47
  48 static RegisterPass<LoopDependenceAnalysis>
  49 R("lda", "Loop Dependence Analysis", false, true);
  50 char LoopDependenceAnalysis::ID = 0;
  51
  52 //===----------------------------------------------------------------------===//
  53 //                             Utility Functions
  54 //===----------------------------------------------------------------------===//
  55
  56 static inline bool IsMemRefInstr(const Value *V) {
  57   const Instruction *I = dyn_cast<const Instruction>(V);
  58   return I && (I->mayReadFromMemory() || I->mayWriteToMemory());
  59 }
  60
  61 static void GetMemRefInstrs(const Loop *L,
  62                             SmallVectorImpl<Instruction*> &Memrefs) {
  63   for (Loop::block_iterator b = L->block_begin(), be = L->block_end();
  64        b != be; ++b)
  65     for (BasicBlock::iterator i = (*b)->begin(), ie = (*b)->end();
  66          i != ie; ++i)
  67       if (IsMemRefInstr(i))
  68         Memrefs.push_back(i);
  69 }
  70
  71 static bool IsLoadOrStoreInst(Value *I) {
  72   return isa<LoadInst>(I) || isa<StoreInst>(I);
  73 }
  74
  75 static Value *GetPointerOperand(Value *I) {
  76   if (LoadInst *i = dyn_cast<LoadInst>(I))
  77     return i->getPointerOperand();
  78   if (StoreInst *i = dyn_cast<StoreInst>(I))
  79     return i->getPointerOperand();
  80   llvm_unreachable("Value is no load or store instruction!");
  81   // Never reached.
  82   return 0;
  83 }
  84
  85 static AliasAnalysis::AliasResult UnderlyingObjectsAlias(AliasAnalysis *AA,
  86                                                          const Value *A,
  87                                                          const Value *B) {
  88   const Value *aObj = A->getUnderlyingObject();
  89   const Value *bObj = B->getUnderlyingObject();
  90   return AA->alias(aObj, AA->getTypeStoreSize(aObj->getType()),
  91                    bObj, AA->getTypeStoreSize(bObj->getType()));
  92 }
  93
  94 static inline const SCEV *GetZeroSCEV(ScalarEvolution *SE) {
  95   return SE->getConstant(Type::Int32Ty, 0L);
  96 }
  97
  98 //===----------------------------------------------------------------------===//
  99 //                             Dependence Testing
 100 //===----------------------------------------------------------------------===//
 101
 102 bool LoopDependenceAnalysis::isDependencePair(const Value *A,
 103                                               const Value *B) const {
 104   return IsMemRefInstr(A) &&
 105          IsMemRefInstr(B) &&
 106          (cast<const Instruction>(A)->mayWriteToMemory() ||
 107           cast<const Instruction>(B)->mayWriteToMemory());
 108 }
 109
 110 bool LoopDependenceAnalysis::findOrInsertDependencePair(Value *A,
 111                                                         Value *B,
 112                                                         DependencePair *&P) {
 113   void *insertPos = 0;
 114   FoldingSetNodeID id;
 115   id.AddPointer(A);
 116   id.AddPointer(B);
 117
 118   P = Pairs.FindNodeOrInsertPos(id, insertPos);
 119   if (P) return true;
 120
 121   P = PairAllocator.Allocate<DependencePair>();
 122   new (P) DependencePair(id, A, B);
 123   Pairs.InsertNode(P, insertPos);
 124   return false;
 125 }
 126
 127 bool LoopDependenceAnalysis::isLoopInvariant(const SCEV *S) const {
 128   for (const Loop *L = this->L; L != 0; L = L->getParentLoop())
 129     if (!S->isLoopInvariant(L))
 130       return false;
 131   return true;
 132 }
 133
 134 bool LoopDependenceAnalysis::isAffine(const SCEV *S) const {
 135   const SCEVAddRecExpr *rec = dyn_cast<SCEVAddRecExpr>(S);
 136   return isLoopInvariant(S) || (rec && rec->isAffine());
 137 }
 138
 139 bool LoopDependenceAnalysis::isZIVPair(const SCEV *A, const SCEV *B) const {
 140   return isLoopInvariant(A) && isLoopInvariant(B);
 141 }
 142
 143 LoopDependenceAnalysis::DependenceResult
 144 LoopDependenceAnalysis::analyseZIV(const SCEV *A,
 145                                    const SCEV *B,
 146                                    Subscript *S) const {
 147   assert(isZIVPair(A, B) && "Attempted to ZIV-test non-ZIV SCEVs!");
 148   return A == B ? Dependent : Independent;
 149 }
 150
 151 LoopDependenceAnalysis::DependenceResult
 152 LoopDependenceAnalysis::analyseSubscript(const SCEV *A,
 153                                          const SCEV *B,
 154                                          Subscript *S) const {
 155   DEBUG(errs() << "  Testing subscript: " << *A << ", " << *B << "\n");
 156
 157   if (A == B) {
 158     DEBUG(errs() << "  -> [D] same SCEV\n");
 159     return Dependent;
 160   }
 161
 162   if (!isAffine(A) || !isAffine(B)) {
 163     DEBUG(errs() << "  -> [?] not affine\n");
 164     return Unknown;
 165   }
 166
 167   if (isZIVPair(A, B))
 168     return analyseZIV(A, B, S);
 169
 170   // TODO: Implement SIV/MIV testers.
