1 //===- BasicAliasAnalysis.cpp - Local Alias Analysis Impl -----------------===//
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 file defines the default implementation of the Alias Analysis interface
11 // that simply implements a few identities (two different globals cannot alias,
12 // etc), but otherwise does no analysis.
13 //
14 //===----------------------------------------------------------------------===//
15
16 #include "llvm/Analysis/AliasAnalysis.h"
17 #include "llvm/Analysis/Passes.h"
18 #include "llvm/Constants.h"
19 #include "llvm/DerivedTypes.h"
20 #include "llvm/Function.h"
21 #include "llvm/GlobalAlias.h"
22 #include "llvm/GlobalVariable.h"
23 #include "llvm/Instructions.h"
24 #include "llvm/IntrinsicInst.h"
25 #include "llvm/Operator.h"
26 #include "llvm/Pass.h"
27 #include "llvm/Analysis/CaptureTracking.h"
28 #include "llvm/Analysis/MemoryBuiltins.h"
29 #include "llvm/Analysis/ValueTracking.h"
30 #include "llvm/Target/TargetData.h"
31 #include "llvm/ADT/SmallPtrSet.h"
32 #include "llvm/ADT/SmallVector.h"
33 #include "llvm/Support/ErrorHandling.h"
34 #include "llvm/Support/GetElementPtrTypeIterator.h"
35 #include <algorithm>
36 using namespace llvm;
37
38 //===----------------------------------------------------------------------===//
39 // Useful predicates
40 //===----------------------------------------------------------------------===//
41
42 /// isKnownNonNull - Return true if we know that the specified value is never
43 /// null.
isKnownNonNull(const Value * V)44 static bool isKnownNonNull(const Value *V) {
45 // Alloca never returns null, malloc might.
46 if (isa<AllocaInst>(V)) return true;
47
48 // A byval argument is never null.
49 if (const Argument *A = dyn_cast<Argument>(V))
50 return A->hasByValAttr();
51
52 // Global values are not null unless extern weak.
53 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
54 return !GV->hasExternalWeakLinkage();
55 return false;
56 }
57
58 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
59 /// object that never escapes from the function.
isNonEscapingLocalObject(const Value * V)60 static bool isNonEscapingLocalObject(const Value *V) {
61 // If this is a local allocation, check to see if it escapes.
62 if (isa<AllocaInst>(V) || isNoAliasCall(V))
63 // Set StoreCaptures to True so that we can assume in our callers that the
64 // pointer is not the result of a load instruction. Currently
65 // PointerMayBeCaptured doesn't have any special analysis for the
66 // StoreCaptures=false case; if it did, our callers could be refined to be
67 // more precise.
68 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
69
70 // If this is an argument that corresponds to a byval or noalias argument,
71 // then it has not escaped before entering the function. Check if it escapes
72 // inside the function.
73 if (const Argument *A = dyn_cast<Argument>(V))
74 if (A->hasByValAttr() || A->hasNoAliasAttr()) {
75 // Don't bother analyzing arguments already known not to escape.
76 if (A->hasNoCaptureAttr())
77 return true;
78 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
79 }
80 return false;
81 }
82
83 /// isEscapeSource - Return true if the pointer is one which would have
84 /// been considered an escape by isNonEscapingLocalObject.
isEscapeSource(const Value * V)85 static bool isEscapeSource(const Value *V) {
86 if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V))
87 return true;
88
89 // The load case works because isNonEscapingLocalObject considers all
90 // stores to be escapes (it passes true for the StoreCaptures argument
91 // to PointerMayBeCaptured).
92 if (isa<LoadInst>(V))
93 return true;
94
95 return false;
96 }
97
98 /// isObjectSmallerThan - Return true if we can prove that the object specified
99 /// by V is smaller than Size.
isObjectSmallerThan(const Value * V,unsigned Size,const TargetData & TD)100 static bool isObjectSmallerThan(const Value *V, unsigned Size,
101 const TargetData &TD) {
102 const Type *AccessTy;
103 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
104 AccessTy = GV->getType()->getElementType();
105 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
106 if (!AI->isArrayAllocation())
107 AccessTy = AI->getType()->getElementType();
108 else
109 return false;
110 } else if (const CallInst* CI = extractMallocCall(V)) {
111 if (!isArrayMalloc(V, &TD))
112 // The size is the argument to the malloc call.
113 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
114 return (C->getZExtValue() < Size);
115 return false;
116 } else if (const Argument *A = dyn_cast<Argument>(V)) {
117 if (A->hasByValAttr())
118 AccessTy = cast<PointerType>(A->getType())->getElementType();
119 else
120 return false;
121 } else {
122 return false;
123 }
124
125 if (AccessTy->isSized())
126 return TD.getTypeAllocSize(AccessTy) < Size;
127 return false;
128 }
129
130 //===----------------------------------------------------------------------===//
131 // NoAA Pass
132 //===----------------------------------------------------------------------===//
133
134 namespace {
135 /// NoAA - This class implements the -no-aa pass, which always returns "I
136 /// don't know" for alias queries. NoAA is unlike other alias analysis
137 /// implementations, in that it does not chain to a previous analysis. As
138 /// such it doesn't follow many of the rules that other alias analyses must.
