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 return isa<LoadInst>(I) || isa<StoreInst>(I);
80 }
81
GetPointerOperand(Value * I)82 static Value *GetPointerOperand(Value *I) {
83 if (LoadInst *i = dyn_cast<LoadInst>(I))
84 return i->getPointerOperand();
85 if (StoreInst *i = dyn_cast<StoreInst>(I))
86 return i->getPointerOperand();
87 llvm_unreachable("Value is no load or store instruction!");
88 // Never reached.
89 return 0;
90 }
91
UnderlyingObjectsAlias(AliasAnalysis * AA,const Value * A,const Value * B)92 static AliasAnalysis::AliasResult UnderlyingObjectsAlias(AliasAnalysis *AA,
93 const Value *A,
94 const Value *B) {
95 const Value *aObj = GetUnderlyingObject(A);
96 const Value *bObj = GetUnderlyingObject(B);
97 return AA->alias(aObj, AA->getTypeStoreSize(aObj->getType()),
98 bObj, AA->getTypeStoreSize(bObj->getType()));
99 }
100
GetZeroSCEV(ScalarEvolution * SE)101 static inline const SCEV *GetZeroSCEV(ScalarEvolution *SE) {
102 return SE->getConstant(Type::getInt32Ty(SE->getContext()), 0L);
103 }
104
105 //===----------------------------------------------------------------------===//
106 // Dependence Testing
107 //===----------------------------------------------------------------------===//
108
isDependencePair(const Value * A,const Value * B) const109 bool LoopDependenceAnalysis::isDependencePair(const Value *A,
110 const Value *B) const {
111 return IsMemRefInstr(A) &&
112 IsMemRefInstr(B) &&
113 (cast<const Instruction>(A)->mayWriteToMemory() ||
114 cast<const Instruction>(B)->mayWriteToMemory());
115 }
116
findOrInsertDependencePair(Value * A,Value * B,DependencePair * & P)117 bool LoopDependenceAnalysis::findOrInsertDependencePair(Value *A,
118 Value *B,
119 DependencePair *&P) {
120 void *insertPos = 0;
121 FoldingSetNodeID id;
122 id.AddPointer(A);
123 id.AddPointer(B);
124
125 P = Pairs.FindNodeOrInsertPos(id, insertPos);
126 if (P) return true;
127
128 P = new (PairAllocator) DependencePair(id, A, B);
129 Pairs.InsertNode(P, insertPos);
130 return false;
131 }
132
getLoops(const SCEV * S,DenseSet<const Loop * > * Loops) const133 void LoopDependenceAnalysis::getLoops(const SCEV *S,
134 DenseSet<const Loop*>* Loops) const {
135 // Refactor this into an SCEVVisitor, if efficiency becomes a concern.
136 for (const Loop *L = this->L; L != 0; L = L->getParentLoop())
137 if (!SE->isLoopInvariant(S, L))
138 Loops->insert(L);
139 }
140
isLoopInvariant(const SCEV * S) const141 bool LoopDependenceAnalysis::isLoopInvariant(const SCEV *S) const {
142 DenseSet<const Loop*> loops;
143 getLoops(S, &loops);
144 return loops.empty();
145 }
146
isAffine(const SCEV * S) const147 bool LoopDependenceAnalysis::isAffine(const SCEV *S) const {
148 const SCEVAddRecExpr *rec = dyn_cast<SCEVAddRecExpr>(S);
149 return isLoopInvariant(S) || (rec && rec->isAffine());
150 }
151
isZIVPair(const SCEV * A,const SCEV * B) const152 bool LoopDependenceAnalysis::isZIVPair(const SCEV *A, const SCEV *B) const {
153 return isLoopInvariant(A) && isLoopInvariant(B);
154 }
155
isSIVPair(const SCEV * A,const SCEV * B) const156 bool LoopDependenceAnalysis::isSIVPair(const SCEV *A, const SCEV *B) const {
157 DenseSet<const Loop*> loops;
158 getLoops(A, &loops);
159 getLoops(B, &loops);
160 return loops.size() == 1;
161 }
162
163 LoopDependenceAnalysis::DependenceResult
analyseZIV(const SCEV * A,const SCEV * B,Subscript * S) const164 LoopDependenceAnalysis::analyseZIV(const SCEV *A,
165 const SCEV *B,
166 Subscript *S) const {
167 assert(isZIVPair(A, B) && "Attempted to ZIV-test non-ZIV SCEVs!");
168 return A == B ? Dependent : Independent;
169 }
170
171 LoopDependenceAnalysis::DependenceResult
analyseSIV(const SCEV * A,const SCEV * B,Subscript * S) const172 LoopDependenceAnalysis::analyseSIV(const SCEV *A,
173 const SCEV *B,
174 Subscript *S) const {
175 return Unknown; // TODO: Implement.
