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      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