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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 }
     95 
     96 static AliasAnalysis::AliasResult UnderlyingObjectsAlias(AliasAnalysis *AA,
     97                                                          const Value *A,
     98                                                          const Value *B) {
     99   const Value *aObj = GetUnderlyingObject(A);
    100   const Value *bObj = GetUnderlyingObject(B);
    101   return AA->alias(aObj, AA->getTypeStoreSize(aObj->getType()),
    102                    bObj, AA->getTypeStoreSize(bObj->getType()));
    103 }
    104 
    105 static inline const SCEV *GetZeroSCEV(ScalarEvolution *SE) {
    106   return SE->getConstant(Type::getInt32Ty(SE->getContext()), 0L);
    107 }
    108 
    109 //===----------------------------------------------------------------------===//
    110 //                             Dependence Testing
    111 //===----------------------------------------------------------------------===//
    112 
    113 bool LoopDependenceAnalysis::isDependencePair(const Value *A,
    114                                               const Value *B) const {
    115   return IsMemRefInstr(A) &&
    116          IsMemRefInstr(B) &&
    117          (cast<const Instruction>(A)->mayWriteToMemory() ||
    118           cast<const Instruction>(B)->mayWriteToMemory());
    119 }
    120 
    121 bool LoopDependenceAnalysis::findOrInsertDependencePair(Value *A,
    122                                                         Value *B,
    123                                                         DependencePair *&P) {
    124   void *insertPos = 0;
    125   FoldingSetNodeID id;
    126   id.AddPointer(A);
    127   id.AddPointer(B);
    128 
    129   P = Pairs.FindNodeOrInsertPos(id, insertPos);
    130   if (P) return true;
    131 
    132   P = new (PairAllocator) DependencePair(id, A, B);
    133   Pairs.InsertNode(P, insertPos);
    134   return false;
    135 }
    136 
    137 void LoopDependenceAnalysis::getLoops(const SCEV *S,
    138                                       DenseSet<const Loop*>* Loops) const {
    139   // Refactor this into an SCEVVisitor, if efficiency becomes a concern.
    140   for (const Loop *L = this->L; L != 0; L = L->getParentLoop())
    141     if (!SE->isLoopInvariant(S, L))
    142       Loops->insert(L);
    143 }
    144 
    145 bool LoopDependenceAnalysis::isLoopInvariant(const SCEV *S) const {
    146   DenseSet<const Loop*> loops;
    147   getLoops(S, &loops);
    148   return loops.empty();
    149 }
    150 
    151 bool LoopDependenceAnalysis::isAffine(const SCEV *S) const {
    152   const SCEVAddRecExpr *rec = dyn_cast<SCEVAddRecExpr>(S);
    153   return isLoopInvariant(S) || (rec && rec->isAffine());
    154 }
    155 
    156 bool LoopDependenceAnalysis::isZIVPair(const SCEV *A, const SCEV *B) const {
    157   return isLoopInvariant(A) && isLoopInvariant(B);
    158 }
    159 
    160 bool LoopDependenceAnalysis::isSIVPair(const SCEV *A, const SCEV *B) const {
    161   DenseSet<const Loop*> loops;
    162   getLoops(A, &loops);
    163   getLoops(B, &loops);
    164   return loops.size() == 1;
    165 }
    166 
    167 LoopDependenceAnalysis::DependenceResult
    168 LoopDependenceAnalysis::analyseZIV(const SCEV *A,
    169                                    const SCEV *B,
    170                                    Subscript *S) const {
    171   assert(isZIVPair(A, B) && "Attempted to ZIV-test non-ZIV SCEVs!");
    172   return A == B ? Dependent : Independent;
    173 }
    174 
    175 LoopDependenceAnalysis::DependenceResult
    176 LoopDependenceAnalysis::analyseSIV(const SCEV *A,
    177                                    const SCEV *B,
    178                                    Subscript *S) const {
    179   return Unknown; // TODO: Implement.
    180 }
    181 
    182 LoopDependenceAnalysis::DependenceResult
    183 LoopDependenceAnalysis::analyseMIV(const SCEV *A,
    184                                    const SCEV *B,
    185                                    Subscript *S) const {
    186   return Unknown; // TODO: Implement.