176 }
177
178 LoopDependenceAnalysis::DependenceResult
analyseMIV(const SCEV * A,const SCEV * B,Subscript * S) const179 LoopDependenceAnalysis::analyseMIV(const SCEV *A,
180 const SCEV *B,
181 Subscript *S) const {
182 return Unknown; // TODO: Implement.
183 }
184
185 LoopDependenceAnalysis::DependenceResult
analyseSubscript(const SCEV * A,const SCEV * B,Subscript * S) const186 LoopDependenceAnalysis::analyseSubscript(const SCEV *A,
187 const SCEV *B,
188 Subscript *S) const {
189 DEBUG(dbgs() << " Testing subscript: " << *A << ", " << *B << "\n");
190
191 if (A == B) {
192 DEBUG(dbgs() << " -> [D] same SCEV\n");
193 return Dependent;
194 }
195
196 if (!isAffine(A) || !isAffine(B)) {
197 DEBUG(dbgs() << " -> [?] not affine\n");
198 return Unknown;
199 }
200
201 if (isZIVPair(A, B))
202 return analyseZIV(A, B, S);
203
204 if (isSIVPair(A, B))
205 return analyseSIV(A, B, S);
206
207 return analyseMIV(A, B, S);
208 }
209
210 LoopDependenceAnalysis::DependenceResult
analysePair(DependencePair * P) const211 LoopDependenceAnalysis::analysePair(DependencePair *P) const {
212 DEBUG(dbgs() << "Analysing:\n" << *P->A << "\n" << *P->B << "\n");
213
214 // We only analyse loads and stores but no possible memory accesses by e.g.
215 // free, call, or invoke instructions.
216 if (!IsLoadOrStoreInst(P->A) || !IsLoadOrStoreInst(P->B)) {
217 DEBUG(dbgs() << "--> [?] no load/store\n");
218 return Unknown;
219 }
220
221 Value *aPtr = GetPointerOperand(P->A);
222 Value *bPtr = GetPointerOperand(P->B);
223
224 switch (UnderlyingObjectsAlias(AA, aPtr, bPtr)) {
225 case AliasAnalysis::MayAlias:
226 case AliasAnalysis::PartialAlias:
227 // We can not analyse objects if we do not know about their aliasing.
228 DEBUG(dbgs() << "---> [?] may alias\n");
229 return Unknown;
230
231 case AliasAnalysis::NoAlias:
232 // If the objects noalias, they are distinct, accesses are independent.
233 DEBUG(dbgs() << "---> [I] no alias\n");
234 return Independent;
235
236 case AliasAnalysis::MustAlias:
237 break; // The underlying objects alias, test accesses for dependence.
238 }
239
240 const GEPOperator *aGEP = dyn_cast<GEPOperator>(aPtr);
241 const GEPOperator *bGEP = dyn_cast<GEPOperator>(bPtr);
242
243 if (!aGEP || !bGEP)
244 return Unknown;
245
246 // FIXME: Is filtering coupled subscripts necessary?
247
248 // Collect GEP operand pairs (FIXME: use GetGEPOperands from BasicAA), adding
249 // trailing zeroes to the smaller GEP, if needed.
250 typedef SmallVector<std::pair<const SCEV*, const SCEV*>, 4> GEPOpdPairsTy;
251 GEPOpdPairsTy opds;
252 for(GEPOperator::const_op_iterator aIdx = aGEP->idx_begin(),
253 aEnd = aGEP->idx_end(),
254 bIdx = bGEP->idx_begin(),
255 bEnd = bGEP->idx_end();
256 aIdx != aEnd && bIdx != bEnd;
257 aIdx += (aIdx != aEnd), bIdx += (bIdx != bEnd)) {
258 const SCEV* aSCEV = (aIdx != aEnd) ? SE->getSCEV(*aIdx) : GetZeroSCEV(SE);
259 const SCEV* bSCEV = (bIdx != bEnd) ? SE->getSCEV(*bIdx) : GetZeroSCEV(SE);
260 opds.push_back(std::make_pair(aSCEV, bSCEV));
261 }
262
263 if (!opds.empty() && opds[0].first != opds[0].second) {
264 // We cannot (yet) handle arbitrary GEP pointer offsets. By limiting
265 //
266 // TODO: this could be relaxed by adding the size of the underlying object
267 // to the first subscript. If we have e.g. (GEP x,0,i; GEP x,2,-i) and we
268 // know that x is a [100 x i8]*, we could modify the first subscript to be
269 // (i, 200-i) instead of (i, -i).