139 ///
140 struct NoAA : public ImmutablePass, public AliasAnalysis {
141 static char ID; // Class identification, replacement for typeinfo
NoAA__anonbfd7c63f0111::NoAA142 NoAA() : ImmutablePass(ID) {}
NoAA__anonbfd7c63f0111::NoAA143 explicit NoAA(char &PID) : ImmutablePass(PID) { }
144
getAnalysisUsage__anonbfd7c63f0111::NoAA145 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
146 }
147
initializePass__anonbfd7c63f0111::NoAA148 virtual void initializePass() {
149 TD = getAnalysisIfAvailable<TargetData>();
150 }
151
alias__anonbfd7c63f0111::NoAA152 virtual AliasResult alias(const Value *V1, unsigned V1Size,
153 const Value *V2, unsigned V2Size) {
154 return MayAlias;
155 }
156
getModRefBehavior__anonbfd7c63f0111::NoAA157 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS) {
158 return UnknownModRefBehavior;
159 }
getModRefBehavior__anonbfd7c63f0111::NoAA160 virtual ModRefBehavior getModRefBehavior(const Function *F) {
161 return UnknownModRefBehavior;
162 }
163
pointsToConstantMemory__anonbfd7c63f0111::NoAA164 virtual bool pointsToConstantMemory(const Value *P) { return false; }
getModRefInfo__anonbfd7c63f0111::NoAA165 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
166 const Value *P, unsigned Size) {
167 return ModRef;
168 }
getModRefInfo__anonbfd7c63f0111::NoAA169 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
170 ImmutableCallSite CS2) {
171 return ModRef;
172 }
173
deleteValue__anonbfd7c63f0111::NoAA174 virtual void deleteValue(Value *V) {}
copyValue__anonbfd7c63f0111::NoAA175 virtual void copyValue(Value *From, Value *To) {}
176
177 /// getAdjustedAnalysisPointer - This method is used when a pass implements
178 /// an analysis interface through multiple inheritance. If needed, it
179 /// should override this to adjust the this pointer as needed for the
180 /// specified pass info.
getAdjustedAnalysisPointer__anonbfd7c63f0111::NoAA181 virtual void *getAdjustedAnalysisPointer(const void *ID) {
182 if (ID == &AliasAnalysis::ID)
183 return (AliasAnalysis*)this;
184 return this;
185 }
186 };
187 } // End of anonymous namespace
188
189 // Register this pass...
190 char NoAA::ID = 0;
191 INITIALIZE_AG_PASS(NoAA, AliasAnalysis, "no-aa",
192 "No Alias Analysis (always returns 'may' alias)",
193 true, true, false);
194
createNoAAPass()195 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
196
197 //===----------------------------------------------------------------------===//
198 // GetElementPtr Instruction Decomposition and Analysis
199 //===----------------------------------------------------------------------===//
200
201 namespace {
202 enum ExtensionKind {
203 EK_NotExtended,
204 EK_SignExt,
205 EK_ZeroExt
206 };
207
208 struct VariableGEPIndex {
209 const Value *V;
210 ExtensionKind Extension;
211 int64_t Scale;
212 };
213 }
214
215
216 /// GetLinearExpression - Analyze the specified value as a linear expression:
217 /// "A*V + B", where A and B are constant integers. Return the scale and offset
218 /// values as APInts and return V as a Value*, and return whether we looked
219 /// through any sign or zero extends. The incoming Value is known to have
220 /// IntegerType and it may already be sign or zero extended.
221 ///
222 /// Note that this looks through extends, so the high bits may not be
223 /// represented in the result.
GetLinearExpression(Value * V,APInt & Scale,APInt & Offset,ExtensionKind & Extension,const TargetData & TD,unsigned Depth)224 static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
225 ExtensionKind &Extension,
226 const TargetData &TD, unsigned Depth) {
227 assert(V->getType()->isIntegerTy() && "Not an integer value");
228
229 // Limit our recursion depth.
230 if (Depth == 6) {
231 Scale = 1;
232 Offset = 0;
233 return V;
234 }
235
236 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
237 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
238 switch (BOp->getOpcode()) {
239 default: break;
240 case Instruction::Or:
241 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't
242 // analyze it.
243 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &TD))
244 break;
245 // FALL THROUGH.
246 case Instruction::Add:
247 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
248 TD, Depth+1);
249 Offset += RHSC->getValue();
250 return V;
251 case Instruction::Mul:
252 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
253 TD, Depth+1);
254 Offset *= RHSC->getValue();
255 Scale *= RHSC->getValue();
256 return V;
257 case Instruction::Shl:
258 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
259 TD, Depth+1);
260 Offset <<= RHSC->getValue().getLimitedValue();
261 Scale <<= RHSC->getValue().getLimitedValue();
262 return V;
263 }
264 }
265 }
266
267 // Since GEP indices are sign extended anyway, we don't care about the high
268 // bits of a sign or zero extended value - just scales and offsets. The
269 // extensions have to be consistent though.
