1 //===- LoopDependenceAnalysis.cpp - LDA Implementation ----------*- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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.
13 // Please note that this is work in progress and the interface is subject to
16 // TODO: adapt as implementation progresses.
18 // TODO: document lingo (pair, subscript, index)
20 //===----------------------------------------------------------------------===//
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"
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");
44 LoopPass *llvm::createLoopDependenceAnalysisPass() {
45 return new LoopDependenceAnalysis();
48 static RegisterPass<LoopDependenceAnalysis>
49 R("lda", "Loop Dependence Analysis", false, true);
50 char LoopDependenceAnalysis::ID = 0;
52 //===----------------------------------------------------------------------===//
54 //===----------------------------------------------------------------------===//
56 static inline bool IsMemRefInstr(const Value *V) {
57 const Instruction *I = dyn_cast<const Instruction>(V);
58 return I && (I->mayReadFromMemory() || I->mayWriteToMemory());
61 static void GetMemRefInstrs(const Loop *L,
62 SmallVectorImpl<Instruction*> &Memrefs) {
63 for (Loop::block_iterator b = L->block_begin(), be = L->block_end();
65 for (BasicBlock::iterator i = (*b)->begin(), ie = (*b)->end();
71 static bool IsLoadOrStoreInst(Value *I) {
72 return isa<LoadInst>(I) || isa<StoreInst>(I);
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!");
85 static AliasAnalysis::AliasResult UnderlyingObjectsAlias(AliasAnalysis *AA,
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()));
94 static inline const SCEV *GetZeroSCEV(ScalarEvolution *SE) {
95 return SE->getConstant(Type::Int32Ty, 0L);
98 //===----------------------------------------------------------------------===//
100 //===----------------------------------------------------------------------===//
102 bool LoopDependenceAnalysis::isDependencePair(const Value *A,
103 const Value *B) const {
104 return IsMemRefInstr(A) &&
106 (cast<const Instruction>(A)->mayWriteToMemory() ||
107 cast<const Instruction>(B)->mayWriteToMemory());
110 bool LoopDependenceAnalysis::findOrInsertDependencePair(Value *A,
112 DependencePair *&P) {
118 P = Pairs.FindNodeOrInsertPos(id, insertPos);
121 P = PairAllocator.Allocate<DependencePair>();
122 new (P) DependencePair(id, A, B);
123 Pairs.InsertNode(P, insertPos);
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))
134 bool LoopDependenceAnalysis::isAffine(const SCEV *S) const {
135 const SCEVAddRecExpr *rec = dyn_cast<SCEVAddRecExpr>(S);
136 return isLoopInvariant(S) || (rec && rec->isAffine());
139 LoopDependenceAnalysis::DependenceResult
140 LoopDependenceAnalysis::analyseSubscript(const SCEV *A,
142 Subscript *S) const {
143 DEBUG(errs() << " Testing subscript: " << *A << ", " << *B << "\n");
146 DEBUG(errs() << " -> [D] same SCEV\n");
150 if (!isAffine(A) || !isAffine(B)) {
151 DEBUG(errs() << " -> [?] not affine\n");
155 // TODO: Implement ZIV/SIV/MIV testers.
157 DEBUG(errs() << " -> [?] cannot analyse subscript\n");
161 LoopDependenceAnalysis::DependenceResult
162 LoopDependenceAnalysis::analysePair(DependencePair *P) const {
163 DEBUG(errs() << "Analysing:\n" << *P->A << "\n" << *P->B << "\n");
165 // We only analyse loads and stores but no possible memory accesses by e.g.
166 // free, call, or invoke instructions.
167 if (!IsLoadOrStoreInst(P->A) || !IsLoadOrStoreInst(P->B)) {
168 DEBUG(errs() << "--> [?] no load/store\n");
172 Value *aPtr = GetPointerOperand(P->A);
173 Value *bPtr = GetPointerOperand(P->B);
175 switch (UnderlyingObjectsAlias(AA, aPtr, bPtr)) {
176 case AliasAnalysis::MayAlias:
177 // We can not analyse objects if we do not know about their aliasing.
178 DEBUG(errs() << "---> [?] may alias\n");
181 case AliasAnalysis::NoAlias:
182 // If the objects noalias, they are distinct, accesses are independent.
183 DEBUG(errs() << "---> [I] no alias\n");
186 case AliasAnalysis::MustAlias:
187 break; // The underlying objects alias, test accesses for dependence.
190 const GEPOperator *aGEP = dyn_cast<GEPOperator>(aPtr);
191 const GEPOperator *bGEP = dyn_cast<GEPOperator>(bPtr);
196 // FIXME: Is filtering coupled subscripts necessary?