270 return Unknown;
271 }
272
273 // Now analyse the collected operand pairs (skipping the GEP ptr offsets).
274 for (GEPOpdPairsTy::const_iterator i = opds.begin() + 1, end = opds.end();
275 i != end; ++i) {
276 Subscript subscript;
277 DependenceResult result = analyseSubscript(i->first, i->second, &subscript);
278 if (result != Dependent) {
279 // We either proved independence or failed to analyse this subscript.
280 // Further subscripts will not improve the situation, so abort early.
281 return result;
282 }
283 P->Subscripts.push_back(subscript);
284 }
285 // We successfully analysed all subscripts but failed to prove independence.
286 return Dependent;
287 }
288
depends(Value * A,Value * B)289 bool LoopDependenceAnalysis::depends(Value *A, Value *B) {
290 assert(isDependencePair(A, B) && "Values form no dependence pair!");
291 ++NumAnswered;
292
293 DependencePair *p;
294 if (!findOrInsertDependencePair(A, B, p)) {
295 // The pair is not cached, so analyse it.
296 ++NumAnalysed;
297 switch (p->Result = analysePair(p)) {
298 case Dependent: ++NumDependent; break;
299 case Independent: ++NumIndependent; break;
300 case Unknown: ++NumUnknown; break;
301 }
302 }
303 return p->Result != Independent;
304 }
305
306 //===----------------------------------------------------------------------===//
307 // LoopDependenceAnalysis Implementation
308 //===----------------------------------------------------------------------===//
309
runOnLoop(Loop * L,LPPassManager &)310 bool LoopDependenceAnalysis::runOnLoop(Loop *L, LPPassManager &) {
311 this->L = L;
312 AA = &getAnalysis<AliasAnalysis>();
313 SE = &getAnalysis<ScalarEvolution>();
314 return false;
315 }
316
releaseMemory()317 void LoopDependenceAnalysis::releaseMemory() {
318 Pairs.clear();
319 PairAllocator.Reset();
320 }
321
getAnalysisUsage(AnalysisUsage & AU) const322 void LoopDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
323 AU.setPreservesAll();
324 AU.addRequiredTransitive<AliasAnalysis>();
325 AU.addRequiredTransitive<ScalarEvolution>();
326 }
327
PrintLoopInfo(raw_ostream & OS,LoopDependenceAnalysis * LDA,const Loop * L)328 static void PrintLoopInfo(raw_ostream &OS,
329 LoopDependenceAnalysis *LDA, const Loop *L) {
330 if (!L->empty()) return; // ignore non-innermost loops
331
332 SmallVector<Instruction*, 8> memrefs;
333 GetMemRefInstrs(L, memrefs);
334
335 OS << "Loop at depth " << L->getLoopDepth() << ", header block: ";
336 WriteAsOperand(OS, L->getHeader(), false);
337 OS << "\n";
338
339 OS << " Load/store instructions: " << memrefs.size() << "\n";
340 for (SmallVector<Instruction*, 8>::const_iterator x = memrefs.begin(),
341 end = memrefs.end(); x != end; ++x)
342 OS << "\t" << (x - memrefs.begin()) << ": " << **x << "\n";
343
344 OS << " Pairwise dependence results:\n";
345 for (SmallVector<Instruction*, 8>::const_iterator x = memrefs.begin(),
346 end = memrefs.end(); x != end; ++x)
347 for (SmallVector<Instruction*, 8>::const_iterator y = x + 1;
348 y != end; ++y)
349 if (LDA->isDependencePair(*x, *y))
350 OS << "\t" << (x - memrefs.begin()) << "," << (y - memrefs.begin())
351 << ": " << (LDA->depends(*x, *y) ? "dependent" : "independent")
352 << "\n";
353 }
354
print(raw_ostream & OS,const Module *) const355 void LoopDependenceAnalysis::print(raw_ostream &OS, const Module*) const {
356 // TODO: doc why const_cast is safe
357 PrintLoopInfo(OS, const_cast<LoopDependenceAnalysis*>(this), this->L);
358 }
359