270 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) ||
271 (isa<ZExtInst>(V) && Extension != EK_SignExt)) {
272 Value *CastOp = cast<CastInst>(V)->getOperand(0);
273 unsigned OldWidth = Scale.getBitWidth();
274 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
275 Scale.trunc(SmallWidth);
276 Offset.trunc(SmallWidth);
277 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt;
278
279 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension,
280 TD, Depth+1);
281 Scale.zext(OldWidth);
282 Offset.zext(OldWidth);
283
284 return Result;
285 }
286
287 Scale = 1;
288 Offset = 0;
289 return V;
290 }
291
292 /// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it
293 /// into a base pointer with a constant offset and a number of scaled symbolic
294 /// offsets.
295 ///
296 /// The scaled symbolic offsets (represented by pairs of a Value* and a scale in
297 /// the VarIndices vector) are Value*'s that are known to be scaled by the
298 /// specified amount, but which may have other unrepresented high bits. As such,
299 /// the gep cannot necessarily be reconstructed from its decomposed form.
300 ///
301 /// When TargetData is around, this function is capable of analyzing everything
302 /// that Value::getUnderlyingObject() can look through. When not, it just looks
303 /// through pointer casts.
304 ///
305 static const Value *
DecomposeGEPExpression(const Value * V,int64_t & BaseOffs,SmallVectorImpl<VariableGEPIndex> & VarIndices,const TargetData * TD)306 DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
307 SmallVectorImpl<VariableGEPIndex> &VarIndices,
308 const TargetData *TD) {
309 // Limit recursion depth to limit compile time in crazy cases.
310 unsigned MaxLookup = 6;
311
312 BaseOffs = 0;
313 do {
314 // See if this is a bitcast or GEP.
315 const Operator *Op = dyn_cast<Operator>(V);
316 if (Op == 0) {
317 // The only non-operator case we can handle are GlobalAliases.
318 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
319 if (!GA->mayBeOverridden()) {
320 V = GA->getAliasee();
321 continue;
322 }
323 }
324 return V;
325 }
326
327 if (Op->getOpcode() == Instruction::BitCast) {
328 V = Op->getOperand(0);
329 continue;
330 }
331
332 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
333 if (GEPOp == 0)
334 return V;
335
336 // Don't attempt to analyze GEPs over unsized objects.
337 if (!cast<PointerType>(GEPOp->getOperand(0)->getType())
338 ->getElementType()->isSized())
339 return V;
340
341 // If we are lacking TargetData information, we can't compute the offets of
342 // elements computed by GEPs. However, we can handle bitcast equivalent
343 // GEPs.
344 if (TD == 0) {
345 if (!GEPOp->hasAllZeroIndices())
346 return V;
347 V = GEPOp->getOperand(0);
348 continue;
349 }
350
351 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
352 gep_type_iterator GTI = gep_type_begin(GEPOp);
353 for (User::const_op_iterator I = GEPOp->op_begin()+1,
354 E = GEPOp->op_end(); I != E; ++I) {
355 Value *Index = *I;
356 // Compute the (potentially symbolic) offset in bytes for this index.
357 if (const StructType *STy = dyn_cast<StructType>(*GTI++)) {
358 // For a struct, add the member offset.
359 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
360 if (FieldNo == 0) continue;
361
362 BaseOffs += TD->getStructLayout(STy)->getElementOffset(FieldNo);
363 continue;
364 }
365
366 // For an array/pointer, add the element offset, explicitly scaled.
367 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
368 if (CIdx->isZero()) continue;
369 BaseOffs += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue();
370 continue;
371 }
372
373 uint64_t Scale = TD->getTypeAllocSize(*GTI);
374 ExtensionKind Extension = EK_NotExtended;
375
376 // If the integer type is smaller than the pointer size, it is implicitly
377 // sign extended to pointer size.
378 unsigned Width = cast<IntegerType>(Index->getType())->getBitWidth();
379 if (TD->getPointerSizeInBits() > Width)
380 Extension = EK_SignExt;
381
382 // Use GetLinearExpression to decompose the index into a C1*V+C2 form.
383 APInt IndexScale(Width, 0), IndexOffset(Width, 0);
384 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension,
385 *TD, 0);
386
387 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
388 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
389 BaseOffs += IndexOffset.getZExtValue()*Scale;
390 Scale *= IndexScale.getZExtValue();
391
392
393 // If we already had an occurrance of this index variable, merge this
394 // scale into it. For example, we want to handle:
395 // A[x][x] -> x*16 + x*4 -> x*20
396 // This also ensures that 'x' only appears in the index list once.
397 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) {
398 if (VarIndices[i].V == Index &&
399 VarIndices[i].Extension == Extension) {
400 Scale += VarIndices[i].Scale;
401 VarIndices.erase(VarIndices.begin()+i);
402 break;
403 }
404 }
405
406 // Make sure that we have a scale that makes sense for this target's
407 // pointer size.
