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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 
createLoopDependenceAnalysisPass()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 
IsMemRefInstr(const Value * V)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 
GetMemRefInstrs(const Loop * L,SmallVectorImpl<Instruction * > & Memrefs)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 
IsLoadOrStoreInst(Value * I)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 
GetPointerOperand(Value * I)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 
UnderlyingObjectsAlias(AliasAnalysis * AA,const Value * A,const Value * B)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 
GetZeroSCEV(ScalarEvolution * SE)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 
isDependencePair(const Value * A,const Value * B) const115 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 
findOrInsertDependencePair(Value * A,Value * B,DependencePair * & P)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 
getLoops(const SCEV * S,DenseSet<const Loop * > * Loops) const139 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 
isLoopInvariant(const SCEV * S) const147 bool LoopDependenceAnalysis::isLoopInvariant(const SCEV *S) const {
148   DenseSet<const Loop*> loops;
149   getLoops(S, &loops);
150   return loops.empty();
151 }
152 
isAffine(const SCEV * S) const153 bool LoopDependenceAnalysis::isAffine(const SCEV *S) const {
154   const SCEVAddRecExpr *rec = dyn_cast<SCEVAddRecExpr>(S);
155   return isLoopInvariant(S) || (rec && rec->isAffine());
156 }
157 
isZIVPair(const SCEV * A,const SCEV * B) const158 bool LoopDependenceAnalysis::isZIVPair(const SCEV *A, const SCEV *B) const {
159   return isLoopInvariant(A) && isLoopInvariant(B);
160 }
161 
isSIVPair(const SCEV * A,const SCEV * B) const162 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
analyseZIV(const SCEV * A,const SCEV * B,Subscript * S) const170 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
analyseSIV(const SCEV * A,const SCEV * B,Subscript * S) const178 LoopDependenceAnalysis::analyseSIV(const SCEV *A,
179                                    const SCEV *B,
180                                    Subscript *S) const {
181   return Unknown; // TODO: Implement.
182 }
183 
184 LoopDependenceAnalysis::DependenceResult
analyseMIV(const SCEV * A,const SCEV * B,Subscript * S) const185 LoopDependenceAnalysis::analyseMIV(const SCEV *A,
186                                    const SCEV *B,
187                                    Subscript *S) const {
188   return Unknown; // TODO: Implement.
189 }
190 
191 LoopDependenceAnalysis::DependenceResult
analyseSubscript(const SCEV * A,const SCEV * B,Subscript * S) const192 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
analysePair(DependencePair * P) const217 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 
depends(Value * A,Value * B)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 
runOnLoop(Loop * L,LPPassManager &)316 bool LoopDependenceAnalysis::runOnLoop(Loop *L, LPPassManager &) {
317   this->L = L;
318   AA = &getAnalysis<AliasAnalysis>();
319   SE = &getAnalysis<ScalarEvolution>();
320   return false;
321 }
322 
releaseMemory()323 void LoopDependenceAnalysis::releaseMemory() {
324   Pairs.clear();
325   PairAllocator.Reset();
326 }
327 
getAnalysisUsage(AnalysisUsage & AU) const328 void LoopDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
329   AU.setPreservesAll();
330   AU.addRequiredTransitive<AliasAnalysis>();
331   AU.addRequiredTransitive<ScalarEvolution>();
332 }
333 
PrintLoopInfo(raw_ostream & OS,LoopDependenceAnalysis * LDA,const Loop * L)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 
print(raw_ostream & OS,const Module *) const361 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