198 // Collect GEP operand pairs (FIXME: use GetGEPOperands from BasicAA), adding
199 // trailing zeroes to the smaller GEP, if needed.
200 typedef SmallVector<std::pair<const SCEV*, const SCEV*>, 4> GEPOpdPairsTy;
202 for(GEPOperator::const_op_iterator aIdx = aGEP->idx_begin(),
203 aEnd = aGEP->idx_end(),
204 bIdx = bGEP->idx_begin(),
205 bEnd = bGEP->idx_end();
206 aIdx != aEnd && bIdx != bEnd;
207 aIdx += (aIdx != aEnd), bIdx += (bIdx != bEnd)) {
208 const SCEV* aSCEV = (aIdx != aEnd) ? SE->getSCEV(*aIdx) : GetZeroSCEV(SE);
209 const SCEV* bSCEV = (bIdx != bEnd) ? SE->getSCEV(*bIdx) : GetZeroSCEV(SE);
210 opds.push_back(std::make_pair(aSCEV, bSCEV));
213 if (!opds.empty() && opds[0].first != opds[0].second) {
214 // We cannot (yet) handle arbitrary GEP pointer offsets. By limiting
216 // TODO: this could be relaxed by adding the size of the underlying object
217 // to the first subscript. If we have e.g. (GEP x,0,i; GEP x,2,-i) and we
218 // know that x is a [100 x i8]*, we could modify the first subscript to be
219 // (i, 200-i) instead of (i, -i).
223 // Now analyse the collected operand pairs (skipping the GEP ptr offsets).
224 for (GEPOpdPairsTy::const_iterator i = opds.begin() + 1, end = opds.end();
227 DependenceResult result = analyseSubscript(i->first, i->second, &subscript);
228 if (result != Dependent) {
229 // We either proved independence or failed to analyse this subscript.
230 // Further subscripts will not improve the situation, so abort early.
233 P->Subscripts.push_back(subscript);
235 // We successfully analysed all subscripts but failed to prove independence.
239 bool LoopDependenceAnalysis::depends(Value *A, Value *B) {
240 assert(isDependencePair(A, B) && "Values form no dependence pair!");
244 if (!findOrInsertDependencePair(A, B, p)) {
245 // The pair is not cached, so analyse it.
247 switch (p->Result = analysePair(p)) {
248 case Dependent: ++NumDependent; break;
249 case Independent: ++NumIndependent; break;
250 case Unknown: ++NumUnknown; break;
253 return p->Result != Independent;
256 //===----------------------------------------------------------------------===//
257 // LoopDependenceAnalysis Implementation
258 //===----------------------------------------------------------------------===//
260 bool LoopDependenceAnalysis::runOnLoop(Loop *L, LPPassManager &) {
262 AA = &getAnalysis<AliasAnalysis>();
263 SE = &getAnalysis<ScalarEvolution>();
267 void LoopDependenceAnalysis::releaseMemory() {
269 PairAllocator.Reset();
272 void LoopDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
273 AU.setPreservesAll();
274 AU.addRequiredTransitive<AliasAnalysis>();
275 AU.addRequiredTransitive<ScalarEvolution>();
278 static void PrintLoopInfo(raw_ostream &OS,
279 LoopDependenceAnalysis *LDA, const Loop *L) {
280 if (!L->empty()) return; // ignore non-innermost loops
282 SmallVector<Instruction*, 8> memrefs;
283 GetMemRefInstrs(L, memrefs);
285 OS << "Loop at depth " << L->getLoopDepth() << ", header block: ";
286 WriteAsOperand(OS, L->getHeader(), false);
289 OS << " Load/store instructions: " << memrefs.size() << "\n";
290 for (SmallVector<Instruction*, 8>::const_iterator x = memrefs.begin(),
291 end = memrefs.end(); x != end; ++x)
292 OS << "\t" << (x - memrefs.begin()) << ": " << **x << "\n";
294 OS << " Pairwise dependence results:\n";
295 for (SmallVector<Instruction*, 8>::const_iterator x = memrefs.begin(),
296 end = memrefs.end(); x != end; ++x)
297 for (SmallVector<Instruction*, 8>::const_iterator y = x + 1;
299 if (LDA->isDependencePair(*x, *y))
300 OS << "\t" << (x - memrefs.begin()) << "," << (y - memrefs.begin())
301 << ": " << (LDA->depends(*x, *y) ? "dependent" : "independent")
305 void LoopDependenceAnalysis::print(raw_ostream &OS, const Module*) const {
306 // TODO: doc why const_cast is safe
307 PrintLoopInfo(OS, const_cast<LoopDependenceAnalysis*>(this), this->L);
310 void LoopDependenceAnalysis::print(std::ostream &OS, const Module *M) const {
311 raw_os_ostream os(OS);