408 if (unsigned ShiftBits = 64-TD->getPointerSizeInBits()) {
409 Scale <<= ShiftBits;
410 Scale >>= ShiftBits;
411 }
412
413 if (Scale) {
414 VariableGEPIndex Entry = {Index, Extension,
415 static_cast<int64_t>(Scale)};
416 VarIndices.push_back(Entry);
417 }
418 }
419
420 // Analyze the base pointer next.
421 V = GEPOp->getOperand(0);
422 } while (--MaxLookup);
423
424 // If the chain of expressions is too deep, just return early.
425 return V;
426 }
427
428 /// GetIndexDifference - Dest and Src are the variable indices from two
429 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
430 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
431 /// difference between the two pointers.
GetIndexDifference(SmallVectorImpl<VariableGEPIndex> & Dest,const SmallVectorImpl<VariableGEPIndex> & Src)432 static void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
433 const SmallVectorImpl<VariableGEPIndex> &Src) {
434 if (Src.empty()) return;
435
436 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
437 const Value *V = Src[i].V;
438 ExtensionKind Extension = Src[i].Extension;
439 int64_t Scale = Src[i].Scale;
440
441 // Find V in Dest. This is N^2, but pointer indices almost never have more
442 // than a few variable indexes.
443 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
444 if (Dest[j].V != V || Dest[j].Extension != Extension) continue;
445
446 // If we found it, subtract off Scale V's from the entry in Dest. If it
447 // goes to zero, remove the entry.
448 if (Dest[j].Scale != Scale)
449 Dest[j].Scale -= Scale;
450 else
451 Dest.erase(Dest.begin()+j);
452 Scale = 0;
453 break;
454 }
455
456 // If we didn't consume this entry, add it to the end of the Dest list.
457 if (Scale) {
458 VariableGEPIndex Entry = { V, Extension, -Scale };
459 Dest.push_back(Entry);
460 }
461 }
462 }
463
464 //===----------------------------------------------------------------------===//
465 // BasicAliasAnalysis Pass
466 //===----------------------------------------------------------------------===//
467
468 #ifndef NDEBUG
getParent(const Value * V)469 static const Function *getParent(const Value *V) {
470 if (const Instruction *inst = dyn_cast<Instruction>(V))
471 return inst->getParent()->getParent();
472
473 if (const Argument *arg = dyn_cast<Argument>(V))
474 return arg->getParent();
475
476 return NULL;
477 }
478
notDifferentParent(const Value * O1,const Value * O2)479 static bool notDifferentParent(const Value *O1, const Value *O2) {
480
481 const Function *F1 = getParent(O1);
482 const Function *F2 = getParent(O2);
483
484 return !F1 || !F2 || F1 == F2;
485 }
486 #endif
487
488 namespace {
489 /// BasicAliasAnalysis - This is the default alias analysis implementation.
490 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
491 /// derives from the NoAA class.
492 struct BasicAliasAnalysis : public NoAA {
493 static char ID; // Class identification, replacement for typeinfo
BasicAliasAnalysis__anonbfd7c63f0311::BasicAliasAnalysis494 BasicAliasAnalysis() : NoAA(ID) {}
495
alias__anonbfd7c63f0311::BasicAliasAnalysis496 virtual AliasResult alias(const Value *V1, unsigned V1Size,
497 const Value *V2, unsigned V2Size) {
498 assert(Visited.empty() && "Visited must be cleared after use!");
499 assert(notDifferentParent(V1, V2) &&
500 "BasicAliasAnalysis doesn't support interprocedural queries.");
501 AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size);
502 Visited.clear();
503 return Alias;
504 }
505
506 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
507 const Value *P, unsigned Size);
508
getModRefInfo__anonbfd7c63f0311::BasicAliasAnalysis509 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
510 ImmutableCallSite CS2) {
511 // The AliasAnalysis base class has some smarts, lets use them.
512 return AliasAnalysis::getModRefInfo(CS1, CS2);
513 }
514
515 /// pointsToConstantMemory - Chase pointers until we find a (constant
516 /// global) or not.
517 virtual bool pointsToConstantMemory(const Value *P);
518
519 /// getModRefBehavior - Return the behavior when calling the given
520 /// call site.
521 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
522
523 /// getModRefBehavior - Return the behavior when calling the given function.
524 /// For use when the call site is not known.
525 virtual ModRefBehavior getModRefBehavior(const Function *F);
526
527 /// getAdjustedAnalysisPointer - This method is used when a pass implements
528 /// an analysis interface through multiple inheritance. If needed, it
529 /// should override this to adjust the this pointer as needed for the
530 /// specified pass info.
getAdjustedAnalysisPointer__anonbfd7c63f0311::BasicAliasAnalysis531 virtual void *getAdjustedAnalysisPointer(const void *ID) {
532 if (ID == &AliasAnalysis::ID)
533 return (AliasAnalysis*)this;
534 return this;
535 }
536
537 private:
538 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
539 SmallPtrSet<const Value*, 16> Visited;
540
541 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
542 // instruction against another.