 171
 172   DEBUG(errs() << "  -> [?] cannot analyse subscript\n");
 173   return Unknown;
 174 }
 175
 176 LoopDependenceAnalysis::DependenceResult
 177 LoopDependenceAnalysis::analysePair(DependencePair *P) const {
 178   DEBUG(errs() << "Analysing:\n" << *P->A << "\n" << *P->B << "\n");
 179
 180   // We only analyse loads and stores but no possible memory accesses by e.g.
 181   // free, call, or invoke instructions.
 182   if (!IsLoadOrStoreInst(P->A) || !IsLoadOrStoreInst(P->B)) {
 183     DEBUG(errs() << "--> [?] no load/store\n");
 184     return Unknown;
 185   }
 186
 187   Value *aPtr = GetPointerOperand(P->A);
 188   Value *bPtr = GetPointerOperand(P->B);
 189
 190   switch (UnderlyingObjectsAlias(AA, aPtr, bPtr)) {
 191   case AliasAnalysis::MayAlias:
 192     // We can not analyse objects if we do not know about their aliasing.
 193     DEBUG(errs() << "---> [?] may alias\n");
 194     return Unknown;
 195
 196   case AliasAnalysis::NoAlias:
 197     // If the objects noalias, they are distinct, accesses are independent.
 198     DEBUG(errs() << "---> [I] no alias\n");
 199     return Independent;
 200
 201   case AliasAnalysis::MustAlias:
 202     break; // The underlying objects alias, test accesses for dependence.
 203   }
 204
 205   const GEPOperator *aGEP = dyn_cast<GEPOperator>(aPtr);
 206   const GEPOperator *bGEP = dyn_cast<GEPOperator>(bPtr);
 207
 208   if (!aGEP || !bGEP)
 209     return Unknown;
 210
 211   // FIXME: Is filtering coupled subscripts necessary?
 212
 213   // Collect GEP operand pairs (FIXME: use GetGEPOperands from BasicAA), adding
 214   // trailing zeroes to the smaller GEP, if needed.
 215   typedef SmallVector<std::pair<const SCEV*, const SCEV*>, 4> GEPOpdPairsTy;
 216   GEPOpdPairsTy opds;
 217   for(GEPOperator::const_op_iterator aIdx = aGEP->idx_begin(),
 218                                      aEnd = aGEP->idx_end(),
 219                                      bIdx = bGEP->idx_begin(),
 220                                      bEnd = bGEP->idx_end();
 221       aIdx != aEnd && bIdx != bEnd;
 222       aIdx += (aIdx != aEnd), bIdx += (bIdx != bEnd)) {
 223     const SCEV* aSCEV = (aIdx != aEnd) ? SE->getSCEV(*aIdx) : GetZeroSCEV(SE);
 224     const SCEV* bSCEV = (bIdx != bEnd) ? SE->getSCEV(*bIdx) : GetZeroSCEV(SE);
 225     opds.push_back(std::make_pair(aSCEV, bSCEV));
 226   }
 227
 228   if (!opds.empty() && opds[0].first != opds[0].second) {
 229     // We cannot (yet) handle arbitrary GEP pointer offsets. By limiting
 230     //
 231     // TODO: this could be relaxed by adding the size of the underlying object
 232     // to the first subscript. If we have e.g. (GEP x,0,i; GEP x,2,-i) and we
 233     // know that x is a [100 x i8]*, we could modify the first subscript to be
 234     // (i, 200-i) instead of (i, -i).
