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"
39using namespace llvm;
40
41STATISTIC(NumAnswered,    "Number of dependence queries answered");
42STATISTIC(NumAnalysed,    "Number of distinct dependence pairs analysed");
43STATISTIC(NumDependent,   "Number of pairs with dependent accesses");
44STATISTIC(NumIndependent, "Number of pairs with independent accesses");
45STATISTIC(NumUnknown,     "Number of pairs with unknown accesses");
46
47LoopPass *llvm::createLoopDependenceAnalysisPass() {
48  return new LoopDependenceAnalysis();
49}
50
51INITIALIZE_PASS_BEGIN(LoopDependenceAnalysis, "lda",
52                "Loop Dependence Analysis", false, true)
53INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
54INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
55INITIALIZE_PASS_END(LoopDependenceAnalysis, "lda",
56                "Loop Dependence Analysis", false, true)
57char LoopDependenceAnalysis::ID = 0;
58
59//===----------------------------------------------------------------------===//
60//                             Utility Functions
61//===----------------------------------------------------------------------===//
62
63static inline bool IsMemRefInstr(const Value *V) {
64  const Instruction *I = dyn_cast<const Instruction>(V);
65  return I && (I->mayReadFromMemory() || I->mayWriteToMemory());
66}
67
68static 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
78static 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
88static 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
96static 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
105static inline const SCEV *GetZeroSCEV(ScalarEvolution *SE) {
106  return SE->getConstant(Type::getInt32Ty(SE->getContext()), 0L);
107}
108
109//===----------------------------------------------------------------------===//
110//                             Dependence Testing
111//===----------------------------------------------------------------------===//
112
113bool 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
121bool 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
137void 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
145bool LoopDependenceAnalysis::isLoopInvariant(const SCEV *S) const {
146  DenseSet<const Loop*> loops;
147  getLoops(S, &loops);
148  return loops.empty();
149}
150
151bool LoopDependenceAnalysis::isAffine(const SCEV *S) const {
152  const SCEVAddRecExpr *rec = dyn_cast<SCEVAddRecExpr>(S);
153  return isLoopInvariant(S) || (rec && rec->isAffine());
154}
155
156bool LoopDependenceAnalysis::isZIVPair(const SCEV *A, const SCEV *B) const {
157  return isLoopInvariant(A) && isLoopInvariant(B);
158}
159
160bool 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
167LoopDependenceAnalysis::DependenceResult
168LoopDependenceAnalysis::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
175LoopDependenceAnalysis::DependenceResult
176LoopDependenceAnalysis::analyseSIV(const SCEV *A,
177                                   const SCEV *B,
178                                   Subscript *S) const {
179  return Unknown; // TODO: Implement.
180}
181
182LoopDependenceAnalysis::DependenceResult
183LoopDependenceAnalysis::analyseMIV(const SCEV *A,
184                                   const SCEV *B,
185                                   Subscript *S) const {
186  return Unknown; // TODO: Implement.
187}
188
189LoopDependenceAnalysis::DependenceResult
190LoopDependenceAnalysis::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
214LoopDependenceAnalysis::DependenceResult
215LoopDependenceAnalysis::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
293bool 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
314bool LoopDependenceAnalysis::runOnLoop(Loop *L, LPPassManager &) {
315  this->L = L;
316  AA = &getAnalysis<AliasAnalysis>();
317  SE = &getAnalysis<ScalarEvolution>();
318  return false;
319}
320
321void LoopDependenceAnalysis::releaseMemory() {
322  Pairs.clear();
323  PairAllocator.Reset();
324}
325
326void LoopDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
327  AU.setPreservesAll();
328  AU.addRequiredTransitive<AliasAnalysis>();
329  AU.addRequiredTransitive<ScalarEvolution>();
330}
331
332static 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
359void 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