543 AliasResult aliasGEP(const GEPOperator *V1, unsigned V1Size,
544 const Value *V2, unsigned V2Size,
545 const Value *UnderlyingV1, const Value *UnderlyingV2);
546
547 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
548 // instruction against another.
549 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize,
550 const Value *V2, unsigned V2Size);
551
552 /// aliasSelect - Disambiguate a Select instruction against another value.
553 AliasResult aliasSelect(const SelectInst *SI, unsigned SISize,
554 const Value *V2, unsigned V2Size);
555
556 AliasResult aliasCheck(const Value *V1, unsigned V1Size,
557 const Value *V2, unsigned V2Size);
558 };
559 } // End of anonymous namespace
560
561 // Register this pass...
562 char BasicAliasAnalysis::ID = 0;
563 INITIALIZE_AG_PASS(BasicAliasAnalysis, AliasAnalysis, "basicaa",
564 "Basic Alias Analysis (default AA impl)",
565 false, true, true);
566
createBasicAliasAnalysisPass()567 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
568 return new BasicAliasAnalysis();
569 }
570
571
572 /// pointsToConstantMemory - Chase pointers until we find a (constant
573 /// global) or not.
pointsToConstantMemory(const Value * P)574 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
575 if (const GlobalVariable *GV =
576 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
577 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
578 // global to be marked constant in some modules and non-constant in others.
579 // GV may even be a declaration, not a definition.
580 return GV->isConstant();
581
582 return NoAA::pointsToConstantMemory(P);
583 }
584
585 /// getModRefBehavior - Return the behavior when calling the given call site.
586 AliasAnalysis::ModRefBehavior
getModRefBehavior(ImmutableCallSite CS)587 BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
588 if (CS.doesNotAccessMemory())
589 // Can't do better than this.
590 return DoesNotAccessMemory;
591
592 ModRefBehavior Min = UnknownModRefBehavior;
593
594 // If the callsite knows it only reads memory, don't return worse
595 // than that.
596 if (CS.onlyReadsMemory())
597 Min = OnlyReadsMemory;
598
599 // The AliasAnalysis base class has some smarts, lets use them.
600 return std::min(AliasAnalysis::getModRefBehavior(CS), Min);
601 }
602
603 /// getModRefBehavior - Return the behavior when calling the given function.
604 /// For use when the call site is not known.
605 AliasAnalysis::ModRefBehavior
getModRefBehavior(const Function * F)606 BasicAliasAnalysis::getModRefBehavior(const Function *F) {
607 if (F->doesNotAccessMemory())
608 // Can't do better than this.
609 return DoesNotAccessMemory;
610 if (F->onlyReadsMemory())
611 return OnlyReadsMemory;
612 if (unsigned id = F->getIntrinsicID())
613 return getIntrinsicModRefBehavior(id);
614
615 return NoAA::getModRefBehavior(F);
616 }
617
618 /// getModRefInfo - Check to see if the specified callsite can clobber the
619 /// specified memory object. Since we only look at local properties of this
620 /// function, we really can't say much about this query. We do, however, use
621 /// simple "address taken" analysis on local objects.
622 AliasAnalysis::ModRefResult
getModRefInfo(ImmutableCallSite CS,const Value * P,unsigned Size)623 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
624 const Value *P, unsigned Size) {
625 assert(notDifferentParent(CS.getInstruction(), P) &&
626 "AliasAnalysis query involving multiple functions!");
627
628 const Value *Object = P->getUnderlyingObject();
629
630 // If this is a tail call and P points to a stack location, we know that
631 // the tail call cannot access or modify the local stack.
632 // We cannot exclude byval arguments here; these belong to the caller of
633 // the current function not to the current function, and a tail callee
634 // may reference them.
635 if (isa<AllocaInst>(Object))
636 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
637 if (CI->isTailCall())
638 return NoModRef;
639
640 // If the pointer is to a locally allocated object that does not escape,
641 // then the call can not mod/ref the pointer unless the call takes the pointer
642 // as an argument, and itself doesn't capture it.
643 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
644 isNonEscapingLocalObject(Object)) {
645 bool PassedAsArg = false;
646 unsigned ArgNo = 0;
647 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
648 CI != CE; ++CI, ++ArgNo) {
649 // Only look at the no-capture pointer arguments.
650 if (!(*CI)->getType()->isPointerTy() ||
651 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
652 continue;
653
654 // If this is a no-capture pointer argument, see if we can tell that it
655 // is impossible to alias the pointer we're checking. If not, we have to
656 // assume that the call could touch the pointer, even though it doesn't
657 // escape.
658 if (!isNoAlias(cast<Value>(CI), UnknownSize, P, UnknownSize)) {
659 PassedAsArg = true;
660 break;
661 }
662 }
663
664 if (!PassedAsArg)
665 return NoModRef;
666 }
667
668 // Finally, handle specific knowledge of intrinsics.