    187 }
    188 
    189 LoopDependenceAnalysis::DependenceResult
    190 LoopDependenceAnalysis::analyseSubscript(const SCEV *A,
    191                                          const SCEV *B,
    192                                          Subscript *S) const {
    193   DEBUG(dbgs() << "  Testing subscript: " << *A << ", " << *B << "\n");
    194 
    195   if (A == B) {
    196     DEBUG(dbgs() << "  -> [D] same SCEV\n");
    197     return Dependent;
    198   }
    199 
    200   if (!isAffine(A) || !isAffine(B)) {
    201     DEBUG(dbgs() << "  -> [?] not affine\n");
    202     return Unknown;
    203   }
    204 
    205   if (isZIVPair(A, B))
    206     return analyseZIV(A, B, S);
    207 
    208   if (isSIVPair(A, B))
    209     return analyseSIV(A, B, S);
    210 
    211   return analyseMIV(A, B, S);
    212 }
    213 
    214 LoopDependenceAnalysis::DependenceResult
    215 LoopDependenceAnalysis::analysePair(DependencePair *P) const {
    216   DEBUG(dbgs() << "Analysing:\n" << *P->A << "\n" << *P->B << "\n");
    217 
    218   // We only analyse loads and stores but no possible memory accesses by e.g.
    219   // free, call, or invoke instructions.
    220   if (!IsLoadOrStoreInst(P->A) || !IsLoadOrStoreInst(P->B)) {
    221     DEBUG(dbgs() << "--> [?] no load/store\n");
    222     return Unknown;
    223   }
    224 
    225   Value *aPtr = GetPointerOperand(P->A);
    226   Value *bPtr = GetPointerOperand(P->B);
    227 
    228   switch (UnderlyingObjectsAlias(AA, aPtr, bPtr)) {
    229   case AliasAnalysis::MayAlias:
    230   case AliasAnalysis::PartialAlias:
    231     // We can not analyse objects if we do not know about their aliasing.
    232     DEBUG(dbgs() << "---> [?] may alias\n");
    233     return Unknown;
    234 
    235   case AliasAnalysis::NoAlias:
    236     // If the objects noalias, they are distinct, accesses are independent.
    237     DEBUG(dbgs() << "---> [I] no alias\n");
    238     return Independent;
    239 
    240   case AliasAnalysis::MustAlias:
    241     break; // The underlying objects alias, test accesses for dependence.
    242   }
    243 
    244   const GEPOperator *aGEP = dyn_cast<GEPOperator>(aPtr);
    245   const GEPOperator *bGEP = dyn_cast<GEPOperator>(bPtr);
    246 
    247   if (!aGEP || !bGEP)
    248     return Unknown;
    249 
    250   // FIXME: Is filtering coupled subscripts necessary?
    251 
    252   // Collect GEP operand pairs (FIXME: use GetGEPOperands from BasicAA), adding
    253   // trailing zeroes to the smaller GEP, if needed.
    254   typedef SmallVector<std::pair<const SCEV*, const SCEV*>, 4> GEPOpdPairsTy;
    255   GEPOpdPairsTy opds;
    256   for(GEPOperator::const_op_iterator aIdx = aGEP->idx_begin(),
    257                                      aEnd = aGEP->idx_end(),
    258                                      bIdx = bGEP->idx_begin(),
    259                                      bEnd = bGEP->idx_end();
    260       aIdx != aEnd && bIdx != bEnd;
    261       aIdx += (aIdx != aEnd), bIdx += (bIdx != bEnd)) {
    262     const SCEV* aSCEV = (aIdx != aEnd) ? SE->getSCEV(*aIdx) : GetZeroSCEV(SE);
    263     const SCEV* bSCEV = (bIdx != bEnd) ? SE->getSCEV(*bIdx) : GetZeroSCEV(SE);
    264     opds.push_back(std::make_pair(aSCEV, bSCEV));
    265   }
    266 
    267   if (!opds.empty() && opds[0].first != opds[0].second) {
    268     // We cannot (yet) handle arbitrary GEP pointer offsets. By limiting
    269     //
    270     // TODO: this could be relaxed by adding the size of the underlying object
    271     // to the first subscript. If we have e.g. (GEP x,0,i; GEP x,2,-i) and we
    272     // know that x is a [100 x i8]*, we could modify the first subscript to be
    273     // (i, 200-i) instead of (i, -i).