 235     return Unknown;
 236   }
 237
 238   // Now analyse the collected operand pairs (skipping the GEP ptr offsets).
 239   for (GEPOpdPairsTy::const_iterator i = opds.begin() + 1, end = opds.end();
 240        i != end; ++i) {
 241     Subscript subscript;
 242     DependenceResult result = analyseSubscript(i->first, i->second, &subscript);
 243     if (result != Dependent) {
 244       // We either proved independence or failed to analyse this subscript.
 245       // Further subscripts will not improve the situation, so abort early.
 246       return result;
 247     }
 248     P->Subscripts.push_back(subscript);
 249   }
 250   // We successfully analysed all subscripts but failed to prove independence.
 251   return Dependent;
 252 }
 253
 254 bool LoopDependenceAnalysis::depends(Value *A, Value *B) {
 255   assert(isDependencePair(A, B) && "Values form no dependence pair!");
 256   ++NumAnswered;
 257
 258   DependencePair *p;
 259   if (!findOrInsertDependencePair(A, B, p)) {
 260     // The pair is not cached, so analyse it.
 261     ++NumAnalysed;
 262     switch (p->Result = analysePair(p)) {
 263     case Dependent:   ++NumDependent;   break;
 264     case Independent: ++NumIndependent; break;
 265     case Unknown:     ++NumUnknown;     break;
 266     }
 267   }
 268   return p->Result != Independent;
 269 }
 270
 271 //===----------------------------------------------------------------------===//
 272 //                   LoopDependenceAnalysis Implementation
 273 //===----------------------------------------------------------------------===//
 274
 275 bool LoopDependenceAnalysis::runOnLoop(Loop *L, LPPassManager &) {
 276   this->L = L;
 277   AA = &getAnalysis<AliasAnalysis>();
 278   SE = &getAnalysis<ScalarEvolution>();
 279   return false;
 280 }
 281
 282 void LoopDependenceAnalysis::releaseMemory() {
 283   Pairs.clear();
 284   PairAllocator.Reset();
 285 }
 286
 287 void LoopDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
 288   AU.setPreservesAll();
 289   AU.addRequiredTransitive<AliasAnalysis>();
 290   AU.addRequiredTransitive<ScalarEvolution>();
 291 }
 292
 293 static void PrintLoopInfo(raw_ostream &OS,
 294                           LoopDependenceAnalysis *LDA, const Loop *L) {
 295   if (!L->empty()) return; // ignore non-innermost loops
 296
 297   SmallVector<Instruction*, 8> memrefs;
 298   GetMemRefInstrs(L, memrefs);
 299
 300   OS << "Loop at depth " << L->getLoopDepth() << ", header block: ";
 301   WriteAsOperand(OS, L->getHeader(), false);
 302   OS << "\n";
 303
 304   OS << "  Load/store instructions: " << memrefs.size() << "\n";
 305   for (SmallVector<Instruction*, 8>::const_iterator x = memrefs.begin(),
 306        end = memrefs.end(); x != end; ++x)
 307     OS << "\t" << (x - memrefs.begin()) << ": " << **x << "\n";
 308
 309   OS << "  Pairwise dependence results:\n";
 310   for (SmallVector<Instruction*, 8>::const_iterator x = memrefs.begin(),
 311        end = memrefs.end(); x != end; ++x)
 312     for (SmallVector<Instruction*, 8>::const_iterator y = x + 1;
 313          y != end; ++y)
 314       if (LDA->isDependencePair(*x, *y))
 315         OS << "\t" << (x - memrefs.begin()) << "," << (y - memrefs.begin())
 316            << ": " << (LDA->depends(*x, *y) ? "dependent" : "independent")
 317            << "\n";
 318 }
 319
 320 void LoopDependenceAnalysis::print(raw_ostream &OS, const Module*) const {
 321   // TODO: doc why const_cast is safe
 322   PrintLoopInfo(OS, const_cast<LoopDependenceAnalysis*>(this), this->L);
 323 }
 324
 325 void LoopDependenceAnalysis::print(std::ostream &OS, const Module *M) const {
 326   raw_os_ostream os(OS);
 327   print(os, M);
 328 }