669 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
670 if (II != 0)
671 switch (II->getIntrinsicID()) {
672 default: break;
673 case Intrinsic::memcpy:
674 case Intrinsic::memmove: {
675 unsigned Len = UnknownSize;
676 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
677 Len = LenCI->getZExtValue();
678 Value *Dest = II->getArgOperand(0);
679 Value *Src = II->getArgOperand(1);
680 if (isNoAlias(Dest, Len, P, Size)) {
681 if (isNoAlias(Src, Len, P, Size))
682 return NoModRef;
683 return Ref;
684 }
685 break;
686 }
687 case Intrinsic::memset:
688 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
689 // will handle it for the variable length case.
690 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
691 unsigned Len = LenCI->getZExtValue();
692 Value *Dest = II->getArgOperand(0);
693 if (isNoAlias(Dest, Len, P, Size))
694 return NoModRef;
695 }
696 break;
697 case Intrinsic::atomic_cmp_swap:
698 case Intrinsic::atomic_swap:
699 case Intrinsic::atomic_load_add:
700 case Intrinsic::atomic_load_sub:
701 case Intrinsic::atomic_load_and:
702 case Intrinsic::atomic_load_nand:
703 case Intrinsic::atomic_load_or:
704 case Intrinsic::atomic_load_xor:
705 case Intrinsic::atomic_load_max:
706 case Intrinsic::atomic_load_min:
707 case Intrinsic::atomic_load_umax:
708 case Intrinsic::atomic_load_umin:
709 if (TD) {
710 Value *Op1 = II->getArgOperand(0);
711 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
712 if (isNoAlias(Op1, Op1Size, P, Size))
713 return NoModRef;
714 }
715 break;
716 case Intrinsic::lifetime_start:
717 case Intrinsic::lifetime_end:
718 case Intrinsic::invariant_start: {
719 unsigned PtrSize =
720 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
721 if (isNoAlias(II->getArgOperand(1), PtrSize, P, Size))
722 return NoModRef;
723 break;
724 }
725 case Intrinsic::invariant_end: {
726 unsigned PtrSize =
727 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
728 if (isNoAlias(II->getArgOperand(2), PtrSize, P, Size))
729 return NoModRef;
730 break;
731 }
732 }
733
734 // The AliasAnalysis base class has some smarts, lets use them.
735 return AliasAnalysis::getModRefInfo(CS, P, Size);
736 }
737
738
739 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
740 /// against another pointer. We know that V1 is a GEP, but we don't know
741 /// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(),
742 /// UnderlyingV2 is the same for V2.
743 ///
744 AliasAnalysis::AliasResult
aliasGEP(const GEPOperator * GEP1,unsigned V1Size,const Value * V2,unsigned V2Size,const Value * UnderlyingV1,const Value * UnderlyingV2)745 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size,
746 const Value *V2, unsigned V2Size,
747 const Value *UnderlyingV1,
748 const Value *UnderlyingV2) {
749 // If this GEP has been visited before, we're on a use-def cycle.
750 // Such cycles are only valid when PHI nodes are involved or in unreachable
751 // code. The visitPHI function catches cycles containing PHIs, but there
752 // could still be a cycle without PHIs in unreachable code.
753 if (!Visited.insert(GEP1))
754 return MayAlias;
755
756 int64_t GEP1BaseOffset;
757 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
758
759 // If we have two gep instructions with must-alias'ing base pointers, figure
760 // out if the indexes to the GEP tell us anything about the derived pointer.
761 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
762 // Do the base pointers alias?
763 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize,
764 UnderlyingV2, UnknownSize);
765
766 // If we get a No or May, then return it immediately, no amount of analysis
767 // will improve this situation.
768 if (BaseAlias != MustAlias) return BaseAlias;
769
770 // Otherwise, we have a MustAlias. Since the base pointers alias each other
771 // exactly, see if the computed offset from the common pointer tells us
772 // about the relation of the resulting pointer.
773 const Value *GEP1BasePtr =
774 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
775
776 int64_t GEP2BaseOffset;
777 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
778 const Value *GEP2BasePtr =
779 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
780
781 // If DecomposeGEPExpression isn't able to look all the way through the
782 // addressing operation, we must not have TD and this is too complex for us
783 // to handle without it.
784 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
785 assert(TD == 0 &&
786 "DecomposeGEPExpression and getUnderlyingObject disagree!");
787 return MayAlias;
788 }
789
790 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
791 // symbolic difference.
792 GEP1BaseOffset -= GEP2BaseOffset;
793 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
794
795 } else {
796 // Check to see if these two pointers are related by the getelementptr
797 // instruction. If one pointer is a GEP with a non-zero index of the other
798 // pointer, we know they cannot alias.
799
800 // If both accesses are unknown size, we can't do anything useful here.