    274     return Unknown;
    275   }
    276 
    277   // Now analyse the collected operand pairs (skipping the GEP ptr offsets).
    278   for (GEPOpdPairsTy::const_iterator i = opds.begin() + 1, end = opds.end();
    279        i != end; ++i) {
    280     Subscript subscript;
    281     DependenceResult result = analyseSubscript(i->first, i->second, &subscript);
    282     if (result != Dependent) {
    283       // We either proved independence or failed to analyse this subscript.
    284       // Further subscripts will not improve the situation, so abort early.
    285       return result;
    286     }
    287     P->Subscripts.push_back(subscript);
    288   }
    289   // We successfully analysed all subscripts but failed to prove independence.
    290   return Dependent;
    291 }
    292 
    293 bool LoopDependenceAnalysis::depends(Value *A, Value *B) {
    294   assert(isDependencePair(A, B) && "Values form no dependence pair!");
    295   ++NumAnswered;
    296 
    297   DependencePair *p;
    298   if (!findOrInsertDependencePair(A, B, p)) {
    299     // The pair is not cached, so analyse it.
    300     ++NumAnalysed;
    301     switch (p->Result = analysePair(p)) {
    302     case Dependent:   ++NumDependent;   break;
    303     case Independent: ++NumIndependent; break;
    304     case Unknown:     ++NumUnknown;     break;
    305     }
    306   }
    307   return p->Result != Independent;
    308 }
    309 
    310 //===----------------------------------------------------------------------===//
    311 //                   LoopDependenceAnalysis Implementation
    312 //===----------------------------------------------------------------------===//
    313 
    314 bool LoopDependenceAnalysis::runOnLoop(Loop *L, LPPassManager &) {
    315   this->L = L;
    316   AA = &getAnalysis<AliasAnalysis>();
    317   SE = &getAnalysis<ScalarEvolution>();
    318   return false;
    319 }
    320 
    321 void LoopDependenceAnalysis::releaseMemory() {
    322   Pairs.clear();
    323   PairAllocator.Reset();
    324 }
    325 
    326 void LoopDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
    327   AU.setPreservesAll();
    328   AU.addRequiredTransitive<AliasAnalysis>();
    329   AU.addRequiredTransitive<ScalarEvolution>();
    330 }
    331 
    332 static void PrintLoopInfo(raw_ostream &OS,
    333                           LoopDependenceAnalysis *LDA, const Loop *L) {
    334   if (!L->empty()) return; // ignore non-innermost loops
    335 
    336   SmallVector<Instruction*, 8> memrefs;
    337   GetMemRefInstrs(L, memrefs);
    338 
    339   OS << "Loop at depth " << L->getLoopDepth() << ", header block: ";
    340   WriteAsOperand(OS, L->getHeader(), false);
    341   OS << "\n";
    342 
    343   OS << "  Load/store instructions: " << memrefs.size() << "\n";
    344   for (SmallVector<Instruction*, 8>::const_iterator x = memrefs.begin(),
    345        end = memrefs.end(); x != end; ++x)
    346     OS << "\t" << (x - memrefs.begin()) << ": " << **x << "\n";
    347 
    348   OS << "  Pairwise dependence results:\n";
    349   for (SmallVector<Instruction*, 8>::const_iterator x = memrefs.begin(),
    350        end = memrefs.end(); x != end; ++x)
    351     for (SmallVector<Instruction*, 8>::const_iterator y = x + 1;
    352          y != end; ++y)
    353       if (LDA->isDependencePair(*x, *y))
    354         OS << "\t" << (x - memrefs.begin()) << "," << (y - memrefs.begin())
    355            << ": " << (LDA->depends(*x, *y) ? "dependent" : "independent")
    356            << "\n";
    357 }
    358 
    359 void LoopDependenceAnalysis::print(raw_ostream &OS, const Module*) const {
    360   // TODO: doc why const_cast is safe
    361   PrintLoopInfo(OS, const_cast<LoopDependenceAnalysis*>(this), this->L);
    362 }
    363