801 if (V1Size == UnknownSize && V2Size == UnknownSize)
802 return MayAlias;
803
804 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, V2, V2Size);
805 if (R != MustAlias)
806 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
807 // If V2 is known not to alias GEP base pointer, then the two values
808 // cannot alias per GEP semantics: "A pointer value formed from a
809 // getelementptr instruction is associated with the addresses associated
810 // with the first operand of the getelementptr".
811 return R;
812
813 const Value *GEP1BasePtr =
814 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
815
816 // If DecomposeGEPExpression isn't able to look all the way through the
817 // addressing operation, we must not have TD and this is too complex for us
818 // to handle without it.
819 if (GEP1BasePtr != UnderlyingV1) {
820 assert(TD == 0 &&
821 "DecomposeGEPExpression and getUnderlyingObject disagree!");
822 return MayAlias;
823 }
824 }
825
826 // In the two GEP Case, if there is no difference in the offsets of the
827 // computed pointers, the resultant pointers are a must alias. This
828 // hapens when we have two lexically identical GEP's (for example).
829 //
830 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
831 // must aliases the GEP, the end result is a must alias also.
832 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
833 return MustAlias;
834
835 // If we have a known constant offset, see if this offset is larger than the
836 // access size being queried. If so, and if no variable indices can remove
837 // pieces of this constant, then we know we have a no-alias. For example,
838 // &A[100] != &A.
839
840 // In order to handle cases like &A[100][i] where i is an out of range
841 // subscript, we have to ignore all constant offset pieces that are a multiple
842 // of a scaled index. Do this by removing constant offsets that are a
843 // multiple of any of our variable indices. This allows us to transform
844 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
845 // provides an offset of 4 bytes (assuming a <= 4 byte access).
846 for (unsigned i = 0, e = GEP1VariableIndices.size();
847 i != e && GEP1BaseOffset;++i)
848 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].Scale)
849 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].Scale;
850
851 // If our known offset is bigger than the access size, we know we don't have
852 // an alias.
853 if (GEP1BaseOffset) {
854 if (GEP1BaseOffset >= (int64_t)V2Size ||
855 GEP1BaseOffset <= -(int64_t)V1Size)
856 return NoAlias;
857 }
858
859 return MayAlias;
860 }
861
862 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
863 /// instruction against another.
864 AliasAnalysis::AliasResult
aliasSelect(const SelectInst * SI,unsigned SISize,const Value * V2,unsigned V2Size)865 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize,
866 const Value *V2, unsigned V2Size) {
867 // If this select has been visited before, we're on a use-def cycle.
868 // Such cycles are only valid when PHI nodes are involved or in unreachable
869 // code. The visitPHI function catches cycles containing PHIs, but there
870 // could still be a cycle without PHIs in unreachable code.
871 if (!Visited.insert(SI))
872 return MayAlias;
873
874 // If the values are Selects with the same condition, we can do a more precise
875 // check: just check for aliases between the values on corresponding arms.
876 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
877 if (SI->getCondition() == SI2->getCondition()) {
878 AliasResult Alias =
879 aliasCheck(SI->getTrueValue(), SISize,
880 SI2->getTrueValue(), V2Size);
881 if (Alias == MayAlias)
882 return MayAlias;
883 AliasResult ThisAlias =
884 aliasCheck(SI->getFalseValue(), SISize,
885 SI2->getFalseValue(), V2Size);
886 if (ThisAlias != Alias)
887 return MayAlias;
888 return Alias;
889 }
890
891 // If both arms of the Select node NoAlias or MustAlias V2, then returns
892 // NoAlias / MustAlias. Otherwise, returns MayAlias.
893 AliasResult Alias =
894 aliasCheck(V2, V2Size, SI->getTrueValue(), SISize);
895 if (Alias == MayAlias)
896 return MayAlias;
897
898 // If V2 is visited, the recursive case will have been caught in the
899 // above aliasCheck call, so these subsequent calls to aliasCheck
900 // don't need to assume that V2 is being visited recursively.
901 Visited.erase(V2);
902
903 AliasResult ThisAlias =
904 aliasCheck(V2, V2Size, SI->getFalseValue(), SISize);
905 if (ThisAlias != Alias)
906 return MayAlias;
907 return Alias;
908 }
909
910 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
911 // against another.
912 AliasAnalysis::AliasResult
aliasPHI(const PHINode * PN,unsigned PNSize,const Value * V2,unsigned V2Size)913 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
914 const Value *V2, unsigned V2Size) {
915 // The PHI node has already been visited, avoid recursion any further.
916 if (!Visited.insert(PN))
917 return MayAlias;
918
919 // If the values are PHIs in the same block, we can do a more precise
920 // as well as efficient check: just check for aliases between the values
921 // on corresponding edges.
922 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
923 if (PN2->getParent() == PN->getParent()) {
924 AliasResult Alias =
925 aliasCheck(PN->getIncomingValue(0), PNSize,
926 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
927 V2Size);
928 if (Alias == MayAlias)
929 return MayAlias;
930 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
931 AliasResult ThisAlias =
932 aliasCheck(PN->getIncomingValue(i), PNSize,
933 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
934 V2Size);
935 if (ThisAlias != Alias)
936 return MayAlias;
937 }
938 return Alias;
939 }
940
941 SmallPtrSet<Value*, 4> UniqueSrc;
942 SmallVector<Value*, 4> V1Srcs;
943 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
944 Value *PV1 = PN->getIncomingValue(i);
945 if (isa<PHINode>(PV1))
946 // If any of the source itself is a PHI, return MayAlias conservatively
947 // to avoid compile time explosion. The worst possible case is if both
948 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
949 // and 'n' are the number of PHI sources.
950 return MayAlias;
951 if (UniqueSrc.insert(PV1))
952 V1Srcs.push_back(PV1);
953 }
954
955 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize);
956 // Early exit if the check of the first PHI source against V2 is MayAlias.
957 // Other results are not possible.
958 if (Alias == MayAlias)
959 return MayAlias;
960
961 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
962 // NoAlias / MustAlias. Otherwise, returns MayAlias.
963 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
964 Value *V = V1Srcs[i];
965
966 // If V2 is visited, the recursive case will have been caught in the
967 // above aliasCheck call, so these subsequent calls to aliasCheck
968 // don't need to assume that V2 is being visited recursively.
969 Visited.erase(V2);
970
971 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
972 if (ThisAlias != Alias || ThisAlias == MayAlias)
973 return MayAlias;
974 }
975
976 return Alias;
977 }
978
979 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
980 // such as array references.
981 //
982 AliasAnalysis::AliasResult
aliasCheck(const Value * V1,unsigned V1Size,const Value * V2,unsigned V2Size)983 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
984 const Value *V2, unsigned V2Size) {
985 // If either of the memory references is empty, it doesn't matter what the
986 // pointer values are.
987 if (V1Size == 0 || V2Size == 0)
988 return NoAlias;
989
990 // Strip off any casts if they exist.
991 V1 = V1->stripPointerCasts();
992 V2 = V2->stripPointerCasts();
993
994 // Are we checking for alias of the same value?
995 if (V1 == V2) return MustAlias;
996
997 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
998 return NoAlias; // Scalars cannot alias each other
999
1000 // Figure out what objects these things are pointing to if we can.
1001 const Value *O1 = V1->getUnderlyingObject();
1002 const Value *O2 = V2->getUnderlyingObject();
1003
1004 // Null values in the default address space don't point to any object, so they
1005 // don't alias any other pointer.
1006 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
1007 if (CPN->getType()->getAddressSpace() == 0)
1008 return NoAlias;
1009 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
1010 if (CPN->getType()->getAddressSpace() == 0)
1011 return NoAlias;
1012
1013 if (O1 != O2) {
1014 // If V1/V2 point to two different objects we know that we have no alias.
1015 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
1016 return NoAlias;
1017
1018 // Constant pointers can't alias with non-const isIdentifiedObject objects.
1019 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
1020 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
1021 return NoAlias;
1022
1023 // Arguments can't alias with local allocations or noalias calls
1024 // in the same function.
1025 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
1026 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))))
1027 return NoAlias;
1028
1029 // Most objects can't alias null.
1030 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
1031 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
1032 return NoAlias;
1033
1034 // If one pointer is the result of a call/invoke or load and the other is a
1035 // non-escaping local object within the same function, then we know the
1036 // object couldn't escape to a point where the call could return it.
1037 //
1038 // Note that if the pointers are in different functions, there are a
1039 // variety of complications. A call with a nocapture argument may still
1040 // temporary store the nocapture argument's value in a temporary memory
1041 // location if that memory location doesn't escape. Or it may pass a
1042 // nocapture value to other functions as long as they don't capture it.
1043 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
1044 return NoAlias;
1045 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
1046 return NoAlias;
1047 }
1048
1049 // If the size of one access is larger than the entire object on the other
1050 // side, then we know such behavior is undefined and can assume no alias.
1051 if (TD)
1052 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) ||
1053 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD)))
1054 return NoAlias;
1055
1056 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
1057 // GEP can't simplify, we don't even look at the PHI cases.
1058 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
1059 std::swap(V1, V2);
1060 std::swap(V1Size, V2Size);
1061 std::swap(O1, O2);
1062 }
1063 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1))
1064 return aliasGEP(GV1, V1Size, V2, V2Size, O1, O2);
1065
1066 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
1067 std::swap(V1, V2);
1068 std::swap(V1Size, V2Size);
1069 }
1070 if (const PHINode *PN = dyn_cast<PHINode>(V1))
1071 return aliasPHI(PN, V1Size, V2, V2Size);
1072
1073 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
1074 std::swap(V1, V2);
1075 std::swap(V1Size, V2Size);
1076 }
1077 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1))
1078 return aliasSelect(S1, V1Size, V2, V2Size);
1079
1080 return NoAA::alias(V1, V1Size, V2, V2Size);
1081 }
1082
1083 // Make sure that anything that uses AliasAnalysis pulls in this file.
1084 DEFINING_FILE_FOR(BasicAliasAnalysis)
1085